WO2024095532A1 - Hot rolled steel sheet - Google Patents

Hot rolled steel sheet Download PDF

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Publication number
WO2024095532A1
WO2024095532A1 PCT/JP2023/024341 JP2023024341W WO2024095532A1 WO 2024095532 A1 WO2024095532 A1 WO 2024095532A1 JP 2023024341 W JP2023024341 W JP 2023024341W WO 2024095532 A1 WO2024095532 A1 WO 2024095532A1
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content
rolling
hot
strength
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PCT/JP2023/024341
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French (fr)
Japanese (ja)
Inventor
睦海 吉武
洋志 首藤
大輔 伊藤
洵 安藤
健介 長井
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日本製鉄株式会社
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to hot-rolled steel sheets.
  • Patent Document 1 describes a hot-rolled steel sheet having a predetermined chemical composition, a structure containing, in terms of area ratio, a total of 80-98% ferrite and bainite, and 2-10% martensite, and, when the boundaries in the structure where the misorientation is 15° or more are defined as grain boundaries, and the regions surrounded by the grain boundaries and having a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains, the proportion of the crystal grains where the misorientation within the grains is 5-14° is 10-60% in terms of area ratio.
  • Patent Document 1 also teaches that by setting the proportion of the above crystal grains where the misorientation within the grains is 5-14° to an area ratio of 10-60%, it is possible to improve stretch flangeability and ductility while maintaining high strength, and further teaches that by controlling the total area ratio of ferrite and bainite in the structure and the area ratio of martensite within a predetermined range, it is possible to improve notch fatigue properties.
  • High-strength steel plates are manufactured by hot rolling cast slabs, and it is known that the hot rolling can cause strength anisotropy between the strength in the rolling direction (L direction) and the strength in the width direction (C direction) perpendicular thereto.
  • L direction strength in the rolling direction
  • C direction width direction
  • the strength anisotropy becomes large, it generally becomes a problem because the workability of the steel plate decreases. Therefore, in order to improve the workability of steel plates, there is a high demand for high-strength steel plates with reduced strength anisotropy in addition to the stretch flangeability, ductility, and notch fatigue properties described in Patent Document 1.
  • the present invention aims to provide a hot-rolled steel sheet that, despite its high strength, has improved stretch flangeability, ductility, and notch fatigue properties, and has reduced strength anisotropy.
  • the inventors conducted research with a particular focus on the metal structure of hot-rolled steel sheet.
  • the inventors discovered that by configuring the metal structure of a hot-rolled steel sheet having a specified chemical composition to contain at least one of ferrite and bainite, and martensite in specific proportions, and further controlling the proportion of crystal grains within a specified range, it is possible to improve stretch flangeability, ductility, and notch fatigue properties, while in addition, by appropriately controlling the texture in the surface layer and center of the steel sheet, it is possible to reduce the anisotropy of strength, thus completing the present invention.
  • the present invention which has achieved the above object, is as follows. (1) In mass%, C: 0.020 to 0.070%, Si: 0.010 to 2.000%, Mn: 0.60 to 2.00%, Ti: 0.015 to 0.200%, sol. Al: 0.010 to 1.000%, P: 0.100% or less, S: 0.030% or less, N: 0.0060% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0 to 0.300%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.000%, B: 0 to 0.0100%, Sb: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, REM: 0 to 0.1000%, Bi: 0 to 0.0100%, As: 0 to 0.0100%, Zr: 0 to 1.00%, Co: 0 to
  • the chemical composition is, in mass%, Nb: 0.001 to 0.050%, V: 0.001 to 0.300%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sb: 0.01 to 1.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, REM: 0.0001 to 0.1000%, Bi: 0.0001 to 0.0100%, As: 0.0001 to 0.0100%, Zr: 0.01 to 1.00%, Co: 0.01 to 1.00%, Zn: 0.01 to 1.00%, W: 0.01 to 1.00%, and Sn: 0.01 to 1.00%
  • the hot-rolled steel sheet according to the above (1) characterized in that it contains at least one of the following:
  • the present invention provides a hot-rolled steel sheet that has high strength, but also has improved stretch flangeability, ductility, and notch fatigue properties, and has reduced strength anisotropy.
  • FIG. 2 is a diagram showing the shape of a saddle-shaped molded product used in a saddle-shaped stretch flange test method.
  • FIG. 2 is a diagram showing the shape of a fatigue test specimen used to evaluate notch fatigue properties.
  • the hot-rolled steel sheet according to the embodiment of the present invention has, in mass%, C: 0.020 to 0.070%, Si: 0.01 to 2.00%, Mn: 0.600 to 2.00%, Ti: 0.015 to 0.200%, sol.
  • the chemical composition satisfies 0.100 ⁇ [Si]+[sol
  • the metal structure of a hot-rolled steel sheet having a predetermined chemical composition is configured to contain at least one of ferrite and bainite and martensite in a specific ratio, more specifically, by configuring it to contain at least one of ferrite and bainite: 80 to 98% in total and martensite: 2 to 10% by area percentage, it is possible to improve strength, stretch flangeability, ductility, and notch fatigue properties in a well-balanced manner.
  • a crystal grain with an orientation difference within the grain of 5 to 14° is effective in improving strength, stretch flangeability, and ductility. Therefore, by appropriately controlling the proportion of these crystal grains, more specifically by controlling it to within the range of 10 to 60% by area, it is possible to further improve the balance between strength, stretch flangeability, and ductility.
  • the anisotropy of strength due to the anisotropic metal structure obtained by hot rolling during steel sheet manufacturing, the tensile strength tends to differ between the rolling direction (L direction) and the width direction (C direction) perpendicular thereto, and generally, the tensile strength of hot-rolled steel sheets tends to be lower in the L direction than in the C direction.
  • the present inventors conducted a study, focusing particularly on the texture of hot-rolled steel sheets, in order to achieve both high strength steel sheets by reducing the anisotropy of strength in addition to improving the stretch flangeability, ductility, and notch fatigue properties.
  • the inventors discovered that by controlling the average pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the region from the surface of the hot-rolled steel plate to the 1/6 position of the plate thickness to 2.50 or more, and controlling the average pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations in the region from the 2/5 position to the 3/5 position of the plate thickness to 7.00 or less, it is possible to significantly reduce the anisotropy of the strength in the tensile strength of the hot-rolled steel plate in the L direction and C direction.
  • the crystal orientation is different between the surface layer part of the plate thickness (i.e., the region from the surface of the hot-rolled steel plate to the 1/6 position of the plate thickness) which is directly affected by rolling, and the center part of the plate thickness (i.e., the region from the 2/5 position of the plate thickness to the 3/5 position of the plate thickness). More specifically, in the surface layer part of the plate thickness, textures of ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations are developed, and it is considered that the strength in the L direction is increased due to the development of such textures.
  • the crystal orientation of the rolled sheet is usually expressed as ⁇ hkl ⁇ or (hkl) for the crystal orientation perpendicular to the rolling surface, and ⁇ uvw> or [uvw] for the crystal orientation parallel to the rolling direction.
  • ⁇ hkl ⁇ and ⁇ uvw> are generic names for equivalent planes and orientations, and (hkl) and [uvw] refer to individual crystal planes.
  • the hot-rolled steel sheet according to the embodiment of the present invention is mainly intended for body-centered cubic structures (bcc structures), so for example, (110), (-110), (1-10), (-1-10), (101), (-101), (10-1), (-10-1), (011), (0-11), (01-1) and (0-1-1) are equivalent and indistinguishable. In the embodiment of the present invention, these orientations are collectively expressed as ⁇ 110 ⁇ .
  • the inventors have determined that the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the plate thickness surface layer portion is increased to a predetermined value or more to increase the strength in the L direction, while the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the plate thickness center portion is decreased to a predetermined value or less to decrease the strength in the C direction, more specifically, from the surface of the hot-rolled steel plate to a position 1/6 of the plate thickness.
  • the anisotropy of strength in the tensile strength in the L direction and the C direction of the hot rolled steel sheet can be significantly reduced by controlling the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the region to 2.50 or more and controlling the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the region from the 2/5 position of the sheet thickness to the 3/5 position of the sheet thickness to 7.00 or less.
  • the pole density refers to the ratio of the accumulation degree in a specific orientation of the test material to that of a standard sample that does not have accumulation in a specific orientation.
  • the metal structure in order to improve the stretch flangeability, ductility and notch fatigue properties, is configured to contain at least one of ferrite and bainite and martensite in a specific ratio, and the area percentage of the specific crystal grains having an intragranular orientation difference of 5 to 14° is controlled to be within the range of 10 to 60%. Therefore, it is extremely difficult to control the pole density of a specific texture in the thickness surface layer and the pole density of a specific texture in the thickness center to within a desired range while maintaining the configuration of the metal structure controlled in this way.
  • the rolling conditions in the hot rolling process appropriate, it is possible to realize a metal structure in which the average value of the pole density in the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the thickness surface layer is 2.50 or more and the average value of the pole density in the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the thickness center is 7.00 or less while maintaining the configuration of the metal structure for improving stretch flangeability, ductility and notch fatigue properties.
  • the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent workability, and is therefore particularly useful in the automotive field, where both properties are required to be achieved.
  • C is an element effective in increasing the strength of steel plate.
  • C forms carbides and/or carbonitrides with Ti and Nb in steel, and contributes to precipitation strengthening based on the formed precipitates and to refinement of the structure due to the pinning effect of the precipitates.
  • the C content is set to 0.020% or more.
  • the C content may be 0.022% or more, 0.025% or more, 0.028% or more, or 0.030% or more.
  • the C content is set to 0.070% or less.
  • the C content may be 0.065% or less, 0.060% or less, 0.055% or less, or 0.050% or less.
  • Si is an element that is effective in increasing strength as a solid solution strengthening element.
  • the Si content is set to 0.010% or more.
  • the Si content may be 0.100% or more, more than 0.100%, 0.110% or more, 0.120% or more, 0.150% or more, 0.180% or more, 0.200% or more, 0.300% or more, 0.500% or more, 0.800% or more, or 1.000% or more.
  • the Si content is set to 2.000% or less.
  • the Si content may be 1.800% or less, 1.600% or less, 1.400% or less, or 1.200% or less.
  • Mn is an element that is effective in increasing strength as a hardenability and solid solution strengthening element. In order to fully obtain these effects, the Mn content is set to 0.60% or more. The Mn content may be 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, if Mn is contained excessively, stretch flangeability may be reduced. Therefore, the Mn content is set to 2.00% or less. The Mn content may be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.
  • Ti is an element that precipitates finely in steel as carbide (TiC) and improves the strength of steel by precipitation strengthening. Ti also forms carbides to fix C and suppresses the formation of cementite, which is harmful to stretch flangeability. In order to fully obtain these effects, the Ti content is set to 0.015% or more. The Ti content may be 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if Ti is contained excessively, the carbides may become coarse and the ductility may decrease. Therefore, the Ti content is set to 0.200% or less. The Ti content may be 0.180% or less, 0.170% or less, 0.150% or less, or 0.120% or less.
  • sol. Al is an element that acts as a deoxidizer for molten steel. In order to fully obtain this effect, the sol. Al content is set to 0.010% or more. On the other hand, if the sol. Al content is excessive, coarse oxides are formed, which reduces toughness and ductility and causes fatigue during rolling. Therefore, the sol. Al content is set to 1.000% or less. The sol. Al content is set to 0.800% or less, 0.600% or less, or 0.400% or less.
  • the term "solubilized aluminum" means acid-soluble aluminum, and indicates solute aluminum that is present in the steel in a solid solution state.
  • the P content is set to 0.100% or less.
  • the P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less.
  • the lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the P content may be 0.001% or more, 0.003% or more, or 0.005% or more.
  • the Si content is set to 0.030% or less.
  • the S content may be 0.020% or less, 0.010% or less, or 0.005% or less.
  • the lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the S content may be 0.001% or more, 0.002% or more, or 0.003% or more.
  • N may form precipitates with Ti preferentially over C, and may reduce the amount of Ti that is effective for fixing C. Therefore, the N content is set to 0.0060% or less.
  • the N content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less.
  • the lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.
  • O is an element that is mixed in during the manufacturing process. If O is contained excessively, coarse inclusions may be formed, which may reduce the toughness of the steel plate. Therefore, the O content is set to 0.0100% or less.
  • the O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • the lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long time for refining, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.
  • the basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.
  • Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to refining the structure by the pinning effect, and thus to increasing the strength of the steel sheet.
  • Nb is also an element that fixes C by forming carbides and/or carbonitrides, and suppresses the formation of cementite that is harmful to stretch flangeability.
  • the Nb content may be 0%, but in order to obtain these effects, the Nb content is preferably 0.001% or more.
  • the Nb content may be 0.005% or more, 0.010% or more, or 0.015% or more.
  • the Nb content is set to 0.050% or less.
  • the Nb content may be 0.040% or less, 0.030% or less, or 0.020% or less.
  • V is an element that contributes to improving strength by precipitation strengthening, etc.
  • the V content may be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more.
  • the V content may be 0.010% or more, 0.030% or more, or 0.050% or more.
  • the V content is preferably 0.300% or less.
  • the V content may be 0.200% or less, 0.100% or less, or 0.080% or less.
  • Cr is an element that enhances the hardenability of steel and contributes to improving strength.
  • the Cr content may be 0%, but in order to obtain such an effect, the Cr content is preferably 0.01% or more.
  • the Cr content may be 0.03% or more or 0.05% or more.
  • the Cr content is preferably 2.00% or less.
  • the Cr content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
  • Ni and Cu are elements that contribute to improving strength by precipitation strengthening or solid solution strengthening.
  • the Ni and Cu contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.01% or more, and may be 0.03% or more or 0.05% or more. On the other hand, even if these elements are contained excessively, the effects are saturated and there is a risk of increasing manufacturing costs. Therefore, the Ni and Cu contents are preferably 2.00% or less, and may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
  • Mo is an element that improves the hardenability of steel and contributes to improving strength.
  • the Mo content may be 0%, but in order to obtain such an effect, the Mo content is preferably 0.001% or more.
  • the Mo content may be 0.010% or more, 0.020% or more, or 0.050% or more.
  • the Mo content is preferably 1.000% or less.
  • the Mo content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.
  • [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness.
  • the B content may be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more.
  • the B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more.
  • the B content is preferably 0.0100% or less.
  • the B content may be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.
  • Sb is an element effective in improving corrosion resistance.
  • the Sb content may be 0%, but in order to obtain such an effect, the Sb content is preferably 0.01% or more.
  • the Sb content may be 0.02% or more or 0.05% or more.
  • excessive Sb content may cause a decrease in toughness. Therefore, the Sb content is preferably 1.00% or less.
  • the Sb content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.
  • Ca, Mg and Hf are elements capable of controlling the morphology of nonmetallic inclusions.
  • the Ca, Mg and Hf contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the Ca, Mg and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • REM is an element capable of controlling the morphology of nonmetallic inclusions.
  • the REM content may be 0%, but in order to obtain such an effect, the REM content is preferably 0.0001% or more.
  • the REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the REM content is preferably 0.1000% or less.
  • the REM content may be 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.
  • Bi and As are elements effective in improving corrosion resistance.
  • the Bi and As contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the Bi and As contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • Zr is an element capable of controlling the form of nonmetallic inclusions.
  • the Zr content may be 0%, but in order to obtain such an effect, the Zr content is preferably 0.01% or more.
  • the Zr content may be 0.05% or more or 0.10% or more.
  • the Zr content is preferably 1.00% or less.
  • the Zr content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
  • Co is an element that contributes to improving hardenability and/or heat resistance.
  • the Co content may be 0%, but in order to obtain these effects, the Co content is preferably 0.01% or more.
  • the Co content may be 0.05% or more or 0.10% or more.
  • the Co content is preferably 1.00% or less.
  • the Co content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
  • Zn is an element effective in controlling the shape of inclusions.
  • the Zn content is preferably 0.01% or more.
  • the Zn content may be 0.05% or more or 0.10% or more.
  • the Zn content is preferably 1.00% or less.
  • the Zn content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
  • W is an element that enhances the hardenability of steel and contributes to improving strength.
  • the W content may be 0%, but in order to obtain such an effect, the W content is preferably 0.01% or more.
  • the W content may be 0.05% or more or 0.10% or more.
  • excessive W content may reduce weldability. Therefore, the W content is preferably 1.00% or less.
  • the W content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
  • Sn is an element effective in improving corrosion resistance.
  • the Sn content may be 0%, but in order to obtain such an effect, the Sn content is preferably 0.01% or more.
  • the Sn content may be 0.02% or more or 0.05% or more.
  • excessive Sn content may cause a decrease in toughness. Therefore, the Sn content is preferably 1.00% or less.
  • the Sn content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.
  • the remainder other than the above elements consists of Fe and impurities.
  • Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing hot-rolled steel sheets.
  • [0.100 ⁇ [Si]+[sol. Al] ⁇ 2.500] The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention must satisfy the following formula. 0.100 ⁇ [Si]+[sol. Al] ⁇ 2.500
  • [Si] and [sol. Al] are the contents (mass%) of each element.
  • the grains are surrounded by a boundary with an orientation difference of 15° or more and have a circle equivalent diameter of 0.3 ⁇ m or more.
  • crystal grains having an intragranular misorientation of 5 to 14° are effective in improving strength and stretch flangeability.
  • the ratio of the crystal grains is controlled within the range of 10 to 60% by area to improve the balance between strength and stretch flangeability.
  • sol. Al is also an element effective in controlling the ratio of crystal grains having an intragranular misorientation of 5 to 14 degrees within the range of 10 to 60%. This is believed to be due to the fact that the temperature of the Ar3 point is increased by the inclusion of Si and sol. Al, and the transformation strain introduced into the grains is reduced.
  • the chemical composition of the hot rolled steel sheet according to the embodiment of the present invention is such that the contents of each element are controlled within the ranges described above, while Si and sol. The total content of Si and sol.
  • Al is controlled to be 0.100% or more, i.e., to satisfy [Si] + [sol. Al] ⁇ 0.100.
  • the total content of Si and sol. Al is controlled to be 0.120%.
  • the total content of Si and sol. Al is set to 2.500% or less, that is, [Si] + [sol. Al] ⁇ 2.500.
  • the content may be 2.000% or less, 1.500% or less, 1.000% or less, or 0.000% or less.
  • the chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention may be measured by a general analytical method.
  • the chemical composition of the hot-rolled steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES).
  • C and S may be measured using the combustion-infrared absorption method
  • N may be measured using the inert gas fusion-thermal conductivity method
  • O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
  • the metal structure of the hot-rolled steel sheet according to the embodiment of the present invention includes, in area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%.
  • the total area ratio of at least one of ferrite and bainite is 80% or more, and may be, for example, 82% or more, 85% or more, 88% or more, or 90% or more.
  • the area ratio of martensite may be 10% or less, and may be, for example, 9% or less, 8% or less, 7% or less, or 6% or less.
  • the total area ratio of at least one of ferrite and bainite is high or the area ratio of martensite is low, the balance between strength and notch fatigue properties may be reduced, and desired properties may not be obtained.
  • the total area ratio of at least one of ferrite and bainite is 98% or less, and may be, for example, 96% or less, 94% or less, or 92% or less.
  • the area ratio of martensite is 2% or more, and may be, for example, 3% or more, 4% or more, or 5% or more.
  • the metal structure of the hot-rolled steel sheet may contain either ferrite or bainite, and preferably contains both ferrite and bainite. Therefore, the area ratio of either ferrite or bainite may be 0%, or may be, for example, 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more, respectively. Similarly, the area ratio of ferrite and bainite may be, for example, 90% or less, 80% or less, 70% or less, 60% or less, or 50% or less, respectively. From the viewpoint of improving the ductility of the hot-rolled steel sheet, the area ratio of bainite is preferably 80% or less, and more preferably 70% or less.
  • the remaining structure other than ferrite, bainite, and martensite may be 0% by area, but when the remaining structure is present, the remaining structure may be at least one of retained austenite and pearlite.
  • the area ratio of the remaining structure is not particularly limited, but may be, for example, 1% or more, 2% or more, or 3% or more. From the viewpoint of further improving the stretch flangeability, the area ratio of the remaining structure is preferably, for example, 10% or less, and may be 8% or less, 6% or less, or 5% or less.
  • Identification of the metal structure and calculation of the area ratio in the hot-rolled steel sheet are performed by optical microscope observation after corrosion using a Nital reagent or a Lepera solution and X-ray diffraction method.
  • the structure observation by an optical microscope is performed on a plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface. Specifically, first, a sample is taken from the hot-rolled steel sheet, and the observation surface of the sample is etched with Nital.
  • image analysis is performed on a structure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope, thereby calculating each area ratio of ferrite and pearlite, and the total area ratio of bainite and martensite.
  • image analysis is performed on a structure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope, thereby calculating the total area ratio of retained austenite and martensite.
  • the volume ratio of retained austenite is calculated by X-ray diffraction measurement. Since the volume fraction of the retained austenite is equivalent to the area fraction, this is taken as the area fraction of the retained austenite.
  • the area fraction of martensite is calculated by subtracting the obtained area fraction of the retained austenite from the total area fraction of the retained austenite and martensite calculated previously.
  • the area fraction of bainite is calculated by subtracting the obtained area fraction of martensite from the total area fraction of bainite and martensite calculated previously.
  • an increase in the dislocation density within the grain improves strength while decreasing workability.
  • the strength can be improved without decreasing workability in crystal grains in which the orientation difference within the grain is controlled to 5 to 14°.
  • crystal grains with an orientation difference within the grain of less than 5° are excellent in workability but difficult to increase in strength.
  • crystal grains with an orientation difference within the grain of more than 14° do not necessarily contribute to improving stretch flangeability because the deformability is different within the crystal grain.
  • the proportion of crystal grains having an intragranular misorientation of 5 to 14° by appropriately controlling the proportion of crystal grains having an intragranular misorientation of 5 to 14°, more specifically, by controlling it to within a range of 10 to 60% in terms of area%, it is possible to improve the stretch flangeability while achieving the desired steel sheet strength, and it is possible to further improve the balance between strength and stretch flangeability. If the proportion of crystal grains having an intragranular misorientation of 5 to 14° is small, the stretch flangeability may be reduced. Therefore, from the viewpoint of improving the stretch flangeability, the proportion of crystal grains having an intragranular misorientation of 5 to 14° may be 15% or more, 18% or more, or 20% or more.
  • the proportion of crystal grains having an intragranular misorientation of 5 to 14° may be 55% or less, 50% or less, 45% or less, or 40% or less.
  • the proportion of crystal grains with an intragranular orientation difference of 5 to 14° is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from the steel sheet so that the plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface is the observation surface. Next, at a depth position of 1/4 of the plate thickness from the surface of the steel sheet, an area of 200 ⁇ m in the rolling direction of the steel sheet and 100 ⁇ m in the normal direction to the rolling surface is analyzed by EBSD analysis at a measurement interval of 0.2 ⁇ m to obtain crystal orientation information.
  • EBSD electron backscattered diffraction
  • the EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL).
  • JSM-7001F thermal field emission scanning electron microscope
  • HTKARI detector HAI detector manufactured by TSL.
  • regions with an orientation difference of 15° or more and a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains
  • the average orientation difference within the crystal grains is calculated
  • the ratio of crystal grains with an orientation difference of 5 to 14° within the grains is obtained.
  • the crystal grains and the average orientation difference within the grains defined as above can be calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device.
  • orientation difference within a grain refers to "Grain Orientation Spread (GOS)", which is the orientation dispersion within a crystal grain.
  • GOS Grain Orientation Spread
  • the value of the orientation difference within a grain is described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD Method and X-ray Diffraction Method", Hidehiko Kimura et al., Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 71, No. 712, 2005, p.
  • the GOS is calculated as an average value of the misorientation between a reference crystal orientation and all measurement points within the same crystal grain.
  • the reference crystal orientation is an average orientation of all measurement points within the same crystal grain.
  • the GOS value can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" that comes with the EBSD analyzer.
  • the average value of the pole densities of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the region from the surface of the hot-rolled steel plate to the 1/6 position of the plate thickness (i.e., the plate thickness surface layer portion) is controlled to 2.50 or more, and the average value of the pole densities of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the region from the 2/5 position of the plate thickness to the 3/5 position of the plate thickness (i.e., the plate thickness center portion) is controlled to 7.00 or less.
  • the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the sheet thickness surface layer portion to 2.50 or more to increase the strength in the L direction
  • the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the sheet thickness center portion to 7.00 or less to reduce the strength in the C direction
  • the difference in tensile strength between the L direction and the C direction of the obtained hot rolled steel sheet can be reduced, and as a result, the anisotropy of the strength in the tensile strength in the L direction and the C direction can be significantly reduced.
  • the larger the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the sheet thickness surface layer portion the more preferable it is, and it may be, for example, 2.80 or more, 3.00 or more, 3.20 or more, or 3.50 or more.
  • the upper limit is not particularly limited, but for example, the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the sheet thickness surface layer portion may be 5.00 or less, 4.80 or less, 4.70 or less, 4.50 or less, 4.20 or less, 4.00 or less, or 3.80 or less.
  • the smaller the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations in the sheet thickness center portion the more preferable, and may be, for example, 6.80 or less, 6.50 or less, 6.20 or less, or 6.00 or less.
  • the lower limit is not particularly limited, but for example, the average pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations in the center part of the sheet thickness may be 3.50 or more, 4.00 or more, 4.20 or more, 4.40 or more, 4.50 or more, or 5.00 or more.
  • a sample is taken from the steel sheet so that the sheet thickness cross section parallel to the rolling direction and perpendicular to the sheet surface becomes the observation surface, and EBSD analysis is performed at measurement intervals of 1 ⁇ m on a rectangular region of the steel sheet, which is 1000 ⁇ m in the rolling direction and 100 ⁇ m in the normal direction to the rolling surface and is centered at a depth position of 1/12 of the sheet thickness from the steel sheet surface, to obtain crystal orientation information of this rectangular region.
  • the EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL).
  • the ODF Orientation Distribution Function
  • the Harmonic Series Expansion spherical harmonic function method
  • the expansion order was set to 16.
  • the calculation was performed taking into account symmetry (orthotropic).
  • the hot rolled steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm, although not particularly limited thereto.
  • the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 5.0 mm or less, or 4.0 mm or less.
  • the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 1470 MPa or less, 1250 MPa or less, 1180 MPa or less, 1080 MPa or less, or 980 MPa or less.
  • the tensile strength is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.
  • the tensile strength thus obtained is also referred to herein as C-direction TS (TSC).
  • Total elongation: El According to the hot-rolled steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the total elongation can be improved, and more specifically, a total elongation of 15.0% or more can be achieved.
  • the total elongation is preferably 18.0% or more, more preferably 20.0% or more, and most preferably 22.0% or more.
  • the upper limit is not particularly limited, but for example, the total elongation may be 40.0% or less or 35.0% or less.
  • the total elongation is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.
  • the method for producing a hot-rolled steel sheet according to an embodiment of the present invention includes: (A) a hot rolling process including heating a slab having the chemical composition described above in relation to the hot rolled steel sheet and then finish rolling the slab, and satisfying the following conditions (A1) to (A5); and (A1) the heating temperature of the slab is a solution temperature (SRTmin) °C or higher represented by the following formula 1 and 1260 °C or lower; (A2) The cumulative strain ( ⁇ eff.) in the last three stages of finish rolling, represented by the following formula 2, is 0.50 to 0.60; (A3) The end temperature of the finish rolling is Ar3+30°C or higher; (A4) In the finish rolling, two or more rolling passes having a shape ratio (X) represented by the following formula 3 of 2.3 or more are performed at 1100 ° C.
  • X shape ratio
  • the rolling temperature of the first three stages of the finish rolling is equal to or higher than the entry temperature of the finish rolling (FT0) - 50 ° C.
  • SRTmin 7000 / ⁇ 2.75 - log ([Ti] x [C]) ⁇ - 273 ...Equation 1
  • [Ti] and [C] are the contents (mass%) of each element in the steel.
  • ⁇ eff. ⁇ i(t,T) ...
  • Q 183200J
  • t indicates the cumulative time (seconds) until just before cooling in the pass
  • T indicates the rolling temperature (° C.) in the pass.
  • a cooling step includes primarily cooling the finish-rolled steel sheet to a temperature range of 650 to 750°C at an average cooling rate of 10°C/s or more, holding the steel sheet in the temperature range for 3.0 to 10.0 seconds, and then secondary cooling to 100°C or less at an average cooling rate of 30°C/s or more. Each step will be described in detail below.
  • the hot-rolled steel sheet according to the embodiment of the present invention contains Ti, and if the heating temperature of the slab is less than the solution temperature (SRTmin) ° C., Ti is not sufficiently dissolved. If Ti is not sufficiently dissolved during slab heating, it is difficult to improve the strength of the steel by precipitation strengthening by finely precipitating Ti as carbide (TiC) in the steel during the cooling process after the hot rolling process. In addition, it is difficult to fix C by forming carbide (TiC) and suppress the generation of cementite, which is harmful to stretch flangeability. On the other hand, if the heating temperature of the slab is more than 1260 ° C., the yield decreases due to scale-off.
  • SiC solution temperature
  • the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
  • the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
  • the cumulative strain in the latter three stages of finish rolling is less than 0.50, the dislocation density of the introduced austenite is insufficient, and the proportion of crystal grains with an intragranular misorientation of 5 to 14° is less than 10%.
  • the cumulative strain in the latter three stages of finish rolling is more than 0.60, recrystallization of austenite occurs during hot rolling, and the accumulated dislocation density during transformation decreases. As a result, the proportion of crystal grains with an intragranular misorientation of 5 to 14° is similarly less than 10%.
  • Q 183200J
  • t indicates the cumulative time (seconds) until just before cooling in the corresponding pass
  • T indicates the rolling temperature (° C.) in the corresponding pass.
  • the end temperature of the finish rolling needs to be Ar3+30°C or higher. If the end temperature of the finish rolling is less than Ar3+30°C, when ferrite is generated in a part of the structure due to the variation of the components in the steel sheet and the rolling temperature, the ferrite may be processed. The processed ferrite may cause a decrease in ductility. In addition, if the end temperature of the finish rolling is less than Ar3+30°C, the ratio of crystal grains having an intragranular misorientation of 5 to 14° may exceed 60% and become excessively high.
  • Ar3 (°C) is calculated based on the chemical composition of the hot-rolled steel sheet by the following formula 4.
  • Ar3 901 - 325 x [C] + 33 x [Si] + 287 x [P] + 40 x [sol. Al] - 92 x ([Mn] + [Mo] + [Cu]) - 46 x ([Cr] + [Ni]) ...Equation 4
  • [C], [Si], [P], [sol. Al], [Mn], [Mo], [Cu], [Cr] and [Ni] are the contents (mass%) of each element in the steel, and are 0 when the element is not contained.
  • the inventors have found that by increasing the shear strain introduced into the plate thickness surface layer portion of the steel plate in finish rolling, the degree of accumulation in the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the plate thickness surface layer portion can be increased, thereby controlling the average value of the pole density of these orientations within a desired range. More specifically, by performing two or more rolling passes at 1100°C or less in the finish rolling such that the shape ratio (X) represented by the following formula 3 is 2.3 or more, it is possible to increase the average pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the sheet thickness surface layer portion to 2.50 or more.
  • the shape ratio (X) means the roll contact arc length ( ⁇ (R(h 0 -h 1 ))) divided by the average plate thickness ((h 0 +h 1 )/2).
  • a shape ratio (X) of 2.3 or more can be achieved, and the shear strain introduced into the plate thickness surface layer portion of the steel plate can be increased.
  • the rolling temperature at that time is 1100 ° C or less, the recovery of the introduced shear strain can be suppressed.
  • the average value of the pole density of the ⁇ 110 ⁇ ⁇ 111> and ⁇ 112 ⁇ ⁇ 111> orientations in the plate thickness surface layer portion can be reliably increased to 2.50 or more.
  • the upper limit of the number of such rolling passes is not particularly limited, and for example, the number of rolling passes may be 5 passes or less.
  • the rolling temperature is more than 1100 ° C, or X is 2.3 or more and the number of rolling passes at 1100 ° C or less is 1 pass or less, sufficient shear strain cannot be introduced into the plate thickness surface layer.
  • the roll radius of the roll used in the rolling mill can be selected from a range in which X is 2.3 or more. Although not particularly limited, for example, the roll radius can be selected from a range of 150 to 400 mm.
  • the rolling temperatures of the first three stages of the finish rolling i.e., the rolling temperature of the first stage of the finish rolling (FT1), the rolling temperature of the second stage of the finish rolling (FT2) and the rolling temperature of the third stage of the finish rolling (FT3), are controlled to be equal to or higher than the entry temperature of the finish rolling (FT0) -50°C, thereby reducing the shear strain introduced in the center of the plate thickness, thereby achieving an average pole density of 7.00 or less in the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations.
  • the shear strain introduced in the plate thickness center portion is smaller than that in the plate thickness surface layer portion that directly contacts the roll, and therefore, by controlling the rolling temperature of the first three stages of the finish rolling to a relatively high temperature as described above, it is possible to sufficiently reduce the introduced shear strain. From the viewpoint of further reducing the anisotropy of strength, it is preferable to control the rolling temperature of the first three stages of the finish rolling to FT0-45 ° C. or higher.
  • the rolling temperature of even one of the first three stages of the rolling is less than FT0-50 ° C., the effect of reducing the shear strain is not sufficient, and the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the plate thickness center portion cannot be reduced to 7.00 or less.
  • the upper limit of the rolling temperature of the first three stages of the finish rolling is not particularly limited, but for example, the rolling temperature of the first three stages of the finish rolling may be 1100 ° C. or less or 1000 ° C. or less.
  • one or more rolling passes under the condition (A4) and one or more rolling passes under the condition (A5) may overlap with each other.
  • the finish-rolled steel sheet is subjected to two-stage cooling in the next cooling step. Specifically, the finish-rolled steel sheet is first cooled to a temperature range of 650 to 750 ° C. at an average cooling rate of 10 ° C./s or more, held in that temperature range for 3.0 to 10.0 seconds, and then secondarily cooled to 100 ° C. or less at an average cooling rate of 30 ° C./s or more.
  • the cooling end temperature of the primary cooling is less than 650 ° C
  • transformation due to para-equilibrium occurs at a temperature lower than the desired temperature range
  • the proportion of crystal grains with an orientation difference of 5 to 14 ° in the grains is less than 10%.
  • the holding time at 650 to 750 ° C is less than 3.0 seconds
  • the proportion of crystal grains with an orientation difference of 5 to 14 ° in the grains is also less than 10%.
  • the holding time at 650 to 750 ° C exceeds 10.0 seconds or the average cooling rate of the secondary cooling is less than 30 ° C / s, cementite that is harmful to stretch flangeability is likely to be generated.
  • the cooling end temperature of the secondary cooling is more than 100 ° C, the area ratio of martensite is less than 2%.
  • the average cooling rate of the primary and secondary cooling may be 200 ° C / s or less in consideration of the equipment capacity of the cooling equipment.
  • Hot-rolled steel sheet manufactured by the above manufacturing method can have a metal structure that contains, by area percentage, at least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%, and where, if boundaries with an orientation difference of 15° or more are defined as grain boundaries, and regions surrounded by such grain boundaries and having a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains, the proportion of crystal grains with an intragranular orientation difference of 5-14° is 10-60% by area percentage.
  • a metal structure that contains, by area percentage, at least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%, and where, if boundaries with an orientation difference of 15° or more are defined as grain boundaries, and regions surrounded by such grain boundaries and having a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains, the proportion of crystal grains with an intragranular orientation difference of 5-14° is 10-60% by area percentage.
  • the average pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the region from the surface to the 1/6 position of the plate thickness is controlled to 2.50 or more, and the average pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations in the region from the 2/5 position to the 3/5 position of the plate thickness is controlled to 7.00 or less, so that the anisotropy of strength in the tensile strength in the L direction and C direction of the hot rolled steel sheet can be significantly reduced. Therefore, according to the hot rolled steel sheet manufactured by the above manufacturing method, it is possible to reliably achieve the contradictory properties of high strength and excellent workability at the same time, and it is particularly useful in the automotive field where both properties are required.
  • hot-rolled steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength, stretch flangeability, ductility, notch fatigue properties, and strength anisotropy of the obtained hot-rolled steel sheets were investigated.
  • molten steel was cast by continuous casting to form slabs having various chemical compositions shown in Tables 1 and 2, and these slabs were heated under the conditions shown in Table 3, and then hot rolling was performed.
  • Hot rolling was performed by performing rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and finish rolling was performed using a tandem rolling mill consisting of seven rolling stands.
  • the entry temperature (F0) of the finish rolling, the rolling temperature of the first stage of the finish rolling (FT1), the rolling temperature of the second stage of the finish rolling (FT2), the rolling temperature of the third stage of the finish rolling (FT3), the end temperature of the finish rolling, and the accumulated strain ( ⁇ eff.) of the last three stages of the finish rolling were as shown in Table 2.
  • finish rolling was performed using rolls having the roll radii shown in Table 3, and the number of rolling passes at 1100°C or less to achieve a shape ratio (X) of 2.3 or more was shown in Table 3.
  • finish-rolled steel plate was subjected to primary and secondary cooling under the conditions shown in Table 3 to obtain a hot-rolled steel plate having the plate thickness shown in Table 2.
  • the properties of the resulting hot-rolled steel sheets were measured and evaluated using the following methods.
  • TSC tensile strength
  • El total elongation
  • the stretch flangeability was evaluated by a saddle-shaped stretch flange test method using a saddle-shaped molded product. Specifically, a molded product of a saddle-shaped shape simulating a stretch flange shape consisting of a straight part and a circular part as shown in FIG. 1 was pressed, and the stretch flangeability was evaluated by the limit forming height at that time.
  • a saddle-shaped stretch flange test method a saddle-shaped molded product with a corner curvature radius R of 50 to 60 mm and an opening angle ⁇ of 120° is used to measure the limit forming height H (mm) when the clearance when punching the corner part is 11%.
  • the clearance indicates the ratio of the gap between the punching die and the punch to the thickness of the test piece. Since the clearance is actually determined by the combination of the punching tool and the plate thickness, 11% means that the range of 10.5 to 11.5% is satisfied.
  • the judgment of the limit forming height H was made by visually observing the presence or absence of cracks having a length of 1/3 or more of the plate thickness after forming, and the limit forming height at which no cracks existed was determined.
  • the product (TSC ⁇ H) of the tensile strength TSC (MPa) and the limit forming height H (mm) was used as an index of stretch flangeability, and the stretch flangeability was evaluated as being improved when TSC ⁇ H ⁇ 19,500 MPa ⁇ mm.
  • the hot-rolled steel sheet was evaluated as having high strength, yet improved stretch flangeability, ductility, and notch fatigue properties, and reduced strength anisotropy. The results are shown in Tables 4 and 5.
  • the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations exceeded 7.00, the strength in the C direction could not be reduced, and the anisotropy of the strength became prominent.
  • the average cooling rate of the first cooling in the cooling step was low, so it is believed that transformation due to paraequilibrium occurred at a relatively high temperature.
  • the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced.
  • the cooling stop temperature of the first cooling was high, so it is believed that transformation due to paraequilibrium occurred at a relatively high temperature.
  • the cooling stop temperature of the first cooling was low, so it is believed that transformation due to paraequilibrium occurred at a temperature lower than the desired temperature range.
  • the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced.
  • the holding time at 650 to 750 ° C in the first cooling was short, so the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced.
  • the cooling stop temperature of the secondary cooling in the cooling process was high, so the area ratio of martensite was less than 2%. As a result, the TSC and notch fatigue properties were reduced.
  • Comparative Examples 31 and 33 the C and Mn contents were high, respectively, and therefore the stretch flangeability was reduced.
  • Comparative Examples 32 and 34 the C and Mn contents were low, respectively, and therefore sufficient strength could not be obtained.
  • Comparative Example 35 the Al content was high, and therefore cracks occurred during rolling, and subsequent testing could not be performed.
  • Comparative Example 36 the total content of Si and sol. Al was high, and therefore ferrite formation was promoted, and the TSC was reduced.
  • Comparative Example 37 the total content of Si and sol. Al was low, and therefore the proportion of crystal grains with an intragranular misorientation of 5 to 14° was less than 10%, and therefore the stretch flangeability was reduced.
  • Comparative Example 38 the Ti content was high, and therefore the carbide (TiC) became coarse, and therefore the ductility was reduced.
  • Comparative Example 39 the Ti content was low, and therefore it is believed that the formation of cementite could not be sufficiently suppressed, and as a result, the stretch flangeability was reduced.

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Abstract

Provided is a hot rolled steel sheet that has a prescribed chemical composition and contains 80 to 98% of at least one of a ferrite and a bainite in total and 2 to10% of a martensite in terms of areal percentage and that has a metallic structure, wherein: given that a boundary having an orientation difference of 15º or more is defined as a grain boundary and a region that is surrounded by the grain boundary and has a circle-equivalent diameter of 0. 3μm or more is defined as a crystal grain, the proportion of crystal grains having an in-grain orientation difference of 5-14º is 10 to 60% in terms of areal percentage; the average pole density in the {110}<111> and {112}<111> orientations in a region from the surface to the 1/6-sheet thickness position is 2.50 or more; and the average pole density in the {100}<011>, {211}<011> , and {332}<113> orientations in a region from the 2/5-sheet thickness position to the 3/5-sheet thickness position is 7.00 or less.

Description

熱間圧延鋼板Hot rolled steel plate
　本発明は、熱間圧延鋼板に関する。 The present invention relates to hot-rolled steel sheets.
　近年、自動車業界では、燃費向上の観点から車体の軽量化が求められている。車体の軽量化と衝突安全性を両立するためには、使用する鋼板の高強度化が有効な方法の一つであり、このような背景から高強度鋼板の開発が進められている。一方で、高強度化とともに鋼板の加工性は一般に低下する。このため、高強度鋼板の開発においては、加工性を一定以上確保しつつ高強度化を図ることが重要である。 In recent years, the automotive industry has been seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency. In order to achieve both a lighter vehicle body and crashworthiness, increasing the strength of the steel plates used is one effective method, and against this background, the development of high-strength steel plates is underway. However, as strength increases, the workability of the steel plate generally decreases. For this reason, when developing high-strength steel plates, it is important to increase strength while maintaining a certain level of workability.
　これに関連して、特許文献１では、所定の化学組成を有し、組織が、面積率で、合計で８０～９８％のフェライト及びベイナイトと、２～１０％のマルテンサイトとを含み、前記組織において、方位差が１５°以上である境界を粒界とし、前記粒界によって囲まれ、かつ円相当直径が０．３μｍ以上である領域を結晶粒と定義した場合、粒内の方位差が５～１４°である前記結晶粒の割合が、面積率で、１０～６０％であることを特徴とする熱延鋼板が記載されている。また、特許文献１では、粒内の方位差が５～１４°である上記結晶粒の割合を面積率で１０～６０％とすることで、高強度でありながら伸びフランジ性及び延性を向上させることができ、さらに組織中のフェライト及びベイナイトの合計面積率並びにマルテンサイトの面積率を所定の範囲内に制御することで切り欠き疲労特性を改善できることが教示されている。 In this regard, Patent Document 1 describes a hot-rolled steel sheet having a predetermined chemical composition, a structure containing, in terms of area ratio, a total of 80-98% ferrite and bainite, and 2-10% martensite, and, when the boundaries in the structure where the misorientation is 15° or more are defined as grain boundaries, and the regions surrounded by the grain boundaries and having a circle equivalent diameter of 0.3 μm or more are defined as crystal grains, the proportion of the crystal grains where the misorientation within the grains is 5-14° is 10-60% in terms of area ratio. Patent Document 1 also teaches that by setting the proportion of the above crystal grains where the misorientation within the grains is 5-14° to an area ratio of 10-60%, it is possible to improve stretch flangeability and ductility while maintaining high strength, and further teaches that by controlling the total area ratio of ferrite and bainite in the structure and the area ratio of martensite within a predetermined range, it is possible to improve notch fatigue properties.
国際公開２０１６／１３３２２２号International Publication No. 2016/133222
　高強度鋼板は、鋳造されたスラブに熱間圧延を施すことにより製造されるが、当該熱間圧延に関連して圧延方向（Ｌ方向）の強度とそれに直角な幅方向（Ｃ方向）の強度との間で強度の異方性を持つ場合があることが知られている。強度の異方性が大きくなると、一般に鋼板の加工性が低下するため問題となる。したがって、鋼板の加工性を改善するために、特許文献１に記載されるような伸びフランジ性、延性及び切り欠き疲労特性に加えて、強度の異方性が低減された高強度鋼板に対して高いニーズがある。 High-strength steel plates are manufactured by hot rolling cast slabs, and it is known that the hot rolling can cause strength anisotropy between the strength in the rolling direction (L direction) and the strength in the width direction (C direction) perpendicular thereto. When the strength anisotropy becomes large, it generally becomes a problem because the workability of the steel plate decreases. Therefore, in order to improve the workability of steel plates, there is a high demand for high-strength steel plates with reduced strength anisotropy in addition to the stretch flangeability, ductility, and notch fatigue properties described in Patent Document 1.
　そこで、本発明は、高強度であるにもかかわらず、伸びフランジ性、延性及び切り欠き疲労特性が改善されかつ強度の異方性が低減された熱間圧延鋼板を提供することを目的とする。 The present invention aims to provide a hot-rolled steel sheet that, despite its high strength, has improved stretch flangeability, ductility, and notch fatigue properties, and has reduced strength anisotropy.
　本発明者らは、上記目的を達成するために、特に熱間圧延鋼板の金属組織に着目して検討を行った。その結果、本発明者らは、所定の化学組成を有する熱間圧延鋼板の金属組織をフェライト及びベイナイトの少なくとも１種並びにマルテンサイトを特定の割合で含むよう構成し、さらに結晶粒の割合を所定の範囲内に制御することで伸びフランジ性、延性及び切り欠き疲労特性を改善しつつ、これに加えて鋼板の板厚表層部と板厚中心部の集合組織を適切に制御することで強度の異方性を低減することができることを見出し、本発明を完成させた。 In order to achieve the above object, the inventors conducted research with a particular focus on the metal structure of hot-rolled steel sheet. As a result, the inventors discovered that by configuring the metal structure of a hot-rolled steel sheet having a specified chemical composition to contain at least one of ferrite and bainite, and martensite in specific proportions, and further controlling the proportion of crystal grains within a specified range, it is possible to improve stretch flangeability, ductility, and notch fatigue properties, while in addition, by appropriately controlling the texture in the surface layer and center of the steel sheet, it is possible to reduce the anisotropy of strength, thus completing the present invention.
　上記目的を達成し得た本発明は下記のとおりである。
　（１）質量％で、
　Ｃ：０．０２０～０．０７０％、
　Ｓｉ：０．０１０～２．０００％、
　Ｍｎ：０．６０～２．００％、
　Ｔｉ：０．０１５～０．２００％、
　ｓｏｌ．Ａｌ：０．０１０～１．０００％、
　Ｐ：０．１００％以下、
　Ｓ：０．０３０％以下、
　Ｎ：０．００６０％以下、
　Ｏ：０．０１００％以下、
　Ｎｂ：０～０．０５０％、
　Ｖ：０～０．３００％、
　Ｃｒ：０～２．００％、
　Ｎｉ：０～２．００％、
　Ｃｕ：０～２．００％、
　Ｍｏ：０～１．０００％、
　Ｂ：０～０．０１００％、
　Ｓｂ：０～１．００％、
　Ｃａ：０～０．０１００％、
　Ｍｇ：０～０．０１００％、
　Ｈｆ：０～０．０１００％、
　ＲＥＭ：０～０．１０００％、
　Ｂｉ：０～０．０１００％、
　Ａｓ：０～０．０１００％、
　Ｚｒ：０～１．００％、
　Ｃｏ：０～１．００％、
　Ｚｎ：０～１．００％、
　Ｗ：０～１．００％、
　Ｓｎ：０～１．００％、並びに
　残部：Ｆｅ及び不純物からなり、
　０．１００≦［Ｓｉ］＋［ｓｏｌ．Ａｌ］≦２．５００を満たし、式中、［Ｓｉ］及び［ｓｏｌ．Ａｌ］は各元素の含有量（質量％）である化学組成を有し、
　面積％で、
　フェライト及びベイナイトの少なくとも１種：合計で８０～９８％、並びに
　マルテンサイト：２～１０％を含み、
　方位差が１５°以上である境界を粒界とし、前記粒界によって囲まれ、かつ円相当直径が０．３μｍ以上である領域を結晶粒と定義した場合に、粒内の方位差が５～１４°である前記結晶粒の割合が、面積％で、１０～６０％であり、
　表面から板厚１／６位置までの領域における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値が２．５０以上であり、
　板厚２／５位置から板厚３／５位置までの領域における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値が７．００以下である金属組織を有することを特徴とする、熱間圧延鋼板。
　（２）前記化学組成が、質量％で、
　Ｎｂ：０．００１～０．０５０％、
　Ｖ：０．００１～０．３００％、
　Ｃｒ：０．０１～２．００％、
　Ｎｉ：０．０１～２．００％、
　Ｃｕ：０．０１～２．００％、
　Ｍｏ：０．００１～１．０００％、
　Ｂ：０．０００１～０．０１００％、
　Ｓｂ：０．０１～１．００％、
　Ｃａ：０．０００１～０．０１００％、
　Ｍｇ：０．０００１～０．０１００％、
　Ｈｆ：０．０００１～０．０１００％、
　ＲＥＭ：０．０００１～０．１０００％、
　Ｂｉ：０．０００１～０．０１００％、
　Ａｓ：０．０００１～０．０１００％、
　Ｚｒ：０．０１～１．００％、
　Ｃｏ：０．０１～１．００％、
　Ｚｎ：０．０１～１．００％、
　Ｗ：０．０１～１．００％、及び
　Ｓｎ：０．０１～１．００％
のうち少なくとも１種を含むことを特徴とする、上記（１）に記載の熱間圧延鋼板。 The present invention, which has achieved the above object, is as follows.
(1) In mass%,
C: 0.020 to 0.070%,
Si: 0.010 to 2.000%,
Mn: 0.60 to 2.00%,
Ti: 0.015 to 0.200%,
sol. Al: 0.010 to 1.000%,
P: 0.100% or less,
S: 0.030% or less,
N: 0.0060% or less,
O: 0.0100% or less,
Nb: 0 to 0.050%,
V: 0 to 0.300%,
Cr: 0 to 2.00%,
Ni: 0 to 2.00%,
Cu: 0 to 2.00%,
Mo: 0 to 1.000%,
B: 0 to 0.0100%,
Sb: 0 to 1.00%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Hf: 0 to 0.0100%,
REM: 0 to 0.1000%,
Bi: 0 to 0.0100%,
As: 0 to 0.0100%,
Zr: 0 to 1.00%,
Co: 0 to 1.00%,
Zn: 0 to 1.00%,
W: 0 to 1.00%,
Sn: 0 to 1.00%, and the balance: Fe and impurities;
The chemical composition satisfies 0.100≦[Si]+[sol. Al]≦2.500, in which [Si] and [sol. Al] are the contents (mass%) of each element,
In area %,
At least one of ferrite and bainite: 80 to 98% in total; and martensite: 2 to 10%;
When a boundary having an orientation difference of 15° or more is defined as a grain boundary, and a region surrounded by the grain boundary and having a circle equivalent diameter of 0.3 μm or more is defined as a crystal grain, the ratio of the crystal grains having an intragranular orientation difference of 5 to 14° is 10 to 60% by area percent,
The average pole density of the {110}<111> and {112}<111> orientations in the region from the surface to the 1/6 position of the sheet thickness is 2.50 or more;
A hot-rolled steel sheet, characterized in that it has a metal structure in which the average pole density of the {100}<011>, {211}<011> and {332}<113> orientations in a region from the 2/5 position of the sheet thickness to the 3/5 position of the sheet thickness is 7.00 or less.
(2) The chemical composition is, in mass%,
Nb: 0.001 to 0.050%,
V: 0.001 to 0.300%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Mo: 0.001 to 1.000%,
B: 0.0001 to 0.0100%,
Sb: 0.01 to 1.00%,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0100%,
Hf: 0.0001 to 0.0100%,
REM: 0.0001 to 0.1000%,
Bi: 0.0001 to 0.0100%,
As: 0.0001 to 0.0100%,
Zr: 0.01 to 1.00%,
Co: 0.01 to 1.00%,
Zn: 0.01 to 1.00%,
W: 0.01 to 1.00%, and Sn: 0.01 to 1.00%
The hot-rolled steel sheet according to the above (1), characterized in that it contains at least one of the following:
　本発明によれば、高強度であるにもかかわらず、伸びフランジ性、延性及び切り欠き疲労特性が改善されかつ強度の異方性が低減された熱間圧延鋼板を提供することができる。 The present invention provides a hot-rolled steel sheet that has high strength, but also has improved stretch flangeability, ductility, and notch fatigue properties, and has reduced strength anisotropy.
鞍型伸びフランジ試験法に用いる、鞍型形状の成形品の形状を示す図である。FIG. 2 is a diagram showing the shape of a saddle-shaped molded product used in a saddle-shaped stretch flange test method. 切り欠き疲労特性を評価するために用いた疲労試験片の形状を示す図である。FIG. 2 is a diagram showing the shape of a fatigue test specimen used to evaluate notch fatigue properties.
＜熱間圧延鋼板＞
　本発明の実施形態に係る熱間圧延鋼板は、質量％で、
　Ｃ：０．０２０～０．０７０％、
　Ｓｉ：０．０１～２．００％、
　Ｍｎ：０．６００～２．００％、
　Ｔｉ：０．０１５～０．２００％、
　ｓｏｌ．Ａｌ：０．１００～１．０００％、
　Ｐ：０．１００％以下、
　Ｓ：０．０３０％以下、
　Ｎ：０．００６０％以下、
　Ｏ：０．０１００％以下、
　Ｎｂ：０～０．０５０％、
　Ｖ：０～０．３００％、
　Ｃｒ：０～２．００％、
　Ｎｉ：０～２．００％、
　Ｃｕ：０～２．００％、
　Ｍｏ：０～１．０００％、
　Ｂ：０～０．０１００％、
　Ｓｂ：０～１．００％、
　Ｃａ：０～０．０１００％、
　Ｍｇ：０～０．０１００％、
　Ｈｆ：０～０．０１００％、
　ＲＥＭ：０～０．１０００％、
　Ｂｉ：０～０．０１００％、
　Ａｓ：０～０．０１００％、
　Ｚｒ：０～１．００％、
　Ｃｏ：０～１．００％、
　Ｚｎ：０～１．００％、
　Ｗ：０～１．００％、
　Ｓｎ：０～１．００％、並びに
　残部：Ｆｅ及び不純物からなり、
　０．１００≦［Ｓｉ］＋［ｓｏｌ．Ａｌ］≦２．５００を満たし、式中、［Ｓｉ］及び［ｓｏｌ．Ａｌ］は各元素の含有量（質量％）である化学組成を有し、
　面積％で、
　フェライト及びベイナイトの少なくとも１種：合計で８０～９８％、並びに
　マルテンサイト：２～１０％を含み、
　方位差が１５°以上である境界を粒界とし、前記粒界によって囲まれ、かつ円相当直径が０．３μｍ以上である領域を結晶粒と定義した場合に、粒内の方位差が５～１４°である前記結晶粒の割合が、面積％で、１０～６０％であり、
　表面から板厚１／６位置までの領域における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値が２．５０以上であり、
　板厚２／５位置から板厚３／５位置までの領域における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値が７．００以下である金属組織を有することを特徴としている。 <Hot-rolled steel sheets>
The hot-rolled steel sheet according to the embodiment of the present invention has, in mass%,
C: 0.020 to 0.070%,
Si: 0.01 to 2.00%,
Mn: 0.600 to 2.00%,
Ti: 0.015 to 0.200%,
sol. Al: 0.100 to 1.000%,
P: 0.100% or less,
S: 0.030% or less,
N: 0.0060% or less,
O: 0.0100% or less,
Nb: 0 to 0.050%,
V: 0 to 0.300%,
Cr: 0 to 2.00%,
Ni: 0 to 2.00%,
Cu: 0 to 2.00%,
Mo: 0 to 1.000%,
B: 0 to 0.0100%,
Sb: 0 to 1.00%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Hf: 0 to 0.0100%,
REM: 0 to 0.1000%,
Bi: 0 to 0.0100%,
As: 0 to 0.0100%,
Zr: 0 to 1.00%,
Co: 0 to 1.00%,
Zn: 0 to 1.00%,
W: 0 to 1.00%,
Sn: 0 to 1.00%, and the balance: Fe and impurities;
The chemical composition satisfies 0.100≦[Si]+[sol. Al]≦2.500, in which [Si] and [sol. Al] are the contents (mass%) of each element,
In area %,
At least one of ferrite and bainite: 80 to 98% in total; and martensite: 2 to 10%;
When a boundary having an orientation difference of 15° or more is defined as a grain boundary, and a region surrounded by the grain boundary and having a circle equivalent diameter of 0.3 μm or more is defined as a crystal grain, the ratio of the crystal grains having an intragranular orientation difference of 5 to 14° is 10 to 60% by area percent,
The average pole density of the {110}<111> and {112}<111> orientations in the region from the surface to the 1/6 position of the sheet thickness is 2.50 or more;
The steel sheet is characterized in having a metal structure in which the average pole density of the {100}<011>, {211}<011> and {332}<113> orientations in the region from the 2/5 position of the sheet thickness to the 3/5 position of the sheet thickness is 7.00 or less.
　先に述べたとおり、鋼板の高強度化とともに伸びフランジ性等の特性が低下することや、鋼板製造時の熱間圧延に関連して圧延方向（Ｌ方向）の強度とそれに直角な幅方向（Ｃ方向）の強度との間で強度の異方性を持つ場合があることが知られている。まず、本発明の実施形態においては、所定の化学組成を有する熱間圧延鋼板の金属組織を、フェライト及びベイナイトの少なくとも１種並びにマルテンサイトを特定の割合で含むよう構成し、より具体的には面積％でフェライト及びベイナイトの少なくとも１種：合計で８０～９８％並びにマルテンサイト：２～１０％を含むよう構成することで、強度と伸びフランジ性、延性及び切り欠き疲労特性とをバランスよく向上させることができる。加えて、方位差１５°以上の境界で囲まれ、円相当直径０．３μｍ以上の領域を結晶粒と定義した場合に、粒内の方位差が５～１４°である結晶粒は、強度と伸びフランジ性及び延性を向上させるのに有効である。このため、当該結晶粒の割合を適切に制御すること、より具体的には面積％で１０～６０％の範囲内に制御することで、強度と伸びフランジ性及び延性のバランスをさらに改善することが可能となる。 As mentioned above, it is known that the strength of steel sheets is increased and that there is anisotropy in strength between the strength in the rolling direction (L direction) and the strength in the width direction (C direction) perpendicular thereto, in relation to the hot rolling during steel sheet manufacturing. First, in an embodiment of the present invention, the metal structure of a hot-rolled steel sheet having a predetermined chemical composition is configured to contain at least one of ferrite and bainite and martensite in a specific ratio, more specifically, by configuring it to contain at least one of ferrite and bainite: 80 to 98% in total and martensite: 2 to 10% by area percentage, it is possible to improve strength, stretch flangeability, ductility, and notch fatigue properties in a well-balanced manner. In addition, when a region surrounded by a boundary with an orientation difference of 15° or more and having a circle equivalent diameter of 0.3 μm or more is defined as a crystal grain, a crystal grain with an orientation difference within the grain of 5 to 14° is effective in improving strength, stretch flangeability, and ductility. Therefore, by appropriately controlling the proportion of these crystal grains, more specifically by controlling it to within the range of 10 to 60% by area, it is possible to further improve the balance between strength, stretch flangeability, and ductility.
　一方で、強度の異方性についてより詳しく説明すると、鋼板製造時の熱間圧延によって得られる非等方的な金属組織に起因して、圧延方向（Ｌ方向）とそれに直角な幅方向（Ｃ方向）との間で引張強さが異なる傾向があり、一般的には、熱間圧延鋼板におけるＬ方向の引張強さがＣ方向の引張強さに比べて低くなるような強度の異方性を示す傾向がある。上記の伸びフランジ性、延性及び切り欠き疲労特性を向上させるとともに、このような強度の異方性を低減することで、自動車等の用途で使用される高強度鋼板の加工性を大きく改善することが可能である。しかしながら、鋼板の高強度化とこれら特性の改善との両立を図ることは一般に非常に困難である。そこで、本発明者らは、伸びフランジ性、延性及び切り欠き疲労特性の向上に加えて強度の異方性を低減することで鋼板の高強度化との両立を実現すべく、特に熱間圧延鋼板の集合組織に着目して検討を行った。その結果、本発明者らは、熱間圧延鋼板の表面から板厚１／６位置までの領域における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値を２．５０以上に制御しかつ板厚２／５位置から板厚３／５位置までの領域における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値を７．００以下に制御することで、熱間圧延鋼板のＬ方向とＣ方向の引張強さにおける強度の異方性を顕著に低減することができることを見出した。 On the other hand, to explain the anisotropy of strength in more detail, due to the anisotropic metal structure obtained by hot rolling during steel sheet manufacturing, the tensile strength tends to differ between the rolling direction (L direction) and the width direction (C direction) perpendicular thereto, and generally, the tensile strength of hot-rolled steel sheets tends to be lower in the L direction than in the C direction. By improving the above-mentioned stretch flangeability, ductility, and notch fatigue properties and reducing such strength anisotropy, it is possible to greatly improve the workability of high-strength steel sheets used in applications such as automobiles. However, it is generally very difficult to achieve both high strength steel sheets and improvements in these properties. Therefore, the present inventors conducted a study, focusing particularly on the texture of hot-rolled steel sheets, in order to achieve both high strength steel sheets by reducing the anisotropy of strength in addition to improving the stretch flangeability, ductility, and notch fatigue properties. As a result, the inventors discovered that by controlling the average pole density of the {110}<111> and {112}<111> orientations in the region from the surface of the hot-rolled steel plate to the 1/6 position of the plate thickness to 2.50 or more, and controlling the average pole density of the {100}<011>, {211}<011>, and {332}<113> orientations in the region from the 2/5 position to the 3/5 position of the plate thickness to 7.00 or less, it is possible to significantly reduce the anisotropy of the strength in the tensile strength of the hot-rolled steel plate in the L direction and C direction.
　より詳しく説明すると、熱間圧延鋼板の板厚断面を解析した場合に、圧延の影響を直接的に受ける板厚表層部（すなわち熱間圧延鋼板の表面から板厚１／６位置までの領域）と板厚中心部（すなわち板厚２／５位置から板厚３／５位置までの領域）とでは結晶の配向性が異なっている。より具体的には、板厚表層部では、｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の集合組織が発達しており、このような集合組織の発達に起因してＬ方向の強度が高められると考えられる。一方で、板厚中心部では、｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の集合組織が発達しており、このような集合組織の発達に起因してＣ方向の強度が高められると考えられる。しかしながら、板厚表層部と板厚中心部とでは、板厚中心部の影響が強く出やすく、それゆえＬ方向の引張強さがＣ方向の引張強さに比べて低くなるような強度の異方性を示すことになると考えられる。ここで、圧延板の結晶方位は、通常、圧延面に垂直な結晶方位を｛ｈｋｌ｝又は（ｈｋｌ）で表示し、圧延方向に平行な結晶方位を＜ｕｖｗ＞又は［ｕｖｗ］で表示する。｛ｈｋｌ｝及び＜ｕｖｗ＞は等価な面及び方位の総称であり、（ｈｋｌ）及び［ｕｖｗ］は個々の結晶面を指す。本発明の実施形態に係る熱間圧延鋼板では、主に体心立方構造（ｂｃｃ構造）を対象としているため、例えば、（１１０）、（－１１０）、（１－１０）、（－１－１０）、（１０１）、（－１０１）、（１０－１）、（－１０－１）、（０１１）、（０－１１）、（０１－１）及び（０－１－１）は等価であり、区別がつかない。本発明の実施形態においては、これらの方位を総称して｛１１０｝と表示する。 To explain in more detail, when the thickness cross section of a hot-rolled steel plate is analyzed, the crystal orientation is different between the surface layer part of the plate thickness (i.e., the region from the surface of the hot-rolled steel plate to the 1/6 position of the plate thickness) which is directly affected by rolling, and the center part of the plate thickness (i.e., the region from the 2/5 position of the plate thickness to the 3/5 position of the plate thickness). More specifically, in the surface layer part of the plate thickness, textures of {110}<111> and {112}<111> orientations are developed, and it is considered that the strength in the L direction is increased due to the development of such textures. On the other hand, in the center part of the plate thickness, textures of {100}<011>, {211}<011>, and {332}<113> orientations are developed, and it is considered that the strength in the C direction is increased due to the development of such textures. However, the surface layer part of the plate thickness and the center part of the plate thickness are more likely to be affected by the center part of the plate thickness, and therefore, it is considered that the strength anisotropy is exhibited such that the tensile strength in the L direction is lower than the tensile strength in the C direction. Here, the crystal orientation of the rolled sheet is usually expressed as {hkl} or (hkl) for the crystal orientation perpendicular to the rolling surface, and <uvw> or [uvw] for the crystal orientation parallel to the rolling direction. {hkl} and <uvw> are generic names for equivalent planes and orientations, and (hkl) and [uvw] refer to individual crystal planes. The hot-rolled steel sheet according to the embodiment of the present invention is mainly intended for body-centered cubic structures (bcc structures), so for example, (110), (-110), (1-10), (-1-10), (101), (-101), (10-1), (-10-1), (011), (0-11), (01-1) and (0-1-1) are equivalent and indistinguishable. In the embodiment of the present invention, these orientations are collectively expressed as {110}.
　そこで、本発明者らは、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値を所定の値以上に大きくしてＬ方向の強度を高める一方で、板厚中心部における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値を所定の値以下に小さくしてＣ方向の強度を低くすること、より具体的には熱間圧延鋼板の表面から板厚１／６位置までの領域における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値を２．５０以上に制御しかつ板厚２／５位置から板厚３／５位置までの領域における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値を７．００以下に制御することで、熱間圧延鋼板のＬ方向とＣ方向の引張強さにおける強度の異方性を顕著に低減することができることを見出した。ここで、極密度とは、特定の方位への集積を持たない標準試料に対する供試材の特定方位への集積度の比をいうものである。本発明の実施形態に係る熱間圧延鋼板では、先に述べたとおり、伸びフランジ性、延性及び切り欠き疲労特性を向上させるために、その金属組織をフェライト及びベイナイトの少なくとも１種並びにマルテンサイトを特定の割合で含むよう構成するとともに、粒内の方位差が５～１４°である特定の結晶粒を面積％で１０～６０％の範囲内に制御している。したがって、このように制御された金属組織の構成を維持しつつ、さらに板厚表層部における特定の集合組織の極密度と板厚中心部の特定の集合組織の極密度をそれぞれ所望の範囲内に制御することは極めて難しい。これに対し、熱間圧延鋼板の製造方法に関連して後で詳しく説明されるように、本発明の実施形態では、熱間圧延工程における圧延条件を適切なものとすることで、伸びフランジ性、延性及び切り欠き疲労特性を向上させるための金属組織の構成を維持しつつ、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値が２．５０以上でありかつ板厚中心部における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値が７．００以下である金属組織の実現を可能としたものである。その結果として、本発明の実施形態によれば、高強度、例えば引張強さ５４０ＭＰａ以上の高強度であるにもかかわらず、伸びフランジ性、延性及び切り欠き疲労特性の向上と強度の異方性の低減を達成することができる。したがって、本発明の実施形態に係る熱間圧延鋼板は、高強度と優れた加工性の相反する特性を確実に両立させることができるので、これらの特性の両立が求められる自動車分野の使用において特に有用である。 Therefore, the inventors have determined that the average value of the pole density of the {110}<111> and {112}<111> orientations in the plate thickness surface layer portion is increased to a predetermined value or more to increase the strength in the L direction, while the average value of the pole density of the {100}<011>, {211}<011> and {332}<113> orientations in the plate thickness center portion is decreased to a predetermined value or less to decrease the strength in the C direction, more specifically, from the surface of the hot-rolled steel plate to a position 1/6 of the plate thickness. It has been found that the anisotropy of strength in the tensile strength in the L direction and the C direction of the hot rolled steel sheet can be significantly reduced by controlling the average value of the pole density of the {110}<111> and {112}<111> orientations in the region to 2.50 or more and controlling the average value of the pole density of the {100}<011>, {211}<011> and {332}<113> orientations in the region from the 2/5 position of the sheet thickness to the 3/5 position of the sheet thickness to 7.00 or less. Here, the pole density refers to the ratio of the accumulation degree in a specific orientation of the test material to that of a standard sample that does not have accumulation in a specific orientation. As described above, in the hot rolled steel sheet according to the embodiment of the present invention, in order to improve the stretch flangeability, ductility and notch fatigue properties, the metal structure is configured to contain at least one of ferrite and bainite and martensite in a specific ratio, and the area percentage of the specific crystal grains having an intragranular orientation difference of 5 to 14° is controlled to be within the range of 10 to 60%. Therefore, it is extremely difficult to control the pole density of a specific texture in the thickness surface layer and the pole density of a specific texture in the thickness center to within a desired range while maintaining the configuration of the metal structure controlled in this way. On the other hand, as will be described in detail later in relation to the manufacturing method of hot-rolled steel sheet, in the embodiment of the present invention, by making the rolling conditions in the hot rolling process appropriate, it is possible to realize a metal structure in which the average value of the pole density in the {110}<111> and {112}<111> orientations in the thickness surface layer is 2.50 or more and the average value of the pole density in the {100}<011>, {211}<011> and {332}<113> orientations in the thickness center is 7.00 or less while maintaining the configuration of the metal structure for improving stretch flangeability, ductility and notch fatigue properties. As a result, according to the embodiment of the present invention, it is possible to achieve improvement in stretch flangeability, ductility and notch fatigue properties and reduction in the anisotropy of strength, despite the high strength, for example, tensile strength of 540 MPa or more. Therefore, the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent workability, and is therefore particularly useful in the automotive field, where both properties are required to be achieved.
　以下、本発明の実施形態に係る熱間圧延鋼板についてより詳しく説明する。以下の説明において、各元素の含有量の単位である「％」は、特に断りがない限り「質量％」を意味するものである。また、本明細書において、数値範囲を示す「～」とは、特に断りがない場合、その前後に記載される数値を下限値及び上限値として含む意味で使用される。 The hot-rolled steel sheet according to an embodiment of the present invention will be described in more detail below. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values before and after it are included as the lower and upper limits, unless otherwise specified.
［Ｃ：０．０２０～０．０７０％］
　Ｃは、鋼板の強度を高めるのに有効な元素である。また、Ｃは、鋼中でＴｉ及びＮｂと炭化物及び／又は炭窒化物を形成し、形成した析出物に基づく析出強化や、当該析出物のピン止め効果による組織の微細化にも寄与する。これらの効果を十分に得るために、Ｃ含有量は０．０２０％以上とする。Ｃ含有量は０．０２２％以上、０．０２５％以上、０．０２８％以上又は０．０３０％以上であってもよい。一方で、Ｃを過度に含有すると、伸びフランジ性や溶接性が低下する場合がある。したがって、Ｃ含有量は０．０７０％以下とする。Ｃ含有量は０．０６５％以下、０．０６０％以下、０．０５５％以下又は０．０５０％以下であってもよい。 [C: 0.020 to 0.070%]
C is an element effective in increasing the strength of steel plate. In addition, C forms carbides and/or carbonitrides with Ti and Nb in steel, and contributes to precipitation strengthening based on the formed precipitates and to refinement of the structure due to the pinning effect of the precipitates. In order to fully obtain these effects, the C content is set to 0.020% or more. The C content may be 0.022% or more, 0.025% or more, 0.028% or more, or 0.030% or more. On the other hand, if C is contained excessively, stretch flangeability and weldability may be reduced. Therefore, the C content is set to 0.070% or less. The C content may be 0.065% or less, 0.060% or less, 0.055% or less, or 0.050% or less.
［Ｓｉ：０．０１０～２．０００％］
　Ｓｉは、固溶強化元素として強度上昇に有効な元素である。このような効果を十分に得るために、Ｓｉ含有量は０．０１０％以上とする。Ｓｉ含有量は０．１００％以上、０．１００％超、０．１１０％以上、０．１２０％以上、０．１５０％以上、０．１８０％以上、０．２００％以上、０．３００％以上、０．５００％以上、０．８００％以上又は１．０００％以上であってもよい。一方で、Ｓｉを過度に含有すると、Ｓｉスケールと呼ばれる表面品質不良を発生する場合がある。したがって、Ｓｉ含有量は２．０００％以下とする。Ｓｉ含有量は１．８００％以下、１．６００％以下、１．４００％以下又は１．２００％以下であってもよい。 [Si: 0.010 to 2.000%]
Si is an element that is effective in increasing strength as a solid solution strengthening element. In order to fully obtain such an effect, the Si content is set to 0.010% or more. The Si content may be 0.100% or more, more than 0.100%, 0.110% or more, 0.120% or more, 0.150% or more, 0.180% or more, 0.200% or more, 0.300% or more, 0.500% or more, 0.800% or more, or 1.000% or more. On the other hand, if Si is contained excessively, a surface quality defect called Si scale may occur. Therefore, the Si content is set to 2.000% or less. The Si content may be 1.800% or less, 1.600% or less, 1.400% or less, or 1.200% or less.
［Ｍｎ：０．６０～２．００％］
　Ｍｎは、焼入れ性及び固溶強化元素として強度上昇に有効な元素である。これらの効果を十分に得るために、Ｍｎ含有量は０．６０％以上とする。Ｍｎ含有量は０．７０％以上、０．８０％以上、０．９０％以上又は１．００％以上であってもよい。一方で、Ｍｎを過度に含有すると、伸びフランジ性が低下する場合がある。したがって、Ｍｎ含有量は２．００％以下とする。Ｍｎ含有量は１．８０％以下、１．６０％以下、１．４０％以下又は１．２０％以下であってもよい。 [Mn: 0.60 to 2.00%]
Mn is an element that is effective in increasing strength as a hardenability and solid solution strengthening element. In order to fully obtain these effects, the Mn content is set to 0.60% or more. The Mn content may be 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, if Mn is contained excessively, stretch flangeability may be reduced. Therefore, the Mn content is set to 2.00% or less. The Mn content may be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.
［Ｔｉ：０．０１５～０．２００％］
　Ｔｉは、炭化物（ＴｉＣ）として鋼中に微細に析出し、析出強化により鋼の強度を向上させる元素である。また、Ｔｉは、炭化物を形成することでＣを固定し、伸びフランジ性にとって有害なセメンタイトの生成を抑制する元素でもある。これらの効果を十分に得るために、Ｔｉ含有量は０．０１５％以上とする。Ｔｉ含有量は０．０２０％以上、０．０３０％以上、０．０４０％以上又は０．０５０％以上であってもよい。一方で、Ｔｉを過度に含有すると、炭化物が粗大となり、延性が低下する場合がある。したがって、Ｔｉ含有量は０．２００％以下とする。Ｔｉ含有量は０．１８０％以下、０．１７０％以下、０．１５０％以下又は０．１２０％以下であってもよい。 [Ti: 0.015 to 0.200%]
Ti is an element that precipitates finely in steel as carbide (TiC) and improves the strength of steel by precipitation strengthening. Ti also forms carbides to fix C and suppresses the formation of cementite, which is harmful to stretch flangeability. In order to fully obtain these effects, the Ti content is set to 0.015% or more. The Ti content may be 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if Ti is contained excessively, the carbides may become coarse and the ductility may decrease. Therefore, the Ti content is set to 0.200% or less. The Ti content may be 0.180% or less, 0.170% or less, 0.150% or less, or 0.120% or less.
［ｓｏｌ．Ａｌ：０．０１０～１．０００％］
　ｓｏｌ．Ａｌは、溶鋼の脱酸剤として作用する元素である。このような効果を十分に得るために、ｓｏｌ．Ａｌ含有量は０．０１０％以上とする。ｓｏｌ．Ａｌ含有量は０．０１２％以上、０．０１５％以上又は０．０２０％以上であってもよい。一方で、ｓｏｌ．Ａｌを過度に含有すると、粗大な酸化物が形成し、靭性や延性が低下して圧延中に破断に至る場合がある。したがって、ｓｏｌ．Ａｌ含有量は１．０００％以下とする。ｓｏｌ．Ａｌ含有量は０．８００％以下、０．６００％以下又は０．４００％以下であってもよい。なお、ｓｏｌ．Ａｌとは酸可溶性Ａｌを意味し、固溶状態で鋼中に存在する固溶Ａｌのことを示す。 [sol. Al: 0.010 to 1.000%]
Sol. Al is an element that acts as a deoxidizer for molten steel. In order to fully obtain this effect, the sol. Al content is set to 0.010% or more. On the other hand, if the sol. Al content is excessive, coarse oxides are formed, which reduces toughness and ductility and causes fatigue during rolling. Therefore, the sol. Al content is set to 1.000% or less. The sol. Al content is set to 0.800% or less, 0.600% or less, or 0.400% or less. The term "solubilized aluminum" means acid-soluble aluminum, and indicates solute aluminum that is present in the steel in a solid solution state.
［Ｐ：０．１００％以下］
　Ｐは、過度に含有すると溶接性などに不利に影響する場合がある。したがって、Ｐ含有量は０．１００％以下とする。Ｐ含有量は０．０５０％以下、０．０３０％以下、０．０２０％以下又は０．０１５％以下であってもよい。Ｐ含有量の下限は特に限定されず０％であってもよいが、過度な低減はコストの上昇を招く。したがって、Ｐ含有量は０．００１％以上、０．００３％以上又は０．００５％以上であってもよい。 [P: 0.100% or less]
If P is contained in an excessive amount, it may adversely affect weldability, etc. Therefore, the P content is set to 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the P content may be 0.001% or more, 0.003% or more, or 0.005% or more.
［Ｓ：０．０３０％以下］
　Ｓは、過度に含有するとＭｎＳが多く生成して靭性を低下させる場合がある。したがって、Ｓｉ含有量は０．０３０％以下とする。Ｓ含有量は０．０２０％以下、０．０１０％以下又は０．００５％以下であってもよい。Ｓ含有量の下限は特に限定されず０％であってもよいが、過度な低減はコストの上昇を招く。したがって、Ｓ含有量は０．００１％以上、０．００２％以上又は０．００３％以上であってもよい。 [S: 0.030% or less]
If S is contained excessively, a large amount of MnS may be generated, which may reduce toughness. Therefore, the Si content is set to 0.030% or less. The S content may be 0.020% or less, 0.010% or less, or 0.005% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the S content may be 0.001% or more, 0.002% or more, or 0.003% or more.
［Ｎ：０．００６０％以下］
　Ｎは、Ｃより優先的にＴｉと析出物を形成し、Ｃの固定に有効なＴｉを減少させる場合がある。したがって、Ｎ含有量は０．００６０％以下とする。Ｎ含有量は０．００５０％以下、０．００４０％以下又は０．００３０％以であってもよい。Ｎ含有量の下限は特に限定されず０％であってもよいが、過度な低減はコストの上昇を招く。したがって、Ｎ含有量は０．０００１％以上又は０．０００５％以上であってもよい。 [N: 0.0060% or less]
N may form precipitates with Ti preferentially over C, and may reduce the amount of Ti that is effective for fixing C. Therefore, the N content is set to 0.0060% or less. The N content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.
［Ｏ：０．０１００％以下］
　Ｏは、製造工程で混入する元素である。Ｏを過度に含有すると、粗大な介在物が形成して鋼板の靭性を低下させる場合がある。したがって、Ｏ含有量は０．０１００％以下とする。Ｏ含有量は０．００８０％以下、０．００６０％以下又は０．００４０％以下であってもよい。Ｏ含有量の下限は特に限定されず０％であってもよいが、０．０００１％未満に低減するためには精錬に時間を要し、生産性の低下を招く。したがって、Ｏ含有量は０．０００１％以上又は０．０００５％以上であってもよい。 [O: 0.0100% or less]
O is an element that is mixed in during the manufacturing process. If O is contained excessively, coarse inclusions may be formed, which may reduce the toughness of the steel plate. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long time for refining, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.
　本発明の実施形態に係る熱間圧延鋼板の基本化学組成は上記のとおりである。さらに、当該熱間圧延鋼板は、必要に応じて、残部のＦｅの一部に代えて以下の任意選択元素のうち少なくとも１種を含有してもよい。 The basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.
［Ｎｂ：０～０．０５０％］
　Ｎｂは、鋼中に炭化物、窒化物及び／又は炭窒化物を形成してピン止め効果により組織の微細化、ひいては鋼板の高強度化に寄与する元素である。また、Ｎｂは、炭化物及び／又は炭窒化物を形成することでＣを固定し、伸びフランジ性にとって有害なセメンタイトの生成を抑制する元素でもある。Ｎｂ含有量は０％であってもよいが、これらの効果を得るためには、Ｎｂ含有量は０．００１％以上であることが好ましい。Ｎｂ含有量は０．００５％以上、０．０１０％以上又は０．０１５％以上であってもよい。一方で、Ｎｂを過度に含有すると、鋼中に粗大な炭化物等が生成して鋼板の延性が低下する場合がある。したがって、Ｎｂ含有量は０．０５０％以下とする。Ｎｂ含有量は０．０４０％以下、０．０３０％以下又は０．０２０％以下であってもよい。 [Nb: 0 to 0.050%]
Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to refining the structure by the pinning effect, and thus to increasing the strength of the steel sheet. In addition, Nb is also an element that fixes C by forming carbides and/or carbonitrides, and suppresses the formation of cementite that is harmful to stretch flangeability. The Nb content may be 0%, but in order to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, if Nb is excessively contained, coarse carbides and the like may be formed in the steel, decreasing the ductility of the steel sheet. Therefore, the Nb content is set to 0.050% or less. The Nb content may be 0.040% or less, 0.030% or less, or 0.020% or less.
［Ｖ：０～０．３００％］
　Ｖは、析出強化等により強度の向上に寄与する元素である。Ｖ含有量は０％であってもよいが、このような効果を得るためには、Ｖ含有量は０．００１％以上であることが好ましい。Ｖ含有量は０．０１０％以上、０．０３０％以上又は０．０５０％以上であってもよい。一方で、Ｖを過度に含有しても効果が飽和し、製造コストの上昇を招く虞がある。したがって、Ｖ含有量は０．３００％以下であることが好ましい。Ｖ含有量は０．２００％以下、０．１００％以下又は０．０８０％以下であってもよい。 [V: 0 to 0.300%]
V is an element that contributes to improving strength by precipitation strengthening, etc. The V content may be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more. The V content may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if V is contained excessively, the effect is saturated and there is a risk of increasing the manufacturing cost. Therefore, the V content is preferably 0.300% or less. The V content may be 0.200% or less, 0.100% or less, or 0.080% or less.
［Ｃｒ：０～２．００％］
　Ｃｒは、鋼の焼入れ性を高め、強度の向上に寄与する元素である。Ｃｒ含有量は０％であってもよいが、このような効果を得るためには、Ｃｒ含有量は０．０１％以上であることが好ましい。Ｃｒ含有量は０．０３％以上又は０．０５％以上であってもよい。一方で、Ｃｒを過度に含有しても効果が飽和し、製造コストの上昇を招く虞がある。したがって、Ｃｒ含有量は２．００％以下であることが好ましい。Ｃｒ含有量は１．５０％以下、１．００％以下、０．５０％以下、０．３０％以下、０．１５％以下又は０．１０％以下であってもよい。 [Cr: 0 to 2.00%]
Cr is an element that enhances the hardenability of steel and contributes to improving strength. The Cr content may be 0%, but in order to obtain such an effect, the Cr content is preferably 0.01% or more. The Cr content may be 0.03% or more or 0.05% or more. On the other hand, even if Cr is contained excessively, the effect is saturated and there is a risk of increasing the manufacturing cost. Therefore, the Cr content is preferably 2.00% or less. The Cr content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
［Ｎｉ：０～２．００％］
［Ｃｕ：０～２．００％］
　Ｎｉ及びＣｕは、析出強化又は固溶強化により強度の向上に寄与する元素である。Ｎｉ及びＣｕ含有量は０％であってもよいが、このような効果を得るためには、これらの元素の含有量はそれぞれ０．０１％以上であることが好ましく、０．０３％以上又は０．０５％以上であってもよい。一方で、これらの元素を過度に含有しても効果が飽和し、製造コストの上昇を招く虞がある。したがって、Ｎｉ及びＣｕ含有量はそれぞれ２．００％以下であることが好ましく、１．５０％以下、１．００％以下、０．５０％以下、０．３０％以下、０．１５％以下又は０．１０％以下であってもよい。 [Ni: 0 to 2.00%]
[Cu: 0 to 2.00%]
Ni and Cu are elements that contribute to improving strength by precipitation strengthening or solid solution strengthening. The Ni and Cu contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.01% or more, and may be 0.03% or more or 0.05% or more. On the other hand, even if these elements are contained excessively, the effects are saturated and there is a risk of increasing manufacturing costs. Therefore, the Ni and Cu contents are preferably 2.00% or less, and may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
［Ｍｏ：０～１．０００％］
　Ｍｏは、鋼の焼入れ性を高め、強度の向上に寄与する元素である。Ｍｏ含有量は０％であってもよいが、このような効果を得るためには、Ｍｏ含有量は０．００１％以上であることが好ましい。Ｍｏ含有量は０．０１０％以上、０．０２０％以上又は０．０５０％以上であってもよい。一方で、Ｍｏを過度に含有すると、熱間加工時の変形抵抗が増大し、設備負荷が大きくなる場合がある。したがって、Ｍｏ含有量は１．０００％以下であることが好ましい。Ｍｏ含有量は０．８００％以下、０．５００％以下、０．２００％以下、０．１００％以下又は０．０８０％以下であってもよい。 [Mo: 0 to 1.000%]
Mo is an element that improves the hardenability of steel and contributes to improving strength. The Mo content may be 0%, but in order to obtain such an effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if Mo is excessively contained, the deformation resistance during hot working may increase, and the equipment load may become large. Therefore, the Mo content is preferably 1.000% or less. The Mo content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.
［Ｂ：０～０．０１００％］
　Ｂは、粒界に偏析して粒界強度を高めることで低温靭性を向上させる。Ｂ含有量は０％であってもよいが、このような効果を得るためには、Ｂ含有量は０．０００１％以上であることが好ましい。Ｂ含有量は０．０００２％以上、０．０００３％以上又は０．０００５％以上であってもよい。一方で、Ｂを過度に含有しても効果が飽和し、製造コストの上昇を招く虞がある。したがって、Ｂ含有量は０．０１００％以下であることが好ましい。Ｂ含有量は０．００５０％以下、０．００３０％以下、０．００１５％以下又は０．００１０％以下であってもよい。 [B: 0 to 0.0100%]
B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. The B content may be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, even if B is contained excessively, the effect becomes saturated and there is a risk of causing an increase in manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.
［Ｓｂ：０～１．００％］
　Ｓｂは、耐食性の向上に有効な元素である。Ｓｂ含有量は０％であってもよいが、このような効果を得るためには、Ｓｂ含有量は０．０１％以上であることが好ましい。Ｓｂ含有量は０．０２％以上又は０．０５％以上であってもよい。一方で、Ｓｂを過度に含有すると、靭性の低下を招く場合がある。したがって、Ｓｂ含有量は１．００％以下であることが好ましい。Ｓｂ含有量は０．８０％以下、０．５０％以下、０．３０％以下、０．１０％以下又は０．０８％以下であってもよい。 [Sb: 0 to 1.00%]
Sb is an element effective in improving corrosion resistance. The Sb content may be 0%, but in order to obtain such an effect, the Sb content is preferably 0.01% or more. The Sb content may be 0.02% or more or 0.05% or more. On the other hand, excessive Sb content may cause a decrease in toughness. Therefore, the Sb content is preferably 1.00% or less. The Sb content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.
［Ｃａ：０～０．０１００％］
［Ｍｇ：０～０．０１００％］
［Ｈｆ：０～０．０１００％］
　Ｃａ、Ｍｇ及びＨｆは、非金属介在物の形態を制御することができる元素である。Ｃａ、Ｍｇ及びＨｆ含有量は０％であってもよいが、このような効果を得るためには、これら元素の含有量はそれぞれ０．０００１％以上であることが好ましく、０．０００５％以上、０．００１０％以上又は０．００１５％以上であってもよい。一方で、これらの元素を過度に含有しても効果が飽和し、必要以上に鋼板中に含有させることは製造コストの上昇を招く。したがって、Ｃａ、Ｍｇ及びＨｆ含有量はそれぞれ０．０１００％以下であることが好ましく、０．００５０％以下、０．００３０％以下又は０．００２０％以下であってもよい。 [Ca: 0 to 0.0100%]
[Mg: 0 to 0.0100%]
[Hf: 0 to 0.0100%]
Ca, Mg and Hf are elements capable of controlling the morphology of nonmetallic inclusions. The Ca, Mg and Hf contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if these elements are contained in excess, the effect is saturated, and containing more than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the Ca, Mg and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
［ＲＥＭ：０～０．１０００％］
　ＲＥＭは、非金属介在物の形態を制御することができる元素である。ＲＥＭ含有量は０％であってもよいが、このような効果を得るためには、ＲＥＭ含有量は０．０００１％以上であることが好ましい。ＲＥＭ含有量は０．０００５％以上、０．００１０％以上又は０．００１５％以上であってもよい。一方で、ＲＥＭを過度に含有しても効果が飽和し、必要以上に鋼板中に含有させることは製造コストの上昇を招く。したがって、ＲＥＭ含有量は０．１０００％以下であることが好ましい。ＲＥＭ含有量は０．０５００％以下、０．０１００％以下、０．００５０％以下、０．００３０％以下又は０．００２０％以下であってもよい。本明細書におけるＲＥＭとは、原子番号２１番のスカンジウム（Ｓｃ）、原子番号３９番のイットリウム（Ｙ）、及びランタノイドである原子番号５７番のランタン（Ｌａ）～原子番号７１番のルテチウム（Ｌｕ）の１７元素の総称であり、ＲＥＭ含有量はこれら元素の合計含有量である。 [REM: 0 to 0.1000%]
REM is an element capable of controlling the morphology of nonmetallic inclusions. The REM content may be 0%, but in order to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if an excessive amount of REM is contained, the effect is saturated, and the inclusion of more than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the REM content is preferably 0.1000% or less. The REM content may be 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.
［Ｂｉ：０～０．０１００％］
［Ａｓ：０～０．０１００％］
　Ｂｉ及びＡｓは、耐食性の向上に有効な元素である。Ｂｉ及びＡｓ含有量は０％であってもよいが、このような効果を得るためには、これらの元素の含有量はそれぞれ０．０００１％以上であることが好ましく、０．０００５％以上、０．００１０％以上又は０．００１５％以上であってもよい。一方で、これらの元素を過度に含有しても効果が飽和し、必要以上に鋼板中に含有させることは製造コストの上昇を招く。したがって、Ｂｉ及びＡｓ含有量はそれぞれ０．０１００％以下であることが好ましく、０．００５０％以下、０．００３０％以下又は０．００２０％以下であってもよい。 [Bi: 0 to 0.0100%]
[As: 0 to 0.0100%]
Bi and As are elements effective in improving corrosion resistance. The Bi and As contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if these elements are contained in excess, the effect is saturated, and containing more than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the Bi and As contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
［Ｚｒ：０～１．００％］
　Ｚｒは、非金属介在物の形態を制御することができる元素である。Ｚｒ含有量は０％であってもよいが、このような効果を得るためには、Ｚｒ含有量は０．０１％以上であることが好ましい。Ｚｒ含有量は０．０５％以上又は０．１０％以上であってもよい。一方で、Ｚｒを過度に含有しても効果が飽和し、必要以上に鋼板中に含有させることは製造コストの上昇を招く。したがって、Ｚｒ含有量は１．００％以下であることが好ましい。Ｚｒ含有量は０．８０％以下、０．５０％以下、０．３０％以下又は０．２０％以下であってもよい。 [Zr: 0 to 1.00%]
Zr is an element capable of controlling the form of nonmetallic inclusions. The Zr content may be 0%, but in order to obtain such an effect, the Zr content is preferably 0.01% or more. The Zr content may be 0.05% or more or 0.10% or more. On the other hand, even if Zr is contained excessively, the effect is saturated, and the inclusion of more Zr than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the Zr content is preferably 1.00% or less. The Zr content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
［Ｃｏ：０～１．００％］
　Ｃｏは、焼入れ性及び／又は耐熱性の向上に寄与する元素である。Ｃｏ含有量は０％であってもよいが、これらの効果を得るためには、Ｃｏ含有量は０．０１％以上であることが好ましい。Ｃｏ含有量は０．０５％以上又は０．１０％以上であってもよい。一方で、Ｃｏを過度に含有すると、熱間加工性が低下する場合があり、原料コストの増加にも繋がる。したがって、Ｃｏ含有量は１．００％以下であることが好ましい。Ｃｏ含有量は０．８０％以下、０．５０％以下、０．３０％以下又は０．２０％以下であってもよい。 [Co: 0 to 1.00%]
Co is an element that contributes to improving hardenability and/or heat resistance. The Co content may be 0%, but in order to obtain these effects, the Co content is preferably 0.01% or more. The Co content may be 0.05% or more or 0.10% or more. On the other hand, excessive Co content may deteriorate hot workability and lead to an increase in raw material costs. Therefore, the Co content is preferably 1.00% or less. The Co content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
［Ｚｎ：０～１．００％］
　Ｚｎは、介在物の形状を制御するのに有効な元素である。このような効果を得るためには、Ｚｎ含有量は０．０１％以上であることが好ましい。Ｚｎ含有量は０．０５％以上又は０．１０％以上であってもよい。一方で、Ｚｎを過度に含有しても効果が飽和し、製造コストの上昇を招く。したがって、Ｚｎ含有量は１．００％以下であることが好ましい。Ｚｎ含有量は０．８０％以下、０．５０％以下、０．３０％以下又は０．２０％以下であってもよい。 [Zn: 0 to 1.00%]
Zn is an element effective in controlling the shape of inclusions. In order to obtain such an effect, the Zn content is preferably 0.01% or more. The Zn content may be 0.05% or more or 0.10% or more. On the other hand, even if Zn is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the Zn content is preferably 1.00% or less. The Zn content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
［Ｗ：０～１．００％］
　Ｗは、鋼の焼入れ性を高め、強度の向上に寄与する元素である。Ｗ含有量は０％であってもよいが、このような効果を得るためには、Ｗ含有量は０．０１％以上であることが好ましい。Ｗ含有量は０．０５％以上又は０．１０％以上であってもよい。一方で、Ｗを過度に含有すると、溶接性が低下する場合がある。したがって、Ｗ含有量は１．００％以下であることが好ましい。Ｗ含有量は０．８０％以下、０．５０％以下、０．３０％以下又は０．２０％以下であってもよい。 [W: 0 to 1.00%]
W is an element that enhances the hardenability of steel and contributes to improving strength. The W content may be 0%, but in order to obtain such an effect, the W content is preferably 0.01% or more. The W content may be 0.05% or more or 0.10% or more. On the other hand, excessive W content may reduce weldability. Therefore, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
［Ｓｎ：０～１．００％］
　Ｓｎは、耐食性の向上に有効な元素である。Ｓｎ含有量は０％であってもよいが、このような効果を得るためには、Ｓｎ含有量は０．０１％以上であることが好ましい。Ｓｎ含有量は０．０２％以上又は０．０５％以上であってもよい。一方で、Ｓｎを過度に含有すると、靭性の低下を招く場合がある。したがって、Ｓｎ含有量は１．００％以下であることが好ましい。Ｓｎ含有量は０．８０％以下、０．５０％以下、０．３０％以下、０．１０％以下又は０．０８％以下であってもよい。 [Sn: 0 to 1.00%]
Sn is an element effective in improving corrosion resistance. The Sn content may be 0%, but in order to obtain such an effect, the Sn content is preferably 0.01% or more. The Sn content may be 0.02% or more or 0.05% or more. On the other hand, excessive Sn content may cause a decrease in toughness. Therefore, the Sn content is preferably 1.00% or less. The Sn content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.
　本発明の実施形態に係る熱間圧延鋼板において、上記の元素以外の残部は、Ｆｅ及び不純物からなる。不純物とは、熱間圧延鋼板を工業的に製造する際に、鉱石やスクラップ等のような原料を始めとして、製造工程の種々の要因によって混入する成分等である。 In the hot-rolled steel sheet according to the embodiment of the present invention, the remainder other than the above elements consists of Fe and impurities. Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing hot-rolled steel sheets.
［０．１００≦［Ｓｉ］＋［ｓｏｌ．Ａｌ］≦２．５００］
　本発明の実施形態に係る熱間圧延鋼板の化学組成は、下記式を満たす必要がある。
　　　０．１００≦［Ｓｉ］＋［ｓｏｌ．Ａｌ］≦２．５００
　式中、［Ｓｉ］及び［ｓｏｌ．Ａｌ］は各元素の含有量（質量％）である。先に述べたとおり、方位差１５°以上の境界で囲まれ、円相当直径０．３μｍ以上の領域を結晶粒と定義した場合に、粒内の方位差が５～１４°である結晶粒は、強度と伸びフランジ性を向上させるのに有効である。このため、本発明の実施形態に係る熱間圧延鋼板では、後で詳しく説明されるように、当該結晶粒の割合を面積％で１０～６０％の範囲内に制御することで強度と伸びフランジ性のバランスを改善させている。Ｓｉ及びｓｏｌ．Ａｌは、個々の元素について説明した効果に加えて、粒内の方位差が５～１４°である結晶粒の割合を１０～６０％の範囲内に制御するのに有効な元素でもある。これは、Ｓｉ及びｓｏｌ．Ａｌを含有することでＡｒ３点の温度が上昇し、粒内に導入される変態ひずみが少なくなることに起因していると考えられる。これらの効果を十分に得るために、本発明の実施形態に係る熱間圧延鋼板の化学組成は、各元素の含有量を先に説明した範囲内に制御しつつ、Ｓｉ及びｓｏｌ．Ａｌの合計の含有量が０．１００％以上、すなわち［Ｓｉ］＋［ｓｏｌ．Ａｌ］≧０．１００を満たすように制御される。Ｓｉ及びｓｏｌ．Ａｌの合計の含有量は０．１２０％以上、０．１５０％以上、０．２００％以上又は０．３００％以上であってもよい。一方で、Ｓｉ及びｓｏｌ．Ａｌの合計の含有量が高すぎると、フェライト生成が促進され強度が低下する場合がある。したがって、Ｓｉ及びｓｏｌ．Ａｌの合計の含有量は２．５００％以下、すなわち［Ｓｉ］＋［ｓｏｌ．Ａｌ］≦２．５００とする。Ｓｉ及びｓｏｌ．Ａｌの合計の含有量は２．０００％以下、１．５００％以下、１．０００％以下又は０．０００％以下であってもよい。 [0.100≦[Si]+[sol. Al]≦2.500]
The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention must satisfy the following formula.
0.100≦[Si]+[sol. Al]≦2.500
In the formula, [Si] and [sol. Al] are the contents (mass%) of each element. As described above, the grains are surrounded by a boundary with an orientation difference of 15° or more and have a circle equivalent diameter of 0.3 μm or more. When the region is defined as a crystal grain, crystal grains having an intragranular misorientation of 5 to 14° are effective in improving strength and stretch flangeability. In the case of hot-rolled steel sheets, as will be described in detail later, the ratio of the crystal grains is controlled within the range of 10 to 60% by area to improve the balance between strength and stretch flangeability. In addition to the effects described for each element, sol. Al is also an element effective in controlling the ratio of crystal grains having an intragranular misorientation of 5 to 14 degrees within the range of 10 to 60%. This is believed to be due to the fact that the temperature of the Ar3 point is increased by the inclusion of Si and sol. Al, and the transformation strain introduced into the grains is reduced. In order to obtain this, the chemical composition of the hot rolled steel sheet according to the embodiment of the present invention is such that the contents of each element are controlled within the ranges described above, while Si and sol. The total content of Si and sol. Al is controlled to be 0.100% or more, i.e., to satisfy [Si] + [sol. Al] ≥ 0.100. The total content of Si and sol. Al is controlled to be 0.120%. On the other hand, if the total content of Si and sol. Al is too high, the formation of ferrite is promoted, and the strength is reduced. Therefore, the total content of Si and sol. Al is set to 2.500% or less, that is, [Si] + [sol. Al] ≦ 2.500. The content may be 2.000% or less, 1.500% or less, 1.000% or less, or 0.000% or less.
　本発明の実施形態に係る熱間圧延鋼板の化学組成は、一般的な分析方法によって測定すればよい。例えば、当該熱間圧延鋼板の化学組成は、誘導結合プラズマ発光分光分析（ＩＣＰ－ＡＥＳ：Ｉｎｄｕｃｔｉｖｅｌｙ　Ｃｏｕｐｌｅｄ　Ｐｌａｓｍａ－Ａｔｏｍｉｃ　Ｅｍｉｓｓｉｏｎ　Ｓｐｅｃｔｒｏｍｅｔｒｙ）を用いて測定すればよい。Ｃ及びＳは燃焼－赤外線吸収法を用い、Ｎは不活性ガス融解－熱伝導度法を用い、Ｏは不活性ガス融解－非分散型赤外線吸収法を用いて測定すればよい。 The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the hot-rolled steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
［金属組織］
［フェライト及びベイナイトの少なくとも１種：合計で８０～９８％、並びにマルテンサイト：２～１０％］
　本発明の実施形態に係る熱間圧延鋼板の金属組織は、面積％で、フェライト及びベイナイトの少なくとも１種：合計で８０～９８％、並びにマルテンサイト：２～１０％を含む。熱間圧延鋼板の金属組織をこれらの組織によって構成することで、強度と伸びフランジ性、延性及び切り欠き疲労特性とをバランスよく向上させることができる。フェライト及びベイナイトの少なくとも１種の合計面積率が低いか又はマルテンサイトの面積率が高いと、特に強度と伸びフランジ性のバランスが低下し、所望の特性が得られない場合がある。したがって、フェライト及びベイナイトの少なくとも１種の合計面積率は８０％以上とし、例えば８２％以上、８５％以上、８８％以上又は９０％以上であってもよい。同様に、マルテンサイトの面積率は１０％以下とし、例えば９％以下、８％以下、７％以下又は６％以下であってもよい。一方で、フェライト及びベイナイトの少なくとも１種の合計面積率が高いか又はマルテンサイトの面積率が低いと、特に強度と切り欠き疲労特性のバランスが低下し、所望の特性が得られない場合がある。したがって、フェライト及びベイナイトの少なくとも１種の合計面積率は９８％以下とし、例えば９６％以下、９４％以下又は９２％以下であってもよい。同様に、マルテンサイトの面積率は２％以上とし、例えば３％以上、４％以上又は５％以上であってもよい。 [Metal structure]
[At least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%]
The metal structure of the hot-rolled steel sheet according to the embodiment of the present invention includes, in area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%. By configuring the metal structure of the hot-rolled steel sheet with these structures, it is possible to improve the strength, stretch flangeability, ductility, and notch fatigue properties in a well-balanced manner. If the total area ratio of at least one of ferrite and bainite is low or the area ratio of martensite is high, the balance between strength and stretch flangeability may be reduced, and desired properties may not be obtained. Therefore, the total area ratio of at least one of ferrite and bainite is 80% or more, and may be, for example, 82% or more, 85% or more, 88% or more, or 90% or more. Similarly, the area ratio of martensite may be 10% or less, and may be, for example, 9% or less, 8% or less, 7% or less, or 6% or less. On the other hand, if the total area ratio of at least one of ferrite and bainite is high or the area ratio of martensite is low, the balance between strength and notch fatigue properties may be reduced, and desired properties may not be obtained. Therefore, the total area ratio of at least one of ferrite and bainite is 98% or less, and may be, for example, 96% or less, 94% or less, or 92% or less. Similarly, the area ratio of martensite is 2% or more, and may be, for example, 3% or more, 4% or more, or 5% or more.
　熱間圧延鋼板の金属組織は、フェライト及びベイナイトのいずれか一方を含んでいればよく、好ましくはフェライト及びベイナイトの両方を含む。したがって、フェライト及びベイナイトの面積率は、いずれか一方が０％であってもよく、例えばそれぞれ２％以上、５％以上、１０％以上、２０％以上、３０％以上又は４０％以上であってもよい。同様に、フェライト及びベイナイトの面積率は、例えばそれぞれ９０％以下、８０％以下、７０％以下、６０％以下又は５０％以下であってもよい。熱間圧延鋼板の延性向上の観点からは、ベイナイトの面積率は８０％以下であることが好ましく、７０％以下であることがより好ましい。 The metal structure of the hot-rolled steel sheet may contain either ferrite or bainite, and preferably contains both ferrite and bainite. Therefore, the area ratio of either ferrite or bainite may be 0%, or may be, for example, 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more, respectively. Similarly, the area ratio of ferrite and bainite may be, for example, 90% or less, 80% or less, 70% or less, 60% or less, or 50% or less, respectively. From the viewpoint of improving the ductility of the hot-rolled steel sheet, the area ratio of bainite is preferably 80% or less, and more preferably 70% or less.
［残部組織］
　フェライト、ベイナイト及びマルテンサイト以外の残部組織は、面積％で０％であってもよいが、残部組織が存在する場合には、当該残部組織は残留オーステナイト及びパーライトの少なくとも１種であってよい。残部組織の面積率は、特に限定されないが、例えば１％以上、２％以上又は３％以上であってもよい。伸びフランジ性をさらに向上させる観点からは、残部組織の面積率は、例えば１０％以下であることが好ましく、８％以下、６％以下又は５％以下であってもよい。 [Remainder structure]
The remaining structure other than ferrite, bainite, and martensite may be 0% by area, but when the remaining structure is present, the remaining structure may be at least one of retained austenite and pearlite. The area ratio of the remaining structure is not particularly limited, but may be, for example, 1% or more, 2% or more, or 3% or more. From the viewpoint of further improving the stretch flangeability, the area ratio of the remaining structure is preferably, for example, 10% or less, and may be 8% or less, 6% or less, or 5% or less.
［金属組織の同定及び面積率の算出］
　熱間圧延鋼板における金属組織の同定及び面積率の算出は、ナイタール試薬又はレペラ液を用いた腐食後の光学顕微鏡観察並びにＸ線回折法によって行われる。光学顕微鏡による組織観察は、圧延方向に平行かつ板面に垂直な方向の板厚断面に対して行われる。具体的には、まず、熱間圧延鋼板から試料を採取し、試料の観察面をナイタールでエッチングする。次いで、光学顕微鏡を用いて板厚の１／４深さ位置において３００μｍ×３００μｍの視野で得られた組織写真に対して画像解析を行うことにより、フェライト及びパーライトの各面積率、並びにベイナイトとマルテンサイトの合計面積率を算出する。次に、観察面をレペラ腐食した試料を用い、同様に光学顕微鏡を用いて板厚の１／４深さ位置において３００μｍ×３００μｍの視野で得られた組織写真に対して画像解析を行うことにより、残留オーステナイトとマルテンサイトの合計面積率を算出する。次に、圧延面法線方向から板厚の１／４深さまで面削した試料を用い、Ｘ線回折測定により残留オーステナイトの体積率を算出する。残留オーステナイトの体積率は面積率と同等であるため、これを残留オーステナイトの面積率とする。得られた残留オーステナイトの面積率を先に算出した残留オーステナイトとマルテンサイトの合計面積率から引算することでマルテンサイトの面積率を算出する。最後に、得られたマルテンサイトの面積率を同様に先に算出したベイナイトとマルテンサイトの合計面積率から引算することでベイナイトの面積率を算出する。 [Identification of metal structure and calculation of area ratio]
Identification of the metal structure and calculation of the area ratio in the hot-rolled steel sheet are performed by optical microscope observation after corrosion using a Nital reagent or a Lepera solution and X-ray diffraction method. The structure observation by an optical microscope is performed on a plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface. Specifically, first, a sample is taken from the hot-rolled steel sheet, and the observation surface of the sample is etched with Nital. Next, image analysis is performed on a structure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 μm × 300 μm using an optical microscope, thereby calculating each area ratio of ferrite and pearlite, and the total area ratio of bainite and martensite. Next, using a sample whose observation surface has been Lepera-etched, image analysis is performed on a structure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 μm × 300 μm using an optical microscope, thereby calculating the total area ratio of retained austenite and martensite. Next, using a sample that has been surface-cut from the normal direction of the rolled surface to a 1/4 depth of the plate thickness, the volume ratio of retained austenite is calculated by X-ray diffraction measurement. Since the volume fraction of the retained austenite is equivalent to the area fraction, this is taken as the area fraction of the retained austenite. The area fraction of martensite is calculated by subtracting the obtained area fraction of the retained austenite from the total area fraction of the retained austenite and martensite calculated previously. Finally, the area fraction of bainite is calculated by subtracting the obtained area fraction of martensite from the total area fraction of bainite and martensite calculated previously.
［粒内の方位差が５～１４°である結晶粒の割合：面積％で１０～６０％］
　本発明の実施形態に係る熱間圧延鋼板の金属組織においては、方位差が１５°以上である境界を粒界とし、当該粒界によって囲まれ、かつ円相当直径が０．３μｍ以上である領域を結晶粒と定義した場合に、粒内の方位差が５～１４°である結晶粒の割合は、面積％で１０～６０％の範囲内に制御される。このような粒内方位差を有する結晶粒は、強度と伸びフランジ性を向上させるのに有効である。何ら特定の理論に束縛されることを意図するものではないが、粒内の結晶方位差は、結晶粒に含まれる転位密度と相関があると考えられる。一般的に、粒内の転位密度の増加は強度の向上をもたらす一方で加工性を低下させる。しかしながら、粒内の方位差が５～１４°に制御された結晶粒においては、加工性を低下させることなく強度を向上させることができると考えられる。これに対し、粒内の方位差が５°未満の結晶粒は加工性に優れるが高強度化が困難である。一方で、粒内の方位差が１４°超の結晶粒は結晶粒内で変形能が異なるため、伸びフランジ性の向上に必ずしも寄与しない。したがって、本発明の実施形態に係る熱間圧延鋼板では、粒内の方位差が５～１４°の結晶粒の割合を適切に制御すること、より具体的には面積％で１０～６０％の範囲内に制御することで、所望の鋼板強度を達成しつつ、伸びフランジ性を改善することができ、強度と伸びフランジ性のバランスをさらに改善することが可能となる。粒内の方位差が５～１４°の結晶粒の割合が小さいと、伸びフランジ性が低下する場合がある。したがって、伸びフランジ性向上の観点から、粒内の方位差が５～１４°の結晶粒の割合は１５％以上、１８％以上又は２０％以上であってもよい。一方で、粒内の方位差が５～１４°の結晶粒の割合が大きいと、延性が低下する場合がある。したがって、延性向上の観点から、粒内の方位差が５～１４°の結晶粒の割合は５５％以下、５０％以下、４５％以下又は４０％以下であってもよい。 [Proportion of crystal grains with intragranular misorientation of 5 to 14°: 10 to 60% by area]
In the metal structure of the hot-rolled steel sheet according to the embodiment of the present invention, when the boundary with an orientation difference of 15° or more is defined as a grain boundary, and the region surrounded by the grain boundary and having a circle equivalent diameter of 0.3 μm or more is defined as a crystal grain, the proportion of crystal grains with an orientation difference of 5 to 14° within the range of 10 to 60% in terms of area %. Crystal grains having such an orientation difference within the grain are effective for improving strength and stretch flangeability. Although it is not intended to be bound by any particular theory, it is believed that the crystal orientation difference within the grain is correlated with the dislocation density contained in the crystal grain. Generally, an increase in the dislocation density within the grain improves strength while decreasing workability. However, it is believed that the strength can be improved without decreasing workability in crystal grains in which the orientation difference within the grain is controlled to 5 to 14°. In contrast, crystal grains with an orientation difference within the grain of less than 5° are excellent in workability but difficult to increase in strength. On the other hand, crystal grains with an orientation difference within the grain of more than 14° do not necessarily contribute to improving stretch flangeability because the deformability is different within the crystal grain. Therefore, in the hot-rolled steel sheet according to the embodiment of the present invention, by appropriately controlling the proportion of crystal grains having an intragranular misorientation of 5 to 14°, more specifically, by controlling it to within a range of 10 to 60% in terms of area%, it is possible to improve the stretch flangeability while achieving the desired steel sheet strength, and it is possible to further improve the balance between strength and stretch flangeability. If the proportion of crystal grains having an intragranular misorientation of 5 to 14° is small, the stretch flangeability may be reduced. Therefore, from the viewpoint of improving the stretch flangeability, the proportion of crystal grains having an intragranular misorientation of 5 to 14° may be 15% or more, 18% or more, or 20% or more. On the other hand, if the proportion of crystal grains having an intragranular misorientation of 5 to 14° is large, the ductility may be reduced. Therefore, from the viewpoint of improving the ductility, the proportion of crystal grains having an intragranular misorientation of 5 to 14° may be 55% or less, 50% or less, 45% or less, or 40% or less.
［粒内の方位差が５～１４°である結晶粒の割合の測定］
　粒内の方位差が５～１４°である結晶粒の割合は、電子線後方散乱回折法（Ｅｌｅｃｔｒｏｎ　ＢａｃｋＳｃａｔｔｅｒｅｄ　Ｄｉｆｆｒａｃｔｉｏｎ、ＥＢＳＤ）によって測定される。より具体的には、まず、圧延方向に平行かつ板面に垂直な方向の板厚断面が観察面となるように鋼板から試料を採取する。次いで、鋼板表面から板厚の１／４深さ位置で、鋼板の圧延方向に２００μｍ、圧延面法線方向に１００μｍの領域を０．２μｍの測定間隔でＥＢＳＤ解析して結晶方位情報を得る。ここで、ＥＢＳＤ解析は、サーマル電界放射型走査電子顕微鏡（ＪＥＯＬ製ＪＳＭ－７００１Ｆ）とＥＢＳＤ検出器（ＴＳＬ製ＨＩＫＡＲＩ検出器）で構成された装置を用い、５０～３００点／秒の解析速度で実施する。次に、得られた結晶方位情報に対して、方位差１５°以上かつ円相当直径で０．３μｍ以上の領域を結晶粒と定義し、結晶粒の粒内の平均方位差を計算し、粒内の方位差が５～１４°である結晶粒の割合を求める。上記のように定義した結晶粒や粒内の平均方位差は、ＥＢＳＤ解析装置に付属のソフトウェア「ＯＩＭ　Ａｎａｌｙｓｉｓ（登録商標）」を用いて算出することができる。本発明において、「粒内の方位差」とは、結晶粒内の方位分散である「Ｇｒａｉｎ　Ｏｒｉｅｎｔａｔｉｏｎ　Ｓｐｒｅａｄ（ＧＯＳ）」を表す。粒内の方位差の値は「ＥＢＳＤ法およびＸ線回折法によるステンレス鋼の塑性変形におけるミスオリエンテーションの解析」、木村英彦他、日本機械学会論文集（Ａ編）、７１巻、７１２号、２００５年、ｐ．１７２２－１７２８に記載されているように、同一結晶粒内において基準となる結晶方位と全ての測定点間のミスオリエンテーションの平均値として求められる。本発明の実施形態において、基準となる結晶方位は、同一結晶粒内の全ての測定点を平均化した方位である。ＧＯＳの値は、ＥＢＳＤ解析装置に付属のソフトウェア「ＯＩＭ　Ａｎａｌｙｓｉｓ（登録商標）Ｖｅｒｓｉｏｎ　７．０．１」を用いて算出することができる。 [Measurement of the percentage of crystal grains with intragranular misorientation of 5 to 14°]
The proportion of crystal grains with an intragranular orientation difference of 5 to 14° is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from the steel sheet so that the plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface is the observation surface. Next, at a depth position of 1/4 of the plate thickness from the surface of the steel sheet, an area of 200 μm in the rolling direction of the steel sheet and 100 μm in the normal direction to the rolling surface is analyzed by EBSD analysis at a measurement interval of 0.2 μm to obtain crystal orientation information. Here, the EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). Next, for the obtained crystal orientation information, regions with an orientation difference of 15° or more and a circle equivalent diameter of 0.3 μm or more are defined as crystal grains, the average orientation difference within the crystal grains is calculated, and the ratio of crystal grains with an orientation difference of 5 to 14° within the grains is obtained. The crystal grains and the average orientation difference within the grains defined as above can be calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. In the present invention, "orientation difference within a grain" refers to "Grain Orientation Spread (GOS)", which is the orientation dispersion within a crystal grain. The value of the orientation difference within a grain is described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD Method and X-ray Diffraction Method", Hidehiko Kimura et al., Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 71, No. 712, 2005, p. As described in IEEE Transactions on Microelectronics, vol. 1722-1728, the GOS is calculated as an average value of the misorientation between a reference crystal orientation and all measurement points within the same crystal grain. In the embodiment of the present invention, the reference crystal orientation is an average orientation of all measurement points within the same crystal grain. The GOS value can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" that comes with the EBSD analyzer.
［板厚表層部における特定方位の極密度の平均値：２．５０以上、及び板厚中心部における特定方位の極密度の平均値：７．００以下］
　本発明の実施形態に係る熱間圧延鋼板の金属組織においては、熱間圧延鋼板の表面から板厚１／６位置までの領域（すなわち板厚表層部）における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値が２．５０以上に制御され、板厚２／５位置から板厚３／５位置までの領域（すなわち板厚中心部）における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値が７．００以下に制御される。板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値を２．５０以上に制御してＬ方向の強度を高める一方で、板厚中心部における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値を７．００以下に制御してＣ方向の強度を低くすることで、得られる熱間圧延鋼板のＬ方向とＣ方向の引張強さの差を小さくすることができ、結果としてＬ方向とＣ方向の引張強さにおける強度の異方性を顕著に低減することができる。強度の異方性をより低減するという観点からは、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値は大きいほど好ましく、例えば２．８０以上、３．００以上、３．２０以上又は３．５０以上であってもよい。上限は特に限定されないが、例えば、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値は５．００以下、４．８０以下、４．７０以下、４．５０以下、４．２０以下、４．００以下又は３．８０以下であってもよい。同様に、強度の異方性をより低減するという観点からは、板厚中心部における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値は小さいほど好ましく、例えば６．８０以下、６．５０以下、６．２０以下又は６．００以下であってもよい。下限は特に限定されないが、例えば、板厚中心部における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値は３．５０以上、４．００以上、４．２０以上、４．４０以上、４．５０以上又は５．００以上であってもよい。 [Average value of pole density in a specific orientation in the plate thickness surface layer: 2.50 or more, and average value of pole density in a specific orientation in the plate thickness center portion: 7.00 or less]
In the metal structure of the hot-rolled steel plate according to the embodiment of the present invention, the average value of the pole densities of the {110}<111> and {112}<111> orientations in the region from the surface of the hot-rolled steel plate to the 1/6 position of the plate thickness (i.e., the plate thickness surface layer portion) is controlled to 2.50 or more, and the average value of the pole densities of the {100}<011>, {211}<011> and {332}<113> orientations in the region from the 2/5 position of the plate thickness to the 3/5 position of the plate thickness (i.e., the plate thickness center portion) is controlled to 7.00 or less. By controlling the average value of the pole density of the {110}<111> and {112}<111> orientations in the sheet thickness surface layer portion to 2.50 or more to increase the strength in the L direction, while controlling the average value of the pole density of the {100}<011>, {211}<011> and {332}<113> orientations in the sheet thickness center portion to 7.00 or less to reduce the strength in the C direction, the difference in tensile strength between the L direction and the C direction of the obtained hot rolled steel sheet can be reduced, and as a result, the anisotropy of the strength in the tensile strength in the L direction and the C direction can be significantly reduced. From the viewpoint of further reducing the anisotropy of the strength, the larger the average value of the pole density of the {110}<111> and {112}<111> orientations in the sheet thickness surface layer portion, the more preferable it is, and it may be, for example, 2.80 or more, 3.00 or more, 3.20 or more, or 3.50 or more. The upper limit is not particularly limited, but for example, the average value of the pole density of the {110}<111> and {112}<111> orientations in the sheet thickness surface layer portion may be 5.00 or less, 4.80 or less, 4.70 or less, 4.50 or less, 4.20 or less, 4.00 or less, or 3.80 or less. Similarly, from the viewpoint of further reducing the anisotropy of the strength, the smaller the average value of the pole density of the {100}<011>, {211}<011>, and {332}<113> orientations in the sheet thickness center portion, the more preferable, and may be, for example, 6.80 or less, 6.50 or less, 6.20 or less, or 6.00 or less. The lower limit is not particularly limited, but for example, the average pole density of the {100}<011>, {211}<011>, and {332}<113> orientations in the center part of the sheet thickness may be 3.50 or more, 4.00 or more, 4.20 or more, 4.40 or more, 4.50 or more, or 5.00 or more.
［板厚表層部及び板厚中心部における特定方位の極密度の平均値の測定］
　板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値並びに板厚中心部における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値はＥＢＳＤによって測定される。より具体的には、板厚表層部の測定については、まず、圧延方向に平行かつ板面に垂直な方向の板厚断面が観察面となるように鋼板から試料を採取し、鋼板表面から板厚の１／１２深さ位置を中心とする鋼板の圧延方向に１０００μｍかつ圧延面法線方向に１００μｍの矩形領域に対して、１μｍの測定間隔でＥＢＳＤ解析を実施して、この矩形領域の結晶方位情報を取得する。ＥＢＳＤ解析は、サーマル電界放射型走査電子顕微鏡（ＪＥＯＬ製ＪＳＭ－７００１Ｆ）とＥＢＳＤ検出器（ＴＳＬ製ＨＩＫＡＲＩ検出器）で構成された装置を用い、５０～３００点／秒の解析速度で実施する。次に、この矩形領域の結晶方位情報から、ＥＢＳＤ解析装置に付属のソフトウェア「ＯＩＭ　Ａｎａｌｙｓｉｓ（登録商標）」を用いて、この矩形領域のＯＤＦ（Ｏｒｉｅｎｔａｔｉｏｎ　Ｄｉｓｔｒｉｂｕｔｉｏｎ　Ｆｕｎｃｔｉｏｎ）を算出する。ＯＤＦを計算する方法としてＨａｒｍｏｎｉｃ　Ｓｅｒｉｅｓ　Ｅｘｐａｎｓｉｏｎ（球面調和関数法）を用いて展開次数を１６とした。また、対称性（ｏｒｔｈｏｔｒｏｐｉｃ）を考慮した計算を実施した。これにより｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞の各結晶方位の極密度を求めることができ、それらの算術平均を「熱間圧延鋼板の表面から板厚１／６位置までの領域（板厚表層部）における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値」として決定する。板厚中心部の測定については、鋼板表面から板厚の１／２深さ位置を中心とする鋼板の圧延方向に１０００μｍかつ圧延面法線方向に１００μｍの矩形領域に対してＥＢＳＤ解析を実施したこと以外は、板厚表層部の測定の場合と同様に測定することで、｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞の各結晶方位の極密度を求めることができ、それらの算術平均を「板厚２／５位置から板厚３／５位置までの領域（板厚中心部）における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値」として決定する。なお、ここでの結晶方位は鋼板表面に垂直な方向の結晶方位を表しているため、解析の際には測定の試料セットの方向を考慮して結晶方位データの測定座標系と試料座標系とを合わせる必要がある。 [Measurement of the average pole density of a specific orientation at the plate thickness surface layer and plate thickness center]
The average pole density of the {110}<111> and {112}<111> orientations in the sheet thickness surface layer portion and the average pole density of the {100}<011>, {211}<011> and {332}<113> orientations in the sheet thickness center portion are measured by EBSD. More specifically, for the measurement of the sheet thickness surface layer portion, first, a sample is taken from the steel sheet so that the sheet thickness cross section parallel to the rolling direction and perpendicular to the sheet surface becomes the observation surface, and EBSD analysis is performed at measurement intervals of 1 μm on a rectangular region of the steel sheet, which is 1000 μm in the rolling direction and 100 μm in the normal direction to the rolling surface and is centered at a depth position of 1/12 of the sheet thickness from the steel sheet surface, to obtain crystal orientation information of this rectangular region. The EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). Next, from the crystal orientation information of this rectangular region, the ODF (Orientation Distribution Function) of this rectangular region is calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. As a method for calculating the ODF, the Harmonic Series Expansion (spherical harmonic function method) was used, and the expansion order was set to 16. In addition, the calculation was performed taking into account symmetry (orthotropic). This makes it possible to determine the pole densities of the {110}<111> and {112}<111> crystal orientations, and the arithmetic average of these is determined as "the average value of the pole densities of the {110}<111> and {112}<111> orientations in the region from the surface of the hot rolled steel plate to the 1/6 position of the plate thickness (plate thickness surface layer portion)." For the measurement of the plate thickness center portion, except that EBSD analysis was performed on a rectangular region of 1000 μm in the rolling direction of the steel plate and 100 μm in the normal direction of the rolling surface from the steel plate surface, centered at the 1/2 depth position of the plate thickness, the pole density of each crystal orientation of {100}<011>, {211}<011>, and {332}<113> can be obtained by measuring in the same manner as for the measurement of the plate thickness surface layer portion, and the arithmetic average of them is determined as "the average value of the pole density of the {100}<011>, {211}<011>, and {332}<113> orientations in the region from the plate thickness 2/5 position to the plate thickness 3/5 position (plate thickness center portion)". Note that since the crystal orientation here represents the crystal orientation in the direction perpendicular to the steel plate surface, it is necessary to align the measurement coordinate system of the crystal orientation data with the sample coordinate system during analysis, taking into account the direction of the sample set for measurement.
［板厚］
　本発明の実施形態に係る熱間圧延鋼板は、特に限定されないが、一般的には１．０～６．０ｍｍの板厚を有する。例えば、板厚は１．２ｍｍ以上、１．６ｍｍ以上若しくは２．０ｍｍ以上であってもよく、及び／又は５．０ｍｍ以下若しくは４．０ｍｍ以下であってもよい。 [Thickness]
The hot rolled steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm, although not particularly limited thereto. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 5.0 mm or less, or 4.0 mm or less.
［機械的特性］
［引張強さ：ＴＳ］
　上記の化学組成及び金属組織を有する熱間圧延鋼板によれば、高い引張強さ、具体的には５４０ＭＰａ以上の引張強さを達成することができる。引張強さは、好ましくは６００ＭＰａ以上、７００ＭＰａ以上、７８０ＭＰａ以上又は８５０ＭＰａ以上である。本発明の実施形態に係る熱間圧延鋼板によれば、このような非常に高い引張強さを有するにもかかわらず、上で説明した化学組成と金属組織の特定の組み合わせにより、伸びフランジ性、延性及び切り欠き疲労特性を改善するとともに、強度の異方性を低減することができる。引張強さの上限は特に限定されないが、例えば、熱間圧延鋼板の引張強さは１４７０ＭＰａ以下、１２５０ＭＰａ以下、１１８０ＭＰａ以下、１０８０ＭＰａ以下又は９８０ＭＰａ以下であってもよい。引張強さは、試験片の長手方向が熱間圧延鋼板の圧延直角方向と平行になる向き（Ｃ方向）からＪＩＳ５号試験片を採取し、ＪＩＳ　Ｚ　２２４１：２０１１に準拠した引張試験を行うことで測定される。このようにして得られた引張強さは、本明細書においてＣ方向ＴＳ（ＴＳＣ）とも称する。 [Mechanical properties]
[Tensile strength: TS]
According to the hot-rolled steel sheet having the above chemical composition and metal structure, a high tensile strength, specifically a tensile strength of 540 MPa or more, can be achieved. The tensile strength is preferably 600 MPa or more, 700 MPa or more, 780 MPa or more, or 850 MPa or more. According to the hot-rolled steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, the specific combination of the chemical composition and metal structure described above can improve the stretch flangeability, ductility, and notch fatigue properties, and reduce the anisotropy of strength. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 1470 MPa or less, 1250 MPa or less, 1180 MPa or less, 1080 MPa or less, or 980 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011. The tensile strength thus obtained is also referred to herein as C-direction TS (TSC).
［全伸び：Ｅｌ］
　上記の化学組成及び金属組織を有する熱間圧延鋼板によれば、高い引張強さに加えて、全伸びを改善することもでき、より具体的には１５．０％以上の全伸びを達成することができる。全伸びは、好ましくは１８．０％以上、より好ましくは２０．０％以上、最も好ましくは２２．０％以上である。上限は特に限定されないが、例えば、全伸びは４０．０％以下又は３５．０％以下であってもよい。全伸びは、試験片の長手方向が熱間圧延鋼板の圧延直角方向と平行になる向き（Ｃ方向）からＪＩＳ５号試験片を採取し、ＪＩＳ　Ｚ　２２４１：２０１１に準拠した引張試験を行うことで測定される。 [Total elongation: El]
According to the hot-rolled steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the total elongation can be improved, and more specifically, a total elongation of 15.0% or more can be achieved. The total elongation is preferably 18.0% or more, more preferably 20.0% or more, and most preferably 22.0% or more. The upper limit is not particularly limited, but for example, the total elongation may be 40.0% or less or 35.0% or less. The total elongation is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.
＜熱間圧延鋼板の製造方法＞
　次に、本発明の実施形態に係る熱間圧延鋼板の好ましい製造方法について説明する。以下の説明は、本発明の実施形態に係る熱間圧延鋼板を製造するための特徴的な方法の例示を意図するものであって、当該熱間圧延鋼板を以下に説明するような製造方法によって製造されるものに限定することを意図するものではない。 <Method of manufacturing hot-rolled steel sheet>
Next, a preferred method for manufacturing the hot-rolled steel sheet according to the embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing the hot-rolled steel sheet according to the embodiment of the present invention, and is not intended to limit the hot-rolled steel sheet to one manufactured by the manufacturing method described below.
　本発明の実施形態に係る熱間圧延鋼板の製造方法は、
　（Ａ）熱間圧延鋼板に関連して上で説明した化学組成を有するスラブを加熱し、次いで仕上げ圧延することを含み、下記（Ａ１）～（Ａ５）の条件を満足する熱間圧延工程、及び
　　（Ａ１）スラブの加熱温度が下記式１で表される溶体化温度（ＳＲＴｍｉｎ）℃以上１２６０℃以下であること、
　　（Ａ２）下記式２で表される仕上げ圧延の後段３段の累積ひずみ（εｅｆｆ．）が０．５０～０．６０であること、
　　（Ａ３）仕上げ圧延の終了温度がＡｒ３＋３０℃以上であること、
　　（Ａ４）仕上げ圧延において下記式３で表される形状比（Ｘ）が２．３以上である圧延パスを１１００℃以下で２パス以上実施すること、
　　（Ａ５）仕上げ圧延の前段３段の圧延温度が仕上げ圧延の入側温度（ＦＴ０）－５０℃以上であること
　ＳＲＴｍｉｎ＝７０００／｛２．７５－ｌｏｇ（［Ｔｉ］×［Ｃ］）｝－２７３
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　・・・式１
　ここで、［Ｔｉ］及び［Ｃ］は鋼中の各元素の含有量（質量％）である。
　εｅｆｆ．＝Σεｉ（ｔ，Ｔ）　　　・・・式２
　ここで、
　εｉ（ｔ，Ｔ）＝εｉ０／ｅｘｐ｛（ｔ／τＲ）2/3｝
　τＲ＝τ０・ｅｘｐ（Ｑ／ＲＴ）
　τ０＝８．４６×１０-6
　Ｑ＝１８３２００Ｊ
　Ｒ＝８．３１４Ｊ／Ｋ・ｍｏｌ
　εｉ０は圧下時の対数ひずみを示し、ｔは当該パスでの冷却直前までの累積時間（秒）を示し、Ｔは当該パスでの圧延温度（℃）を示す。
　Ｘ＝２√（Ｒ（ｈ0－ｈ1））／（ｈ0＋ｈ1）　　　・・・式３
　ここで、
　Ｒ：圧延機のロール半径（ｍｍ）
　ｈ0：入側板厚（ｍｍ）
　ｈ1：出側板厚（ｍｍ）
　（Ｂ）仕上げ圧延された鋼板を１０℃／ｓ以上の平均冷却速度で６５０～７５０℃の温度域まで１次冷却し、前記温度域で３．０～１０．０秒間保持し、次いで１００℃以下まで３０℃／ｓ以上の平均冷却速度で２次冷却することを含む冷却工程
を含むことを特徴としている。以下、各工程について詳しく説明する。 The method for producing a hot-rolled steel sheet according to an embodiment of the present invention includes:
(A) a hot rolling process including heating a slab having the chemical composition described above in relation to the hot rolled steel sheet and then finish rolling the slab, and satisfying the following conditions (A1) to (A5); and (A1) the heating temperature of the slab is a solution temperature (SRTmin) °C or higher represented by the following formula 1 and 1260 °C or lower;
(A2) The cumulative strain (εeff.) in the last three stages of finish rolling, represented by the following formula 2, is 0.50 to 0.60;
(A3) The end temperature of the finish rolling is Ar3+30°C or higher;
(A4) In the finish rolling, two or more rolling passes having a shape ratio (X) represented by the following formula 3 of 2.3 or more are performed at 1100 ° C. or less;
(A5) The rolling temperature of the first three stages of the finish rolling is equal to or higher than the entry temperature of the finish rolling (FT0) - 50 ° C. SRTmin = 7000 / {2.75 - log ([Ti] x [C])} - 273
...Equation 1
Here, [Ti] and [C] are the contents (mass%) of each element in the steel.
εeff. =Σεi(t,T) ... Equation 2
here,
εi(t,T)＝εi0/exp{(t/τR) 2/3 }
τR=τ0·exp(Q/RT)
τ0=8.46× 10
Q = 183200J
R = 8.314 J/K mol
εi0 indicates the logarithmic strain during rolling, t indicates the cumulative time (seconds) until just before cooling in the pass, and T indicates the rolling temperature (° C.) in the pass.
X=2√(R(h 0 −h 1 ))/(h 0 +h 1 ) Equation 3
here,
R: Roll radius of rolling mill (mm)
h0 : Entry plate thickness (mm)
h1 : Exit plate thickness (mm)
(B) A cooling step includes primarily cooling the finish-rolled steel sheet to a temperature range of 650 to 750°C at an average cooling rate of 10°C/s or more, holding the steel sheet in the temperature range for 3.0 to 10.0 seconds, and then secondary cooling to 100°C or less at an average cooling rate of 30°C/s or more. Each step will be described in detail below.
［（Ａ）熱間圧延工程］
［（Ａ１）スラブの加熱温度］
　まず、熱間圧延鋼板に関連して上で説明した化学組成を有するスラブが加熱される。使用するスラブは、生産性の観点から連続鋳造法において鋳造することが好ましいが、造塊法又は薄スラブ鋳造法によって製造してもよい。スラブの加熱温度は、下記式１で表される溶体化温度（ＳＲＴｍｉｎ）℃以上１２６０℃以下とする必要がある。
　ＳＲＴｍｉｎ＝７０００／｛２．７５－ｌｏｇ（［Ｔｉ］×［Ｃ］）｝－２７３
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　・・・式１
　ここで、［Ｔｉ］及び［Ｃ］は鋼中の各元素の含有量（質量％）である。
　本発明の実施形態に係る熱間圧延鋼板はＴｉを含有しており、スラブの加熱温度が溶体化温度（ＳＲＴｍｉｎ）℃未満であると、Ｔｉが十分に溶体化しない。スラブ加熱時にＴｉが十分に溶体化しないと、熱間圧延工程後の冷却工程などにおいてＴｉを炭化物（ＴｉＣ）として鋼中に微細析出させて析出強化により鋼の強度を向上させることが困難となる。加えて、炭化物（ＴｉＣ）を形成することでＣを固定し、伸びフランジ性にとって有害なセメンタイトの生成を抑制することも困難となる。一方、スラブの加熱温度が１２６０℃超であると、スケールオフにより歩留まりが低下する。 [(A) Hot rolling process]
[(A1) Slab heating temperature]
First, a slab having the chemical composition described above in relation to the hot rolled steel sheet is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may also be produced by an ingot casting method or a thin slab casting method. The heating temperature of the slab needs to be equal to or higher than the solution temperature (SRTmin) °C, which is expressed by the following formula 1, and equal to or lower than 1260 °C.
SRTmin = 7000 / {2.75 - log ([Ti] x [C])} - 273
...Equation 1
Here, [Ti] and [C] are the contents (mass%) of each element in the steel.
The hot-rolled steel sheet according to the embodiment of the present invention contains Ti, and if the heating temperature of the slab is less than the solution temperature (SRTmin) ° C., Ti is not sufficiently dissolved. If Ti is not sufficiently dissolved during slab heating, it is difficult to improve the strength of the steel by precipitation strengthening by finely precipitating Ti as carbide (TiC) in the steel during the cooling process after the hot rolling process. In addition, it is difficult to fix C by forming carbide (TiC) and suppress the generation of cementite, which is harmful to stretch flangeability. On the other hand, if the heating temperature of the slab is more than 1260 ° C., the yield decreases due to scale-off.
［粗圧延］
　本製造方法では、例えば、加熱されたスラブに対し、板厚調整等のために、仕上げ圧延の前に粗圧延を施してもよい。粗圧延は、所望のシートバー寸法が確保できればよく、その条件は特に限定されない。 [Rough rolling]
In the present manufacturing method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
［（Ａ２）仕上げ圧延の後段３段の累積ひずみ（εｅｆｆ．）：０．５０～０．６０］
　加熱されたスラブ又はそれに加えて必要に応じて粗圧延されたスラブは、次に仕上げ圧延を施される。本製造方法では、５基以上の圧延スタンドからなるタンデム圧延機を用いて仕上げ圧延することが好ましい。５基の圧延スタンドからなるタンデム圧延機を用いる場合には、（Ａ２）の仕上げ圧延の後段３段と、後で説明する（Ａ５）の仕上げ圧延の前段３段との間で圧延パスが一部重複することになる。しかしながら、（Ａ２）及び（Ａ５）の条件を満足する限り、（Ａ２）の条件における圧延パスと（Ａ５）の条件における圧延パスは互いに部分的に重複していてもよい。本製造方法では、粒内の方位差が５～１４°である結晶粒の割合を面積％で１０～６０％の範囲内に制御するために、加熱されたスラブに対して行われる仕上げ圧延において、後段３段（最終３パス）の累積ひずみ（εｅｆｆ．）を０．５０～０．６０とした上で、後述する冷却工程を実施する必要がある。これは以下の理由による。粒内の方位差が５～１４°である結晶粒は、比較的低温にてパラ平衡状態で変態することにより生成する。このため、熱間圧延工程において変態前のオーステナイトの転位密度をある範囲に限定するとともに、その後の冷却工程において冷却速度をある範囲に限定することによって粒内の方位差が５～１４°である結晶粒の生成を制御することが可能となる。すなわち、仕上げ圧延の後段３段の累積ひずみ及びその後の冷却を制御することで、粒内の方位差が５～１４°である結晶粒の核生成頻度及びその後の成長速度を制御することができる。その結果、冷却後に得られる熱間圧延鋼板における粒内の方位差が５～１４°の結晶粒の面積率を制御することができる。より具体的には、仕上げ圧延によって導入されるオーステナイトの転位密度が主に核生成頻度に関わり、仕上げ圧延後の冷却速度が主に成長速度に関わる。 [(A2) Cumulative strain in the last three stages of finish rolling (εeff.): 0.50 to 0.60]
The heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling. In this manufacturing method, it is preferable to use a tandem rolling mill consisting of five or more rolling stands for finish rolling. When using a tandem rolling mill consisting of five rolling stands, the rolling passes will overlap between the last three stages of the finish rolling of (A2) and the first three stages of the finish rolling of (A5) described later. However, as long as the conditions of (A2) and (A5) are satisfied, the rolling passes under the conditions of (A2) and (A5) may overlap each other partially. In this manufacturing method, in order to control the ratio of crystal grains having an intragranular orientation difference of 5 to 14° to within a range of 10 to 60% in terms of area percentage, in the finish rolling performed on the heated slab, it is necessary to set the cumulative strain (εeff.) of the last three stages (final three passes) to 0.50 to 0.60 and then perform the cooling process described later. This is for the following reasons. The crystal grains with an intragranular misorientation of 5 to 14° are generated by transformation in a para-equilibrium state at a relatively low temperature. Therefore, it is possible to control the generation of crystal grains with an intragranular misorientation of 5 to 14° by limiting the dislocation density of austenite before transformation to a certain range in the hot rolling process and limiting the cooling rate to a certain range in the subsequent cooling process. That is, by controlling the accumulated strain in the latter three stages of finish rolling and the subsequent cooling, it is possible to control the nucleation frequency and the subsequent growth rate of crystal grains with an intragranular misorientation of 5 to 14°. As a result, it is possible to control the area ratio of crystal grains with an intragranular misorientation of 5 to 14° in the hot rolled steel sheet obtained after cooling. More specifically, the dislocation density of austenite introduced by finish rolling is mainly related to the nucleation frequency, and the cooling rate after finish rolling is mainly related to the growth rate.
　仕上げ圧延の後段３段の累積ひずみが０．５０未満では、導入されるオーステナイトの転位密度が十分でなく、粒内の方位差が５～１４°である結晶粒の割合が１０％未満となる。一方で、仕上げ圧延の後段３段の累積ひずみが０．６０超であると、熱間圧延中にオーステナイトの再結晶が起こり、変態時の蓄積転位密度が低下する。その結果、同様に粒内の方位差が５～１４°である結晶粒の割合が１０％未満となってしまう。本製造方法において、仕上げ圧延の後段３段の累積ひずみ（εｅｆｆ．）は、下記式２によって求められる。
　εｅｆｆ．＝Σεｉ（ｔ，Ｔ）　　　・・・式２
　ここで、
　εｉ（ｔ，Ｔ）＝εｉ０／ｅｘｐ｛（ｔ／τＲ）2/3｝
　τＲ＝τ０・ｅｘｐ（Ｑ／ＲＴ）
　τ０＝８．４６×１０-6
　Ｑ＝１８３２００Ｊ
　Ｒ＝８．３１４Ｊ／Ｋ・ｍｏｌ
　εｉ０は圧下時の対数ひずみを示し、ｔは当該パスでの冷却直前までの累積時間（秒）を示し、Ｔは当該パスでの圧延温度（℃）を示す。 If the cumulative strain in the latter three stages of finish rolling is less than 0.50, the dislocation density of the introduced austenite is insufficient, and the proportion of crystal grains with an intragranular misorientation of 5 to 14° is less than 10%. On the other hand, if the cumulative strain in the latter three stages of finish rolling is more than 0.60, recrystallization of austenite occurs during hot rolling, and the accumulated dislocation density during transformation decreases. As a result, the proportion of crystal grains with an intragranular misorientation of 5 to 14° is similarly less than 10%. In this manufacturing method, the cumulative strain (εeff.) in the latter three stages of finish rolling is calculated by the following formula 2.
εeff. =Σεi(t,T) ... Equation 2
here,
εi(t,T)＝εi0/exp{(t/τR) 2/3 }
τR=τ0·exp(Q/RT)
τ0=8.46× 10
Q = 183200J
R = 8.314 J/K mol
εi0 indicates the logarithmic strain during rolling, t indicates the cumulative time (seconds) until just before cooling in the corresponding pass, and T indicates the rolling temperature (° C.) in the corresponding pass.
［（Ａ３）仕上げ圧延の終了温度：Ａｒ３＋３０℃以上］
　本製造方法においては、仕上げ圧延の終了温度は、Ａｒ３＋３０℃以上とする必要がある。仕上げ圧延の終了温度がＡｒ３＋３０℃未満であると、鋼板中の成分及び圧延温度のばらつきに起因して、組織の一部においてフェライトが生じている場合に、フェライトへ加工が加えられる虞がある。加工されたフェライトは延性低下の原因となる場合がある。加えて、仕上げ圧延の終了温度がＡｒ３＋３０℃未満であると、粒内の方位差が５～１４°である結晶粒の割合が６０％を超えて過度に高くなる場合がある。本製造方法においては、Ａｒ３（℃）は、熱間圧延鋼板の化学組成に基づいて、下記式４により求められる。
　Ａｒ３＝９０１－３２５×［Ｃ］＋３３×［Ｓｉ］＋２８７×［Ｐ］＋４０×［ｓｏｌ．Ａｌ］－９２×（［Ｍｎ］＋［Ｍｏ］＋［Ｃｕ］）－４６×（［Ｃｒ］＋［Ｎｉ］）
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　・・・式４
　ここで、［Ｃ］、［Ｓｉ］、［Ｐ］、［ｓｏｌ．Ａｌ］、［Ｍｎ］、［Ｍｏ］、［Ｃｕ］、［Ｃｒ］及び［Ｎｉ］は、鋼中の各元素の含有量（質量％）であり、元素を含有しない場合は０である。 [(A3) End temperature of finish rolling: Ar3+30°C or higher]
In this manufacturing method, the end temperature of the finish rolling needs to be Ar3+30°C or higher. If the end temperature of the finish rolling is less than Ar3+30°C, when ferrite is generated in a part of the structure due to the variation of the components in the steel sheet and the rolling temperature, the ferrite may be processed. The processed ferrite may cause a decrease in ductility. In addition, if the end temperature of the finish rolling is less than Ar3+30°C, the ratio of crystal grains having an intragranular misorientation of 5 to 14° may exceed 60% and become excessively high. In this manufacturing method, Ar3 (°C) is calculated based on the chemical composition of the hot-rolled steel sheet by the following formula 4.
Ar3 = 901 - 325 x [C] + 33 x [Si] + 287 x [P] + 40 x [sol. Al] - 92 x ([Mn] + [Mo] + [Cu]) - 46 x ([Cr] + [Ni])
...Equation 4
Here, [C], [Si], [P], [sol. Al], [Mn], [Mo], [Cu], [Cr] and [Ni] are the contents (mass%) of each element in the steel, and are 0 when the element is not contained.
［（Ａ４）形状比（Ｘ）が２．３以上の圧延パスを１１００℃以下で２パス以上］
　強度の異方性を低減するためには、先に説明したように、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値を所定の値以上に大きくしてＬ方向の強度を高める一方で、板厚中心部における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値を所定の値以下に小さくしてＣ方向の強度を低くする必要がある。そこで、本発明者らは、仕上げ圧延において鋼板の板厚表層部に導入する剪断ひずみを大きくすることで、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位への集積度を高め、それによってこれらの方位の極密度の平均値を所望の範囲内に制御できることを見出した。より具体的には、仕上げ圧延において下記式３で表される形状比（Ｘ）が２．３以上である圧延パスを１１００℃以下で２パス以上実施することで、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値を２．５０以上に高めることが可能となる。
　Ｘ＝２√（Ｒ（ｈ0－ｈ1））／（ｈ0＋ｈ1）　　　・・・式３
　ここで、
　Ｒ：圧延機のロール半径（ｍｍ）
　ｈ0：入側板厚（ｍｍ）
　ｈ1：出側板厚（ｍｍ） [(A4) 2 or more rolling passes with a shape ratio (X) of 2.3 or more at 1100° C. or less]
In order to reduce the anisotropy of strength, as explained above, it is necessary to increase the average value of the pole density of the {110}<111> and {112}<111> orientations in the plate thickness surface layer portion to a predetermined value or more to increase the strength in the L direction, while decreasing the average value of the pole density of the {100}<011>, {211}<011> and {332}<113> orientations in the plate thickness center portion to a predetermined value or less to decrease the strength in the C direction. Therefore, the inventors have found that by increasing the shear strain introduced into the plate thickness surface layer portion of the steel plate in finish rolling, the degree of accumulation in the {110}<111> and {112}<111> orientations in the plate thickness surface layer portion can be increased, thereby controlling the average value of the pole density of these orientations within a desired range. More specifically, by performing two or more rolling passes at 1100°C or less in the finish rolling such that the shape ratio (X) represented by the following formula 3 is 2.3 or more, it is possible to increase the average pole density of the {110}<111> and {112}<111> orientations in the sheet thickness surface layer portion to 2.50 or more.
X=2√(R(h 0 −h 1 ))/(h 0 +h 1 ) Equation 3
here,
R: Roll radius of rolling mill (mm)
h0 : Entry plate thickness (mm)
h1 : Exit plate thickness (mm)
　形状比（Ｘ）は、ロール接触弧長（√（Ｒ（ｈ0－ｈ1）））を平均板厚（（ｈ0＋ｈ1）／２）で除したものを意味する。本製造方法では、圧延機において適切なロール半径を有するロールを使用して適切な圧下率で圧延を施すことにより、２．３以上の形状比（Ｘ）を達成して鋼板の板厚表層部に導入する剪断ひずみを大きくすることができる。さらに、その際の圧延温度を１１００℃以下に制限することで、導入した剪断ひずみが回復するのを抑制することができる。したがって、このような圧延パスを２パス以上実施することで板厚表層部に十分な剪断ひずみを導入することができ、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値を確実に２．５０以上に高めることが可能となる。強度の異方性をさらに低減する観点からは、Ｘが２．３以上でかつ１１００℃以下の圧延パスは３パス以上実施することが好ましい。このような圧延パスの回数の上限は特に限定されず、例えば、当該圧延パスの回数は５パス以下であってよい。一方で、Ｘが２．３未満であるか、圧延温度が１１００℃超であるか、又はＸが２．３以上でかつ１１００℃以下の圧延パスが１パス以下である場合には、板厚表層部に十分な剪断ひずみを導入することができない。その結果として、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値を２．５０以上に高めることができなくなる。圧延機において使用されるロールのロール半径は、Ｘが２．３以上となる範囲内において適切な値を選択することができる。特に限定されないが、例えば、ロール半径は１５０～４００ｍｍの範囲内から選択することができる。 The shape ratio (X) means the roll contact arc length (√(R(h 0 -h 1 ))) divided by the average plate thickness ((h 0 +h 1 )/2). In the present manufacturing method, by performing rolling at an appropriate reduction rate using rolls having an appropriate roll radius in a rolling mill, a shape ratio (X) of 2.3 or more can be achieved, and the shear strain introduced into the plate thickness surface layer portion of the steel plate can be increased. Furthermore, by limiting the rolling temperature at that time to 1100 ° C or less, the recovery of the introduced shear strain can be suppressed. Therefore, by performing such rolling passes for two or more passes, sufficient shear strain can be introduced into the plate thickness surface layer portion, and the average value of the pole density of the {110} <111> and {112} <111> orientations in the plate thickness surface layer portion can be reliably increased to 2.50 or more. From the viewpoint of further reducing the anisotropy of strength, it is preferable to perform three or more rolling passes in which X is 2.3 or more and 1100 ° C or less. The upper limit of the number of such rolling passes is not particularly limited, and for example, the number of rolling passes may be 5 passes or less. On the other hand, if X is less than 2.3, the rolling temperature is more than 1100 ° C, or X is 2.3 or more and the number of rolling passes at 1100 ° C or less is 1 pass or less, sufficient shear strain cannot be introduced into the plate thickness surface layer. As a result, the average value of the pole density of the {110} <111> and {112} <111> orientations in the plate thickness surface layer cannot be increased to 2.50 or more. The roll radius of the roll used in the rolling mill can be selected from a range in which X is 2.3 or more. Although not particularly limited, for example, the roll radius can be selected from a range of 150 to 400 mm.
［（Ａ５）仕上げ圧延の前段３段の圧延温度：ＦＴ０－５０℃以上］
　板厚中心部では、板厚表層部とは反対に、仕上げ圧延によって導入されるひずみを小さくして、｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値を７．００以下に制限する必要がある。そこで、本製造方法では、仕上げ圧延の前段３段の圧延温度、すなわち仕上げ圧延１段目の圧延温度（ＦＴ１）、仕上げ圧延２段目の圧延温度（ＦＴ２）及び仕上げ圧延３段目の圧延温度（ＦＴ３）を仕上げ圧延の入側温度（ＦＴ０）－５０℃以上に制御することで板厚中心部に導入される剪断ひずみを軽減し、それによって｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値７．００以下を達成するようにしている。板厚中心部は、ロールと直接的に接触する板厚表層部の場合と比較して導入される剪断ひずみが小さく、それゆえ仕上げ圧延の前段３段の圧延温度を上記のように比較的高い温度に制御することで、導入される剪断ひずみを十分に軽減することが可能である。強度の異方性をさらに低減する観点からは、仕上げ圧延の前段３段の圧延温度をＦＴ０－４５℃以上に制御することが好ましい。一方で、圧延の前段３段のうち１段でも圧延温度がＦＴ０－５０℃未満になると、剪断ひずみの十分な軽減効果が得られず、板厚中心部における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値を７．００以下に低減することができなくなる。仕上げ圧延の前段３段の圧延温度の上限は特に限定されないが、例えば、仕上げ圧延の前段３段の圧延温度は１１００℃以下又は１０００℃以下であってもよい。 [(A5) Rolling temperature of the first three stages of finish rolling: FT0-50°C or higher]
In the center of the plate thickness, in contrast to the surface layer of the plate thickness, it is necessary to reduce the strain introduced by the finish rolling and limit the average pole density of the {100}<011>, {211}<011> and {332}<113> orientations to 7.00 or less. Therefore, in the present manufacturing method, the rolling temperatures of the first three stages of the finish rolling, i.e., the rolling temperature of the first stage of the finish rolling (FT1), the rolling temperature of the second stage of the finish rolling (FT2) and the rolling temperature of the third stage of the finish rolling (FT3), are controlled to be equal to or higher than the entry temperature of the finish rolling (FT0) -50°C, thereby reducing the shear strain introduced in the center of the plate thickness, thereby achieving an average pole density of 7.00 or less in the {100}<011>, {211}<011> and {332}<113> orientations. The shear strain introduced in the plate thickness center portion is smaller than that in the plate thickness surface layer portion that directly contacts the roll, and therefore, by controlling the rolling temperature of the first three stages of the finish rolling to a relatively high temperature as described above, it is possible to sufficiently reduce the introduced shear strain. From the viewpoint of further reducing the anisotropy of strength, it is preferable to control the rolling temperature of the first three stages of the finish rolling to FT0-45 ° C. or higher. On the other hand, if the rolling temperature of even one of the first three stages of the rolling is less than FT0-50 ° C., the effect of reducing the shear strain is not sufficient, and the average value of the pole density of the {100}<011>, {211}<011> and {332}<113> orientations in the plate thickness center portion cannot be reduced to 7.00 or less. The upper limit of the rolling temperature of the first three stages of the finish rolling is not particularly limited, but for example, the rolling temperature of the first three stages of the finish rolling may be 1100 ° C. or less or 1000 ° C. or less.
　本製造方法においては、（Ａ４）及び（Ａ５）の条件を満足する限り、（Ａ４）の条件における１つ又は複数の圧延パスと（Ａ５）の条件における１つ又は複数の圧延パスは互いに重複していてもよい。 In this manufacturing method, as long as the conditions (A4) and (A5) are satisfied, one or more rolling passes under the condition (A4) and one or more rolling passes under the condition (A5) may overlap with each other.
［（Ｂ）冷却工程］
　本製造方法では、仕上げ圧延された鋼板に対し、次の冷却工程において２段階冷却が行われる。具体的には、まず、仕上げ圧延された鋼板が１０℃／ｓ以上の平均冷却速度で６５０～７５０℃の温度域まで１次冷却され、当該温度域で３．０～１０．０秒間保持され、次いで１００℃以下まで３０℃／ｓ以上の平均冷却速度で２次冷却される。熱間圧延工程における（Ａ２）及び（Ａ３）等の条件と組み合わせて、このような２段階冷却を行うことで、所望の比較的低い温度域にてパラ平衡による変態が起こり、それによって粒内の方位差が５～１４°である結晶粒の割合を面積％で１０～６０％の範囲内に確実に制御することが可能となる。これに対し、１次冷却の平均冷却速度が１０℃／ｓ未満であるか又は１次冷却の冷却停止温度が７５０℃超であると、比較的高温でパラ平衡による変態が起こり、粒内の方位差が５～１４°である結晶粒の割合が１０％未満となってしまう。また、１次冷却の冷却停止温度が６５０℃未満であると、所望の温度域よりも低温でパラ平衡による変態が起こり、同様に粒内の方位差が５～１４°である結晶粒の割合が１０％未満となる。さらに、６５０～７５０℃での保持時間が３．０秒未満であっても、同様に粒内の方位差が５～１４°である結晶粒の割合が１０％未満となってしまう。一方で、６５０～７５０℃での保持時間が１０．０秒を超えるか又は２次冷却の平均冷却速度が３０℃／ｓ未満であると、伸びフランジ性に有害なセメンタイトが生成しやすくなる。また、２次冷却の冷却停止温度が１００℃超であると、マルテンサイトの面積率が２％未満となってしまう。１次及び２次冷却の平均冷却速度の上限は特に限定されないが、例えば、１次及び２次冷却の平均冷却速度は、冷却設備の設備能力を考慮して２００℃／ｓ以下としてもよい。 [(B) Cooling step]
In this manufacturing method, the finish-rolled steel sheet is subjected to two-stage cooling in the next cooling step. Specifically, the finish-rolled steel sheet is first cooled to a temperature range of 650 to 750 ° C. at an average cooling rate of 10 ° C./s or more, held in that temperature range for 3.0 to 10.0 seconds, and then secondarily cooled to 100 ° C. or less at an average cooling rate of 30 ° C./s or more. By performing such two-stage cooling in combination with the conditions (A2) and (A3) in the hot rolling step, a para-equilibrium transformation occurs in a desired relatively low temperature range, thereby making it possible to reliably control the proportion of crystal grains having an intragranular misorientation of 5 to 14 ° within a range of 10 to 60% in terms of area %. On the other hand, if the average cooling rate of the primary cooling is less than 10 ° C./s or the cooling stop temperature of the primary cooling is more than 750 ° C., a para-equilibrium transformation occurs at a relatively high temperature, and the proportion of crystal grains having an intragranular misorientation of 5 to 14 ° is less than 10%. In addition, if the cooling end temperature of the primary cooling is less than 650 ° C, transformation due to para-equilibrium occurs at a temperature lower than the desired temperature range, and similarly, the proportion of crystal grains with an orientation difference of 5 to 14 ° in the grains is less than 10%. Furthermore, even if the holding time at 650 to 750 ° C is less than 3.0 seconds, the proportion of crystal grains with an orientation difference of 5 to 14 ° in the grains is also less than 10%. On the other hand, if the holding time at 650 to 750 ° C exceeds 10.0 seconds or the average cooling rate of the secondary cooling is less than 30 ° C / s, cementite that is harmful to stretch flangeability is likely to be generated. In addition, if the cooling end temperature of the secondary cooling is more than 100 ° C, the area ratio of martensite is less than 2%. There is no particular limit to the upper limit of the average cooling rate of the primary and secondary cooling, but for example, the average cooling rate of the primary and secondary cooling may be 200 ° C / s or less in consideration of the equipment capacity of the cooling equipment.
　上記の製造方法によって製造された熱間圧延鋼板によれば、面積％で、フェライト及びベイナイトの少なくとも１種：合計で８０～９８％、並びにマルテンサイト：２～１０％を含み、方位差が１５°以上である境界を粒界とし、当該粒界によって囲まれ、かつ円相当直径が０．３μｍ以上である領域を結晶粒と定義した場合に、粒内の方位差が５～１４°である結晶粒の割合が面積％で１０～６０％である金属組織を得ることができる。その結果として、高強度であるにもかかわらず、伸びフランジ性、延性及び切り欠き疲労特性を顕著に改善することが可能となる。加えて、当該金属組織においては、表面から板厚１／６位置までの領域における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値が２．５０以上に制御され、板厚２／５位置から板厚３／５位置までの領域における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値が７．００以下に制御されていることから、熱間圧延鋼板のＬ方向とＣ方向の引張強さにおける強度の異方性を顕著に低減することができる。したがって、上記の製造方法によって製造された熱間圧延鋼板によれば、高強度と優れた加工性の相反する特性を確実に両立させることができるので、これらの特性の両立が求められる自動車分野の使用において特に有用である。 Hot-rolled steel sheet manufactured by the above manufacturing method can have a metal structure that contains, by area percentage, at least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%, and where, if boundaries with an orientation difference of 15° or more are defined as grain boundaries, and regions surrounded by such grain boundaries and having a circle equivalent diameter of 0.3 μm or more are defined as crystal grains, the proportion of crystal grains with an intragranular orientation difference of 5-14° is 10-60% by area percentage. As a result, despite the high strength, it is possible to significantly improve stretch flangeability, ductility, and notch fatigue properties. In addition, in the metal structure, the average pole density of the {110}<111> and {112}<111> orientations in the region from the surface to the 1/6 position of the plate thickness is controlled to 2.50 or more, and the average pole density of the {100}<011>, {211}<011>, and {332}<113> orientations in the region from the 2/5 position to the 3/5 position of the plate thickness is controlled to 7.00 or less, so that the anisotropy of strength in the tensile strength in the L direction and C direction of the hot rolled steel sheet can be significantly reduced. Therefore, according to the hot rolled steel sheet manufactured by the above manufacturing method, it is possible to reliably achieve the contradictory properties of high strength and excellent workability at the same time, and it is particularly useful in the automotive field where both properties are required.
　以下、実施例によって本発明をより詳細に説明するが、本発明はこれらの実施例に何ら限定されるものではない。 The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.
　以下の実施例では、本発明の実施形態に係る熱間圧延鋼板を種々の条件下で製造し、得られた熱間圧延鋼板の引張強さ、伸びフランジ性、延性、切り欠き疲労特性、及び強度の異方性について調べた。 In the following examples, hot-rolled steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength, stretch flangeability, ductility, notch fatigue properties, and strength anisotropy of the obtained hot-rolled steel sheets were investigated.
　まず、溶鋼を連続鋳造法にて鋳造して表１及び２に示す種々の化学組成を有するスラブを形成し、これらのスラブを表３に示す条件下で加熱し、次いで熱間圧延を行った。熱間圧延は、粗圧延と仕上げ圧延を行うことにより実施した。より具体的には、粗圧延は全ての実施例及び比較例で同じ条件であり、仕上げ圧延は７基の圧延スタンドからなるタンデム圧延機を用いて実施し、仕上げ圧延の入側温度（Ｆ０）、仕上げ圧延１段目の圧延温度（ＦＴ１）、仕上げ圧延２段目の圧延温度（ＦＴ２）、仕上げ圧延３段目の圧延温度（ＦＴ３）、仕上げ圧延の終了温度、及び仕上げ圧延の後段３段の累積ひずみ（εｅｆｆ．）は表２に示すとおりであった。また、仕上げ圧延は、表３に示すロール半径を有するロールを用いて、形状比（Ｘ）が２．３以上となる圧延パスを１１００℃以下で表３に示す回数実施した。次に、仕上げ圧延された鋼板を、まず表３に示す条件下で１次冷却及び２次冷却を施し、表２に示す板厚を有する熱間圧延鋼板を得た。 First, molten steel was cast by continuous casting to form slabs having various chemical compositions shown in Tables 1 and 2, and these slabs were heated under the conditions shown in Table 3, and then hot rolling was performed. Hot rolling was performed by performing rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and finish rolling was performed using a tandem rolling mill consisting of seven rolling stands. The entry temperature (F0) of the finish rolling, the rolling temperature of the first stage of the finish rolling (FT1), the rolling temperature of the second stage of the finish rolling (FT2), the rolling temperature of the third stage of the finish rolling (FT3), the end temperature of the finish rolling, and the accumulated strain (εeff.) of the last three stages of the finish rolling were as shown in Table 2. In addition, the finish rolling was performed using rolls having the roll radii shown in Table 3, and the number of rolling passes at 1100°C or less to achieve a shape ratio (X) of 2.3 or more was shown in Table 3. Next, the finish-rolled steel plate was subjected to primary and secondary cooling under the conditions shown in Table 3 to obtain a hot-rolled steel plate having the plate thickness shown in Table 2.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　得られた熱間圧延鋼板の特性は以下の方法によって測定及び評価した。 The properties of the resulting hot-rolled steel sheets were measured and evaluated using the following methods.
［引張強さ（ＴＳＣ）及び全伸び（Ｅｌ）］
　引張強さ（ＴＳＣ）及び全伸び（Ｅｌ）は、試験片の長手方向が熱間圧延鋼板の圧延直角方向と平行になる向き（Ｃ方向）からＪＩＳ５号試験片を採取し、ＪＩＳ　Ｚ　２２４１：２０１１に準拠した引張試験を行うことで測定した。 [Tensile strength (TSC) and total elongation (El)]
The tensile strength (TSC) and total elongation (El) were measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the hot-rolled steel plate (C direction) and performing a tensile test in accordance with JIS Z 2241:2011.
［伸びフランジ性の評価］
　伸びフランジ性は、鞍型成型品を用いた鞍型伸びフランジ試験法によって評価した。具体的には、図１に示すような直線部と円弧部とからなる伸びフランジ形状を模擬した鞍型形状の成型品をプレス加工し、そのときの限界成形高さで伸びフランジ性を評価した。鞍型伸びフランジ試験法では、コーナーの曲率半径Ｒを５０～６０ｍｍ、開き角θを１２０°とした鞍型成型品を用いて、コーナー部を打ち抜く際のクリアランスを１１％とした時の限界成形高さＨ（ｍｍ）を測定する。ここで、クリアランスとは打ち抜きダイスとパンチの間隙と、試験片の厚さとの比を示す。クリアランスは実際には打ち抜き工具と板厚の組み合わせによって決まるため、１１％とは、１０．５～１１．５％の範囲を満足することを意味する。限界成形高さＨの判定は、成形後に目視にて板厚の１／３以上の長さを有するクラックの存在の有無を観察し、クラックが存在しない限界の成形高さとした。引張強さＴＳＣ（ＭＰａ）と限界成形高さＨ（ｍｍ）との積（ＴＳＣ×Ｈ）を伸びフランジ性の指標として評価を行い、ＴＳＣ×Ｈ≧１９５００ＭＰａ・ｍｍの場合に、伸びフランジ性が改善されたとして評価した。 [Evaluation of stretch flangeability]
The stretch flangeability was evaluated by a saddle-shaped stretch flange test method using a saddle-shaped molded product. Specifically, a molded product of a saddle-shaped shape simulating a stretch flange shape consisting of a straight part and a circular part as shown in FIG. 1 was pressed, and the stretch flangeability was evaluated by the limit forming height at that time. In the saddle-shaped stretch flange test method, a saddle-shaped molded product with a corner curvature radius R of 50 to 60 mm and an opening angle θ of 120° is used to measure the limit forming height H (mm) when the clearance when punching the corner part is 11%. Here, the clearance indicates the ratio of the gap between the punching die and the punch to the thickness of the test piece. Since the clearance is actually determined by the combination of the punching tool and the plate thickness, 11% means that the range of 10.5 to 11.5% is satisfied. The judgment of the limit forming height H was made by visually observing the presence or absence of cracks having a length of 1/3 or more of the plate thickness after forming, and the limit forming height at which no cracks existed was determined. The product (TSC×H) of the tensile strength TSC (MPa) and the limit forming height H (mm) was used as an index of stretch flangeability, and the stretch flangeability was evaluated as being improved when TSC×H≧19,500 MPa·mm.
［延性の評価］
　ＴＳＣ（ＭＰａ）とＥｌ（％）との積（ＴＳＣ×Ｅｌ）がＴＳＣ×Ｅｌ≧１３５００ＭＰａ・％を満たす場合に、延性が改善されたとして評価した。 [Evaluation of ductility]
When the product (TSC×El) of TSC (MPa) and El (%) satisfied TSC×El≧13500 MPa·%, the ductility was evaluated as being improved.
［切り欠き疲労特性の評価］
　切り欠き疲労特性は、以下のようにして評価した。具体的には、引張試験片採取位置と同様の位置から、圧延直角方向と平行になる向き（Ｃ方向）が長辺になるように図２に示す形状の疲労試験片を採取して疲労試験を行った。疲労試験片は最表層より０．０５ｍｍ程度の深さまで研削した。応力比Ｒ＝０．１、周波数５Ｈｚで応力制御軸疲労試験を行い、１０００万回後に破断しない応力を切り欠き疲労限（ＦＬ）と定義し、切り欠き疲労特性を評価した。試験の結果、ＦＬ／ＴＳＣ≧０．２５を満たす場合に、切り欠き疲労特性が改善されたとして評価した。 [Evaluation of notch fatigue properties]
The notch fatigue properties were evaluated as follows. Specifically, fatigue test pieces were taken from the same position as the tensile test piece taking position, so that the long side was parallel to the rolling direction (C direction), and fatigue tests were performed. The fatigue test pieces were ground to a depth of about 0.05 mm from the outermost layer. A stress-controlled axial fatigue test was performed with a stress ratio R = 0.1 and a frequency of 5 Hz, and the stress at which the specimen did not break after 10 million cycles was defined as the notch fatigue limit (FL), and the notch fatigue properties were evaluated. When the test results satisfied FL/TSC ≥ 0.25, the notch fatigue properties were evaluated as having been improved.
［強度の異方性の評価］
　強度の異方性は、まず、試験片の長手方向が熱間圧延鋼板の圧延方向と平行になる向き（Ｌ方向）からＪＩＳ５号試験片を採取し、ＪＩＳ　Ｚ　２２４１：２０１１に準拠した引張試験を行うことでＬ方向の引張強さ、すなわちＬ方向ＴＳ（ＴＳＬ）を測定した。次に、得られたＬ方向ＴＳと先に求めたＣ方向ＴＳ（ＴＳＣ）がＴＳＬ／ＴＳＣ≧０．９５を満たす場合に、強度の異方性が低減されたものとして評価した。 [Evaluation of strength anisotropy]
As for the anisotropy of strength, first, a JIS No. 5 test piece was taken from a direction (L direction) in which the longitudinal direction of the test piece was parallel to the rolling direction of the hot-rolled steel sheet, and the tensile strength in the L direction, i.e., L direction TS (TSL) was measured by performing a tensile test in accordance with JIS Z 2241: 2011. Next, when the obtained L direction TS and the previously obtained C direction TS (TSC) satisfied TSL/TSC≧0.95, it was evaluated that the anisotropy of strength was reduced.
　引張強さＴＳＣが５４０ＭＰａ以上であり、かつＴＳＣ×Ｈ≧１９５００ＭＰａ・ｍｍ、ＴＳＣ×Ｅｌ≧１３５００ＭＰａ・％、ＦＬ／ＴＳＣ≧０．２５、及びＴＳＬ／ＴＳＣ≧０．９５である場合を、高強度であるにもかかわらず、伸びフランジ性、延性及び切り欠き疲労特性が改善されかつ強度の異方性が低減された熱間圧延鋼板として評価した。その結果を表４及び５に示す。 　If the tensile strength TSC is 540 MPa or more, and TSC x H ≥ 19,500 MPa·mm, TSC x El ≥ 13,500 MPa·%, FL/TSC ≥ 0.25, and TSL/TSC ≥ 0.95, the hot-rolled steel sheet was evaluated as having high strength, yet improved stretch flangeability, ductility, and notch fatigue properties, and reduced strength anisotropy. The results are shown in Tables 4 and 5.
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
　表１～５を参照すると、比較例４は、仕上げ圧延の後段３段の累積ひずみ（εｅｆｆ．）が高かったために、熱間圧延中にオーステナイトの再結晶が起こり、変態時の蓄積転位密度が低下したものと考えられる。その結果として、粒内の方位差が５～１４°である結晶粒の割合が１０％未満となり、伸びフランジ性が低下した。比較例５は、εｅｆｆ．が低かったために、導入されるオーステナイトの転位密度が十分でなかったと考えられる。その結果として、同様に粒内の方位差が５～１４°である結晶粒の割合が１０％未満となり、伸びフランジ性が低下した。比較例６は、仕上げ圧延の終了温度が低かったために、粒内の方位差が５～１４°である結晶粒の割合が６０％を超えてしまい、延性が低下した。比較例７及び８は、１１００℃以下における形状比（Ｘ）２．３以上の圧延パスの回数が少なかったために、板厚表層部に十分な剪断ひずみを導入することができなかったと考えられる。その結果として、板厚表層部における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値が２．５０未満となり、Ｌ方向の強度を高めることができず、強度の異方性が顕著となった。とりわけ、比較例７は、他の実施例と同様のロール半径を有するロールを用いて仕上げ圧延を実施したが板厚が比較的厚かったために十分な形状比を確保できなかった。一方、比較例８は、他の実施例と同様の板厚であったが、ロール半径が１５０ｍｍと比較的小さかったために十分な形状比を確保できなかった。比較例９は、仕上げ圧延３段目の圧延温度（ＦＴ３）が低かったために、板厚中心部に導入される剪断ひずみを軽減することができなかったと考えられる。その結果として、｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値が７．００超となり、Ｃ方向の強度を低減することができず、強度の異方性が顕著となった。比較例１０は、冷却工程における１次冷却の平均冷却速度が低かったために、比較的高温でパラ平衡による変態が起こったと考えられる。その結果として、粒内の方位差が５～１４°である結晶粒の割合が１０％未満となり、伸びフランジ性が低下した。比較例１１は、１次冷却の冷却停止温度が高かったために、同様に比較的高温でパラ平衡による変態が起こったと考えられる。その結果として、粒内の方位差が５～１４°である結晶粒の割合が１０％未満となり、伸びフランジ性が低下した。比較例１２は、１次冷却の冷却停止温度が低かったために、所望の温度域よりも低温でパラ平衡による変態が起こったと考えられる。その結果として、同様に粒内の方位差が５～１４°である結晶粒の割合が１０％未満となり、伸びフランジ性が低下した。比較例１３は、１次冷却における６５０～７５０℃での保持時間が短かったために、同様に粒内の方位差が５～１４°である結晶粒の割合が１０％未満となり、伸びフランジ性が低下した。比較例１４は、冷却工程における２次冷却の冷却停止温度が高かったために、マルテンサイトの面積率が２％未満となってしまった。その結果として、ＴＳＣ及び切り欠き疲労特性が低下した。 Referring to Tables 1 to 5, in Comparative Example 4, the accumulated strain (εeff.) in the last three stages of finish rolling was high, so austenite recrystallization occurred during hot rolling, and the accumulated dislocation density during transformation is thought to have decreased. As a result, the proportion of crystal grains with an intragranular misorientation of 5 to 14° was less than 10%, and stretch flangeability was reduced. In Comparative Example 5, the dislocation density of the introduced austenite was thought to be insufficient because εeff. was low. As a result, the proportion of crystal grains with an intragranular misorientation of 5 to 14° was similarly less than 10%, and stretch flangeability was reduced. In Comparative Example 6, the end temperature of finish rolling was low, so the proportion of crystal grains with an intragranular misorientation of 5 to 14° exceeded 60%, and ductility was reduced. In Comparative Examples 7 and 8, the number of rolling passes with a shape ratio (X) of 2.3 or more at 1100°C or less was small, so it is thought that sufficient shear strain could not be introduced into the plate thickness surface layer. As a result, the average value of the pole density of the {110}<111> and {112}<111> orientations in the plate thickness surface layer portion was less than 2.50, the strength in the L direction could not be increased, and the anisotropy of the strength became prominent. In particular, in Comparative Example 7, the finish rolling was performed using rolls having the same roll radius as in the other Examples, but the plate thickness was relatively thick, so a sufficient shape ratio could not be ensured. On the other hand, in Comparative Example 8, the plate thickness was similar to that of the other Examples, but the roll radius was relatively small at 150 mm, so a sufficient shape ratio could not be ensured. In Comparative Example 9, it is considered that the rolling temperature (FT3) of the third stage of the finish rolling was low, so the shear strain introduced into the plate thickness center portion could not be reduced. As a result, the average value of the pole density of the {100}<011>, {211}<011>, and {332}<113> orientations exceeded 7.00, the strength in the C direction could not be reduced, and the anisotropy of the strength became prominent. In Comparative Example 10, the average cooling rate of the first cooling in the cooling step was low, so it is believed that transformation due to paraequilibrium occurred at a relatively high temperature. As a result, the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced. In Comparative Example 11, the cooling stop temperature of the first cooling was high, so it is believed that transformation due to paraequilibrium occurred at a relatively high temperature. As a result, the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced. In Comparative Example 12, the cooling stop temperature of the first cooling was low, so it is believed that transformation due to paraequilibrium occurred at a temperature lower than the desired temperature range. As a result, the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced. In Comparative Example 13, the holding time at 650 to 750 ° C in the first cooling was short, so the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced. In Comparative Example 14, the cooling stop temperature of the secondary cooling in the cooling process was high, so the area ratio of martensite was less than 2%. As a result, the TSC and notch fatigue properties were reduced.
　比較例３１及び３３は、それぞれＣ及びＭｎ含有量が高かったために伸びフランジ性が低下した。比較例３２及び３４は、それぞれＣ及びＭｎ含有量が低かったために十分な強度を得ることができなかった。比較例３５は、Ａｌ含有量が高かったために、圧延中に割れが発生し、その後の試験を行うことができなかった。比較例３６は、Ｓｉ及びｓｏｌ．Ａｌの合計の含有量が高かったために、フェライト生成が促進されてしまい、ＴＳＣが低下した。比較例３７は、Ｓｉ及びｓｏｌ．Ａｌの合計の含有量が低かったために、粒内の方位差が５～１４°である結晶粒の割合が１０％未満となり、伸びフランジ性が低下した。比較例３８は、Ｔｉ含有量が高かったために、炭化物（ＴｉＣ）が粗大となり、延性が低下した。比較例３９は、Ｔｉ含有量が低かったために、セメンタイトの生成を十分に抑制することができなかったと考えられ、その結果として伸びフランジ性が低下した。 In Comparative Examples 31 and 33, the C and Mn contents were high, respectively, and therefore the stretch flangeability was reduced. In Comparative Examples 32 and 34, the C and Mn contents were low, respectively, and therefore sufficient strength could not be obtained. In Comparative Example 35, the Al content was high, and therefore cracks occurred during rolling, and subsequent testing could not be performed. In Comparative Example 36, the total content of Si and sol. Al was high, and therefore ferrite formation was promoted, and the TSC was reduced. In Comparative Example 37, the total content of Si and sol. Al was low, and therefore the proportion of crystal grains with an intragranular misorientation of 5 to 14° was less than 10%, and therefore the stretch flangeability was reduced. In Comparative Example 38, the Ti content was high, and therefore the carbide (TiC) became coarse, and therefore the ductility was reduced. In Comparative Example 39, the Ti content was low, and therefore it is believed that the formation of cementite could not be sufficiently suppressed, and as a result, the stretch flangeability was reduced.
　これとは対照的に、全ての発明例に係る熱間圧延鋼板において、所定の化学組成を有し、さらに製造方法における各条件を適切に制御することで、面積％で、フェライト及びベイナイトの少なくとも１種：合計で８０～９８％、並びにマルテンサイト：２～１０％を含み、粒内の方位差が５～１４°である結晶粒の割合が面積％で１０～６０％であり、表面から板厚１／６位置までの領域における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値が２．５０以上であり、板厚２／５位置から板厚３／５位置までの領域における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値が７．００以下である金属組織を有する熱間圧延鋼板を得ることができた。また、その結果として、引張強さ５４０ＭＰａ以上の高強度であるにもかかわらず、伸びフランジ性、延性及び切り欠き疲労特性を向上させるとともに、Ｌ方向とＣ方向の引張強さにおける強度の異方性を顕著に低減することができた。 In contrast to this, in all of the hot-rolled steel sheets according to the examples of the invention, it was possible to obtain a hot-rolled steel sheet having a metal structure with a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, which contains, by area percentage, at least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%, in which the proportion of crystal grains with an intragranular orientation misorientation of 5-14° is 10-60%, by area percentage, and in which the average pole density of the {110}<111> and {112}<111> orientations in the region from the surface to the 1/6 position of the plate thickness is 2.50 or more, and the average pole density of the {100}<011>, {211}<011> and {332}<113> orientations in the region from the 2/5 position of the plate thickness to the 3/5 position of the plate thickness is 7.00 or less. As a result, despite the high tensile strength of 540 MPa or more, it was possible to improve stretch flangeability, ductility, and notch fatigue properties, while also significantly reducing the anisotropy of tensile strength in the L and C directions.

Claims (2)

  1. 　質量％で、
    　Ｃ：０．０２０～０．０７０％、
    　Ｓｉ：０．０１０～２．０００％、
    　Ｍｎ：０．６０～２．００％、
    　Ｔｉ：０．０１５～０．２００％、
    　ｓｏｌ．Ａｌ：０．０１０～１．０００％、
    　Ｐ：０．１００％以下、
    　Ｓ：０．０３０％以下、
    　Ｎ：０．００６０％以下、
    　Ｏ：０．０１００％以下、
    　Ｎｂ：０～０．０５０％、
    　Ｖ：０～０．３００％、
    　Ｃｒ：０～２．００％、
    　Ｎｉ：０～２．００％、
    　Ｃｕ：０～２．００％、
    　Ｍｏ：０～１．０００％、
    　Ｂ：０～０．０１００％、
    　Ｓｂ：０～１．００％、
    　Ｃａ：０～０．０１００％、
    　Ｍｇ：０～０．０１００％、
    　Ｈｆ：０～０．０１００％、
    　ＲＥＭ：０～０．１０００％、
    　Ｂｉ：０～０．０１００％、
    　Ａｓ：０～０．０１００％、
    　Ｚｒ：０～１．００％、
    　Ｃｏ：０～１．００％、
    　Ｚｎ：０～１．００％、
    　Ｗ：０～１．００％、
    　Ｓｎ：０～１．００％、並びに
    　残部：Ｆｅ及び不純物からなり、
    　０．１００≦［Ｓｉ］＋［ｓｏｌ．Ａｌ］≦２．５００を満たし、式中、［Ｓｉ］及び［ｓｏｌ．Ａｌ］は各元素の含有量（質量％）である化学組成を有し、
    　面積％で、
    　フェライト及びベイナイトの少なくとも１種：合計で８０～９８％、並びに
    　マルテンサイト：２～１０％を含み、
    　方位差が１５°以上である境界を粒界とし、前記粒界によって囲まれ、かつ円相当直径が０．３μｍ以上である領域を結晶粒と定義した場合に、粒内の方位差が５～１４°である前記結晶粒の割合が、面積％で、１０～６０％であり、
    　表面から板厚１／６位置までの領域における｛１１０｝＜１１１＞及び｛１１２｝＜１１１＞方位の極密度の平均値が２．５０以上であり、
    　板厚２／５位置から板厚３／５位置までの領域における｛１００｝＜０１１＞、｛２１１｝＜０１１＞及び｛３３２｝＜１１３＞方位の極密度の平均値が７．００以下である金属組織を有することを特徴とする、熱間圧延鋼板。     In mass percent,
    C: 0.020 to 0.070%,
    Si: 0.010 to 2.000%,
    Mn: 0.60 to 2.00%,
    Ti: 0.015 to 0.200%,
    sol. Al: 0.010 to 1.000%,
    P: 0.100% or less,
    S: 0.030% or less,
    N: 0.0060% or less,
    O: 0.0100% or less,
    Nb: 0 to 0.050%,
    V: 0 to 0.300%,
    Cr: 0 to 2.00%,
    Ni: 0 to 2.00%,
    Cu: 0 to 2.00%,
    Mo: 0 to 1.000%,
    B: 0 to 0.0100%,
    Sb: 0 to 1.00%,
    Ca: 0 to 0.0100%,
    Mg: 0 to 0.0100%,
    Hf: 0 to 0.0100%,
    REM: 0 to 0.1000%,
    Bi: 0 to 0.0100%,
    As: 0 to 0.0100%,
    Zr: 0 to 1.00%,
    Co: 0 to 1.00%,
    Zn: 0 to 1.00%,
    W: 0 to 1.00%,
    Sn: 0 to 1.00%, and the balance: Fe and impurities;
    The chemical composition satisfies 0.100≦[Si]+[sol. Al]≦2.500, in which [Si] and [sol. Al] are the contents (mass%) of each element,
    In area %,
    At least one of ferrite and bainite: 80 to 98% in total; and martensite: 2 to 10%;
    When a boundary having an orientation difference of 15° or more is defined as a grain boundary, and a region surrounded by the grain boundary and having a circle equivalent diameter of 0.3 μm or more is defined as a crystal grain, the ratio of the crystal grains having an intragranular orientation difference of 5 to 14° is 10 to 60% by area percent,
    The average pole density of the {110}<111> and {112}<111> orientations in the region from the surface to the 1/6 position of the sheet thickness is 2.50 or more;
    A hot-rolled steel sheet, characterized in that it has a metal structure in which the average pole density of the {100}<011>, {211}<011> and {332}<113> orientations in a region from the 2/5 position of the sheet thickness to the 3/5 position of the sheet thickness is 7.00 or less.
  2. 　前記化学組成が、質量％で、
    　Ｎｂ：０．００１～０．０５０％、
    　Ｖ：０．００１～０．３００％、
    　Ｃｒ：０．０１～２．００％、
    　Ｎｉ：０．０１～２．００％、
    　Ｃｕ：０．０１～２．００％、
    　Ｍｏ：０．００１～１．０００％、
    　Ｂ：０．０００１～０．０１００％、
    　Ｓｂ：０．０１～１．００％、
    　Ｃａ：０．０００１～０．０１００％、
    　Ｍｇ：０．０００１～０．０１００％、
    　Ｈｆ：０．０００１～０．０１００％、
    　ＲＥＭ：０．０００１～０．１０００％、
    　Ｂｉ：０．０００１～０．０１００％、
    　Ａｓ：０．０００１～０．０１００％、
    　Ｚｒ：０．０１～１．００％、
    　Ｃｏ：０．０１～１．００％、
    　Ｚｎ：０．０１～１．００％、
    　Ｗ：０．０１～１．００％、及び
    　Ｓｎ：０．０１～１．００％
    のうち少なくとも１種を含むことを特徴とする、請求項１に記載の熱間圧延鋼板。     The chemical composition, in mass%,
    Nb: 0.001 to 0.050%,
    V: 0.001 to 0.300%,
    Cr: 0.01 to 2.00%,
    Ni: 0.01 to 2.00%,
    Cu: 0.01 to 2.00%,
    Mo: 0.001 to 1.000%,
    B: 0.0001 to 0.0100%,
    Sb: 0.01 to 1.00%,
    Ca: 0.0001 to 0.0100%,
    Mg: 0.0001 to 0.0100%,
    Hf: 0.0001 to 0.0100%,
    REM: 0.0001 to 0.1000%,
    Bi: 0.0001 to 0.0100%,
    As: 0.0001 to 0.0100%,
    Zr: 0.01 to 1.00%,
    Co: 0.01 to 1.00%,
    Zn: 0.01 to 1.00%,
    W: 0.01 to 1.00%, and Sn: 0.01 to 1.00%
    The hot-rolled steel sheet according to claim 1, characterized in that it contains at least one of the following:
PCT/JP2023/024341 2022-11-02 2023-06-30 Hot rolled steel sheet WO2024095532A1 (en)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2005298956A (en) * 2004-04-16 2005-10-27 Sumitomo Metal Ind Ltd Hot rolled steel sheet and its production method
JP2009132988A (en) * 2007-04-19 2009-06-18 Nippon Steel Corp Steel sheet, hot dip galvanized steel sheet, hot dip galvannealed steel sheet and steel pipe having low yield ratio and high young's modulus, and method for producing them
WO2016133222A1 (en) * 2015-02-20 2016-08-25 新日鐵住金株式会社 Hot-rolled steel sheet
WO2020195605A1 (en) * 2019-03-26 2020-10-01 日本製鉄株式会社 Steel sheet, method for manufacturing same and plated steel sheet

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2005298956A (en) * 2004-04-16 2005-10-27 Sumitomo Metal Ind Ltd Hot rolled steel sheet and its production method
JP2009132988A (en) * 2007-04-19 2009-06-18 Nippon Steel Corp Steel sheet, hot dip galvanized steel sheet, hot dip galvannealed steel sheet and steel pipe having low yield ratio and high young's modulus, and method for producing them
WO2016133222A1 (en) * 2015-02-20 2016-08-25 新日鐵住金株式会社 Hot-rolled steel sheet
WO2020195605A1 (en) * 2019-03-26 2020-10-01 日本製鉄株式会社 Steel sheet, method for manufacturing same and plated steel sheet

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