US20240167133A1 - Hot-rolled steel sheet and method for manufacturing same - Google Patents

Hot-rolled steel sheet and method for manufacturing same Download PDF

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US20240167133A1
US20240167133A1 US18/285,239 US202118285239A US2024167133A1 US 20240167133 A1 US20240167133 A1 US 20240167133A1 US 202118285239 A US202118285239 A US 202118285239A US 2024167133 A1 US2024167133 A1 US 2024167133A1
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cooling
martensite
hot
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Naoko Katou
Eisaku Sakurada
Hitoshi Nikaido
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Nippon Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties 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
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    • 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
    • C22C38/02—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
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
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    • 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
    • C22C38/38—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
    • C22C38/48—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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to a hot-rolled steel sheet and a method for manufacturing the same.
  • chassis parts represented by lower arms or the like have complicated shapes in order to perform part functions. Therefore, in order to secure both the strength and processability, a high-strength hot-rolled steel sheet having a sheet thickness of 2.0 to 6.0 mm may be applied to chassis parts.
  • hot-rolled steel sheets used in automobile parts are processed into complicated shapes as described above and thus, within processability, ductility and hole expandability are particularly required.
  • high-strength hot-rolled steel sheets to be applied to automobile parts high-strength hot-rolled steel sheets having a tensile strength of 780 MPa or more and excellent ductility and hole expandability have been required.
  • Ti and Nb are elements that precipitate fine alloy carbides in ferrite, and these fine alloy carbides contribute to improving the strength.
  • Si may be added to the hot-rolled steel sheet n order to strengthen the ferrite by precipitation with such Ti and. Nb.
  • Si may be added in many cases.
  • the hot-rolled steel sheet contains Si, there is a risk of a scale pattern being formed on the surface of the steel sheet, impairing the appearance and also deteriorating fatigue properties.
  • Patent Document 1 discloses a steel sheet having a structure mainly composed of ferrite-bainite to which 0.25 mass % or less of Si and Al are added, and a method for manufacturing the same.
  • Patent Document 2 discloses a high-strength steel sheet having both elongation (ductility) and hole expandability and improved fatigue strength using a structure mainly composed of ferrite in the metallographic structure and reducing the area ratio of martensite.
  • the present invention has been made in view of the above circumstances and an object of the present invention is to provide a hot-rolled steel sheet having excellent strength, ductility, hole expandability and elongation-flangeability and a method for manufacturing the same.
  • the gist of the present invention based on the above findings is as follows.
  • a hot-rolled steel sheet according to one aspect of the present invention having a chemical composition containing, in mass %,
  • FIG. 1 is a graph showing the relationship between molding height of an elongation-flanging molded part and breaking limit strain according to a side bend test in a conventional hot-rolled steel sheet having a tensile strength of 340 to 780 MPa.
  • FIG. 2 is a schematic view of a test piece used in the side bend test.
  • FIG. 3 is an image of cracks occurring on a sheared end surface that had been subjected to elongation-flanging molding, captured with a microscope.
  • FIG. 4 A is a cross-sectional image of the sheared end surface that had not been subjected to elongation-flanging molding.
  • FIG. 4 B is an SEM image of the vicinity of cracks formed on the sheared end surface shown in FIG. 4 A .
  • FIG. 5 is a diagram showing the relationship between breaking limit strain and coverage in this example.
  • FIG. 6 is a diagram showing the relationship between breaking limit strain and an average diameter dM of martensite in this example.
  • FIG. 7 is a diagram showing the relationship between coverage and water density in a quaternary cooling process in this example.
  • FIG. 8 is a diagram showing the relationship between an average diameter dM of martensite and a codling time in the quaternary cooling process in this example.
  • FIG. 9 is a diagram showing the relationship between coverage and an air cooling time in this example.
  • FIG. 10 A is a structure image (SEM image) of Test No. 21 in this example.
  • FIG. 10 B is a structure image (SEM image) of the vicinity of a sheared end surface after shearing was performed on Test No. 21 in this example.
  • FIG. 11 A is a structure image (SEM image) of Test No. 17 in this example.
  • FIG. 11 B is an enlarged view of an area A shown in FIG. 11 A .
  • the inventors will describe the results of examination of factors that influence elongation-flangeability in hot-rolled steel sheets, and new findings they obtained regarding the relationship between elongation-flangeability and a metallographic structure.
  • the inventors have found that, in conventional hot-rolled steel sheets, even though they had favorable ductility and hole expandability, fine cracks on the sheared end surface during shearing, and molding cracks at the elongation-flanged part caused by these fine cracks occur, and as a result, sufficient elongation-flangeability may not be obtained. Therefore, the inventors examined factors that caused molding cracks at the elongation-flanged part and an index indicating elongation-flangeability.
  • FIG. 3 is an image of cracks occurring on the sheared end surface that had been subjected to elongation-flanging molding, captured with a microscope. As shown in FIG. 3 , within the sheared end surface after shearing, on the sheared end surface subjected to elongation-flanging molding in the direction perpendicular to the rolling direction, cracks parallel to the sheet surface occurred. That is, the hot-rolled steel sheets manufactured under certain cooling conditions are speculated to have broken as described above because cracks penetrated in the sheet thickness direction starting from cracks parallel to the sheet surface.
  • FIG. 4 A is a cross-sectional in rage of the sheared end surface that had not been subjected to elongation-flanging molding. As shown in FIG. 4 A , it was found that cracks parallel to the sheet surface were already formed on the sheared end surface during a shearing stage.
  • FIG. 4 B shows the examination results of the relationship between cracks parallel to the sheet surface and the metallographic structure.
  • FIG. 4 B is an SEM image of the vicinity of cracks formed on the sheared end surface shown in FIG. 4 A .
  • FIG. 4 B is an SEM image captured after corrosion with LePera in which white areas indicate martensite, gray areas indicate ferrite, and black areas indicate voids formed by cracking in martensite.
  • LePera white areas indicate martensite
  • gray areas indicate ferrite
  • black areas indicate voids formed by cracking in martensite.
  • bainite acts effectively against martensite molding cracks.
  • the breaking limit strain evaluated by the side bend test is used as an index of elongation-flangeability. That is, the term “elongation-flangeability” as used in the present invention refers to the breaking limit strain (hereinafter simply referred to as limit strain) when the open cross section of the steel sheet is subjected to in-plane deformation and cracks penetrate in the sheet thickness direction. In the present invention, elongation-flangeability is evaluated based on the breaking limit strain by this side bend test.
  • the chemical composition and the metallographic structure of the hot-rolled steel sheet according to the present embodiment (hereinafter simply referred to as a steel sheet) will be described below in more. detail.
  • the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present invention.
  • a limited numerical value range indicated by “to” includes the lower limit value and the upper limit value, Numerical values of “less than” or “more than” are not included in the numerical value range.
  • % related to the chemical composition of the steel sheet is mass % unless otherwise specified.
  • the hot-rolled steel sheet according to the present embodiment has a chemical composition containing, in mass %, C: 0.035% or more and 0.085% or less, Si: 0.001% or more and 0.15% or less, Mn: 0.70% or more and 1.80% or less, P: 0.020% or less, S: 0.005% or less. Ti: 0.075% or more and 0.170% or less. Nb; 0.003% or more and 0,050% or less, Al: 0.10% or lore and 0.40% or less, and N: 0.008% or less, with the remainder: Fe and impurities. hereinafter, respective elements will be described in detail.
  • the C (carbon) content is 0.033% or more and 0.085% or less. If the C content is less than 0.035%, it is difficult to secure a sufficient area ratio of martensite. Therefore, the C content is 0.035% or more. In addition, it is not preferable to excessively reduce the C content in consideration of steelmaking cost. Accordingly, the C content is preferably 0.037% or more and shore preferably 0.040% or more. On the other hand, if the C content is more than 0.085%, there is a risk of the area ratio of martensite becoming excessively large. Therefore, the C content is 0.085% or less. In addition, it is preferable to minimize the C content in order to reduce the incidence of slab cracking in the casting process. Accordingly, the C content is preferably 0.065% or less.
  • Si 0.001% or More and 0.15% or Less
  • a lower Si (silicon) content is preferable in consideration of the appearance of the steel sheet.
  • the Si content is more than 0.15%, there is a risk of the area ratio of ferrite becoming excessively large. Therefore, the Si content is 0.15% or less.
  • a lower Si content is preferable in order to reduce costs in the pickling process for removing scale formed in the hot rolling process. Accordingly, the Si content is preferably 0.07% or less.
  • the Si content is preferably 0.003% or more.
  • Mn manganese
  • the Mn content is preferably 0.80% or more and more preferably 0.90% or more.
  • the Mn content is 1.80% or less.
  • the Mn content is preferably 1.75% or less, or 1.70% or less.
  • the Mn content is less than 1.20%, an ear wrinkle pattern may be formed at the width direction edge of the steel sheet or steel sheet coil. Generally, such an ear-wrinkle pattern part is trimmed, which leads to a decrease in yield. Therefore, the Mn content is preferably 1.20% or more.
  • P phosphorus
  • P is an element that is generally contained as an impurity, but has a function of increasing the strength of the hot-rolled steel sheet according to solid solution strengthening. Therefore, P may be intentionally included, but P is an element that segregates at grain boundaries, and also has a function of causing a decrease in ductility.
  • the P content is 0.020% or less.
  • the P content is preferably 0.015% or less. It is not necessary to particularly specify the lower limit of the P content, and a lower P content is preferable.
  • the lower limit of the P content can include 0%. However, if the P content is less than 0.001%, the refining cost in the steelmaking process becomes extremely high. Therefore, the P content is preferably 0.001% or more.
  • S sulfur
  • S is an element, that is contained as an impurity, and is an element that forms non-metallic inclusions to reduce ductility of the hot-rolled steel sheet.
  • the S content is 0.0050% or less.
  • the S content is preferably 0.0040% or less. It is not necessary to particularly specify the lower limit of the S content, and a lower S content is preferable.
  • the lower limit of the S content can include 0%. However, if the S content is less than 0.0001%, the refining cost in the steelmaking process increases. The S content is preferably 0.0001% or more in consideration of the refining cost.
  • Ti titanium is an element that precipitates fine alloy carbides in ferrite and has a function of increasing the strength. If the Ti content is less than 0.075%, a sufficient strength cannot be Obtained. Therefore, the Ti content is 0.075% or more, in addition, Ti is an effective element for improving hole expandability. In order to obtain these effects, the Ti content is preferably 0.090% or more. On the other hand, if the Ti content is more than 0.170%, the cold slab piece may crack. Therefore, the Ti content is 0.170% or less. The Ti content is preferably 0.150% or less.
  • Nb 0.003% or More and 0.050% or Less
  • Nb niobium
  • Nb is an element that precipitates fine alloy carbides.
  • Nb has a function of minimizing austenite crystal grain growth during hot rolling, and minimizing coarsening of the crystal grain size of ferrite that is transformed and formed thereafter. When these functions are exhibited, it is possible to increase the strength of the steel sheet. In order to obtain this effect, the Nb content is 0.003% or more.
  • Nb has a function of minimizing coarsening of crystal grains in the heat-affected zone during arc welding and minimizing softening of the heat-affected zone. In order to exhibit these effects, the Nb content is preferably 0.010% or more.
  • the Nb content is inure than 0.050%, the toughness of the hot-rolled slab decreases, and as a result, cracks and flaws may occur in the rolling process. Therefore, the Nb content is 0.050% or less.
  • the Nb content is preferably 0.045% or less.
  • Al is an element that is effective in deoxidizing steel and making the steel sheet sound.
  • Al is an element that effectively acts to improve the area ratio of ferrite. If the Al content is less than 0.10%, the area ratio of ferrite becomes insufficient. Therefore, the Al content is 0.10% or more.
  • Al also has a function of lowering the melting point of scale formed on the surface of the steel sheet in the hot rolling process, and thus it is possible to easily remove scale during heating. In order to obtain these effects, the Al content is preferably 0.20% or more. On the other hand, if the Al content is more than 0.40%, the area ratio of ferrite becomes excessively large, and the strength becomes insufficient. Therefore, the Al content is 0.40% or less.
  • the Al content is preferably 0.35% or less.
  • N nitrogen
  • Nb is an element that forms a nitride with Ti, Nb or Al. These nitrides lower the toughness of the hot-rolled slab and cause flaws and cracks, particularly, cracks at the corner parts (corner cracks) of the casting slab, in the rolling process. Therefore, in consideration f manufacturing, a lower N content is preferable, and the N content is 0.0080% or less.
  • the lower limit of the N content can include 0%.
  • the N content is preferably 0.0005% or more and more preferably 0.0010% or more.
  • the remainder of the chemical composition of hot-rolled steel sheet according to the present embodiment may include Fe and impurities.
  • impurities are elements that are mixed in from ores or scrap as raw materials or a manufacturing environment or the like, or elements that are intentionally added in very small amounts, and have a meaning that they 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 addition to the above elements in order to improve the strength, ductility or other properties. That is, in place of some Fe, one, two or more of Cr, B, Ca, Mo, Ni, Cu and REM may be contained as optional elements within the ranges to be described below. The lower limit of the content when these optional elements are not contained is 0%.
  • respective optional elements will be described in detail.
  • the Cr content is preferably 0.06% or more.
  • the Cr content is more preferably 0.10%.
  • the Cr content is preferably 0.27% or less.
  • the Cr content is more preferably 0.25% or less.
  • the B content has a function of increasing the tensile strength of the steel sheet.
  • the B content is preferably 0.0003% or more.
  • the B content is more preferably 0.0005% or more.
  • the B content is preferably 0.0050% or less.
  • the B content is more preferably 0.0040% or less.
  • Ca (calcium) has a function of spheroidizing non-metallic inclusions and increasing ductility.
  • the Ca content is preferably 0.0003% or more.
  • the Ca content is more preferably 0.0005% or more.
  • the Ca content is preferably 0.0050% or less.
  • the Ca content is more preferably 0.0040% or less.
  • Mo has a function of increasing the tensile strength of the steel sheet.
  • the Mo content is preferably 0.01% or more.
  • the Mo content is more preferably 0.03% or more.
  • the Mo content is more preferably 0.40% or less.
  • the Mo content is more preferably 0.35% or less.
  • Ni 0.01% or More and 0.50% or Less
  • Ni has a function of increasing the tensile strength of the steel sheet.
  • the Ni content is preferably 0.01% or more.
  • the Ni content is more preferably 0.08% or more.
  • the Ni content is preferably 0.50% or less.
  • the Ni content is more preferably 0.40% or less.
  • the Cu has a function of increasing the tensile strength of the steel sheet.
  • the Cu content is preferably 0.01% or mare.
  • the Cu content is more preferably 0.08% or more.
  • the Cu content is preferably 0.50% or less.
  • the Cu content is more preferably 0.40% or less.
  • the REM are elements that have a function of reducing the size of inclusions and are elements that contribute to improve hole expandability and ductility (elongation at break). If the REM content is less than 0.0003%, it is not possible to obtain a sufficient effect of these functions. Therefore, the REM content is preferably 0.0003% or more. The REM content is more preferably 0.0005% or more. On the other hand, if the REM content is more than 0.0300%, since castability and hot processability may deteriorate, the REM content is preferably 0.0300% or less.
  • REM refers to a total of 17 elements composed of Sc, Y and lanthanides
  • the REM content refers to a total content of these elements.
  • lanthanides they are industrially added in the form of misch metals.
  • the chemical composition of the hot-rolled steel sheet described above may be measured by TCP optical emission spectroscopy using chips according to JIS G 1201:2014.
  • ICP-AES inductively coupled plasma-atomic emission spectrometry .
  • C and S may be measured using a combustion-infrared absorption method
  • N may be measured using an inert gas fusion-thermal conductivity method.
  • the metallographic structure contains, in area %, 53.0% or more and 76.0% or less of ferrite, 3.0% or more and 10.0% or less of martensite, 14.0% or more and 39.0% or less of bainite, and 2.6% or less of pearlite.
  • the average diameter of martensite is 0.26 ⁇ m or more and 0.70 ⁇ m or less, and among all interfaces of martensite, the total length of interfaces between martensite and bainite is 75.0% or more with respect to the total length of all interfaces of martensite.
  • the metallographic structure at a depth of 1 ⁇ 4 of the sheet thickness from the surface of the sheet thickness cross section parallel to the rolling direction and at the center position in the sheet width direction is specified.
  • the reason for this is that the metallographic structure at this position shows a typical metallographic structure of the hot-rolled steel sheet.
  • ferrite is a soft structure, it is a metallographic structure that is mainly responsible for deformation.
  • a dual-phase steel sheet containing martensite such as the hot-rolled steel sheet of the present embodiment
  • the area ratio of ferrite is 53.0% or more.
  • the area ratio of ferrite is preferably 57.0% or lore and more preferably 60.0% or more.
  • the area ratio of ferrite is more than 76.0%, a desired strength may not be obtained. Therefore, the area ratio of ferrite is 76.0% or less.
  • the area ratio of ferrite is preferably 73.0% or less, and more preferably 70.0% or less.
  • martensite Since martensite is a rigid structure, it contributes to improving the strength of the hot-rolled steel sheet. If the area ratio of martensite is less than 3.0%, a desired strength may not be obtained. Therefore, the area ratio of martensite is 3.0% or more. The area ratio of martensite is preferably 4.0% or more. On the other hand, if the area ratio of martensite is more than 10.0%, hole expandability ay significantly deteriorate. Therefore, the area ratio of martensite is 10.0% or less. The area ratio of martensite is preferably 9.0% or less, more preferably 8.0 or less, and still more preferably 7.0% or less.
  • Bainite 14.0% or More and 39.0% or Less
  • Bainite is a structure that improves the strength and ductility of the hot-rolled steel sheet.
  • the metallographic structure is arranged so that martensite is surrounded by bainite, it is possible to improve elongation-flangeability. If the area ratio of bainite is less than 14.0%, it is difficult to arrange the above metallographic structure, and a desired elongation-flangeability cannot be obtained. Therefore, the area ratio of bainite is 1.4.0% or more.
  • the area ratio of bainite is preferably 17.0% or more, more preferably 20.0% or more, and still more preferably 25.0% or more.
  • the area ratio of bainite is 39.0% or less.
  • the area ratio of bainite is preferably 35.0% or less, more preferably 30.0% or less, and still more preferably 28.0% or less.
  • the area ratio of pearlite is more than 2.6%, hole expandability may deteriorate. Therefore, the area ratio of pearlite is 2.6% or less, preferably 1.7% or less, and more preferably 1.2% or less.
  • the area ratio of pearlite may be 0%.
  • retained austenite may be included. However, if the area ratio of retained austenite is more than 4.0%, the toughness may decrease. Therefore, if retained austenite is included, the area ratio of retained austenite is preferably 4.0% or less and more preferably 3.0% or less. The area ratio of retained austenite may be 0%.
  • the ratio (area %) of each structure can be measured by the following method.
  • a value measured from metallographic structure information such as a metallographic structure image captured with a scanning electron microscope in the cross section parallel to the rolling direction of the hot-rolled steel sheet may be used.
  • the metallographic structure information such as a metallographic structure image may be obtained by cutting out a width center position in the direction perpendicular to the rolling direction and the sheet thickness direction in parallel to the rolling direction and using an observation field including a depth of 3 ⁇ 8 of the sheet thickness from the surface in the sheet thickness direction at the center. Three or more observation fields are set, and the average value of area ratios of the metallographic structures measured in respective fields of view can be used as the value of the area ratio of the typical metallographic structure of the steel sheet.
  • the area ratios of ferrite, pearlite, bainite, martensite, and retained austenite, the average diameter of martensite, and the coverage are measured in the same field of view.
  • the area ratio of ferrite (hereinafter referred to as V ⁇ ) is an area ratio of the ferrite structure determined by an electron backscatter diffraction (EBSD) method.
  • EBSD electron backscatter diffraction
  • crystal orientation mapping data crystal orientation mapping data measured by the EBSD method is obtained.
  • JSM-7001F thermal field emission scanning electron microscope
  • Hikari detector EBSD detector
  • the crystal orientation mapping data can be obtained using software “OIM Analysis (registered trademark)” bundled in the EBSD analysis device.
  • the degree of vacuum in the device may be 9.6 ⁇ 10 ⁇ 5 Pa or less, and the accelerating voltage may be 20 kv.
  • crystal grains are defined from crystal orientation mapping data.
  • the crystal grain refers to an area surrounded by boundaries in which a crystal orientation difference between an arbitrary measurement point in the crystal orientation mapping data and a measurement point adjacent thereto is 15° or more, that is, grain boundaries.
  • the crystal grains defined in the first step are crystal grains of ferrite.
  • a local misorientation average (GAM value) is used for this method for determining ferrite. This GAM value is a value indicating misorientation of crystal grains. If the GAM value of crystal grains to be determined is within 0.35°, the crystal grains are determined to be ferrite.
  • the third step determination in the second step is performed for all crystal grains recorded in the crystal orientation mapping data. Then, a ratio of the number of measurement points belonging to the crystal grains determined to be ferrite to the total number of measurement points of the crystal orientation mapping data is calculated. This ratio is defined as an area ratio of ferrite.
  • Crystal orientation mapping data may include a total of 1,000 crystal grains so that the error due to the measurement position in the area ratio of ferrite can be sufficiently reduced.
  • the measurement magnification when crystal orientation mapping data is obtained by EBSD analysis is set so that the field of view includes 1,000 crystal grains.
  • the measurement magnification may be 250.
  • an area of 500 ⁇ m ⁇ 500 ⁇ m is measured at a magnification of 250.
  • the measurement range of the area ratio of ferrite is a quadrangle having sides in the sheet thickness direction and the rolling direction.
  • the length of the side in the sheet thickness direction is 500 ⁇ m, and the length of the side in the rolling direction may be the same as that of the side in the sheet thickness direction.
  • the area ratio of ferrite is measured in a range including a position of 3 ⁇ 8 of the sheet thickness from the surface in the sheet thickness direction.
  • the crystal orientation measurement interval within the measurement range is 0.3 ⁇ m. If the measurement interval is less than 0.03 ⁇ m, the electron beam interference range may overlap. On the other hand, if the measurement interval is more than 0.03 ⁇ m, the number of crystal orientation measurement points included in the crystal grain is insufficient, and measurement errors are likely to occur.
  • a ferrite measurement sample may be cut out so that the width center position in the direction perpendicular to the rolling direction and the sheet thickness direction is parallel to the rolling direction, and may be observed in the direction perpendicular to the rolling direction and the sheet thickness direction.
  • the area ratio of martensite (hereinafter referred to as VM) is a value measured from the metallographic structure exposed by corrosion with LePera. Within the metallographic structure exposed due to corrosion with LePera, the metallographic structure observed with white contrast is identified as martensite. The ratio of the white contrast area within the entire area in the observation field, that is, the area of the metallographic structure identified as martensite, is the area ratio VM of martensite.
  • the metallographic structure may be imaged at a magnification of 5,000.
  • the magnification is preferably 5,000.
  • an area of 500 ⁇ m ⁇ 500 ⁇ m is observed at a magnification of 5,000, and the area ratio of martensite is measured.
  • the accelerating voltage when electron beam is emitted is ire a range of 10.0 kV or more and 15.0 kV or less. If the accelerating voltage is more than 15.0 kV, grain boundaries may become blurred. On the other hand, if the accelerating voltage is less than 10.0 kV, since the resolution decreases, it is unsuitable for observation.
  • the area ratio VM of martensite is obtained from the measurement range in which the total number of crystal grains is 600 or more.
  • the total number of crystal grains within the measurement range may be 1,000.
  • the measurement range is a range including a position of 3 ⁇ 8 of the sheet thickness from the surface in the sheet thickness direction.
  • the measurement range is 500 ⁇ m in the sheet thickness direction and is a range of 500 ⁇ m in the rolling direction.
  • the area ratio VB of the bainite structure is defined as the remainder obtained by subtracting a sum of the area ratios of ferrite and martensite obtained by the above method, retained austenite to be described below, and pearlite from 100%.
  • the area ratio of pearlite (hereinafter referred to as VP) is a value measured frons the metallographic structure exposed due to nital corrosion.
  • a pearlite measurement sample may be cut out so that the width center position in the direction perpendicular to the rolling direction and the sheet thickness direction is parallel to the rolling direction, and may be observed in the direction perpendicular to the rolling direction and the sheet thickness direction.
  • the collected pearlite measurement sample a metallographic structure image is obtained in the measurement range centered at a position of 3 ⁇ 8 in the sheet thickness direction from the surface of the steel sheet.
  • the pearlite measurement sample is the same as the sample for measuring the area ratios of ferrite and martensite.
  • a metallographic structure image for measuring area ratio of pearlite is obtained to use a scanning electron microscope.
  • the accelerating voltage when an electron beam is emitted is in a range of 10.0 kV or more and 15.0 kV. If the accelerating voltage is more than 15.0 kV, grain boundaries may become blurred. On the other hand, if the accelerating voltage is less than 10.0 kV, since the resolution decreases, it is unsuitable for observation.
  • the metallographic structure may be imaged at a magnification of 2,000 or more. Here, if the magnification is 10,000 or less, one or more pearlite grains can be imaged within one field of view.
  • the magnification may be 10,000 or less.
  • the magnification is preferably 5,000.
  • the measurement range is a range of 10 ⁇ m or more and 40 ⁇ m or less in the sheet thickness direction and 10 ⁇ m or more and 55 ⁇ m or less in the rolling direction.
  • the area ratio of retained austenite (hereinafter referred to as V ⁇ ) is a value obtained by dividing the number of crystal orientation measurement points in which the crystal structure is determined to be fcc among the crystal orientation mapping data used when the above area ratio V ⁇ of ferrite is obtained by the total number of measurement points of the crystal orientation mapping data.
  • the crystal orientation mapping data used for measuring the area ratio V ⁇ of retained austenite is the same data used for measuring the area ratio V ⁇ of ferrite. That is, the measurement range, the measurement magnification, and the field of view may be the same as in the method for measuring the area ratio of ferrite.
  • the metallographic structure As described above, set the area ratio to be within a desired range and set the ratio of the interface length between martensite and bainite to the total length of interfaces of martensite, and the average diameter dM of martensite to be within a desired range.
  • the ratio of the interface length between martensite and bainite to the total length of interfaces of martensite (hereinafter referred to as coverage) is 75.0% or more. It is thought that, since bainite is a metallographic structure with an intermediate strength between ferrite and martensite, it has a role of reducing the difference in deformation between ferrite and martensite, that is, a role such as a cushion. If the coverage of martensite with bainite is less than 75.0%, the role as the cushion becomes insufficient, and fine cracks occur on the sheared end surface. Then, as a result, it is difficult to obtain excellent elongation-flangeability.
  • the ratio of the interface length between martensite and bainite to the total length of interfaces of martensite is preferably as high as possible, and preferably 78% or more.
  • the upper limit of the ratio of the length of the interface between martensite and the bainite tea the total length of interfaces of martensite is not particularly defined, and may be 100%.
  • the average diameter dM of martensite is 0.26 ⁇ m or more and 0.70 ⁇ m or less in order to reduce the occurrence of voids. If the average diameter dM is set to be within the above range, it is possible to minimize the occurrence of fine cracks on the sheared end surface, and as a result, it s possible to obtain high elongation-flangeability, if the average diameter dM of martensite is more than 0.70 ⁇ m, deformation concentrates at the interfaces between maartsite and bainite due to a difference in hardness. As a result, even if the coverage with bainite is satisfactory, there is a risk of voids being formed in the vicinity of the interface between martensite and bainite.
  • the average diameter dM of martensite is 0.70 ⁇ m or less.
  • the average diameter dM of martensite Is preferably 0.65 ⁇ m or less, and more preferably 0.60 ⁇ m or less.
  • the average diameter dM of martensite is less than 0.26 ⁇ m there is a risk of martensite not contributing to the strength.
  • the average diameter dlvi of martensite is less than 0.26 ⁇ m, the coverage may decrease. Therefore, the average diameter dM of martensite is 0.26 ⁇ m or more.
  • the average diameter dM of martensite is 0.30 ⁇ m or more.
  • the ratio of the interface length between martensite and bainite to the total length of interfaces of martensite is the ratio of a total boundary length (interface length) between martensite and bainite to a total boundary length (interface length) between martensite and other metallographic structures adjacent thereto.
  • a method for determining this ratio will be described below.
  • the total length of interfaces of martensite is a sum value obtained by measuring the lengths of boundaries (interface length) between martensite identified by the above method and other metallographic structures adjacent thereto.
  • the total length of interfaces of martensite may be determined using a metallographic structure image captured by the same method as the method for measuring the average diameter dM of martensite to be described below. Specifically, 300 martensite grains are selected from the captured metallographic structure image, and the interface length of these grains is determined.
  • the lengths of boundaries between martensite and other metallographic structures adjacent thereto are measured, and a sum of all length values is a sum of boundary lengths between martensite and other metallographic structures adjacent thereto, that is, the total length of interfaces of martensite.
  • the sum of boundary lengths between martensite and bainite is a sum of values obtained by measuring the lengths of boundaries between martensite identified by the above method and bainite in contact therewith. This value is a value measured using the same martensite as a measurement target when the total length of interfaces of martensite is measured, and the number of measurements is also the same. That is, the “boundary between martensite and bainite” means the boundary between martensite and bainite among the boundaries between martensite identified by the above method and other metallographic. structures adjacent thereto, and a sum of the, lengths of boundaries is “the boundary length between martensite and bainite.”
  • a value obtained by dividing a sum of boundary lengths between martensite and bainite obtained by the above method by a sum of boundary lengths between n martensite and other metallographic structures adjacent thereto is a coverage of martensite with bainite, that is, a ratio of the interface length between martensite and bainite to the total length of interfaces of martensite.
  • the martensite of the hot-rolled steel sheet of the present embodiment has a sheet-like form. Therefore, martensite crystal grains are approximated to an ellipse, their long diameters and short diameters are measured, and the average value thereof is used as the average diameter of measured martensite grains. Then, the average value of all measured martensite diameters is calculated, and the average value of these values is defined as the average diameter dM of martensite of the hot-rolled steel sheet.
  • the number of martensite grains whose average diameter dM is measured is 300.
  • most martensite grains to be measured are fine (with a diameter of several ⁇ m or less). Therefore, it is preferable to perform measurement using a metallographic structure image captured at a magnification of 5,000.
  • the field of view of the metallographic structure image used when the average diameter dM is measured and the measurement sample are the same as the field of view used when the area ratio of martensite is measured.
  • the hot-rolled steel sheet according to the present embodiment may have properties of a tensile strength of 780 MPa or more, a ductility (elongation at break) of 15.0% or more, and a hole expandability (hole expansion rate) of 60% or more.
  • the breaking limit strain by the side bend test to be described below may be 0.5 or more.
  • the hot-rolled steel sheet according to the present embodiment may have a tensile strength of 780 MPa or more. If the tensile strength is set to 780 MPa or more, it is possible to contribute to reducing the weight of automobile bodies and parts. It is not necessary to particularly specify the upper limit, and the upper limit may be 950 MPa or less.
  • the hot-rolled steel sheet according to the present embodiment may have an elongation at break of 15.0% or more.
  • the hot-rolled steel sheet according to the present embodiment may have a hole expandability (hole expansion rate) of 60% or more.
  • the tensile strength and the elongation at break are measured according to JIS Z 2241:2011 using No. 5 test piece of JIS Z 2241:2011.
  • a tensile test piece is taken in the direction perpendicular to the rolling direction and the sheet thickness direction (sheet width direction) so that it includes a 1 ⁇ 4 part from the edge of the steel sheet.
  • the tensile test piece is taken with the direction perpendicular to the rolling direction as the longitudinal direction.
  • the crosshead speed in the tensile test may be set under conditions in which the strain rate is constant at 0.005 s ⁇ 1 .
  • Hole expandability is evaluated by the hole expansion rate ( ⁇ ) specified in JIS Z 2256:2010. Specifically, using a ⁇ 10 mm punch, a die diameter is selected so that the clearance becomes 12.5%, and holes are punched out. Then, a hole expansion test is performed using a conical die with a tip angle of 60° with the burr on the outside and at a stroke speed of 10 mm/min. The test is stopped when cracks formed around the hole penetrate through the sheet thickness, and the hole diameters before and after the hole expansion test are compared.
  • the breaking limit strain evaluated by the side bend test to be described below is used as an index of elongation-flangeability.
  • the breaking limit strain of the hot-rolled steel sheet of the present embodiment may be 0.5 or more.
  • the inventors examined the relationship between the molding height of the elongation-flanging molded part and the breaking limit strain according to the side bend test using a conventional hot-rolled steel sheet having a tensile strength of 340 to 780 MPa. The results are shown in FIG. 1 .
  • white symbols indicate cases in which molding was possible, and black symbols indicate case in which cracks occurred.
  • 780 material indicates a 780 MPa material.
  • the breaking limit strain obtained by the following side bend test may be 0.5 or more, and is preferably 0.6 or more.
  • the breaking limit strain which is an index of elongation-flanging moldability, is a value measured in the following side bend test.
  • side bend test in the present embodiment methods described in “Nippon Steel Technical Report No. 393, (2012) p. 18 to 24” and “Japanese Unexamined Patent Application, First Publication No. 2009-14538” are used.
  • the shape of the test piece for the side bend test is the shape shown in FIG. 2 .
  • the semicircular part of the test piece may be formed by shearing. Specifically, first, a sheet of 35 mm ⁇ 100 mm is cut out front a steel sheet. Then, a semicircular hole is punched out in the sheet using a (1)30 mm punch under conditions in which the sheet thickness clearance (a value obtained by dividing a gap between a punch and a die by a sheet thickness) is 12.5%. A test piece shown in FIG. 2 is prepared according to these processes. In addition, the radius of the semicircular part, of the test piece is 15 mm.
  • a test piece is deformed at a stroke speed of 10 mm/min, and formation of a crack at the edge of the hole is defined as “breakage.”
  • breakage After breaking, the breaking limit strain with a gauge length of 6.0 mm is measured using a total of three elements including a determined element and elements adjacent thereto.
  • the sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 1.6 to 8.0 mm. Particularly, in consideration of application to automobile chassis, the sheet thickness may be 1.6 mm or more in ninny cases. Therefore, the sheet thickness of the hot-rolled steel sheet according to the present embodiment may be 1.6 mm or more, and is preferably 1.8 mm or more, 2.0 mm or more. In addition, if the sheet thickness is set to 8.0 mm or less, the metallographic structure can be easily relined, and it is possible to easily secure the above metallographic structure. Therefore, the sheet thickness may be 8.0 mm or less, and is preferably 7.0 mm or less.
  • the hot-rolled steel sheet according to the present embodiment having the chemical composition and metallographic structure described above may have a plating layer on the surface, in order to improve corrosion resistance, and may be used as a surface-treated steel sheet.
  • the plating layer may be an electroplating layer or a melt plating layer.
  • electroplating layers include electrogalvanizing and electro Zn—Ni alloy plating.
  • melt plating layers include melt galvanizing, alloyed melt galvanizing, melt aluminum plating, melt Zn—Al alloy plating, melt Zn—Al—Mg alloy plating, and melt 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.
  • an appropriate chemical conversion treatment for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying
  • the temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
  • the temperature of the hot-rolled steel sheet is measured with a contact type or non-contact type thermometer if it is the endmost part in the sheet width direction. If it is a part other than the endmost part of the hot-rolled steel sheet in the sheet width direction, it is measured by a thermocouple or calculated by heat transfer analysis.
  • a method for manufacturing a hot-rolled steel sheet according to the present embodiment that is performed on a slab having the above chemical composition including a hot rolling process in which rolling is performed under conditions in which a final finishing temperature is 880° C. or higher and 950° C. or lower; a primary cooling process in which cooling is performed to a primary cooling stop temperature of 680° C. or higher and 760° C.
  • a secondary cooling process in which cooling is performed at an average cooling rate of 20° C./sec or slower for 1.6 seconds or longer and 6.3 seconds or shorter, after the primary cooling process
  • a tertiary cooling process in which cooling is performed to a tertiary cooling stop temperature of 195° C. or higher and 440° C.
  • a quaternary cooling process in which water cooling is performed at a water density of 2.0 m 3 /min/mm 2 or more and 7.2 m 3 /min/mm 2 or less for 0.33 seconds or longer and 1.50 seconds or shorter, after the tertiary cooling process; a quinary cooling process in which air cooling is performed for 3.0 seconds or longer and 5.0 seconds or shorter, after the quaternary cooling process; and a winding process in which winding is performed at lower than 180° C., after the quinary cooling process.
  • a hot slab having the above chemical composition is subjected to rough rolling, and final rolling is then performed under conditions in which the final rolling outlet temperature (final finishing temperature) is 880° C. or higher and 950° C. or lower.
  • the area ratio of ferrite can be set to be within an appropriate range. If the final finishing temperature is lower than 880° C., the area ratio of ferrite becomes excessively large. In addition, if the final finishing temperature is higher than 950° C., it is difficult to secure a sufficient area ratio of ferrite. Therefore, the final finishing temperature is 880° C. or higher and 950° C. or lower, and the final finishing temperature is preferably 890° C. or higher and 940° C. or lower.
  • the average cooling rate in the primary cooling process is preferably 65° C./sec or faster.
  • the upper limit of the average cooling rate in the primary cooling process is not particularly specified, and may be 150° C./see or slower or 110° C./sec or slower.
  • the cooling stop temperature (primary cooling stop temperature) in the primary cooling process may be 680° C. or higher and 760° C. or lower.
  • the primary cooling stop temperature is lower than 680° C., there is a risk of the area ratio of ferrite becoming insufficient. In addition, even if the primary cooling stop temperature is higher than 760° C., there s a risk of the area ratio of ferrite becoming insufficient and the area ratio of bainite increasing.
  • cooling is performed at an average cooling rate of 20° C./sec or slower for 1.6 seconds or longer and 6.3 seconds or shorter (secondary cooling process). If the average cooling rate in the secondary cooling process is faster than 20° C./sec, there is a risk of the area ratio of ferrite becoming insufficient. Therefore, the average cooling rate in the secondary cooling process is 20° C./sec or slower, and preferably 18° C./sec or slower. In addition, if the cooling time in the secondary cooling process is shorter than 1.6 seconds, there is a risk of the area ratio of ferrite becoming insufficient and additionally the area ratio of bainite increasing.
  • the cooling tinge in the secondary cooling process is preferably 1.8 seconds or longer and 6.1 seconds or shorter.
  • cooling s performed at an average cooling rate of 60° C./sec or faster and 130° C./sec or slower to a tertiary cooling stop temperature of 195° C. or higher and 440° C. or lower.
  • a desired metallographic structure morphology is obtained. Therefore, the tertiary cooling process and the quaternary cooling process are important processes in order to secure elongation-flangeability.
  • the average cooling rate is 60° C./sec or faster, 130° C./sec or slower, a desired amount of bainite can be secured. If the average cooling rate in the tertiary cooling process is slower than 60° C./sec, a sufficient degree of supercooling cannot be secured, and a large amount of bainite is formed only from specific grain boundaries. As a result, after the quaternary cooling process, it is difficult to obtain a sufficient coverage of martensite with bainite. Therefore, in the tertiary cooling process, the average cooling rate is 60° C./sec or faster, preferably 65° C./sec or faster, and more preferably 70° C./sec or faster.
  • the average cooling rate in the tertiary cooling process is faster than 130° C./sec, formation of bainite does not proceed sufficiently, and it is difficult to obtain a sufficient coverage of martensite with bainite after the quaternary cooling process. Therefore, in the tertiary cooling process, the average cooling rate is 130° C./sec or slower, preferably 125° C./sec or slower, and more preferably 120° C./sec or slower.
  • the temperature at which the tertiary cooling process is completed (tertiary cooling stop temperature) 195° C. or higher and 440° C. or lower. If the tertiary cooling stop temperature is lower than 195° C., the area ratio of bainite becomes insufficient. Therefore, the tertiary cooling stop temperature is preferably 220° C. or higher and more preferably 250° C. or higher. On the other hand, if the tertiary cooling stop temperature is higher than 440° C., the area ratio of bainite increases, and it is difficult to obtain a favorable elongation at break. Therefore, the tertiary cooling stop temperature is preferably 420° C. or lower and more preferably 400° C. or lower.
  • water cooling is performed at a water density of 2.0 m 3 /min/mm 2 or more and 7.2 m 3 /min/mm 2 or less for 0.33 seconds or longer and 1.50 seconds or shorter.
  • this quaternary cooling process is an important process in controlling the coverage of martensite with bainite, the average diameter of martensite and the area ratio of martensite.
  • the coverage of martensite with bainite can be increased by setting the water density in the quaternary cooling process to 2.0 m 3 min/mm 2 or more.
  • the upper limit of the water density is not particularly specified, but if the upper limit is 7.2 m 3 /min/mm 2 or more, sheet deformation due to a water pressure may occur. Therefore, the water density is less than 7.2 m 3 /min/mm 2 , preferably 7.0 m 3 /min/mm 2 or less, and more preferably 6.8 m 3 /min/mm 2 or less.
  • the water density is set to be within the above range, and the cooling time is set to 0.33 seconds or longer and 1.50 seconds or shorter.
  • the average diameter dM of martensite changed depending on the cooling time in the quaternary cooling process. Specifically, if the cooling time is shorter than 0.33 seconds, the average diameter dM of martensite becomes excessively small. On the other hand, if the cooling time is longer than 1.50 seconds, the average diameter dM of martensite becomes excessively large. Therefore, the cooling time in the quaternary cooling process is 0.33 seconds or longer and 1.50 seconds or shorter, and preferably 0.40 seconds or longer and 1.40 seconds or shorter.
  • the air cooling time is shorter than 3.0 seconds or longer than 5.0 seconds, the coverage of martensite with bainite is less than 75.0%.
  • the air cooling time may be 4.0 seconds or longer and 4.8 seconds.
  • the winding temperature is 180° C. or higher, the area ratio of martensite becomes insufficient and it is difficult to obtain excellent strength. Therefore, the winding temperature is preferably lower than 180° C.
  • the hot-rolled steel sheet according to the present embodiment can be manufactured.
  • the sheet passing speed of the steel sheet n the quaternary cooling process may be 360 to 790 mpm (meter per minute).
  • hot-rolled steel sheet is wound into a hot-rolled coil, and hot-rolled coil is then unwound and may be pickled in order to remove the oxide film.
  • skin pass rolling may be applied to the extent that ductility does not deteriorate.
  • a device for performing the above cooling processes is not limited. Industrially, it is preferable to use a water spray device that can precisely control the water density.
  • a water spray device may be arranged between transport rollers that transport a steel sheet, and cooling may be performed by spraying a predetermined amount of water from above and below the steel sheet.
  • the thermal history of the supercooling process as described above can be achieved by controlling the density of the water amount to be sprayed or changing the opening/closing position of the valve.
  • Steel sheet coils having a width of 800 mm to 1,080 mm were manufactured using casting slabs having chemical compositions shown in Table 1A and Table 1B under conditions shown in Table 2A to Table 2C.
  • the sheet passing speed was in a range of 360 to 780 mpm (meter per minute).
  • a predetermined thermal history was obtained by changing the opening/closing position of the valve on the run out table (ROT).
  • the sheet thickness of the hot-rolled steel sheet was within a range of 2.0 mm to 6.0 mm.
  • “FT” indicates the final finishing temperature during final rolling in the hot rolling process.
  • the tensile strength (TS) of the hot-rolled steel sheet was determined using a No. 5 test piece of JIS Z 2241:2011 and according to a test method described in JIS Z 2241:2011.
  • a tensile test piece was taken in the direction perpendicular to the rolling direction and the sheet thickness direction (sheet width direction) so that it included a 1 ⁇ 4 part from the edge of the steel sheet. In this case, the tensile test piece was taken with the direction perpendicular to the rolling direction as the longitudinal direction.
  • the crosshead speed in the tensile test was set under conditions in which the strain rate was constant at 0.005 s ⁇ 1 . If the tensile strength was 780 MPa or more, it was determined to be satisfactory, and if the tensile strength was less than 780 MPa, it was determined to be unsatisfactory.
  • the elongation at break which is an index of ductility, was determined according to the test method described in JIS Z 2241:2011 in the same manner as the method for evaluating the tensile strength (TS). If the elongation at break (%) was 15.0% or more, it was determined to be satisfactory, and if the elongation at break was less than 1 5.0%, it was determined to be unsatisfactory.
  • the hole expandability was evaluated by the hole expansion rate ⁇ (%) measured according to JIS Z 2256:2010. Specifically, using a ⁇ 10 min punch, the die diameter was selected so that the clearance became 12.5%, and holes were punched out. Then, a hole expansion test as performed using a conical die with a tip angle of 60° with the burr on the outside and at a stroke speed of 10 mm/min. The test was stopped when cracks formed around the hole penetrated through the sheet thickness, and the hole diameters before and after the hole expansion test were compared, and the hole expansion rate (%) was calculated. If the hole expansion rate (%) was 60% or more, it was determined to be satisfactory, and if the hole expansion rate was less than 60%, it was determined to be unsatisfactory.
  • the hot-rolled steel sheet was sheared, and the occurrence of fine cracks on the sheared end surface was visually observed. Specifically, for shearing, the edge of the punched semicircular part of the following side bend test piece was observed using a microscope at a magnification of 50, and cracks that did not penetrate the sheet thickness present only at the punched edge were defined as fine cracks. In this test, cracks that penetrated the sheet thickness were not occurred. The punched clearance in this case was 12.5%,
  • a sheet of 35 mm ⁇ 100 ram was cut out from a steel sheet. Then, a semicircular hole was punched out in the sheet using a ⁇ 30 mm punch under conditions in which the sheet thickness clearance (a value obtained by dividing a gap between a punch and a die by a sheet thickness) was 12.5%.
  • the sheet thickness clearance (a value obtained by dividing a gap between a punch and a die by a sheet thickness) was 12.5%.
  • a test piece shown in FIG. 2 was prepared according to these processes.
  • the test piece was deformed at a stroke speed of 10 min/min, and formation of a crack at the edge of the hole was defined as “breakage.”
  • the breaking limit strain with a gauge length of 6.0 mm was measured using a total of three elements including a determined element and elements adjacent thereto.
  • three side bend test pieces were prepared, the above breaking limit strains of the test pieces were measured, and the average value of these breaking limit strains was evaluated. If the breaking limit strain was 0.5 or more, it was determined to be satisfactory, and if the breaking limit strain was less than 0.5, it was determined to be unsatisfactory.
  • Test Nos. 11 to 25, and 37 to 41 and Test Nos. 56 to 68 which were invention examples, had high strength, and excellent ductility, hole expandability and elongation-flangeability.
  • FIG. 5 shows the relationship between the breaking limit strain and the coverage in Test Nos. 2, 4, 5, 8, 9, and 11 to 25.
  • All of Test Nos. 11 to 25 had the average diameter of martensite and the area ratio of bainite within the scope of the present invention.
  • FIG. 5 it can be understood that, when a hot-rolled steel sheet in which the average diameter of martensite and the area ratio of bainite were within the scope of the present invention, and the coverage of martensite with bainite was 75.0% or more was used, the breaking limit strain according to the side bend could be 0,5 or more.
  • FIG. 6 shows the relationship between the breaking limit strain and the average diameter dM of martensite in Test Nos. 6, 7, and 11 to 25.
  • Test Nos. 11 to 25 the area ratio of bainite and the coverage of martensite with bainite were within the scope of the present invention.
  • the breaking limit strain according to the side bend could be 0.5 or more.
  • the breaking limit strain according to the side bend could be 0.5 or more.
  • FIG. 7 shows the relationship between the coverage and the water density in the quaternary cooling process in Test Nos. 4, 5, and 11 to 25.
  • manufacturing conditions other than the water density in the quaternary cooling process were within the scope of the present invention.
  • FIG. 7 it can be understood that, when a hot-rolled steel sheet was manufactured under conditions in which all manufacturing conditions including the water density in the quaternary cooling process were within the scope of the present invention, it was possible to sufficiently improve the coverage of the hot-rolled steel sheet.
  • FIG. 8 shows the relationship between the average diameter dM of martensite arid the cooling time in the quaternary cooling process in Test Nos. 6, 7, and 11 to 25.
  • 11 to 25 manufacturing conditions other than the cooling time in the quaternary cooling process were within the scope of the present invention.
  • FIG. 8 it can be understood that, when a hot-rolled steel sheet was manufactured under conditions in which all manufacturing conditions including the cooling time in the quaternary cooling process were within the scope of the present invention, it was possible to sufficiently improve the average diameter dM of martensite.
  • FIG. 9 shows the relationship between the coverage of martensite with bainite and the air cooling time in Test Nos. 8, 9, and 11 to 25.
  • manufacturing conditions other than the air cooling time were within the scope of the present invention.
  • NG. 9 can be understood that, when a hot-rolled steel sheet was manufactured under conditions in which all manufacturing conditions including the air cooling time were within the scope of the present invention, it was possible to sufficiently improve the coverage.
  • the manufacturing conditions other than the air cooling time were within the scope of the present invention, if the air cooling time was outside the scope, it was difficult to increase the coverage of martensite with bainite.
  • Test Nos. 26 to 36 which were comparative examples, although the coverage of martensite with bainite and the average diameter dM of martensite were within the scope of the invention, other requirements for the metallographic structure were not satisfied, any one or more properties were not satisfied.
  • Test No. 27 using a steel type G having an appropriate chemical composition since the final finishing temperature during final rolling s higher than 950° C., the area ratio of ferrite was less than 53.0%. As a result, the ductility decreased.
  • Test No. 50 which was a comparative example, was manufactured under manufacturing conditions within the scope of the present invention, since the Al content in the chemical composition was large, the area ratio of ferrite increased. As a result, the tensile strength was less than 780 MPa.
  • the breaking limit strain according to the side bend reached a desired value. Accordingly, it can be understood that it was very important to control conditions in the quaternary cooling process to be within the scope of the present invention in order to reduce fine cracks on the sheared end surface, and accordingly, improve the breaking limit strain according to the side bend.
  • FIG. 11 A shows a structure image (SEM image) of Test No. 6, which was a comparative example in which a holding time was shorter than 0.33 seconds.
  • FIG. 11 B shows an enlarged view of an area A indicated by FIG. 11 A .
  • the contrast was considered to be the interface with austenite after bainite transformation according to tertiary cooling was completed. It can be understood that martensite transformation proceeded at the interface between austenite and ferrite and a part close thereto, and as a result, the coverage of martensite with bainite decreased.

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