EP4368736A1 - Hot-rolled steel sheet - Google Patents

Hot-rolled steel sheet Download PDF

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Publication number
EP4368736A1
EP4368736A1 EP22837230.6A EP22837230A EP4368736A1 EP 4368736 A1 EP4368736 A1 EP 4368736A1 EP 22837230 A EP22837230 A EP 22837230A EP 4368736 A1 EP4368736 A1 EP 4368736A1
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EP
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Prior art keywords
hot
less
present
steel sheet
rolled steel
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EP22837230.6A
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German (de)
French (fr)
Inventor
Shunsuke Kobayashi
Hiroshi Shuto
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Nippon Steel Corp
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Nippon Steel Corp
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Publication of EP4368736A1 publication Critical patent/EP4368736A1/en
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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
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties 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 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
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C21D2211/002—Bainite
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a hot-rolled steel sheet, and specifically, to a hot-rolled steel sheet that is used after being molded into various shapes by press processing or the like, and particularly, to a hot-rolled steel sheet having high strength and little deterioration in the crack arresting property after plastic deformation.
  • Non-Patent Document 1 describes that plastic deformation becomes more difficult as the strength of the steel sheet increases, and the crack arresting property generally deteriorates.
  • Patent Document 1 discloses a steel sheet for large structures having an excellent crack arresting property after plastic deformation by strictly controlling impurity elements and also setting the ferrite grain size in the surface layer to 3 ⁇ m or less.
  • Patent Document 2 discloses a steel sheet for large structures having an excellent crack arresting property after plastic deformation wherein the steel sheet has a structure containing ferrite crystal grains having a flatness of 2 or more and a minor axis diameter of 5 ⁇ m or less and subgrains having an equivalent circle diameter of 3 ⁇ m or less in the ferrite crystals.
  • Non-Patent Document 1 Yuzo Takahashi, Osamu Kono, Kosaku Ushioda, Shuji Awaihara: Iron and Steel, 99, (2013), 4, 312-321
  • Patent Documents 1 and 2 are both techniques related to a steel sheet for large structures and are not intended for hot-rolled steel sheets. In addition, both have a structure design mainly composed of a ferrite structure, and the steel sheet has a strength of 450 to 700 MPa, and thus it may be difficult to apply the techniques disclosed in Patent Documents 1 and 2 to high-strength hot-rolled steel sheets of 980 MPa or more, which are mainly composed of bainite and martensite.
  • the present invention has been made in view of the above circumstances in the related art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and little deterioration in the crack arresting property after plastic deformation.
  • the gist of the present invention made based on the above findings is as follows.
  • the hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile members, mechanical structural members and also building members.
  • the hot-rolled steel sheet according to the present embodiment contains, in mass%, C: 0.040 to 0.400%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, and O: 0.0100% or less, with the remainder: Fe and impurities.
  • C 0.040 to 0.400%
  • Si 0.05 to 3.00%
  • Mn 1.00 to 4.00%
  • Al 0.001 to 0.500%
  • P 0.100% or less
  • S 0.0300% or less
  • N 0.1000% or less
  • O 0.0100% or less
  • the C increases the fraction of the hard phase and lowers the transformation point of the hard phase, and thus increases the strength of the hot-rolled steel sheet. If the C content is less than 0.040%, it becomes difficult to obtain a desired strength. Therefore, the C content is 0.040% or more.
  • the C content is preferably 0.060% or more, more preferably 0.070% or more, and still more preferably 0.080% or more.
  • the C content is 0.400% or less.
  • the C content is preferably 0.300% or less, more preferably 0.250% or less, and still more preferably 0.150% or less.
  • Si has a function of solid-solution strengthening at room temperature and increasing the strength of the hot-rolled steel sheet and a function of solid-solution softening at a low temperature and improving the toughness of the hot-rolled steel sheet.
  • Si has a function of minimizing flaws in steel (minimizing the occurrence of defects such as blowholes in steel) by deacidification. If the Si content is less than 0.05%, it is not possible to obtain the effect of the above function. Therefore, the Si content is 0.05% or more.
  • the Si content is preferably 0.50% or more, and more preferably 0.80% or more.
  • the Si content is 3.00% or less.
  • the Si content is preferably 2.70% or less and more preferably 2.50% or less.
  • Mn has a function of inhibiting ferrite transformation and increasing the strength of the hot-rolled steel sheet and a function of solid-solution softening at a low temperature and improving the toughness of the hot-rolled steel sheet. If the Mn content is less than 1.00%, it is not possible to obtain a desired tensile strength. Therefore, the Mn content is 1.00% or more.
  • the Mn content is preferably 1.30% or more and more preferably 1.50% or more.
  • the Mn content is 4.00% or less.
  • the Mn content is preferably 3.70% or less, and more preferably 3.50% or less.
  • Al has a function of deacidifying steel and minimizing flaws in steel and a function of exhibiting solid-solution softening at a low temperature and increasing the toughness of the hot-rolled steel sheet. If the sol. Al content is less than 0.001%, it is not possible to obtain the effect of the above function. In addition, if the sol. Al content is less than 0.001%, it is not possible to obtain a desired Rcf value. Therefore, the sol. Al content is 0.001% or more. The sol. Al content is preferably 0.010% or more.
  • the sol. Al content is more than 0.500%, since the above effect is maximized and this is economically unfavorable, the sol. Al content is 0.500% or less.
  • the sol. Al content is preferably 0.300% or less, and more preferably 0.100% or less.
  • sol. Al is acid-soluble Al, and indicates solid solution Al present in steel in a solid solution state.
  • P is an element that is generally contained as an impurity, and is an element having a function of increasing the strength of the hot-rolled steel sheet according to solid-solution strengthening. Therefore, P may be actively contained, but P is an element that easily segregates, and if the P content is more than 0.100%, the grain boundary strength decreases significantly due to grain boundary segregation, and grain boundary fracture is likely to occur. Therefore, the P content is 0.100% or less.
  • the P content is preferably 0.030% or less.
  • the lower limit of the P content is preferably 0.001% or more in consideration of refining costs.
  • S is an element contained as an impurity, and forms a sulfide-based inclusion in steel and promotes the occurrence of cracks. If the S content is more than 0.0300%, the occurrence of cracks during plastic deformation becomes significant, and the crack arresting property after plastic deformation significantly deteriorates. Therefore, the S content is 0.0300% or less. The S content is preferably 0.0050% or less.
  • the lower limit of the S content is preferably 0.0001% or more in consideration of refining costs.
  • N is an element that is contained in steel as an impurity and has a function of promoting the occurrence of cracks starting from impurities. If the N content is more than 0.1000%, the occurrence of cracks during plastic deformation becomes significant, and the crack arresting property after plastic deformation significantly deteriorates. Therefore, the N content is 0.1000% or less.
  • the N content is preferably 0.0800% or less, more preferably 0.0700% or less, and still more preferably 0.0100% or less.
  • the lower limit of the N content may be 0.0001% or more.
  • the N content is preferably 0.0010% or more and more preferably 0.0020% or more.
  • the O content is 0.0100% or less.
  • the O content is preferably 0.0080% or less, and more preferably 0.0050% or less.
  • the O content may be 0.0005% or more, or 0.0010% or more.
  • the remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be composed of Fe and impurities.
  • impurities are elements that are mixed in from ores or scraps as raw materials or a production environment or the like and/or are allowable as long as they do not adversely affect the hot-rolled steel sheet according to the present embodiment.
  • the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements in place of some Fe.
  • the lower limit of the content when the optional elements are not contained is 0%.
  • the optional elements will be described in detail.
  • Ti, Nb and V are elements that precipitate finely in steel as carbides and nitrides and improve the strength of steel according to precipitation strengthening. Therefore, one, two or more of these elements may be contained. In order to obtain this effect more reliably, each content of Ti, Nb and V is preferably 0.010% or more. Here, it is not necessary to contain all of Ti, Nb and V, and any one of them may have a content of 0.010% or more. Each content of Ti, Nb and V is preferably 0.060% or more, and more preferably 0.080% or more.
  • each content of Ti, Nb and V is 1.000% or less, preferably 0.800% or less and more preferably 0.500% or less.
  • Cu, Cr, Mo, Ni and B all have a function of improving hardenability of the hot-rolled steel sheet.
  • Ni has a function of effectively minimizing grain boundary cracks of the slab caused by Cu. Therefore, one, two or more of these elements may be contained.
  • the Cu content is preferably 0.01% or more and more preferably 0.05% or more.
  • the Cu content is 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 0.01% or more and more preferably 0.05% or more.
  • the Cr content is 2.00% or less.
  • Mo has a function of improving hardenability of the hot-rolled steel sheet and a function of precipitating in steel as carbides and increasing the strength of the hot-rolled steel sheet.
  • the Mo content is preferably 0.01% or more and more preferably 0.02% or more.
  • the Mo content is 1.00% or less.
  • the Mo content is preferably 0.50% or less and more preferably 0.20% or less.
  • Ni has a function of improving hardenability of the hot-rolled steel sheet.
  • Ni has a function of effectively minimizing grain boundary cracks of the slab caused by Cu.
  • the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically unfavorable to contain a large amount of Ni. Therefore, the Ni content is 2.00% or less.
  • B has a function of improving hardenability of the hot-rolled steel sheet.
  • the B content is preferably 0.0001% or more and more preferably 0.0002% or more.
  • the B content is 0.0100% or less.
  • the B content is preferably 0.0050% or less.
  • Ca, Mg and REMs all have a function of improving the crack arresting property of the hot-rolled steel sheet by adjusting the shape of inclusions in steel to a preferable shape.
  • Bi has a function of improving the crack arresting property of the hot-rolled steel sheet according to refining of the solidification structure. Therefore, one, two or more of these elements may be contained.
  • the content of any one or more of Ca, Mg, REM and Bi is preferably 0.0005% or more.
  • the Ca content or the Mg content is more than 0.0200% or the REM content is more than 0.1000%, inclusions are excessively formed in steel, and thus the crack arresting property of the hot-rolled steel sheet may deteriorate.
  • the Ca content and the Mg content are 0.0200% or less
  • the REM content is 0.1000% or less
  • the Bi content is 0.020% or less.
  • the Bi content is preferably 0.010% or less.
  • REM refers to a total of 17 elements composed of Sc, Y and lanthanides, and the REM content refers to a total amount of these elements.
  • lanthanides they are industrially added in the form of misch metals.
  • the inventors confirmed that, even if a total amount of 1.00% or less of these elements is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. Therefore, a total amount of 1.00% or less of one, two or more of Zr, Co, Zn and W may be contained.
  • the inventors confirmed that, even if a small amount of Sn is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. However, if a large amount of Sn is contained, flaws may occur during hot rolling and thus the Sn content is 0.05% or less.
  • the chemical composition of the hot-rolled steel sheet described above may be measured by a general analysis method.
  • ICP-AES inductively coupled plasmaatomic emission spectrometry
  • sol. Al may be measured using a filtrate after thermally decomposing a sample with an acid through ICP-AES.
  • 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-nondispersive infrared absorption method.
  • the microstructure contains, in area%, less than 3.00% of retained austenite, and the Rcf value indicating a ratio between average fracture surface units before and after plastic deformation is 2.0 or more.
  • the hot-rolled steel sheet according to the present embodiment can have high strength and an excellent crack arresting property after plastic deformation.
  • the structure fraction and the Rcf value in the microstructure at a depth of 1/4 of the sheet thickness from the surface are specified.
  • the reason for this is that the microstructure at that position is a typical microstructure of the steel sheet.
  • Retained austenite is a microstructure that is present as fcc at room temperature. Retained austenite has concentrated carbon in the surrounding structure, transforms to hard martensite during plastic deformation, and thus can become a starting point for the occurrence of cracks. If the area proportion of retained austenite is 3.00% or more, the above function becomes apparent, and the crack arresting property after plastic deformation significantly deteriorates. Therefore, the area proportion of retained austenite is less than 3.00%.
  • the area proportion of retained austenite is preferably 2.00% or less, less than 1.50% or 1.00% or less, and more preferably less than 1.00% or less than 0.50%. Since it is preferable that the amount of retained austenite be as small as possible, the area proportion of retained austenite may be 0.00%.
  • Methods of measuring the area proportion of retained austenite include X-ray diffraction, electron back scattering diffraction pattern (EBSP) analysis, and magnetic measurement methods.
  • the area proportion of retained austenite is measured by X-ray diffraction.
  • the microstructure of the hot-rolled steel sheet according to the present embodiment may contain ferrite, martensite, bainite and pearlite in addition to retained austenite.
  • the area proportion of ferrite may be 60.00% or less, 50.00% or less, or 45.00% or less. In addition, the area proportion of ferrite may be 0.00% or more, or 0.05% or more.
  • the area proportion of martensite may be 100.00% or less, or 99.00% or less. In addition, the area proportion of martensite may be 0.00% or more, 1.00% or more, or 1.50% or more.
  • the area proportion of bainite may be 100.00% or less, or 96.00% or less. In addition, the area proportion of bainite may be 0.00% or more, or 0.01% or more.
  • the area proportions of the above retained austenite, ferrite, bainite and martensite are area proportions that apply to all three steel types to be described below (DP steel, bainite steel and martensite steel).
  • the microstructure contains, in area proportion, less than 3.00% of retained austenite, and ferrite, martensite and a very small amount of the residual structure, preferably, the area proportion of ferrite is 15.00 to 60.00%, the area proportion of martensite is 40.00 to 85.00%, and the area proportion of the residual structure is less than 45.00%. If the area proportion of each structure is set as above, it is possible to improve the strength and ductility in a well-balanced manner.
  • Ferrite is a structure that is formed when fcc transforms to bcc at a relatively high temperature. Since ferrite has a high processing hardening rate, it has a function of improving strength-ductility balance of the hot-rolled steel sheet. If the area proportion of ferrite is 15.00% or more, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of ferrite is preferably 15.00% or more. The area proportion of ferrite is more preferably 20.00% or more, 25.00% or more, or 30.00% or more.
  • the area proportion of ferrite is preferably 60.00% or less.
  • the area proportion of ferrite is more preferably 55.00% or less, and still more preferably 50.00% or less.
  • Martensite is a structure that is formed when fcc transforms to bcc at a relatively low temperature. Martensite is a structure composed of fine crystal grains with a high dislocation density and has a function of increasing the strength of the hot-rolled steel sheet. If the area proportion of martensite is 40.00% or more, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of martensite is preferably 40.00% or more. The area proportion of martensite is preferably 50.00% or more.
  • the area proportion of martensite is preferably 85.00% or less.
  • the area proportion of martensite is more preferably 80.00% or less, and still more preferably 75.00% or less, or 70.00% or less.
  • a total amount of less than 45.00% of pearlite and bainite may be contained as the residual structure.
  • the area proportion of the residual structure may be 10.00% or less, or 5.00% or less.
  • the residual structure may not be contained and a total area proportion may be 0.00%.
  • the microstructure contains, in area proportion, less than 3.00% of retained austenite, and bainite and a very small amount of the residual structure, preferably, the area proportion of bainite is 50.00% or more, and the area proportion of the residual structure is less than 50.00%. If the area proportion of each structure is set as above, it is possible to improve the strength, ductility and hole expandability at the same time.
  • Bainite is a structure that is formed when fcc transforms to bcc at a low temperature. Bainite is a structure composed of fine crystal grains and carbides and has a function of improving the strength, ductility and hole expandability of the hot-rolled steel sheet in a well-balanced manner. If the area proportion of bainite is 50.00% or more, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of bainite is preferably 50.00% or more. The area proportion of bainite is more preferably 80.00% or more, 85.00% or more, or 90.00% or more.
  • the upper limit is not particularly specified, and may be 100.00% or less.
  • the residual structure less than 50.00%
  • a total amount of less than 50.00% of pearlite, ferrite and martensite may be contained as the residual structure.
  • the area proportion of the residual structure is more preferably 20.00% or less, 15.00% or less, or 10.00% or less.
  • the residual structure may not be contained and a total area proportion may be 0.00%.
  • a total area proportion of martensite is 85.00% or more, and the area proportion of the residual structure is preferably less than 15.00%. If the area proportion of each structure is set as above, it is possible to improve the strength and hole expandability at the same time.
  • martensite has a function of increasing the strength of the hot-rolled steel sheet.
  • martensite since martensite has a random crystal orientation structure, it has a function of improving hole expandability of the hot-rolled steel sheet. If the area proportion of martensite is more than 85.00%, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of martensite is preferably more than 85.00%.
  • the area proportion of martensite is more preferably 90.00% or more, 93.00% or more, or 95.00% or more.
  • the upper limit is not particularly specified, and may be 100.00% or less.
  • the residual structure less than 15.00%
  • a total amount of less than 15.00% of pearlite, ferrite and bainite may be contained as the residual structure.
  • the area proportion of the residual structure is more preferably 10.00% or less, 7.00% or less, or 5.00% or less.
  • the residual structure may not be contained and a total area proportion may be 0.00%.
  • Pearlite is a lamellar microstructure in which cementite precipitates in layers between ferrites, and is a softer microstructure than bainite and martensite. If the area proportion of pearlite is 5.00% or more, carbon is consumed by cementite contained in pearlite, the strength of martensite and bainite decreases, and it may not be possible to obtain a tensile strength of 980 MPa or more. Therefore, in any of the aspects, the area proportion of pearlite may be less than 5.00%. The area proportion of pearlite is more preferably 3.00% or less. In order to improve the elongation-flangeability of the hot-rolled steel sheet, it is preferable that the area proportion of pearlite be reduced as much as possible, and the area proportion of pearlite is more preferably 0.00%.
  • the area proportion of pearlite here is an area proportion that applies to all of the above three steel types (DP steel, bainite steel and martensite steel).
  • Structures other than retained austenite are measured by the following method.
  • the area proportion of ferrite and of pearlite is measured by the following method.
  • a sample is collected so that the depth of 1/4 of the sheet thickness from the surface (the region of a depth of 1/8 from the surface to a depth of 3/8 from the surface) and the center position in the sheet width direction can be observed.
  • the cross section of the sample parallel to the rolling direction is mirror-finished and polished with colloidal silica containing no alkaline solution at room temperature for 8 minutes to remove the strain introduced into the surface layer of the sample.
  • a region with a length of 50 ⁇ m and a position of a depth of 1/4 of the sheet thickness from the surface (region of a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface) and a center position in the sheet width direction is measured at 0.1 ⁇ m measurement intervals by an electron back scattering diffraction method to obtain crystal orientation information.
  • the number of measurement points is at least 500 points.
  • an EBSD device composed of a thermal field emission scanning electron microscope (JSM-7001F commercially available from JEOL) and an EBSD detector (DVC5 type detector commercially available from TSL) is used.
  • the degree of vacuum in the EBSD device is 9.6 ⁇ 10 -5 Pa or less
  • the acceleration voltage is 15 kV
  • the irradiation current level is13
  • the electron beam irradiation level is 62.
  • a reflected electron image is captured in the same field of view.
  • crystal grains in which ferrite and cementite precipitate in layers are identified from the reflected electron image, and when the area proportion of the crystal grains is calculated, the area proportion of pearlite is obtained.
  • crystal orientation information for crystal grains other than crystal grains determined as pearlite, in the obtained crystal orientation information, using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" bundled in the EBSD analysis device, a region in which the Grain Average Misorientation value is 1.0° or less is determined as ferrite. If the area proportion of the region determined as ferrite is determined, the area proportion of ferrite is obtained.
  • nital corrosion is performed, and using an optical microscope and a scanning electron microscope (SEM), at least three 30 ⁇ m ⁇ 30 ⁇ m regions to a position of 1/4 of the sheet thickness from the surface (region of a depth of 1/8 in the sheet thickness direction from the surface to a depth of 3/8 in the sheet thickness direction from the surface) are observed.
  • Image analysis is performed on the structure image obtained by this structure observation, and thus the area proportion of bainite is obtained.
  • repeller corrosion is performed on the same observation position, structure observation is then performed using an optical microscope and a scanning electron microscope, image analysis is performed on the obtained structure image, and thus the area proportion of martensite is calculated.
  • Martensite is a structure having a high dislocation density and substructures such as blocks and packets within grains, and can be distinguished from other microstructures according to an electron channeling contrast image using a scanning electron microscope.
  • Bainite is an aggregate of lath-shaped crystal grains, which is a non-martensite structure among structures that do not contain Fe-based carbides having a major diameter of 20 nm or more inside the structure or a structure which contains Fe-based carbides having a major diameter of 20 nm or more inside the structure, and in which the Fe-based carbides have a single variant, that is, Fe-based carbides extend in the same direction.
  • Fe-based carbides elongated in the same direction are Fe-based carbides with a difference of 5° or less in the elongation direction.
  • the rolling direction of the hot-rolled steel sheet is determined by the following method.
  • a test piece is collected so that the sheet thickness of the hot-rolled steel sheet cross section can be observed.
  • the cross section of the collected test piece in the sheet thickness is mirror-polished, and then observed using an optical microscope.
  • the observation range is the entire sheet thickness, and the direction parallel to the elongation direction of crystal grains is determined as the rolling direction.
  • the Ref value indicates a ratio between average fracture surface units before and after plastic deformation, and the following formula is represented using the Cf1 value, which is the average fracture surface unit before undergoing plastic deformation, and the Cf2 value, which is the average fracture surface unit after undergoing plastic deformation.
  • the crack arresting property deteriorates.
  • the crack arresting property is also affected by the fracture surface unit, and as the fracture surface unit is finer, the propagation path is bent, and the crack propagation is minimized. Therefore, in order to exhibit a favorable crack arresting property even after plastic deformation, it is necessary to increase the Ref value.
  • the Rcf value is 2.00 or more, and preferably 2.20 or more, and more preferably 2.30 or more.
  • the upper limit is not particularly specified, and may be 5.00 or less, 4.00 or less, or 3.00 or less.
  • the Cf1 value and the Cf2 value can be obtained by the following method.
  • brittle fracture in order to calculate the Cf1 value and the Cf2 value, it is necessary to cause brittle fracture.
  • a test method for causing brittle fracture for example, according to JIS Z 2242: 2018, a 2.5 mm sub-sized V notch test piece in which the width direction (C direction) of the hot-rolled steel sheet is the longitudinal direction of the test piece is prepared, and a Charpy impact test may be performed at -196°C. If the sheet thickness of the hot-rolled steel sheet is less than 2.5 mm, the test may be performed on the entire thickness.
  • the rolling direction of the hot-rolled steel sheet is determined by the above method, and the direction perpendicular to the rolling direction is determined as the width direction of the hot-rolled steel sheet.
  • the SEM image capturing region that is captured in order to calculate the Cf1 value and the Cf2 value is a position from the surface of the steel sheet to a depth of 1/4 of the sheet thickness (region of a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface) and a center position in the sheet width direction in the cross section parallel to the rolling direction.
  • SU-6600 Schottky-emission electron gun commercially available from Hitachi High-Tech Corporation
  • a tungsten emitter are used, and in a vacuum of 9.6 ⁇ 10 -5 Pa or less, the acceleration voltage is 1.5 kV.
  • the imaging magnification is 1,000x, and the number of imaging fields is 3 or more.
  • a ductile fracture part called a tear ridge is imaged with a bright contrast.
  • the region surrounded by the tear ridge is defined as one cleavage facet, and the equivalent circle diameter is obtained from the area of each cleavage facet, and is used as the fracture surface unit of each cleavage facet. From the obtained fracture surface unit, the area average diameter weighted by the area of each cleavage facet is determined and used as the average fracture surface unit.
  • the above processing is performed on the hot-rolled steel sheet before plastic deformation and the hot-rolled steel sheet after plastic deformation, and thus the Cf1 value and the Cf2 value are calculated.
  • a JIS No. 5 tensile test piece in which the width direction (C direction) of the hot-rolled steel sheet is the longitudinal direction of the test piece is prepared, a compressive pre-strain of 10% is applied to the steel material in the longitudinal direction of the test piece and various test pieces are then collected.
  • the standard deviation of the Mn concentration may be 0.60 mass% or less.
  • the standard deviation of the Mn concentration is preferably 0.50 mass% or less and more preferably 0.47 mass% or less. In order to minimize a decrease in fracture energy, a smaller value of the lower limit of the standard deviation of the Mn concentration is more desirable, and due to restrictions on the producing process, 0.10 mass% is the substantial lower limit.
  • a position from the surface of the steel sheet to a depth of 1/4 of the sheet thickness (region of a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface) and a center position in the sheet width direction are measured with an electron probe micro-analyzer (EPMA), and the standard deviation of the Mn concentration is measured.
  • the acceleration voltage is 15 kV and the magnification is 5,000x (the region of a depth of 1/8 from the surface to a depth of 3/8 from the surface), and a distribution image in a range of 20 ⁇ m in the sheet thickness direction of the sample is measured. More specifically, measurement intervals are 0.1 ⁇ m, and the Mn concentration is measured at 40,000 or more points. Then, the standard deviation is calculated based on the Mn concentrations obtained from all the measurement points, and thus the standard deviation of the Mn concentration is obtained.
  • the tensile property (tensile strength) is evaluated according to JIS Z 2241: 2011.
  • the test piece is a No. 5 test piece according to JIS Z 2241: 2011.
  • the tensile test piece is collected at a position a quarter from the edge in the sheet width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction.
  • the tensile (maximum) strength of the hot-rolled steel sheet according to the present embodiment is 980 MPa or more, and preferably 1000 MPa or more. If the tensile strength is less than 980 MPa, application parts are limited, and contribution to vehicle body weight reduction is small.
  • the upper limit is not necessarily particularly limited, and may be 1,780 MPa or less, 1,500 MPa or less, or 1,300 MPa or less in order to reduce mold wear.
  • the sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, and may be 1.20 to 8.00 mm. If the sheet thickness of the hot-rolled steel sheet is less than 1.20 mm, it may difficult to secure the rolling completion temperature, the rolling load may become excessive, and hot rolling may become difficult. Therefore, the sheet thickness of the hot-rolled steel sheet according to the present embodiment may be 1.20 mm or more, and is preferably 1.40 mm or more. On the other hand, if the sheet thickness is more than 8.00 mm, the effect of the standard deviation of the Mn concentration becomes significant, and it becomes difficult to obtain a desired Rcf value. Therefore, the sheet thickness may be 8.00 mm or less, and is preferably 6.00 mm or less, or 3.00 mm or less.
  • the hot-rolled steel sheet having the above chemical composition and microstructure according to the present embodiment may be a surface-treated steel sheet that has a plating layer on the surface in order to improve the corrosion resistance.
  • the plating layer may be an electroplating layer or a melting plating layer.
  • electroplating layers include zinc electroplating and electro Zn-Ni alloy plating.
  • melting plating layers include melting zinc plating, alloying melting zinc plating, melting aluminum plating, melting Zn-Al alloy plating, melting Zn-Al-Mg alloy plating, and melting Zn-Al-Mg-Si alloy plating.
  • the amount of plating adhered is not particularly limited, and may be the same as in the related art.
  • it is possible to further improve the corrosion resistance by applying appropriate chemical conversion (for example, applying a silicate-based chromium-free chemical conversion solution and drying) after plating.
  • a preferable method of producing the hot-rolled steel sheet having the above chemical composition and microstructure according to the present embodiment is as follows.
  • the slab is heated under predetermined conditions and then hot-rolled, and accelerated cooling to a predetermined temperature range is performed, and then slow-cooling is performed as necessary, and it is effective to control a cooling history until coiling.
  • the following processes (1) to (7) are sequentially performed.
  • the temperature of the slab and the temperature of the steel sheet in the present embodiment are the surface temperature of the slab and the surface temperature of the steel sheet.
  • a slab to be hot-rolled For a slab to be hot-rolled, a slab obtained by continuous casting or a slab obtained by casting and blooming can be used, and as necessary, one obtained by performing hot processing or cold processing on a slab can be used.
  • a slab to be hot-rolled is preferably heated and held in a temperature range of 1,100°C or higher for 6,000 sec or longer.
  • the steel sheet temperature may vary in a temperature range of 1,100°C or higher or may be constant.
  • the austenite grains can be made uniform when the slab is heated.
  • the slab is held in a temperature range of 700 to 850°C for 900 sec or longer, and then additionally heated, and may be held in a temperature range of 1,100°C or higher for 6,000 sec or longer.
  • the steel sheet temperature may vary in a temperature range or may be constant.
  • Mn is distributed between ferrite and austenite, and when its transformation time is prolonged, Mn can diffuse in the ferrite region. Thereby, Mn microsegregation unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. If the standard deviation of the Mn concentration is large, a region in which Mn is locally concentrated and the fracture energy decreases develops, the occurrence of cracks during plastic deformation is promoted, and thus it may not possible to obtain a desired Rcf value.
  • a reverse mill or a tandem mill for multipass rolling.
  • the total rolling reduction rate in a temperature range of 850 to 1,100°C is less than 90%, the standard deviation of the Mn concentration increases, it is not possible to inhibit development of a region in which Mn is locally concentrated and fracture energy decreases, and the occurrence of cracks during plastic deformation may not be promoted. Thereby, it may not be possible to obtain a desired Ref value.
  • the sheet thickness reduction in a temperature range of 850 to 1,100°C can be represented by ⁇ (t0-t1)/t0 ⁇ 100(%) where the inlet sheet thickness before first rolling in rolling in this temperature range is t0, and the outlet sheet thickness after final stage rolling in rolling in this temperature range is t1.
  • Hot rolling start temperature 850°C or higher and lower than 930°C
  • rolling temperature from the first stage of hot rolling to the stage two stages before the final stage 850°C or higher and lower than 950°C
  • rolling reduction rate for the rolling less than 30%
  • the hot rolling start temperature is 850°C or higher and lower than 930°C
  • the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is 850°C or higher and lower than 950°C
  • the rolling reduction rate from the first stage of hot rolling to the stage two stages before the final stage is less than 30%.
  • the hot rolling start temperature is set to a relatively low temperature
  • the temperature of the first stage of hot rolling is set to low, and rolling is performed at a low rolling reduction rate, it is possible to minimize recrystallization at the first stage of hot rolling and accumulate strains in the austenite grains.
  • unrecrystallized austenite with a high dislocation density can be maintained within grains up to the latter stage of rolling.
  • the rolling start temperature is 930°C or higher
  • the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is 950°C or higher, or the rolling reduction rate for the rolling is 30% or more, it is not possible to minimize recrystallization of austenite at the first stage to the stage two stages before the final stage of hot rolling, and as a result, it may not be possible to obtain a desired Ref value.
  • the hot rolling start temperature is lower than 850°C, or the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is lower than 850°C, it is difficult to set the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage to 930°C or higher, and as a result, it may not be possible to obtain a desired Rcf value.
  • the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage is 930°C or higher and lower than 1,010°C
  • the rolling reduction rate at the final stage and the stage one stage before the final stage is 50% or more
  • the rolling completion temperature is 950°C or higher.
  • Recrystallization at the first stage of rolling is minimized, recrystallization of crystal grains with a high dislocation density is caused at the latter stage of rolling, and thus orientation difference between structures generated from the recrystallized austenite increases.
  • the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage is lower than 930°C, the rolling reduction rate at the final stage and the stage one stage before the final stage is less than 50%, or the rolling completion temperature is lower than 950°C, recrystallization of austenite is insufficient, and it may not be possible to obtain a desired Rcf value.
  • the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage and the rolling completion temperature are lower than 1,010°C, it is possible to refine the structure by preventing coarsening of the austenite grain size. Thereby, it is possible to minimize the occurrence of cracks during plastic deformation and to increase the Ref value.
  • cooling is performed at an average cooling rate of 50°C/sec or faster within 1.0 sec after hot rolling is completed, and the cooling start temperature is 850°C or higher and lower than 960°C.
  • cooling is performed at a high average cooling rate immediately after hot rolling is completed, for example, cooling water may be sprayed onto the surface of the steel sheet.
  • the cooling start temperature is 850°C or higher and lower than 960°C and cooling is performed at an average cooling rate of 50°C/sec or faster within 1.0 sec after hot rolling is completed. Thereby, it is possible to minimize the occurrence of cracks during plastic deformation and to increase the Ref value.
  • the cooling start temperature used here is a temperature immediately before cooling is performed at an average cooling rate of 50°C/sec or faster, for example, a temperature immediately before cooling water is sprayed onto the surface of the steel sheet.
  • the average cooling rate is a value obtained by dividing the temperature drop range of the steel sheet from when accelerated cooling starts (when the steel sheet is introduced into the cooling facility) until accelerated cooling is completed (when the steel sheet is taken out of the cooling facility) by the time required from when accelerated cooling starts until accelerated cooling is completed.
  • Cooling is performed to a temperature range of 600 to 730°C at an average cooling rate of 50°C/sec or faster, and slow cooling with an average cooling rate of slower than 5°C/s is performed in a temperature range of 600 to 730°C for 2.0 sec or longer. Then, cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/s or faster.
  • slow cooling may not be performed, and when slow cooling is not performed, cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/s or faster.
  • the average cooling rate herein is a value obtained by dividing the temperature drop range of the steel sheet from the cooling stop temperature of accelerated cooling to the slow cooling stop temperature by the time required from when accelerated cooling stops until slow cooling stops.
  • the average cooling rate from the cooling stop temperature of slow cooling to the coiling temperature is preferably 50°C/sec or faster.
  • the average cooling rate herein is a value obtained by dividing the temperature drop range of the steel sheet from the cooling stop temperature of slow cooling with an average cooling rate of slower than 5°C/s to the coiling temperature by the time required from when slow cooling with an average cooling rate of slower than 5°C/s stops until coiling.
  • the upper limit of the time for which slow cooling is performed is determined by the facility layout, and may be generally shorter than 10.0 sec.
  • the lower limit of the average cooling rate for slow cooling is not particularly set, but it may be 0°C/s or faster because increasing the temperature without cooling involves a large investment in facility.
  • slow cooling may not be performed.
  • accelerated cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/sec or faster without performing slow cooling, it is possible to inhibit formation of ferrite and pearlite with a low strength and promote formation of martensite.
  • the structure is refined, it is possible to stably produce the above martensite steel, and it is possible to achieve both the strength and the crack arresting property of the hot-rolled steel sheet.
  • the average cooling rate herein is a value obtained by dividing the temperature drop range of the steel sheet from when accelerated cooling starts (when the steel sheet is introduced into the cooling facility) until accelerated cooling is completed (when the steel sheet is taken out of the cooling facility) by the time required from when accelerated cooling starts until accelerated cooling is completed.
  • the upper limit value of the cooling rate is not particularly specified, but if the cooling rate increases, the cooling facility will be large-scaled and the facility cost increases. Therefore, 300°C/sec or slower is preferable in consideration of facility costs.
  • the coiling temperature is 350°C or lower.
  • the coiling temperature is 350°C or lower, it is possible to reduce the amount of iron carbides precipitated and reduce the variation in hardness distribution in the hard phase.
  • martensite steel was obtained by performing cooling to a temperature range of 350°C or lower at a desired average cooling rate without performing slow cooling.
  • DP steel was obtained by performing slow cooling in a high temperature range (660 to 730°C) within the temperature range of 600 to 730°C
  • bainite steel was obtained by performing slow cooling in a low temperature range (600°C or higher, lower than 660°C) within the temperature range of 600 to 730°C.
  • the area proportion of the microstructure, the Rcf value, the standard deviation of the Mn concentration and the tensile strength TS of the obtained hot-rolled steel sheets were obtained by the above methods.
  • the obtained measurement results are shown in Table 5A and Table 5B.
  • the tensile strength TS was 980 MPa or more, it was determined as satisfactory because the hot-rolled steel sheet had high strength. On the other hand, if the tensile strength TS was less than 980 MPa, it was determined as unsatisfactory because the hot-rolled steel sheet did not have high strength.
  • the crack arresting property was evaluated by a Charpy impact test.
  • a Charpy impact test According to JIS Z 2242: 2018, a 2.5 mm sub-sized V notch test piece in which the width direction (C direction) of the hot-rolled steel sheet was the longitudinal direction of the test piece was prepared, and the Charpy impact test was performed at -196°C. If the sheet thickness of the hot-rolled steel sheet was less than 2.5 mm, the test was performed on the entire thickness.
  • a JIS No. 5 B tensile test piece in which the width direction (C direction) of the hot-rolled steel sheet was the longitudinal direction of the test piece was prepared, a compressive pre-strain of 10% was applied to the steel material in the longitudinal direction of the test piece and the V notch test piece was then collected.
  • the test piece was subjected to the Charpy impact test at -196°C by the above method and thus the absorbed energy after plastic deformation was obtained.
  • the hot-rolled steel sheets according to examples of the present invention had high strength and little deterioration in the crack arresting property after plastic deformation.
  • the hot-rolled steel sheets according to the comparative examples did not have one or more of the above properties.

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  • Heat Treatment Of Sheet Steel (AREA)

Abstract

This hot-rolled steel sheet has a predetermined chemical composition and a microstructure containing, in area%, less than 3.00% of retained austenite with an Ref value indicating a ratio between average fracture surface units before and after plastic deformation being 2.00 or more and a tensile strength of 980 MPa or more.

Description

    [Technical Field]
  • The present invention relates to a hot-rolled steel sheet, and specifically, to a hot-rolled steel sheet that is used after being molded into various shapes by press processing or the like, and particularly, to a hot-rolled steel sheet having high strength and little deterioration in the crack arresting property after plastic deformation.
  • Priority is claimed on Japanese Patent Application No. 2021-113549, filed July 8, 2021 , the content of which is incorporated herein by reference.
    • [Background Art]
  • In recent years, in consideration of global environment protection, efforts have been made to reduce the amount of carbon dioxide gas emitted in many fields. Automobile manufacturers are actively developing techniques for reducing the weight of vehicle bodies in order to reduce fuel consumption. However, it is not easy to reduce the weight of vehicle bodies because emphasis is also placed on improving collision resistance in order to secure safety of passengers.
  • In order to reduce greenhouse gas emission according to vehicle body weight reduction, thinning members using high-strength steel sheet is being examined. Therefore, there is a strong demand for steel sheets having both high strength and excellent moldability, and several techniques have been proposed in the related art in response to this requirement. On the other hand, Non-Patent Document 1 describes that plastic deformation becomes more difficult as the strength of the steel sheet increases, and the crack arresting property generally deteriorates.
  • In addition, strongly processed parts such as bent parts undergo large plastic deformation during press molding, and the strength increases due to processing hardening, and thus the crack arresting property further deteriorates, and press cracks may occur in parts undergoing large plastic deformation. The deterioration in the crack arresting property after plastic deformation has been a problem in thick sheet materials used for ships and structural steels in the related art, but with recent increases in strength, it has become necessary to study molding of hot-rolled steel sheets which are materials for automobiles.
  • Regarding the technique for improving toughness after plastic deformation, for example, Patent Document 1 discloses a steel sheet for large structures having an excellent crack arresting property after plastic deformation by strictly controlling impurity elements and also setting the ferrite grain size in the surface layer to 3 µm or less.
  • Patent Document 2 discloses a steel sheet for large structures having an excellent crack arresting property after plastic deformation wherein the steel sheet has a structure containing ferrite crystal grains having a flatness of 2 or more and a minor axis diameter of 5 µm or less and subgrains having an equivalent circle diameter of 3 µm or less in the ferrite crystals.
    • [Citation List] [Patent Document]
    • [Patent Document 1]
      Japanese Patent No. 3499085
    • [Patent Document 2]
      Japanese Patent No. 3467767
    [Non Patent Document]
  • [Non-Patent Document 1]
    Yuzo Takahashi, Osamu Kono, Kosaku Ushioda, Shuji Awaihara: Iron and Steel, 99, (2013), 4, 312-321
    • [Summary of the Invention] [Problems to be Solved by the Invention]
  • The techniques disclosed in Patent Documents 1 and 2 are both techniques related to a steel sheet for large structures and are not intended for hot-rolled steel sheets. In addition, both have a structure design mainly composed of a ferrite structure, and the steel sheet has a strength of 450 to 700 MPa, and thus it may be difficult to apply the techniques disclosed in Patent Documents 1 and 2 to high-strength hot-rolled steel sheets of 980 MPa or more, which are mainly composed of bainite and martensite.
  • The present invention has been made in view of the above circumstances in the related art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and little deterioration in the crack arresting property after plastic deformation.
    • [Means for Solving the Problem]
  • In view of the above circumstances, the inventors conducted extensive studies regarding the relationship between the chemical composition of the hot-rolled steel sheet and the microstructure and mechanical properties, and as a result, the following findings (a) to (e) were obtained, and the present invention was completed.
    1. (a) In order to obtain excellent tensile (maximum) strength, it is preferable to utilize a hard structure. That is, it is preferable that the microstructure contain martensite or bainite.
    2. (b) However, since the hard structure is a structure having a poor crack arresting property, it is not possible to secure an excellent crack arresting property simply by forming a microstructure mainly composed of these.
    3. (c) In addition, since processing hardening occurs due to plastic deformation, the crack arresting property after plastic deformation further deteriorates.
    4. (d) In order to provide a high-strength hot-rolled steel sheet with an excellent crack arresting property and minimize deterioration in the crack arresting property after plastic deformation, it is effective to obtain a fine structure in which the propagation path of cracks bends after plastic deformation.
      Specifically, refining the fracture surface unit after plastic deformation is effective in reducing deterioration in the crack arresting property after plastic deformation.
    5. (e) In order to refine the fracture surface unit after plastic deformation, it is effective to control slab heating conditions, hot rolling conditions and cooling conditions after hot rolling. Thereby, it is possible to form fine austenite grains in which the orientation difference between structures generated during bcc transformation increases, and it is possible to refine the fracture surface unit after plastic deformation.
  • The gist of the present invention made based on the above findings is as follows.
    1. (1) A hot-rolled steel sheet according to one aspect of the present invention consisting of, as a chemical composition, in mass%,
      • C: 0.040 to 0.400%,
      • Si: 0.05 to 3.00%,
      • Mn: 1.00 to 4.00%,
      • sol. Al: 0.001 to 0.500%,
      • P: 0.100% or less,
      • S: 0.0300% or less,
      • N: 0.1000% or less,
      • O: 0.0100% or less,
      • Ti: 0 to 1.000%,
      • V: 0 to 1.000%,
      • Nb: 0 to 1.000%,
      • Cu: 0 to 2.00%,
      • Cr: 0 to 2.00%,
      • Mo: 0 to 1.00%,
      • Ni: 0 to 2.00%,
      • B: 0 to 0.0100%,
      • Ca: 0 to 0.0200%,
      • Mg: 0 to 0.0200%,
      • REM: 0 to 0.1000%,
      • Bi: 0 to 0.020%,
      • one, two or more of Zr, Co, Zn and W: a total amount of 0 to 1.00%,
      • Sn: 0 to 0.05%, and
      • a remainder of Fe and impurities,
      • wherein a microstructure contains, in area%,
      • less than 3.00% of retained austenite,
      • an Rcf value indicating a ratio between average fracture surface units before and after plastic deformation is 2.00 or more, and
      • a tensile strength is 980 MPa or more.
    2. (2) In the hot-rolled steel sheet according to (1), the chemical composition may contains, in mass%, one, two or more selected from the group of
      • Ti: 0.010 to 1.000%,
      • V: 0.010 to 1.000%,
      • Nb: 0.010 to 1.000%,
      • Cu: 0.01 to 2.00%,
      • Cr: 0.01 to 2.00%,
      • Mo: 0.01 to 1.00%,
      • Ni: 0.02 to 2.00%,
      • B: 0.0001 to 0.0100%,
      • Ca: 0.0005 to 0.0200%,
      • Mg: 0.0005 to 0.0200%,
      • REM: 0.0005 to 0.1000%, and
      • Bi: 0.0005 to 0.020%.
    3. (3) In the hot-rolled steel sheet according to (1) or (2), the microstructure may contain, in area%, 15.00 to 60.00% of ferrite and 40.00 to 85.00% of martensite.
    4. (4) In the hot-rolled steel sheet according to (1) or (2), the microstructure may contain, in area%, 50.00% or more of bainite.
    5. (5) In the hot-rolled steel sheet according to (1) or (2), the microstructure may contain, in area%, more than 85.00% of martensite.
    [Effects of the Invention]
  • According to the above aspect of the present invention, it is possible to obtain a hot-rolled steel sheet having high strength and little deterioration in the crack arresting property after plastic deformation.
  • The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile members, mechanical structural members and also building members.
    • [Embodiment(s) for implementing the Invention]
  • A chemical composition and a microstructure of a hot-rolled steel sheet according to the present embodiment will be described below in more detail. However, the present invention is not limited only to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present invention.
  • Hereinafter, numerical values limiting a range indicated by "to" include both the lower limit value and the upper limit value. Numerical values indicated by "less than" or "more than" are not included in this numerical value range. In the following description, % related to the chemical composition of the steel sheet is mass% unless otherwise specified.
    • 1. Chemical composition
  • The hot-rolled steel sheet according to the present embodiment contains, in mass%, C: 0.040 to 0.400%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, and O: 0.0100% or less, with the remainder: Fe and impurities. Hereinafter, respective elements will be described in detail.
    • (1-1) C: 0.040 to 0.400%
  • C increases the fraction of the hard phase and lowers the transformation point of the hard phase, and thus increases the strength of the hot-rolled steel sheet. If the C content is less than 0.040%, it becomes difficult to obtain a desired strength. Therefore, the C content is 0.040% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and still more preferably 0.080% or more.
  • On the other hand, if the C content is more than 0.400%, a large amount of carbides are formed in the structure, and thus the occurrence of internal defects during plastic deformation is promoted, and deterioration in the crack arresting property after plastic deformation becomes large. As a result, it is not possible to obtain a desired Rcf value. Therefore, the C content is 0.400% or less. The C content is preferably 0.300% or less, more preferably 0.250% or less, and still more preferably 0.150% or less.
    • (1-2) Si: 0.05 to 3.00%
  • Si has a function of solid-solution strengthening at room temperature and increasing the strength of the hot-rolled steel sheet and a function of solid-solution softening at a low temperature and improving the toughness of the hot-rolled steel sheet. In addition, Si has a function of minimizing flaws in steel (minimizing the occurrence of defects such as blowholes in steel) by deacidification. If the Si content is less than 0.05%, it is not possible to obtain the effect of the above function. Therefore, the Si content is 0.05% or more. The Si content is preferably 0.50% or more, and more preferably 0.80% or more.
  • However, if the Si content is more than 3.00%, surface properties and chemical convertibility of the steel sheet, as well as ductility and weldability, significantly deteriorate, and the surface energy of fracture decreases. Thereby, the occurrence and propagation of cracks during plastic deformation are facilitated, and deterioration in the crack arresting property after plastic deformation is large. As a result, it is not possible to obtain a desired Rcf value. Therefore, the Si content is 3.00% or less. The Si content is preferably 2.70% or less and more preferably 2.50% or less.
    • (1-3) Mn: 1.00 to 4.00%
  • Mn has a function of inhibiting ferrite transformation and increasing the strength of the hot-rolled steel sheet and a function of solid-solution softening at a low temperature and improving the toughness of the hot-rolled steel sheet. If the Mn content is less than 1.00%, it is not possible to obtain a desired tensile strength. Therefore, the Mn content is 1.00% or more. The Mn content is preferably 1.30% or more and more preferably 1.50% or more.
  • On the other hand, if the Mn content is more than 4.00%, the function of lowering the surface energy of fracture becomes strong and thus the occurrence and propagation of cracks during plastic deformation are facilitated, and deterioration in the crack arresting property after plastic deformation is large. As a result, it is not possible to obtain a desired Ref value. Therefore, the Mn content is 4.00% or less. The Mn content is preferably 3.70% or less, and more preferably 3.50% or less.
    • (1-4) sol. Al: 0.001 to 0.500%
  • Like Si, Al has a function of deacidifying steel and minimizing flaws in steel and a function of exhibiting solid-solution softening at a low temperature and increasing the toughness of the hot-rolled steel sheet. If the sol. Al content is less than 0.001%, it is not possible to obtain the effect of the above function. In addition, if the sol. Al content is less than 0.001%, it is not possible to obtain a desired Rcf value. Therefore, the sol. Al content is 0.001% or more. The sol. Al content is preferably 0.010% or more.
  • On the other hand, if the sol. Al content is more than 0.500%, since the above effect is maximized and this is economically unfavorable, the sol. Al content is 0.500% or less. The sol. Al content is preferably 0.300% or less, and more preferably 0.100% or less.
  • Here, sol. Al is acid-soluble Al, and indicates solid solution Al present in steel in a solid solution state.
    • (1-5) P: 0.100% or less
  • P is an element that is generally contained as an impurity, and is an element having a function of increasing the strength of the hot-rolled steel sheet according to solid-solution strengthening. Therefore, P may be actively contained, but P is an element that easily segregates, and if the P content is more than 0.100%, the grain boundary strength decreases significantly due to grain boundary segregation, and grain boundary fracture is likely to occur. Therefore, the P content is 0.100% or less. The P content is preferably 0.030% or less.
  • It is not particularly necessary to specify the lower limit of the P content, and the lower limit is preferably 0.001% or more in consideration of refining costs.
    • (1-6) S: 0.0300% or less
  • S is an element contained as an impurity, and forms a sulfide-based inclusion in steel and promotes the occurrence of cracks. If the S content is more than 0.0300%, the occurrence of cracks during plastic deformation becomes significant, and the crack arresting property after plastic deformation significantly deteriorates. Therefore, the S content is 0.0300% or less. The S content is preferably 0.0050% or less.
  • It is not particularly necessary to specify the lower limit of the S content, and the lower limit is preferably 0.0001% or more in consideration of refining costs.
    • (1-7) N: 0.1000% or less
  • N is an element that is contained in steel as an impurity and has a function of promoting the occurrence of cracks starting from impurities. If the N content is more than 0.1000%, the occurrence of cracks during plastic deformation becomes significant, and the crack arresting property after plastic deformation significantly deteriorates. Therefore, the N content is 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less, and still more preferably 0.0100% or less.
  • It is not particularly necessary to specify the lower limit of the N content, and the lower limit may be 0.0001% or more. In addition, when one, two or more of Ti, Nb and V are contained to refine the microstructure, in order to promote precipitation of carbonitrides, the N content is preferably 0.0010% or more and more preferably 0.0020% or more.
    • (1-8) O: 0.0100% or less
  • When a large amount of O is contained in steel, a coarse oxide that acts as a starting point for fracture is formed, which causes brittle fracture or hydrogen-induced cracking. Therefore, the O content is 0.0100% or less. The O content is preferably 0.0080% or less, and more preferably 0.0050% or less.
  • In order to disperse a large number of fine oxides during deacidification of molten steel, the O content may be 0.0005% or more, or 0.0010% or more.
  • The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be composed of Fe and impurities. In the present embodiment, impurities are elements that are mixed in from ores or scraps as raw materials or a production environment or the like and/or are allowable as long as they do not adversely affect the hot-rolled steel sheet according to the present embodiment.
  • The hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements in place of some Fe. The lower limit of the content when the optional elements are not contained is 0%. Hereinafter, the optional elements will be described in detail.
    • (1-9) Ti: 0.010 to 1.000%, Nb: 0.010 to 1.000%, and V: 0.010 to 1.000%
  • Ti, Nb and V are elements that precipitate finely in steel as carbides and nitrides and improve the strength of steel according to precipitation strengthening. Therefore, one, two or more of these elements may be contained. In order to obtain this effect more reliably, each content of Ti, Nb and V is preferably 0.010% or more. Here, it is not necessary to contain all of Ti, Nb and V, and any one of them may have a content of 0.010% or more. Each content of Ti, Nb and V is preferably 0.060% or more, and more preferably 0.080% or more.
  • On the other hand, if the content of any one of Ti, Nb and V is more than 1.000%, the processability of the hot-rolled steel sheet deteriorates. Therefore, each content of Ti, Nb and V is 1.000% or less, preferably 0.800% or less and more preferably 0.500% or less.
    • (1-10) Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Ni: 0.02 to 2.00%, and B: 0.0001 to 0.0100%
  • Cu, Cr, Mo, Ni and B all have a function of improving hardenability of the hot-rolled steel sheet. In addition, when Cu is contained, Ni has a function of effectively minimizing grain boundary cracks of the slab caused by Cu. Therefore, one, two or more of these elements may be contained.
  • As described above, Cu has a function of improving the hardenability of the hot-rolled steel sheet. In order to obtain the effect of the above function more reliably, the Cu content is preferably 0.01% or more and more preferably 0.05% or more. However, if the Cu content is more than 2.00%, grain boundary cracks of the slab may occur. Therefore, the Cu content is 2.00% or less. The Cu content is preferably 1.50% or less, and more preferably 1.00% or less.
  • As described above, Cr has a function of improving hardenability of the hot-rolled steel sheet. In order to obtain the effect of the above function more reliably, the Cr content is preferably 0.01% or more and more preferably 0.05% or more. However, if the Cr content is more than 2.00%, the chemical convertibility of the hot-rolled steel sheet significantly deteriorates. Therefore, the Cr content is 2.00% or less.
  • As described above, Mo has a function of improving hardenability of the hot-rolled steel sheet and a function of precipitating in steel as carbides and increasing the strength of the hot-rolled steel sheet. In order to obtain the effect of the above function more reliably, the Mo content is preferably 0.01% or more and more preferably 0.02% or more. However, if the Mo content is more than 1.00%, the effect of the above function is maximized, which is economically unfavorable. Therefore, the Mo content is 1.00% or less. The Mo content is preferably 0.50% or less and more preferably 0.20% or less.
  • As described above, Ni has a function of improving hardenability of the hot-rolled steel sheet. In addition, when Cu is contained, Ni has a function of effectively minimizing grain boundary cracks of the slab caused by Cu. In order to obtain the effect of the above function more reliably, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically unfavorable to contain a large amount of Ni. Therefore, the Ni content is 2.00% or less.
  • As described above, B has a function of improving hardenability of the hot-rolled steel sheet. In order to obtain the effect of the above function more reliably, the B content is preferably 0.0001% or more and more preferably 0.0002% or more. However, if the B content is more than 0.0100%, since the moldability of the hot-rolled steel sheet significantly deteriorates, the B content is 0.0100% or less. The B content is preferably 0.0050% or less.
    • (1-11) Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, REM: 0.0005 to 0.1000% and Bi: 0.0005 to 0.020%
  • Ca, Mg and REMs all have a function of improving the crack arresting property of the hot-rolled steel sheet by adjusting the shape of inclusions in steel to a preferable shape. In addition, Bi has a function of improving the crack arresting property of the hot-rolled steel sheet according to refining of the solidification structure. Therefore, one, two or more of these elements may be contained. In order to obtain the effect of the above function more reliably, the content of any one or more of Ca, Mg, REM and Bi is preferably 0.0005% or more. However, if the Ca content or the Mg content is more than 0.0200% or the REM content is more than 0.1000%, inclusions are excessively formed in steel, and thus the crack arresting property of the hot-rolled steel sheet may deteriorate. In addition, if the Bi content is more than 0.020%, the effect of the above function is maximized, which is economically unfavorable. Therefore, the Ca content and the Mg content are 0.0200% or less, the REM content is 0.1000% or less, and the Bi content is 0.020% or less. The Bi content is preferably 0.010% or less.
  • Here, REM refers to a total of 17 elements composed of Sc, Y and lanthanides, and the REM content refers to a total amount of these elements. In the case of lanthanides, they are industrially added in the form of misch metals.
    • (1-12) one, two or more of Zr, Co, Zn and W: a total amount of 0 to 1.00%, and Sn: 0 to 0.05%
  • Regarding Zr, Co, Zn and W, the inventors confirmed that, even if a total amount of 1.00% or less of these elements is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. Therefore, a total amount of 1.00% or less of one, two or more of Zr, Co, Zn and W may be contained.
  • In addition, the inventors confirmed that, even if a small amount of Sn is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. However, if a large amount of Sn is contained, flaws may occur during hot rolling and thus the Sn content is 0.05% or less.
  • The chemical composition of the hot-rolled steel sheet described above may be measured by a general analysis method. For example, inductively coupled plasmaatomic emission spectrometry (ICP-AES) may be used for measurement. Here, sol. Al may be measured using a filtrate after thermally decomposing a sample with an acid through ICP-AES. C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-nondispersive infrared absorption method.
    • 2. Microstructure of hot-rolled steel sheet
  • Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.
  • In the hot-rolled steel sheet according to the present embodiment, the microstructure contains, in area%, less than 3.00% of retained austenite, and the Rcf value indicating a ratio between average fracture surface units before and after plastic deformation is 2.0 or more.
  • Therefore, the hot-rolled steel sheet according to the present embodiment can have high strength and an excellent crack arresting property after plastic deformation.
  • Here, in the present embodiment, in the cross section parallel to the rolling direction, the structure fraction and the Rcf value in the microstructure at a depth of 1/4 of the sheet thickness from the surface (the region of a depth of 1/8 from the surface to a depth of 3/8 from the surface) and at center position in the sheet width direction are specified. The reason for this is that the microstructure at that position is a typical microstructure of the steel sheet.
    • (2-1) Area proportion of retained austenite: less than 3.00%
  • Retained austenite is a microstructure that is present as fcc at room temperature. Retained austenite has concentrated carbon in the surrounding structure, transforms to hard martensite during plastic deformation, and thus can become a starting point for the occurrence of cracks. If the area proportion of retained austenite is 3.00% or more, the above function becomes apparent, and the crack arresting property after plastic deformation significantly deteriorates. Therefore, the area proportion of retained austenite is less than 3.00%. The area proportion of retained austenite is preferably 2.00% or less, less than 1.50% or 1.00% or less, and more preferably less than 1.00% or less than 0.50%. Since it is preferable that the amount of retained austenite be as small as possible, the area proportion of retained austenite may be 0.00%.
  • Methods of measuring the area proportion of retained austenite include X-ray diffraction, electron back scattering diffraction pattern (EBSP) analysis, and magnetic measurement methods. In the present embodiment, the area proportion of retained austenite is measured by X-ray diffraction.
  • In the present embodiment, in measurement of the area proportion of retained austenite by X-ray diffraction, first, in the cross section parallel to the rolling direction at a depth of 1/4 of the sheet thickness of the hot-rolled steel sheet (the region of a depth of 1/8 from the surface to a depth of 3/8 from the surface) and a center position in the sheet width direction, using Co-Kα rays, an integrated intensity of a total of 7 peaks of α(110), α(200), α(211), γ(111), γ(200), and γ(220) is obtained, and an intensity average method is used for calculation, and thus the area proportion of retained austenite is obtained.
  • The microstructure of the hot-rolled steel sheet according to the present embodiment may contain ferrite, martensite, bainite and pearlite in addition to retained austenite.
  • The area proportion of ferrite may be 60.00% or less, 50.00% or less, or 45.00% or less. In addition, the area proportion of ferrite may be 0.00% or more, or 0.05% or more.
  • The area proportion of martensite may be 100.00% or less, or 99.00% or less. In addition, the area proportion of martensite may be 0.00% or more, 1.00% or more, or 1.50% or more.
  • The area proportion of bainite may be 100.00% or less, or 96.00% or less. In addition, the area proportion of bainite may be 0.00% or more, or 0.01% or more.
  • Here, the area proportions of the above retained austenite, ferrite, bainite and martensite are area proportions that apply to all three steel types to be described below (DP steel, bainite steel and martensite steel).
  • Hereinafter, preferable area proportions of respective structures of three steel types (DP steel, bainite steel and martensite steel) will be described.
    • (2-2) DP steel
  • When the microstructure contains, in area proportion, less than 3.00% of retained austenite, and ferrite, martensite and a very small amount of the residual structure, preferably, the area proportion of ferrite is 15.00 to 60.00%, the area proportion of martensite is 40.00 to 85.00%, and the area proportion of the residual structure is less than 45.00%. If the area proportion of each structure is set as above, it is possible to improve the strength and ductility in a well-balanced manner.
  • Respective structures in this aspect will be described below.
    • Area proportion of ferrite: 15.00 to 60.00%
  • Ferrite is a structure that is formed when fcc transforms to bcc at a relatively high temperature. Since ferrite has a high processing hardening rate, it has a function of improving strength-ductility balance of the hot-rolled steel sheet. If the area proportion of ferrite is 15.00% or more, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of ferrite is preferably 15.00% or more. The area proportion of ferrite is more preferably 20.00% or more, 25.00% or more, or 30.00% or more.
  • On the other hand, since ferrite has low strength, if the area proportion thereof becomes excessive, it is not possible to obtain a desired tensile strength. Therefore, the area proportion of ferrite is preferably 60.00% or less. The area proportion of ferrite is more preferably 55.00% or less, and still more preferably 50.00% or less.
    • Area proportion of martensite: 40.00 to 85.00%
  • Martensite is a structure that is formed when fcc transforms to bcc at a relatively low temperature. Martensite is a structure composed of fine crystal grains with a high dislocation density and has a function of increasing the strength of the hot-rolled steel sheet. If the area proportion of martensite is 40.00% or more, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of martensite is preferably 40.00% or more. The area proportion of martensite is preferably 50.00% or more.
  • On the other hand, martensite has poor ductility, and if the area proportion thereof is excessive, the ductility of the hot-rolled steel sheet may be lowered. Therefore, the area proportion of martensite is preferably 85.00% or less. The area proportion of martensite is more preferably 80.00% or less, and still more preferably 75.00% or less, or 70.00% or less.
    • Area proportion of the residual structure: less than 45.00%
  • In this aspect, a total amount of less than 45.00% of pearlite and bainite may be contained as the residual structure. The area proportion of the residual structure may be 10.00% or less, or 5.00% or less. The residual structure may not be contained and a total area proportion may be 0.00%.
    • (2-3) Bainite steel
  • When the microstructure contains, in area proportion, less than 3.00% of retained austenite, and bainite and a very small amount of the residual structure, preferably, the area proportion of bainite is 50.00% or more, and the area proportion of the residual structure is less than 50.00%. If the area proportion of each structure is set as above, it is possible to improve the strength, ductility and hole expandability at the same time.
  • Respective structures in this aspect will be described below.
    • Area proportion of bainite: 50.00% or more
  • Bainite is a structure that is formed when fcc transforms to bcc at a low temperature. Bainite is a structure composed of fine crystal grains and carbides and has a function of improving the strength, ductility and hole expandability of the hot-rolled steel sheet in a well-balanced manner. If the area proportion of bainite is 50.00% or more, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of bainite is preferably 50.00% or more. The area proportion of bainite is more preferably 80.00% or more, 85.00% or more, or 90.00% or more.
  • The upper limit is not particularly specified, and may be 100.00% or less.
    • The residual structure: less than 50.00%
  • In this aspect, a total amount of less than 50.00% of pearlite, ferrite and martensite may be contained as the residual structure. The area proportion of the residual structure is more preferably 20.00% or less, 15.00% or less, or 10.00% or less. The residual structure may not be contained and a total area proportion may be 0.00%.
    • (2-4) Martensite steel
  • When the microstructure contains, in area proportion, less than 3.00% of retained austenite, martensite, and a very small amount of the residual structure, a total area proportion of martensite is 85.00% or more, and the area proportion of the residual structure is preferably less than 15.00%. If the area proportion of each structure is set as above, it is possible to improve the strength and hole expandability at the same time.
  • Respective structures in this aspect will be described below.
    • Total area proportion of martensite: more than 85.00%
  • As described above, martensite has a function of increasing the strength of the hot-rolled steel sheet. In addition, since martensite has a random crystal orientation structure, it has a function of improving hole expandability of the hot-rolled steel sheet. If the area proportion of martensite is more than 85.00%, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of martensite is preferably more than 85.00%. The area proportion of martensite is more preferably 90.00% or more, 93.00% or more, or 95.00% or more. The upper limit is not particularly specified, and may be 100.00% or less.
    • The residual structure: less than 15.00%
  • In this aspect, a total amount of less than 15.00% of pearlite, ferrite and bainite may be contained as the residual structure. The area proportion of the residual structure is more preferably 10.00% or less, 7.00% or less, or 5.00% or less. The residual structure may not be contained and a total area proportion may be 0.00%.
    • (2-5) Area proportion of pearlite: less than 5.00%
  • Pearlite is a lamellar microstructure in which cementite precipitates in layers between ferrites, and is a softer microstructure than bainite and martensite. If the area proportion of pearlite is 5.00% or more, carbon is consumed by cementite contained in pearlite, the strength of martensite and bainite decreases, and it may not be possible to obtain a tensile strength of 980 MPa or more. Therefore, in any of the aspects, the area proportion of pearlite may be less than 5.00%. The area proportion of pearlite is more preferably 3.00% or less. In order to improve the elongation-flangeability of the hot-rolled steel sheet, it is preferable that the area proportion of pearlite be reduced as much as possible, and the area proportion of pearlite is more preferably 0.00%.
  • In addition, the area proportion of pearlite here is an area proportion that applies to all of the above three steel types (DP steel, bainite steel and martensite steel).
  • Structures other than retained austenite are measured by the following method.
  • The area proportion of ferrite and of pearlite is measured by the following method. In the cross section parallel to the rolling direction, a sample is collected so that the depth of 1/4 of the sheet thickness from the surface (the region of a depth of 1/8 from the surface to a depth of 3/8 from the surface) and the center position in the sheet width direction can be observed. The cross section of the sample parallel to the rolling direction is mirror-finished and polished with colloidal silica containing no alkaline solution at room temperature for 8 minutes to remove the strain introduced into the surface layer of the sample.
  • At an arbitrary position on the cross section of the sample in the longitudinal direction, a region with a length of 50 µm and a position of a depth of 1/4 of the sheet thickness from the surface (region of a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface) and a center position in the sheet width direction is measured at 0.1 µm measurement intervals by an electron back scattering diffraction method to obtain crystal orientation information. The number of measurement points is at least 500 points. For measurement, an EBSD device composed of a thermal field emission scanning electron microscope (JSM-7001F commercially available from JEOL) and an EBSD detector (DVC5 type detector commercially available from TSL) is used. In this case, the degree of vacuum in the EBSD device is 9.6×10-5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is13, and the electron beam irradiation level is 62.
  • In addition, a reflected electron image is captured in the same field of view. First, crystal grains in which ferrite and cementite precipitate in layers are identified from the reflected electron image, and when the area proportion of the crystal grains is calculated, the area proportion of pearlite is obtained. Then, for crystal grains other than crystal grains determined as pearlite, in the obtained crystal orientation information, using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" bundled in the EBSD analysis device, a region in which the Grain Average Misorientation value is 1.0° or less is determined as ferrite. If the area proportion of the region determined as ferrite is determined, the area proportion of ferrite is obtained.
  • After the same observation surface as in the above measurement is polished, nital corrosion is performed, and using an optical microscope and a scanning electron microscope (SEM), at least three 30 µm×30 µm regions to a position of 1/4 of the sheet thickness from the surface (region of a depth of 1/8 in the sheet thickness direction from the surface to a depth of 3/8 in the sheet thickness direction from the surface) are observed. Image analysis is performed on the structure image obtained by this structure observation, and thus the area proportion of bainite is obtained. Then, repeller corrosion is performed on the same observation position, structure observation is then performed using an optical microscope and a scanning electron microscope, image analysis is performed on the obtained structure image, and thus the area proportion of martensite is calculated.
  • In the above structure observation, respective structures are identified by the following method.
  • Martensite is a structure having a high dislocation density and substructures such as blocks and packets within grains, and can be distinguished from other microstructures according to an electron channeling contrast image using a scanning electron microscope.
  • Bainite is an aggregate of lath-shaped crystal grains, which is a non-martensite structure among structures that do not contain Fe-based carbides having a major diameter of 20 nm or more inside the structure or a structure which contains Fe-based carbides having a major diameter of 20 nm or more inside the structure, and in which the Fe-based carbides have a single variant, that is, Fe-based carbides extend in the same direction. Here, Fe-based carbides elongated in the same direction are Fe-based carbides with a difference of 5° or less in the elongation direction.
  • Here, the rolling direction of the hot-rolled steel sheet is determined by the following method.
  • First, a test piece is collected so that the sheet thickness of the hot-rolled steel sheet cross section can be observed. The cross section of the collected test piece in the sheet thickness is mirror-polished, and then observed using an optical microscope. The observation range is the entire sheet thickness, and the direction parallel to the elongation direction of crystal grains is determined as the rolling direction.
    • Ref value indicating a ratio between average fracture surface units before and after plastic deformation: 2.00 or more
  • Generally, since the strength increases due to introduction of dislocation during plastic deformation, the crack arresting property of the hot-rolled steel sheet after plastic deformation deteriorates. Since cracks are formed on cleavage fracture surfaces called fracture surface units, it is important to refine the fracture surface units and bend the propagation path in order to restrict crack propagation. In the present embodiment, when the ratio of cleavage facet (Rcf) value indicating a ratio between fracture surface units before and after plastic deformation is controlled, deterioration in the crack arresting property after plastic deformation is reduced.
  • The Ref value indicates a ratio between average fracture surface units before and after plastic deformation, and the following formula is represented using the Cf1 value, which is the average fracture surface unit before undergoing plastic deformation, and the Cf2 value, which is the average fracture surface unit after undergoing plastic deformation.
    • Rcf=Cf1/Cf2
  • In the hot-rolled steel sheet subjected to plastic deformation, since the strength increases due to processing hardening according to introduction of dislocation, the crack arresting property deteriorates. On the other hand, the crack arresting property is also affected by the fracture surface unit, and as the fracture surface unit is finer, the propagation path is bent, and the crack propagation is minimized. Therefore, in order to exhibit a favorable crack arresting property even after plastic deformation, it is necessary to increase the Ref value.
  • It is thought that, if the Rcf value is less than 2.00, the effect of increasing the strength due to introduction of dislocation is stronger than the effect obtained by refining the fracture surface unit, and thus the crack arresting property deteriorates. Therefore, the Rcf value is 2.00 or more, and preferably 2.20 or more, and more preferably 2.30 or more.
  • Since a higher Rcf value is preferable, the upper limit is not particularly specified, and may be 5.00 or less, 4.00 or less, or 3.00 or less.
  • The Cf1 value and the Cf2 value can be obtained by the following method.
  • In the present embodiment, in order to calculate the Cf1 value and the Cf2 value, it is necessary to cause brittle fracture. As a test method for causing brittle fracture, for example, according to JIS Z 2242: 2018, a 2.5 mm sub-sized V notch test piece in which the width direction (C direction) of the hot-rolled steel sheet is the longitudinal direction of the test piece is prepared, and a Charpy impact test may be performed at -196°C. If the sheet thickness of the hot-rolled steel sheet is less than 2.5 mm, the test may be performed on the entire thickness.
  • Here, the rolling direction of the hot-rolled steel sheet is determined by the above method, and the direction perpendicular to the rolling direction is determined as the width direction of the hot-rolled steel sheet.
  • The SEM image capturing region that is captured in order to calculate the Cf1 value and the Cf2 value is a position from the surface of the steel sheet to a depth of 1/4 of the sheet thickness (region of a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface) and a center position in the sheet width direction in the cross section parallel to the rolling direction. For SEM image capturing, SU-6600 Schottky-emission electron gun (commercially available from Hitachi High-Tech Corporation), and a tungsten emitter are used, and in a vacuum of 9.6×10-5 Pa or less, the acceleration voltage is 1.5 kV. The imaging magnification is 1,000x, and the number of imaging fields is 3 or more.
  • In the captured SEM image, a ductile fracture part called a tear ridge is imaged with a bright contrast. The region surrounded by the tear ridge is defined as one cleavage facet, and the equivalent circle diameter is obtained from the area of each cleavage facet, and is used as the fracture surface unit of each cleavage facet. From the obtained fracture surface unit, the area average diameter weighted by the area of each cleavage facet is determined and used as the average fracture surface unit.
  • The above processing is performed on the hot-rolled steel sheet before plastic deformation and the hot-rolled steel sheet after plastic deformation, and thus the Cf1 value and the Cf2 value are calculated.
  • Here, for the Charpy impact test after plastic deformation, a JIS No. 5 tensile test piece in which the width direction (C direction) of the hot-rolled steel sheet is the longitudinal direction of the test piece is prepared, a compressive pre-strain of 10% is applied to the steel material in the longitudinal direction of the test piece and various test pieces are then collected.
    • Standard deviation of Mn concentration: 0.60 mass% or less
  • In the hot-rolled steel sheet according to the present embodiment, at a position from the surface to a depth of 1/4 of the sheet thickness (the region of a depth of 1/8 from the surface to a depth of 3/8 from the surface) and a center position in the sheet width direction, the standard deviation of the Mn concentration may be 0.60 mass% or less. Thereby, the development of the region in which Mn is locally concentrated and the fracture energy decreases is inhibited, and it is possible to further reduce the occurrence of local cracks during plastic deformation and deterioration of the crack arresting property after plastic deformation.
  • The standard deviation of the Mn concentration is preferably 0.50 mass% or less and more preferably 0.47 mass% or less. In order to minimize a decrease in fracture energy, a smaller value of the lower limit of the standard deviation of the Mn concentration is more desirable, and due to restrictions on the producing process, 0.10 mass% is the substantial lower limit.
  • After the cross section (L cross section) parallel to the rolling direction of the hot-rolled steel sheet is mirror-polished, a position from the surface of the steel sheet to a depth of 1/4 of the sheet thickness (region of a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface) and a center position in the sheet width direction are measured with an electron probe micro-analyzer (EPMA), and the standard deviation of the Mn concentration is measured. In measurement conditions, the acceleration voltage is 15 kV and the magnification is 5,000x (the region of a depth of 1/8 from the surface to a depth of 3/8 from the surface), and a distribution image in a range of 20 µm in the sheet thickness direction of the sample is measured. More specifically, measurement intervals are 0.1 µm, and the Mn concentration is measured at 40,000 or more points. Then, the standard deviation is calculated based on the Mn concentrations obtained from all the measurement points, and thus the standard deviation of the Mn concentration is obtained.
    • 3. Tensile property
  • Among mechanical properties of the hot-rolled steel sheet, the tensile property (tensile strength) is evaluated according to JIS Z 2241: 2011. The test piece is a No. 5 test piece according to JIS Z 2241: 2011. The tensile test piece is collected at a position a quarter from the edge in the sheet width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction.
  • The tensile (maximum) strength of the hot-rolled steel sheet according to the present embodiment is 980 MPa or more, and preferably 1000 MPa or more. If the tensile strength is less than 980 MPa, application parts are limited, and contribution to vehicle body weight reduction is small. The upper limit is not necessarily particularly limited, and may be 1,780 MPa or less, 1,500 MPa or less, or 1,300 MPa or less in order to reduce mold wear.
    • 4. Sheet thickness
  • The sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, and may be 1.20 to 8.00 mm. If the sheet thickness of the hot-rolled steel sheet is less than 1.20 mm, it may difficult to secure the rolling completion temperature, the rolling load may become excessive, and hot rolling may become difficult. Therefore, the sheet thickness of the hot-rolled steel sheet according to the present embodiment may be 1.20 mm or more, and is preferably 1.40 mm or more. On the other hand, if the sheet thickness is more than 8.00 mm, the effect of the standard deviation of the Mn concentration becomes significant, and it becomes difficult to obtain a desired Rcf value. Therefore, the sheet thickness may be 8.00 mm or less, and is preferably 6.00 mm or less, or 3.00 mm or less.
    • 5. Others (5-1) Plating layer
  • The hot-rolled steel sheet having the above chemical composition and microstructure according to the present embodiment may be a surface-treated steel sheet that has a plating layer on the surface in order to improve the corrosion resistance. The plating layer may be an electroplating layer or a melting plating layer. Examples of electroplating layers include zinc electroplating and electro Zn-Ni alloy plating. Examples of melting plating layers include melting zinc plating, alloying melting zinc plating, melting aluminum plating, melting Zn-Al alloy plating, melting Zn-Al-Mg alloy plating, and melting Zn-Al-Mg-Si alloy plating. The amount of plating adhered is not particularly limited, and may be the same as in the related art. In addition, it is possible to further improve the corrosion resistance by applying appropriate chemical conversion (for example, applying a silicate-based chromium-free chemical conversion solution and drying) after plating.
    • 6. Production conditions
  • A preferable method of producing the hot-rolled steel sheet having the above chemical composition and microstructure according to the present embodiment is as follows.
  • In order to obtain the hot-rolled steel sheet according to the present embodiment, the slab is heated under predetermined conditions and then hot-rolled, and accelerated cooling to a predetermined temperature range is performed, and then slow-cooling is performed as necessary, and it is effective to control a cooling history until coiling.
  • In the preferable method of producing the hot-rolled steel sheet according to the present embodiment, the following processes (1) to (7) are sequentially performed. Here, the temperature of the slab and the temperature of the steel sheet in the present embodiment are the surface temperature of the slab and the surface temperature of the steel sheet.
    1. (1) a slab is heated and held in a temperature range of 1,100°C or higher for 6,000 sec or longer. Here, more preferably, during heating, the slab is held in a temperature range of 700 to 850°C for 900 sec or longer and then additionally heated and held in a temperature range of 1,100°C or higher for 6,000 sec or longer.
    2. (2) hot rolling is performed in a temperature range of 850 to 1,100°C so that a total sheet thickness reduction is 90% or more.
    3. (3) the hot rolling start temperature is 850°C or higher and lower than 930°C, the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is 850°C or higher and lower than 950°C, and the rolling reduction rate for the rolling is less than 30%.
    4. (4) the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage are 930°C or higher and lower than 1,010°C, the rolling reduction rate for the rolling is 50% or more, and the rolling completion temperature is 950°C or higher and lower than 1,010°C.
    5. (5) within 1.0 sec after hot rolling is completed, cooling is performed at an average cooling rate of 50°C/sec or faster, and the cooling start temperature is 850°C or higher and lower than 960°C.
    6. (6) cooling is performed to a temperature range of 600 to 730°C at an average cooling rate of 50°C/sec or faster, and slow cooling with an average cooling rate of slower than 5°C/s is performed to a temperature range of 600 to 730°C for 2.0 sec or longer. Then, cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/s or faster.
      Here, slow cooling may not be performed, and when slow cooling is not performed, cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/s or faster.
    7. (7) Coiling is performed to a temperature range of 350°C or lower.
    (6-1) Slab, slab temperature and holding time during hot rolling
  • For a slab to be hot-rolled, a slab obtained by continuous casting or a slab obtained by casting and blooming can be used, and as necessary, one obtained by performing hot processing or cold processing on a slab can be used. A slab to be hot-rolled is preferably heated and held in a temperature range of 1,100°C or higher for 6,000 sec or longer. In addition, during holding at 1,100°C or higher, the steel sheet temperature may vary in a temperature range of 1,100°C or higher or may be constant. When the slab is held in a temperature range of 1,100°C or higher for 6,000 sec or longer, the austenite grains can be made uniform when the slab is heated. When austenite grains are made uniform, it is possible to minimize recrystallization of austenite at the first stage of hot rolling to be described below (first stage of hot rolling to the stage two stages before the final stage), and as a result, and it is possible to obtain a desired Rcf value. If the holding temperature is lower than 1,100°C or the holding time is shorter than 6,000 sec, it is difficult to make austenite grains uniform, it is not possible to minimize recrystallization of austenite at the first stage of hot rolling to be described below, and as a result, it may not be possible to obtain a desired Rcf value.
  • In addition, during slab heating, the slab is held in a temperature range of 700 to 850°C for 900 sec or longer, and then additionally heated, and may be held in a temperature range of 1,100°C or higher for 6,000 sec or longer. Here, during holding in a temperature range of 700 to 850°C, the steel sheet temperature may vary in a temperature range or may be constant. In austenite transformation in a temperature range of 700 to 850°C, Mn is distributed between ferrite and austenite, and when its transformation time is prolonged, Mn can diffuse in the ferrite region. Thereby, Mn microsegregation unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. If the standard deviation of the Mn concentration is large, a region in which Mn is locally concentrated and the fracture energy decreases develops, the occurrence of cracks during plastic deformation is promoted, and thus it may not possible to obtain a desired Rcf value.
  • For hot rolling, it is preferable to use a reverse mill or a tandem mill for multipass rolling. In particular, in consideration of industrial productivity and a stress load on the steel sheet during rolling, it is more preferable to perform hot rolling using a tandem mill for at least the final two stages.
    • (6-2) Rolling reduction rate for hot rolling: a total sheet thickness reduction of 90% or more in a temperature range of 850 to 1,100°C
  • When hot rolling is performed in a temperature range of 850 to 1,100°C so that a total sheet thickness reduction is 90% or more, mainly, recrystallized austenite grains are refined, and accumulation of strain energy in unrecrystallized austenite grains is promoted. Then, the recrystallization of austenite is promoted, atomic diffusion of Mn is promoted, and the standard deviation of the Mn concentration can be reduced. As a result, the occurrence of cracks during plastic deformation is promoted, and it is possible to obtain a desired Rcf value. Therefore, it is preferable to perform hot rolling in a temperature range of 850 to 1,100°C so that a total sheet thickness reduction is 90% or more. If the total rolling reduction rate in a temperature range of 850 to 1,100°C is less than 90%, the standard deviation of the Mn concentration increases, it is not possible to inhibit development of a region in which Mn is locally concentrated and fracture energy decreases, and the occurrence of cracks during plastic deformation may not be promoted. Thereby, it may not be possible to obtain a desired Ref value.
  • Here, the sheet thickness reduction in a temperature range of 850 to 1,100°C can be represented by {(t0-t1)/t0}×100(%) where the inlet sheet thickness before first rolling in rolling in this temperature range is t0, and the outlet sheet thickness after final stage rolling in rolling in this temperature range is t1.
    • (6-3) Hot rolling start temperature: 850°C or higher and lower than 930°C, rolling temperature from the first stage of hot rolling to the stage two stages before the final stage: 850°C or higher and lower than 950°C, and rolling reduction rate for the rolling: less than 30%
  • Preferably, the hot rolling start temperature is 850°C or higher and lower than 930°C, the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is 850°C or higher and lower than 950°C, and the rolling reduction rate from the first stage of hot rolling to the stage two stages before the final stage is less than 30%. When the hot rolling start temperature is set to a relatively low temperature, the temperature of the first stage of hot rolling is set to low, and rolling is performed at a low rolling reduction rate, it is possible to minimize recrystallization at the first stage of hot rolling and accumulate strains in the austenite grains. As a result, unrecrystallized austenite with a high dislocation density can be maintained within grains up to the latter stage of rolling. Thereby, it is possible to obtain a desired Ref value. If the rolling start temperature is 930°C or higher, the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is 950°C or higher, or the rolling reduction rate for the rolling is 30% or more, it is not possible to minimize recrystallization of austenite at the first stage to the stage two stages before the final stage of hot rolling, and as a result, it may not be possible to obtain a desired Ref value. In addition, if the hot rolling start temperature is lower than 850°C, or the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is lower than 850°C, it is difficult to set the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage to 930°C or higher, and as a result, it may not be possible to obtain a desired Rcf value.
    • (6-4) The rolling temperature at the final stage of hot rolling and the stage one stage before the final stage: 930°C or higher and lower than 1,010°C, rolling reduction rate for the rolling: 50% or more, and rolling completion temperature: 950°C or higher and lower than 1,010°C
  • Preferably, the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage is 930°C or higher and lower than 1,010°C, the rolling reduction rate at the final stage and the stage one stage before the final stage is 50% or more, and the rolling completion temperature (temperature after final stage rolling) is 950°C or higher. When the rolling reduction rate at the final stage of hot rolling and the stage one stage before the final stage is 50% or more, and the rolling completion temperature is 950°C or higher, it is possible to promote recrystallization of austenite at the latter stage of rolling. Recrystallization at the first stage of rolling is minimized, recrystallization of crystal grains with a high dislocation density is caused at the latter stage of rolling, and thus orientation difference between structures generated from the recrystallized austenite increases. Thereby, it is possible to increase the Ref value and it is possible to obtain a desired Ref value. It is thought that, between structures with a large orientation difference, crystal rotation occurs during plastic deformation, and thus the crack arresting property at the interface is improved, and the Ref value is improved. If the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage is lower than 930°C, the rolling reduction rate at the final stage and the stage one stage before the final stage is less than 50%, or the rolling completion temperature is lower than 950°C, recrystallization of austenite is insufficient, and it may not be possible to obtain a desired Rcf value.
  • In addition, the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage and the rolling completion temperature are lower than 1,010°C, it is possible to refine the structure by preventing coarsening of the austenite grain size. Thereby, it is possible to minimize the occurrence of cracks during plastic deformation and to increase the Ref value.
    • (6-5) Average cooling rate for cooling within 1.0 sec after hot rolling is completed: 50°C/sec or faster, cooling start temperature: 850°C or higher and lower than 960°C
  • In order to inhibit the growth of austenite crystal grains refined by hot rolling, preferably, cooling is performed at an average cooling rate of 50°C/sec or faster within 1.0 sec after hot rolling is completed, and the cooling start temperature is 850°C or higher and lower than 960°C. In order to perform cooling at an average cooling rate of 50°C/sec or faster within 1.0 sec after hot rolling is completed, cooling is performed at a high average cooling rate immediately after hot rolling is completed, for example, cooling water may be sprayed onto the surface of the steel sheet. When the cooling start temperature is 850°C or higher and lower than 960°C and cooling is performed at an average cooling rate of 50°C/sec or faster within 1.0 sec after hot rolling is completed, austenite crystal grains, as well as structures formed subsequently, can be refined. Thereby, it is possible to minimize the occurrence of cracks during plastic deformation and to increase the Ref value.
  • The cooling start temperature used here is a temperature immediately before cooling is performed at an average cooling rate of 50°C/sec or faster, for example, a temperature immediately before cooling water is sprayed onto the surface of the steel sheet.
  • In addition, the average cooling rate is a value obtained by dividing the temperature drop range of the steel sheet from when accelerated cooling starts (when the steel sheet is introduced into the cooling facility) until accelerated cooling is completed (when the steel sheet is taken out of the cooling facility) by the time required from when accelerated cooling starts until accelerated cooling is completed.
  • (6-6) Cooling is performed to a temperature range of 600 to 730°C at an average cooling rate of 50°C/sec or faster, and slow cooling with an average cooling rate of slower than 5°C/s is performed in a temperature range of 600 to 730°C for 2.0 sec or longer. Then, cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/s or faster.
  • Here, slow cooling may not be performed, and when slow cooling is not performed, cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/s or faster.
  • After the cooling, when cooling is performed to a temperature range of 600 to 730°C at an average cooling rate of 50°C/sec or faster, it is possible to inhibit formation of ferrite and pearlite with low strength. Thereby, the strength of the hot-rolled steel sheet is improved.
  • During cooling, when slow cooling with an average cooling rate of slower than 5°C/s is performed in a temperature range of 600 to 730°C for 2.0 sec or longer, bainite can be sufficiently precipitated. Thereby, since the structure is refined, it is possible to achieve both the strength and the crack arresting property of the hot-rolled steel sheet. In addition, the average cooling rate herein is a value obtained by dividing the temperature drop range of the steel sheet from the cooling stop temperature of accelerated cooling to the slow cooling stop temperature by the time required from when accelerated cooling stops until slow cooling stops.
  • Here, when slow cooling is performed in a high temperature range (660 to 730°C) within the temperature range of 600 to 730°C, it is possible to stably produce the above DP steel. In addition, when slow cooling is performed in a low temperature range (600°C or higher and lower than 660°C) within the temperature range of 600 to 730°C, it is possible to stably produce the above bainite steel.
  • In order to reduce the area proportion of pearlite and obtain a desired tensile strength, the average cooling rate from the cooling stop temperature of slow cooling to the coiling temperature is preferably 50°C/sec or faster. Thereby, a base phase structure can be made hard.
  • In addition, the average cooling rate herein is a value obtained by dividing the temperature drop range of the steel sheet from the cooling stop temperature of slow cooling with an average cooling rate of slower than 5°C/s to the coiling temperature by the time required from when slow cooling with an average cooling rate of slower than 5°C/s stops until coiling.
  • The upper limit of the time for which slow cooling is performed is determined by the facility layout, and may be generally shorter than 10.0 sec. In addition, the lower limit of the average cooling rate for slow cooling is not particularly set, but it may be 0°C/s or faster because increasing the temperature without cooling involves a large investment in facility.
  • Here, slow cooling may not be performed. When accelerated cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/sec or faster without performing slow cooling, it is possible to inhibit formation of ferrite and pearlite with a low strength and promote formation of martensite. Thereby, the structure is refined, it is possible to stably produce the above martensite steel, and it is possible to achieve both the strength and the crack arresting property of the hot-rolled steel sheet.
  • The average cooling rate herein is a value obtained by dividing the temperature drop range of the steel sheet from when accelerated cooling starts (when the steel sheet is introduced into the cooling facility) until accelerated cooling is completed (when the steel sheet is taken out of the cooling facility) by the time required from when accelerated cooling starts until accelerated cooling is completed.
  • The upper limit value of the cooling rate is not particularly specified, but if the cooling rate increases, the cooling facility will be large-scaled and the facility cost increases. Therefore, 300°C/sec or slower is preferable in consideration of facility costs.
    • (6-7) Coiling temperature: 350°C or lower
  • The coiling temperature is 350°C or lower. When the coiling temperature is 350°C or lower, it is possible to reduce the amount of iron carbides precipitated and reduce the variation in hardness distribution in the hard phase. As a result, it is possible to reduce the number of starting points and propagation paths of cracks, it is possible to minimize the occurrence of cracks during plastic deformation, and it is possible to obtain a desired Rcf value.
    • [Examples]
  • Next, effects of one aspect of the present invention will be described in more detail with reference to examples, but conditions in the examples are one condition example used for confirming the feasibility and effects of the present invention, and the present invention is not limited to this one condition example. In the present invention, various conditions can be used without departing from the gist of the present invention and as long as the object of the present invention can be achieved.
  • Steels having chemical compositions shown in Table 1 and Table 2 were melted, and slabs with a thickness of 240 to 300 mm were produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets shown in Table 5A and Table 5B were obtained under production conditions shown in Table 3A to Table 4.
  • Here, martensite steel was obtained by performing cooling to a temperature range of 350°C or lower at a desired average cooling rate without performing slow cooling. In addition, regarding the example in which slow cooling was performed, DP steel was obtained by performing slow cooling in a high temperature range (660 to 730°C) within the temperature range of 600 to 730°C, and bainite steel was obtained by performing slow cooling in a low temperature range (600°C or higher, lower than 660°C) within the temperature range of 600 to 730°C.
  • The area proportion of the microstructure, the Rcf value, the standard deviation of the Mn concentration and the tensile strength TS of the obtained hot-rolled steel sheets were obtained by the above methods. The obtained measurement results are shown in Table 5A and Table 5B.
    • Method of evaluating properties of hot-rolled steel sheet (1) tensile property
  • If the tensile strength TS was 980 MPa or more, it was determined as satisfactory because the hot-rolled steel sheet had high strength. On the other hand, if the tensile strength TS was less than 980 MPa, it was determined as unsatisfactory because the hot-rolled steel sheet did not have high strength.
    • (2) crack arresting property
  • The crack arresting property was evaluated by a Charpy impact test.
    According to JIS Z 2242: 2018, a 2.5 mm sub-sized V notch test piece in which the width direction (C direction) of the hot-rolled steel sheet was the longitudinal direction of the test piece was prepared, and the Charpy impact test was performed at -196°C. If the sheet thickness of the hot-rolled steel sheet was less than 2.5 mm, the test was performed on the entire thickness.
  • In addition, a JIS No. 5 B tensile test piece in which the width direction (C direction) of the hot-rolled steel sheet was the longitudinal direction of the test piece was prepared, a compressive pre-strain of 10% was applied to the steel material in the longitudinal direction of the test piece and the V notch test piece was then collected. The test piece was subjected to the Charpy impact test at -196°C by the above method and thus the absorbed energy after plastic deformation was obtained.
  • If the rate of reduction in the absorbed energy after plastic deformation (("absorbed energy before plastic deformation"-"absorbed energy after plastic deformation")/"absorbed energy before plastic deformation") was 30.00% or less, it was determined as satisfactory because deterioration in the crack arresting property before and after plastic deformation was little. On the other hand, if the rate of reduction in the absorbed energy after plastic deformation was more than 30.00%, it was determined as unsatisfactory because the deterioration in the crack arresting property before and after plastic deformation was large. [Table 1]
    Steel No. mass%, remainder of Fe and impurities Note
    C Si Mn sol. Al P S N O
    A 0.080 1.20 2.50 0.030 0.001 0.0002 0.0032 0.0038 Steel of present invention
    B 0.388 1.12 2.03 0.037 0.017 0.0040 0.0052 0.0037 Steel of present invention
    C 0.043 1.08 2.46 0.022 0.011 0.0039 0.0030 0.0036 Steel of present invention
    D 0.098 2.91 2.10 0.045 0.004 0.0012 0.0049 0.0037 Steel of present invention
    E 0.092 0.06 2.45 0.051 0.024 0.0035 0.0053 0.0042 Steel of present invention
    F 0.094 0.94 3.93 0.027 0.013 0.0025 0.0041 0.0037 Steel of present invention
    C 0.095 1.24 1.13 0.056 0.026 0.0038 0.0025 0.0049 Steel of present invention
    H 0.065 1.16 2.33 0.495 0.004 0.0033 0.0026 0.0031 Steel of present invention
    I 0.100 0.83 2.09 0.003 0.010 0.0034 0.0040 0.0022 Steel of present invention
    J 0.091 1.01 2.11 0.030 0.025 0.0033 0.0053 0.0049 Steel of present invention
    K 0.064 0.96 2.13 0.039 0.004 0.0011 0.0042 0.0019 Steel of present invention
    L 0.063 1.20 2.03 0.029 0.013 0.0014 0.0030 0.0012 Steel of present invention
    M 0.068 1.06 2.22 0.056 0.019 0.0027 0.0034 0.0040 Steel of present invention
    N 0.088 0.91 2.59 0.022 0.029 0.0025 0.0056 0.0044 Steel of present invention
    O 0.085 0.94 2.46 0.047 0.013 0.0040 0.0052 0.0013 Steel of present invention
    P 0.078 0.90 2.67 0.022 0.021 0.0006 0.0021 0.0030 Steel of present invention
    Q 0.075 1.25 2.23 0.022 0.023 0.0016 0.0027 0.0011 Steel of present invention
    R 0.067 1.06 2.28 0.035 0.029 0.0023 0.0022 0.0039 Steel of present invention
    S 0.081 1.16 2.01 0.045 0.023 0.0039 0.0051 0.0045 Steel of present invention
    T 0.415 1.08 2.65 0.044 0.025 0.0008 0.0042 0.0016 Comparative steel
    U 0.038 0.84 2.20 0.020 0.005 0.0021 0.0025 0.0035 Comparative steel
    V 0.097 3.15 2.35 0.057 0.027 0.0035 0.0043 0.0010 Comparative steel
    W 0.064 0.04 2.22 0.028 0.006 0.0031 0.0033 0.0032 Comparative steel
    X 0.069 0.99 4.21 0.028 0.014 0.0040 0.0057 0.0018 Comparative steel
    Y 0.086 1.16 0.94 0.048 0.002 0.0028 0.0025 0.0027 Comparative steel
    Z 0.074 1.27 2.61 0.000 0.009 0.0037 0.0055 0.0016 Comparative steel
    The underline indicates outside the scope of the present invention.
    [Table 2]
    Steel No. mass%, remainder of Fe and impurities Note
    Ti V Nb Cu Cr Mo Ni B Ca Mg REM Bi Zr Co Zn W Sn
    A Steel of present invention
    B Steel of present invention
    C Steel of present invention
    D Steel of present invention
    E Steel of present invention
    F Steel of present invention
    G Steel of present invention
    H Steel of present invention
    I Steel of present invention
    J 0.0017 0.0035 Steel of present invention
    K 0.0029 Steel of present invention
    L 0.006 0.14 Steel of present invention
    M 0.04 Steel of present invention
    N 0.086 0.156 0.093 Steel of present invention
    O 0.25 0.31 Steel of present invention
    P 0.68 Steel of present invention
    Q 0.17 0.03 Steel of present invention
    R 0.59 Steel of present invention
    S 0.183 0.0024 0.06 Steel of present invention
    T Comparative steel
    U Comparative steel
    V Comparative steel
    W Comparative steel
    X Comparative steel
    Y Comparative steel
    Z Comparative steel
    [Table 3A]
    Producti on No. Steel No. Slab heating Hot rolling Note
    Holding time in temperat ure range of 700 to 850°C Holding time in temperatu re range of 1,100°C or higher Total sheet thicknes s reductio n in range of 850 to 1,100°C Start temperatu re Lowest rolling temperatu re from first stage to stage two stages before final stage Highest rolling temperatu re from first stage to stage two stages before final stage Maximu m reductio n rate from first stage to stage two stages before final stage Rolling temperat e at stage one stage before final stage Reductio n rate at stage one stage before final stage Rolling temperatu re of final stage Reductio n rate of final stage Rolling completio n temperatu re
    s s % °C °C °C % °C % °C % °C
    1 A 1615 8760 93 918 926 946 18 943 54 956 62 974 Example of present invention
    2 A 1719 6624 97 852 869 947 28 946 51 952 51 970 Example of present invention
    3 A 862 9657 96 858 867 931 19 932 51 946 61 962 Example of present invention
    4 A 1908 5657 97 885 895 947 21 949 59 961 58 978 Comparati ve example
    5 A 1609 8414 88 903 917 937 26 957 50 974 52 990 Comparati ve example
    6 A 1981 9843 94 837 856 893 23 905 57 923 50 936 Comparati ve example
    7 A 1557 7768 97 851 836 857 17 896 59 917 62 935 Comparati ve example
    8 A 1867 6416 94 953 978 987 16 1013 54 1025 60 1042 Comparati ve example
    9 A 1754 6759 96 926 935 967 29 996 59 1014 64 1032 Comparati ve example
    10 A 1695 8400 91 856 864 909 36 949 60 953 65 961 Comparati ve example
    11 A 1819 7051 92 875 892 896 17 902 53 914 59 933 Comparati ve example
    12 A 1452 8593 96 914 930 945 19 932 51 921 65 936 Comparati ve example
    13 A 1884 7963 94 864 878 942 19 933 55 935 56 940 Comparati ve example
    14 A 1795 7414 97 925 930 947 24 967 60 990 65 1023 Comparati ve example
    15 A 1573 8686 97 903 909 924 17 953 32 967 61 981 Comparati ve example
    16 A 1455 7471 91 879 885 947 15 955 57 958 37 969 Comparati ve example
    17 A 1578 7809 96 905 917 930 16 948 58 962 57 970 Comparati ve example
    18 A 1801 7830 97 886 904 930 22 939 52 942 52 956 Comparati ve example
    19 A 1833 9243 94 926 935 946 23 967 56 982 60 1002 Comparati ve example
    20 A 1747 7085 93 880 880 925 18 933 50 940 56 954 Comparati ve example
    21 A 1706 6729 96 900 919 931 20 932 57 940 63 951 Comparati ve example
    22 A 1784 9173 91 886 890 922 19 958 58 958 60 970 Comparati ve example
    23 B 1725 6825 97 868 882 917 18 935 53 938 50 951 Example of present invention
    24 C 1752 9976 90 879 879 914 24 931 57 944 65 962 Example of present invention
    25 D 1589 7780 96 859 860 906 28 960 52 979 52 985 Example of present invention
    26 E 1848 8293 96 900 900 922 17 951 56 967 52 982 Example of present invention
    27 F 786 8119 95 907 912 940 19 946 52 965 53 977 Example of present invention
    The underline indicates that production conditions are not preferable.
    [Table 3B]
    Productio n No. Stee l No. Slab heating Hot rolling Note
    Holding time in temperatu re range of 700 to 850°C Holding time in temperatu re range of 1,100°C or higher Total sheet thicknes s reductio n in range of 850 to 1,100°C Start temperatu re Lowest rolling temperatu re from first stage to stage two stages before final stage Highest rolling temperatu re from first stage to stage two stages before final stage Maximu m reductio n rate from first stage to stage two stages before final stage Rolling temperat e at stage one stage before final stage Reductio n rate at stage one stage before final stage Rolling temperatu re of final stage Reductio n rate of final stage Rolling completio n temperatu re
    s s % °C °C °C % °C % °C % °C
    28 G 1619 9102 91 897 903 913 23 944 50 948 59 963 Example of present invention
    29 H 1666 9885 94 908 908 923 17 954 53 963 53 976 Example of present invention
    30 I 1807 6164 93 883 894 909 21 954 59 971 55 971 Example of present invention
    31 J 1457 8780 94 867 880 917 16 941 59 948 63 955 Example of present invention
    32 K 1927 6628 92 907 917 923 25 944 60 947 54 952 Example of present invention
    33 L 1607 8591 97 868 880 930 25 933 57 934 62 955 Example of present invention
    34 M 975 8131 91 850 864 903 23 958 55 976 51 978 Example of present invention
    35 N 1777 8700 91 926 941 949 25 959 53 965 55 981 Example of present invention
    36 O 1674 6123 97 865 871 926 16 936 53 948 50 968 Example of present invention
    37 P 833 7742 90 912 915 947 15 943 51 946 55 959 Example of present invention
    38 Q 1624 8350 93 915 924 932 18 944 55 961 51 969 Example of present invention
    39 R 1938 8151 96 858 860 899 27 935 51 938 59 953 Example of present invention
    40 s 1802 9402 96 886 904 913 25 943 60 949 51 950 Example of present invention
    41 T 1795 6034 97 858 863 873 16 954 60 965 65 970 Comparati ve example
    42 U 1487 7606 95 878 880 884 23 932 53 948 57 964 Comparati ve example
    43 V 1851 7713 96 912 922 925 19 938 54 949 61 969 Comparati ve example
    44 W 1456 7017 91 859 871 881 27 936 56 942 57 951 Comparati ve example
    45 X 1404 9862 91 904 914 928 20 936 53 949 56 962 Comparati ve example
    46 Y 1994 7492 93 859 862 868 24 947 59 959 65 965 Comparati ve example
    47 Z 1687 9885 93 926 926 935 21 952 50 966 53 967 Comparati ve example
    48 E 1726 8569 96 905 905 927 17 945 56 952 52 963 Example of present invention
    49 F 1658 8211 95 910 913 937 19 942 52 947 53 955 Example of present invention
    50 B 1866 7634 93 871 880 911 18 933 53 935 50 950 Example of present invention
    51 A 1769 7969 93 926 926 943 18 941 54 955 62 968 Example of present invention
    52 E 1639 4368 95 922 923 934 21 944 51 951 55 964 Comparati ve example
    53 B 1934 7752 94 928 923 941 18 937 31 946 35 953 Comparati ve example
    The underline indicates that production conditions are not preferable.
    [Table 4]
    Production No. Steel No. Cooling Coiling Note
    Time until cooling starts Average cooling rate Cooling start temperature Slow cooling time in temperature range of 600 to 730°C Average cooling rate until coiling Coiling temperature
    s °C/s °C s °C °C
    1 A 0.7 58 910 0.0 56 268 Example of present invention
    2 A 0.6 66 951 3.6 51 225 Example of present invention
    3 A 0.6 76 900 0.0 66 280 Example of present invention
    4 A 0.6 58 893 0.0 67 306 Comparative example
    5 A 0.8 69 910 0.0 68 336 Comparative example
    6 A 0.6 53 940 0.0 53 317 Comparative example
    7 A 0.7 72 873 0.0 67 244 Comparative example
    8 A 0.8 56 867 0.0 59 337 Comparative example
    9 A 0.8 51 916 0.0 53 272 Comparative example
    10 A 0.9 69 871 0.0 78 345 Comparative example
    11 A 0.7 58 935 0.0 52 233 Comparative example
    12 A 0.5 56 911 0.0 55 343 Comparative example
    13 A 0.7 60 860 0.0 65 326 Comparative example
    14 A 0.9 79 926 0.0 70 344 Comparative example
    15 A 1.0 71 860 0.0 72 232 Comparative example
    16 A 0.7 75 956 0.0 55 245 Comparative example
    17 A 2.6 55 899 0.0 54 292 Comparative example
    18 A 1.0 61 723 0.0 68 254 Comparative example
    19 A 0.7 57 976 0.0 66 310 Comparative example
    20 A 0.5 38 938 0.0 66 337 Comparative example
    21 A 1.0 67 857 0.0 35 350 Comparative example
    22 A 0.6 76 943 0.0 64 426 Comparative example
    23 B 1.0 63 950 5.2 63 313 Example of present invention
    24 C 0.8 51 891 0.0 62 330 Example of present invention
    25 D 0.8 76 863 0.0 66 237 Example of present invention
    26 E 0.8 68 919 5.0 62 312 Example of present invention
    27 F 0.5 77 860 4.0 50 267 Example of present invention
    28 G 0.7 71 935 0.0 52 252 Example of present invention
    29 H 0.9 78 949 0.0 66 347 Example of present invention
    30 I 0.6 55 921 4.4 65 237 Example of present invention
    31 J 0.9 77 892 0.0 61 276 Example of present invention
    32 K 0.9 64 889 0.0 75 296 Example of present invention
    33 L 0.8 65 938 2.5 70 209 Example of present invention
    34 M 0.7 56 862 2.4 56 337 Example of present invention
    35 N 0.5 61 878 5.7 52 238 Example of present invention
    36 O 1.0 75 922 0.0 67 225 Example of present invention
    37 P 1.0 67 880 5.0 60 346 Example of present invention
    38 Q 0.7 58 870 0.0 78 298 Example of present invention
    39 R 0.7 60 862 5.7 66 297 Example of present invention
    40 S 0.8 66 886 0.0 71 236 Example of present invention
    41 I 0.6 58 930 0.0 70 238 Comparative example
    42 U 0.5 68 892 0.0 61 337 Comparative example
    43 V 0.9 56 937 0.0 58 260 Comparative example
    44 W 0.8 69 947 0.0 80 316 Comparative example
    45 X 0.6 62 861 0.0 75 303 Comparative example
    46 Y 0.7 63 937 0.0 52 226 Comparative example
    47 Z 0.9 71 936 0.0 74 319 Comparative example
    48 E 0.7 69 952 2.1 63 341 Example of present invention
    49 F 0.6 75 938 6.2 52 286 Example of present invention
    50 B 0.7 80 929 2.2 63 256 Example of present invention
    51 A 0.9 61 932 0.0 71 296 Example of present invention
    52 E 0.8 73 958 5.5 63 241 Comparative example
    53 B 0.8 77 956 4.6 68 298 Comparative example
    The underline indicates that production conditions are not preferable.
    [Table 5A]
    Production No. Steel No. Sheet thickness Retained γ Ferrite Martensite Bainite Pearlite Ref value Standard deviation of Mn concentration Tensile strength TS Rate of reduction in absorbed energy after plastic deformation Note
    mm area% area% area% area% area% - mass% MPa %
    1 A 2.91 0.05 0.56 96.14 2.98 0.27 2.53 0.48 1068 6.99 Example of present invention
    2 A 2.91 1.06 35.97 61.19 1.34 0.44 2.39 0.47 1090 10.26 Example of present invention
    3 A 2.89 0.33 0.67 98.78 0.10 0.12 2.13 0.62 1075 21.19 Example of present invention
    4 A 2.91 0.36 0.14 98.80 0.58 0.12 1.68 0.63 1082 51.39 Comparative example
    5 A 4.37 0.13 0.59 97.64 1.62 0.02 1.79 0.63 988 46.64 Comparative example
    6 A 2.89 0.20 0.14 97.53 1.74 0.39 1.78 0.48 1099 33.52 Comparative example
    7 A 2.95 0.11 0.08 98.03 1.72 0.06 1.70 0.53 1076 45.79 Comparative example
    8 A 2.90 0.29 0.38 95.49 3.54 0.30 1.67 0.52 1020 59.47 Comparative example
    9 A 2.91 0.14 0.31 97.84 1.40 0.31 1.62 0.51 1017 52.15 Comparative example
    10 A 2.89 0.00 0.50 98.18 0.99 0.33 1.75 0.45 1086 37.18 Comparative example
    11 A 2.91 0.33 0.48 95.55 3.21 0.43 1.70 0.54 1118 53.80 Comparative example
    12 A 2.89 0.09 0.99 97.48 1.20 0.24 1.79 0.49 1044 55.81 Comparative example
    13 A 2.95 0.25 0.92 95.39 3.27 0.17 1.67 0.48 1035 34,82 Comparative example
    14 A 2.90 0.32 0.17 98.53 0.71 0.27 1.53 0.52 984 57.15 Comparative example
    15 A 2.94 0.16 0.65 96.85 2.22 0.12 1.79 0.46 1045 50.67 Comparative example
    16 A 2.89 0.31 1.00 95.93 2.50 0.26 1.68 0.53 1042 46.37 Comparative example
    17 A 2.93 0.37 0.30 98.78 0.28 0.27 1.65 0.49 923 39.46 Comparative example
    18 A 2.91 0.43 0.87 90.82 2.58 5.30 1.77 0.53 903 50.93 Comparative example
    19 A 2.95 0.42 0.23 98.83 0.18 0.34 1.67 0.54 917 31.04 Comparative example
    20 A 2.94 0.43 0.80 97.29 1.25 0.23 1.74 0.50 975 55.82 Comparative example
    21 A 2.90 0.03 0.97 92.53 0.49 5.98 2.31 0.50 905 15.48 Comparative example
    22 A 2.92 0.48 0.54 98.60 0.01 0.37 1.56 0.47 996 34.83 Comparative example
    23 B 2.85 2.43 3.89 4.19 84.89 4.60 2.16 0.47 999 25.11 Example of present invention
    24 C 2.89 0.44 1.00 97.43 1.01 0.12 2.25 0.45 992 18.85 Example of present invention
    25 D 2.89 2.29 0.17 93.47 3.99 0.08 2.02 0.45 1150 20.55 Example of present invention
    26 E 2.87 1.59 42.60 49.86 3.85 2.10 2.49 0.47 989 19.79 Example of present invention
    27 F 2.88 0.51 43.21 53.77 2.09 0.42 2.11 0.63 1069 21.46 Example of present invention
    The underline indicates outside the scope of the present invention or indicates that properties are not preferable
    [Table 5B]
    Production No. Steel No. Sheet thickness Retained γ Ferrite Martensite Bainite Pearlite Ref value Standard deviation of Mn concentration Tensile strength TS Rate of reduction in absorbed energy after plastic deformation Note
    mm area% area% area% area% area% - mass% MPa %
    28 G 2.93 0.45 0.96 96.23 1.15 1.21 2.21 0.53 988 16.74 Example of present invention
    29 H 2.86 0.16 0.77 95.77 3.27 0.03 2.42 0.53 1244 11.39 Example of present invention
    30 I 2.91 1.35 5.20 3.10 89.98 0.37 2.14 0.47 999 28.93 Example of present invention
    31 J 2.92 0.17 0.50 97.17 2.08 0.08 2.49 0.50 1191 16.40 Example of present invention
    32 K 2.93 0.24 0.95 97.98 0.68 0.15 2.30 0.47 1218 13.02 Example of present invention
    33 L 2.86 1.43 1.87 4.76 91.79 0.15 2.28 0.46 1057 18.88 Example of present invention
    34 M 2.87 0.57 1.49 1.99 95.46 0.49 2.49 0.56 1111 19.89 Example of present invention
    35 N 2.87 0.82 31.93 64.86 2.24 0.15 2.36 0.53 989 13.31 Example of present invention
    36 O 2.90 0.36 0.51 98.46 0.52 0.15 2.25 0.53 1286 19.52 Example of present invention
    37 P 2.88 0.75 3.49 6.42 89.21 0.13 2.04 0.64 1092 25.94 Example of present invention
    38 Q 2.89 0.36 0.38 96.49 2.56 0.21 2.27 0.47 1187 19.28 Example of present invention
    39 R 2.91 1.65 34.26 62.54 1.39 0.16 2.27 0.52 982 16.90 Example of present invention
    40 S 2.85 0.26 0.09 97.17 2.05 0.43 2.33 0.49 1224 12.66 Example of present invention
    41 T 2.87 3.18 0.85 91.00 0.47 4.50 1.57 0.45 1138 59.14 Comparative example
    42 U 2.89 0.48 0.57 95.29 3.55 0.11 2.21 0.52 900 13.87 Comparative example
    43 V 2.92 3.56 0.25 92.15 3.62 0.42 1.57 0.52 1159 48.99 Comparative example
    44 W 2.87 0.17 0.24 90.17 3.61 5.81 2.18 0.47 942 22.43 Comparative example
    45 X 2.86 0.19 0.16 98.35 1.17 0.13 1.55 0.61 1292 57.14 Comparative example
    46 Y 2.86 0.11 0.23 92.81 1.14 5.71 2.40 0.46 966 17.20 Comparative example
    47 Z 2.89 0.29 0.75 97.11 1.52 0.33 1.58 0.53 1000 50.70 Comparative example
    48 E 2.89 0.63 21.84 74.43 2.65 0.45 2.39 0.55 1186 15.63 Example of present invention
    49 F 2.93 1.22 54.36 41.18 3.13 0.11 2.43 0.53 985 17.55 Example of present invention
    50 B 2.95 0.33 2.65 33.96 62.80 0.26 2.34 0.46 1069 23.73 Example of present invention
    51 A 2.88 0.53 0.68 87.68 10.26 0.85 2.58 0.56 986 26.30 Example of present invention
    52 E 2.87 1.12 29.66 62.64 5.33 1.25 1.63 0.58 985 55.76 Comparative example
    53 B 2.93 2.14 4.26 7.98 82.36 3.26 1.48 0.51 993 58.62 Comparative example
    The underline indicates outside the scope of the present invention or indicates that properties are not preferable
  • Based on Table 5A and Table 5B, it can be understood that the hot-rolled steel sheets according to examples of the present invention had high strength and little deterioration in the crack arresting property after plastic deformation. On the other hand, it can be understood that the hot-rolled steel sheets according to the comparative examples did not have one or more of the above properties.

Claims (5)

  1. A hot-rolled steel sheet consisting of, as a chemical composition, in mass%,
    C: 0.040 to 0.400%,
    Si: 0.05 to 3.00%,
    Mn: 1.00 to 4.00%,
    sol. Al: 0.001 to 0.500%,
    P: 0.100% or less,
    S: 0.0300% or less,
    N: 0.1000% or less,
    O: 0.0100% or less,
    Ti: 0 to 1.000%,
    V: 0 to 1.000%,
    Nb: 0 to 1.000%,
    Cu: 0 to 2.00%,
    Cr: 0 to 2.00%,
    Mo: 0 to 1.00%,
    Ni: 0 to 2.00%,
    B: 0 to 0.0100%,
    Ca: 0 to 0.0200%,
    Mg: 0 to 0.0200%,
    REM: 0 to 0.1000%,
    Bi: 0 to 0.020%,
    one, two or more of Zr, Co, Zn and W: a total amount of 0 to 1.00%,
    Sn: 0 to 0.05%, and
    a remainder of Fe and impurities,
    wherein a microstructure contains, in area%,
    less than 3.00% of retained austenite,
    an Ref value indicating a ratio between average fracture surface units before and after plastic deformation is 2.00 or more, and
    a tensile strength is 980 MPa or more.
  2. The hot-rolled steel sheet according to claim 1,
    wherein the chemical composition contains, in mass%, one, two or more selected from the group of
    Ti: 0.010 to 1.000%,
    V: 0.010 to 1.000%,
    Nb: 0.010 to 1.000%,
    Cu: 0.01 to 2.00%,
    Cr: 0.01 to 2.00%,
    Mo: 0.01 to 1.00%,
    Ni: 0.02 to 2.00%,
    B: 0.0001 to 0.0100%,
    Ca: 0.0005 to 0.0200%,
    Mg: 0.0005 to 0.0200%,
    REM: 0.0005 to 0.1000%, and
    Bi: 0.0005 to 0.020%.
  3. The hot-rolled steel sheet according to claim 1 or 2,
    wherein the microstructure contains, in area%, 15.00 to 60.00% of ferrite and 40.00 to 85.00% of martensite.
  4. The hot-rolled steel sheet according to claim 1 or 2,
    wherein the microstructure contains, in area%, 50.00% or more of bainite.
  5. The hot-rolled steel sheet according to claim 1 or 2,
    wherein the microstructure contains, in area%, more than 85.00% of martensite.
EP22837230.6A 2021-07-08 2022-03-02 Hot-rolled steel sheet Pending EP4368736A1 (en)

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JPS499085B1 (en) 1970-07-02 1974-03-01
JP3499085B2 (en) 1996-06-28 2004-02-23 新日本製鐵株式会社 Low Yield Ratio High Tensile Steel for Construction Excellent in Fracture Resistance and Manufacturing Method Thereof
JP3467767B2 (en) 1998-03-13 2003-11-17 Ｊｆｅスチール株式会社 Steel with excellent brittle crack arrestability and method of manufacturing the same
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JP6108116B2 (en) * 2014-03-26 2017-04-05 Ｊｆｅスチール株式会社 Steel plates for marine, marine structures and hydraulic iron pipes with excellent brittle crack propagation stopping properties and methods for producing the same
EP3378961B1 (en) * 2015-11-19 2021-12-29 Nippon Steel Corporation High strength hot-rolled steel sheet and manufacturing method thereof
JP6860420B2 (en) * 2017-05-24 2021-04-14 株式会社神戸製鋼所 High-strength steel sheet and its manufacturing method
KR20220099570A (en) * 2019-12-23 2022-07-13 닛폰세이테츠 가부시키가이샤 hot rolled steel
JP7312362B2 (en) 2020-01-21 2023-07-21 マツダ株式会社 engine system

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