WO2024096073A1 - Hot-rolled coil - Google Patents

Hot-rolled coil Download PDF

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Publication number
WO2024096073A1
WO2024096073A1 PCT/JP2023/039477 JP2023039477W WO2024096073A1 WO 2024096073 A1 WO2024096073 A1 WO 2024096073A1 JP 2023039477 W JP2023039477 W JP 2023039477W WO 2024096073 A1 WO2024096073 A1 WO 2024096073A1
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hot
precipitates
rolled coil
grain size
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PCT/JP2023/039477
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French (fr)
Japanese (ja)
Inventor
大輔 伊藤
龍雄 横井
栄作 桜田
章文 榊原
洵 安藤
貴昭 堤
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日本製鉄株式会社
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Publication of WO2024096073A1 publication Critical patent/WO2024096073A1/en

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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
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the present invention relates to hot-rolled coils.
  • Patent Document 1 describes a high-strength hot-rolled steel sheet having a predetermined composition and a ferrite phase with an area ratio of 95% or more, in which fine carbides are precipitated in the ferrite phase at 1.0 x 1022 particles/ m3 or more, cementite particles are precipitated at 10 particles/10,000 ⁇ m2 or more, the average particle size of the fine carbides is 10 nm or less, and the difference ⁇ HV0.025 between the hardness HV1 /2t at the center position of the sheet thickness and the hardness HV1 / 4t at the 1/4 position of the sheet thickness or the hardness HV3/4t at the 3/4 position of the sheet thickness is 20 HV or less.
  • Patent Document 1 also describes that according to the above configuration, a high-strength hot-rolled steel sheet having a high strength of tensile strength: 780 MPa or more and excellent workability such as ductility and hole expandability can be easily manufactured, which is an industrially significant effect. Furthermore, in Patent Document 1, Ti contributes to the formation of fine carbides (Ti carbides) having a size of 10 nm or less, and the desired high strength is ensured by precipitation strengthening. In order to ensure the desired hole expandability, the Ti content is set in the range of 0.070 to 0.220%.
  • Patent Document 2 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, an average grain size of 8 ⁇ m or less, a segregation amount of C to high-angle grain boundaries with an orientation difference of 15° or more of 4 to 15 atoms/nm 2 , the number of TiC precipitates with a sphere-equivalent diameter of 3 nm or more on the high-angle grain boundaries is less than 0.01/nm 2 , and the ratio of grains having a number density of TiC precipitates with a sphere-equivalent diameter of 0.8 nm to 2 nm or less in the grains of 8 ⁇ 10 16 /cm 3 or more is 10 to 70%.
  • Patent Document 2 also describes that the above configuration can provide a high-strength hot-rolled steel sheet with a tensile strength of 740 MPa or more that is excellent in formability and low-temperature toughness, and that this is an extremely significant contribution to industry. Patent Document 2 also teaches that by setting the sphere-equivalent diameter of TiC precipitates in the grains to 0.8 nm to 2 nm, precipitation strengthening can be efficiently imparted, which is effective in increasing strength.
  • the paper describes a highly formable, high-tensile steel sheet with excellent strength stability and a tensile strength of 550 MPa or more, characterized in that the steel satisfies the above C requirements and has a ferritic structure in which precipitates smaller than 10 nm containing Ti and one or more of Mo and W are dispersed.
  • Patent Documents 1 and 2 teach the use of precipitation strengthening by Ti carbide as described above in order to achieve high strength exceeding 700 MPa while improving workability, etc.
  • precipitation strengthening to increase the strength of steel materials, for example, variations in strength may occur due to differences in the precipitation state of precipitates in the longitudinal and transverse directions of the hot-rolled coil.
  • Patent Document 3 teaches that in a steel in which a single-phase ferritic structure is strengthened with fine precipitates containing Ti and one or more of Mo and W, the material variation in the longitudinal direction within the coil, particularly the strength variation, can be reduced by setting Ex. C, which is C that does not bond with Ti, Mo, or W, to 0.015% or less and Mn to 0.2 ⁇ Mn1.7-30 ⁇ Ex. C.
  • Ex. C is C that does not bond with Ti, Mo, or W
  • Mn to 0.2 ⁇ Mn1.7-30 ⁇ Ex. C.
  • Patent Document 3 mainly considers the reduction of strength variation from the perspective of the chemical composition of the steel sheet, but does not necessarily fully consider the precipitation state of the fine precipitates in the steel sheet as being appropriate. Therefore, the invention described in Patent Document 3 still has room for improvement in terms of suppressing strength variation.
  • the present invention was made in consideration of these circumstances, and its purpose is to provide a hot-rolled coil with high strength and reduced strength variation through a new configuration.
  • the present invention which has achieved the above object, is as follows. (1) In mass%, C: 0.050 to 0.100%, Si: 0.01 to 0.30%, Mn: 1.30 to 2.10%, Ti: 0.080 to 0.150%, Nb: 0.020 to 0.050%, Al: 0.001 to 0.050%, P: 0.100% or less, S: 0.050% or less, N: 0.0050% or less, O: 0.0050% or less, B: 0 to 0.0050%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Sn: 0 to 0.10%, Cr: 0 to 0.40%, Mo: 0 to 0.200%, V: 0 to 0.100%, As: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.100%, Bi: 0 to 0.020%, Co: 0 to 0.20%, W: 0 to 0.20%, Zn: 0 to 0.20%,
  • the chemical composition is, in mass%, B: 0.0001 to 0.0050%, Cu: 0.01 to 0.20%, Ni: 0.01 to 0.20%, Sn: 0.01 to 0.10%, Cr: 0.01 to 0.40%, Mo: 0.001 to 0.200%, V: 0.001 to 0.100%, As: 0.001 to 0.100%, Zr: 0.001 to 0.100%, Ca: 0.0001 to 0.0050%, Mg: 0.001 to 0.100%, Bi: 0.001 to 0.020%, Co: 0.01 to 0.20%, W: 0.01 to 0.20%, Zn: 0.01 to 0.20%, and REM: 0.0001 to 0.1000%
  • the hot-rolled coil according to the above (1) characterized in that it contains at least one of the following: (3)
  • the present invention makes it possible to provide hot-rolled coils that are high in strength and have reduced strength variation.
  • FIG. 2 is a schematic diagram showing a hot-rolled coil in a wound state.
  • the hot rolled coil according to the embodiment of the present invention has, in mass%, C: 0.050 to 0.100%, Si: 0.01 to 0.30%, Mn: 1.30 to 2.10%, Ti: 0.080 to 0.150%, Nb: 0.020 to 0.050%, Al: 0.001 to 0.050%, P: 0.100% or less, S: 0.050% or less, N: 0.0050% or less, O: 0.0050% or less, B: 0 to 0.0050%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Sn: 0 to 0.10%, Cr: 0 to 0.40%, Mo: 0 to 0.200%, V: 0 to 0.100%, As: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.100%, Bi: 0 to 0.020%, Co: 0 to 0.20%, W: 0 to 0.20%, Zn:
  • the strength of the front and rear ends of the hot-rolled coil in the longitudinal direction decreases, they are cut and only the range with the desired strength is used as a product, resulting in a drop in yield and a decrease in productivity. Furthermore, even if the hot-rolled coil has the desired strength at its leading and trailing ends in the longitudinal direction, if the precipitation state of the precipitates differs in the longitudinal direction, causing variations in strength, there is a problem that, for example, forming defects such as cracks are likely to occur during press processing. On the other hand, the central part of the hot-rolled coil in the longitudinal direction is not directly exposed to the atmosphere, so it is difficult to cool and is maintained at a relatively high temperature.
  • the precipitates may become coarse, and the peak aging at which a high effect of precipitation strengthening is obtained may be exceeded, resulting in an overaging state, and similarly a decrease in strength.
  • the desired strength may not be obtained or variations in strength may occur, causing a decrease in productivity and forming defects during press processing.
  • the precipitation state of the precipitates in the longitudinal direction and the width direction of the hot-rolled coil is considered to be closely related to each other. For this reason, for example, even if only the central part in the longitudinal direction of the hot-rolled coil after coiling is appropriately cooled, if the front end and/or the tail end are not appropriately cooled, the desired precipitation state of the precipitates cannot be reliably obtained even in the central part due to the influence of the above.
  • the front end, central part and tail end of the hot-rolled coil in the longitudinal direction after coiling are appropriately cooled, for example, if the cooling in the width direction, more specifically, the cooling in the width direction from after hot rolling to before coiling, or the cooling in the width direction after coiling is not performed uniformly, the precipitation state of the precipitates in the longitudinal direction will also be affected due to the temperature deviation in the width direction. As a result, the desired strength may not be obtained in the hot-rolled coil obtained in the end, or the strength may vary significantly.
  • the precipitation state of the precipitates changes over the entire length (total length of the hot-rolled coil in the rolling direction) and width (total length of the hot-rolled coil in the width direction) of the hot-rolled coil depending on the temperature history. Therefore, it is generally very difficult to effectively utilize the precipitation strengthening caused by the precipitates to achieve the desired strength while suppressing or reducing the variation in strength in the longitudinal and width directions of the hot-rolled coil.
  • the longitudinal direction in relation to a hot-rolled coil when referring to the longitudinal direction in relation to a hot-rolled coil, the “longitudinal direction” means the “rolling direction” as shown in Figure 1.
  • the width direction when referring to the width direction in relation to a hot-rolled coil, the “width direction” means the “direction perpendicular to the rolling direction and the plate thickness direction” as shown in Figure 1.
  • the inventors therefore conducted a study focusing on the metal structure of the hot-rolled coil in addition to making the chemical composition of the hot-rolled coil appropriate. To explain in more detail, first, the inventors discovered that the desired high strength can be achieved by utilizing fine grain strengthening by adding Nb and the like in addition to precipitation strengthening by precipitates such as Ti carbides.
  • the inventors discovered that the strength improving effect by precipitation strengthening can be fully exerted by controlling the average grain size of precipitates such as Ti carbides in the longitudinal center portion of the hot-rolled coil to within a range of 3.0 to 9.5 nm, and that the strength improving effect by fine grain strengthening can be added by controlling the average grain size of crystal grains due to the addition of Nb and the like to within a range of 5.0 to 8.0 ⁇ m, and as a result, high strength, for example, a high strength of tensile strength of 780 MPa or more can be reliably achieved.
  • the inventors conducted further studies focusing particularly on the precipitation states of precipitates in the width direction of the longitudinal center part of the hot-rolled coil and in the longitudinal direction of the width direction central part.
  • the inventors found that, as will be described in detail later in the manufacturing method of the hot-rolled coil, the variation in the grain size of precipitates in the width direction of the longitudinal center part of the hot-rolled coil and in the longitudinal direction of the width direction central part can be controlled within a predetermined range by appropriate cooling treatment in the cooling step after hot rolling and the cooling treatment in the subsequent coiling step.
  • the inventors discovered that by appropriately performing the cooling process after hot rolling and the cooling treatment in the coiling process, it is possible to control the difference between the maximum and minimum grain sizes of the precipitates in the width direction of the longitudinal center of the hot rolled coil and in the longitudinal direction of the width direction of the central part to 15.0% or less of the average grain size of the precipitates, and as a result, it is possible to significantly suppress or reduce the strength variation in the longitudinal and width directions of the hot rolled coil.
  • the grain size of the precipitates is on the order of nanometers, and observation of the precipitates requires a high-precision measuring device such as a transmission electron microscope (TEM) or a three-dimensional atom probe. Therefore, in order to reduce the strength variation over the entire length and width of the hot-rolled coil, for example, analyzing the grain size of the precipitates over the entire length and width of the hot-rolled coil and feeding it back to the manufacturing conditions requires a lot of time and cost, and is not necessarily realistic.
  • TEM transmission electron microscope
  • the grain size variation of the precipitates in the width direction of the central part of the longitudinal direction of the hot-rolled coil and in the longitudinal direction of the central part of the width direction can be controlled within a predetermined range as described above, and the strength variation in the longitudinal direction and width direction of the hot-rolled coil can be significantly suppressed or reduced.
  • the strength variation in the longitudinal direction and width direction of the hot-rolled coil is significantly suppressed or reduced as described above, so that the risk of forming defects occurring during press processing can be reduced, and productivity can be significantly improved.
  • the hot-rolled coil according to the embodiment of the present invention is particularly useful in the automotive field, but can also be used very effectively in other fields.
  • the term "hot-rolled coil” is not necessarily limited to a completely wound coil as shown in FIG. 1, but may be partially or completely unwound, and may also include a case where the coil has at least a partially plate-like shape (i.e., a hot-rolled steel sheet).
  • C is an element effective in increasing the strength of the steel plate.
  • the C content is set to 0.050% or more.
  • the C content may be 0.055% or more, 0.060% or more, 0.065% or more, or 0.070% or more.
  • the C content is set to 0.100% or less.
  • the C content may be 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.
  • Si is an element that is effective in increasing strength as a solid solution strengthening element.
  • the Si content is set to 0.01% or more.
  • the Si content may be 0.03% or more, 0.05% or more, 0.08% or more, 0.12% or more, or 0.15% or more.
  • the Si content is set to 0.30% or less.
  • the Si content may be 0.28% or less, 0.25% or less, 0.22% or less, or 0.20% or less.
  • Mn is an element that is effective in increasing strength as an element for hardenability and solid solution strengthening. In order to fully obtain these effects, the Mn content is set to 1.30% or more. The Mn content may be 1.40% or more, 1.50% or more, 1.60% or more, or 1.70% or more. On the other hand, if Mn is contained excessively, a large amount of MnS may be generated, which may reduce toughness. Therefore, the Mn content is set to 2.10% or less. The Mn content may be 2.00% or less, 1.90% or less, or 1.80% or less.
  • Ti is an element that precipitates finely in steel as Ti carbides such as TiC, and contributes to improving strength by precipitation strengthening.
  • the Ti content is set to 0.080% or more.
  • the Ti content may be 0.090% or more, 0.095% or more, 0.100% or more, 0.105% or more, or 0.110% or more.
  • the Ti content is set to 0.150% or less.
  • the Ti content may be 0.140% or less, 0.135% or less, 0.130% or less, 0.125% or less, or 0.120% or less.
  • Nb is an element that forms carbides, nitrides and/or carbonitrides in steel to refine crystal grains through a pinning effect, and contributes to high strength of steel plate through fine grain strengthening.
  • the Nb content is set to 0.020% or more.
  • the Nb content may be 0.025% or more, 0.028% or more, 0.030% or more, or 0.032% or more.
  • excessive Nb content may generate coarse carbides in the steel, reducing the toughness of the steel plate. Therefore, the Nb content is set to 0.050% or less.
  • the Nb content may be 0.045% or less, 0.042% or less, 0.040% or less, or 0.038% or less.
  • Al 0.001 to 0.050%
  • Al is an element that acts as a deoxidizer. In order to fully obtain such an effect, the Al content is set to 0.001% or more.
  • the Al content may be 0.010% or more, 0.020% or more, or 0.030% or more.
  • the Al content is set to 0.050% or less.
  • the Al content may be 0.045% or less, or 0.040% or less.
  • the P content is set to 0.100% or less.
  • the P content may be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% 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 0.0001% or more, 0.0005% or more, or 0.001% or more.
  • the Si content is set to 0.050% 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 cost. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.
  • N 0.0050% or less
  • N may form coarse nitrides and reduce toughness.
  • N may combine with Ti in the steel to form titanium nitride (TiN), thereby reducing the effective Ti amount capable of forming precipitates such as Ti carbides, and may reduce the strength improvement effect by precipitation strengthening. Therefore, the lower the N content, the more preferable it is, and it is set to 0.0050% or less.
  • the N content may be 0.0045% or less, 0.0040% or less, or 0.0035% 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 cost. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • the O content is set to 0.0050% or less.
  • the O content may be 0.0040% or less, 0.0035% or less, or 0.0030% 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, 0.0005% or more, or 0.0010% or more.
  • the hot-rolled coil may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.
  • B is an element that enhances the hardenability of steel and contributes to improving strength.
  • 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 0.0050% or less.
  • the B content may be 0.0030% or less, 0.0015% or less, 0.0012% or less, or 0.0008% or less.
  • Cu is an element that contributes to improving strength and/or corrosion resistance.
  • the Cu content may be 0%, but in order to obtain these effects, the Cu content is preferably 0.01% or more.
  • the Cu content may be 0.03% or more or 0.05% or more.
  • excessive Cu content may cause deterioration of toughness and weldability. Therefore, the Cu content is preferably 0.20% or less.
  • the Cu content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.
  • Ni is an element that enhances the hardenability of steel and contributes to improving strength and/or corrosion resistance.
  • the Ni content may be 0%, but in order to obtain these effects, the Ni content is preferably 0.01% or more.
  • the Ni content may be 0.03% or more or 0.05% or more.
  • the Ni content is preferably 0.20% or less.
  • the Ni content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% 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.
  • excessive Sn content may cause a decrease in toughness. Therefore, the Sn content is preferably 0.10% or less.
  • the Sn content may be 0.08% or less, 0.06% or less, or 0.04% or less.
  • Cr is an element that enhances the hardenability of steel and contributes to improving strength and/or corrosion resistance.
  • the Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.01% or more.
  • the Cr content may be 0.05% or more or 0.10% or more.
  • the Cr content is preferably 0.40% or less.
  • the Cr content may be 0.30% or less, 0.20% or less, 0.15% or less, or 0.12% or less.
  • Mo is an element that enhances the hardenability of steel, contributes to improving strength, and also contributes to improving corrosion resistance.
  • the Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more.
  • the Mo content may be 0.010% or more, 0.030% or more, or 0.050% or more.
  • the Mo content is preferably 0.200% or less.
  • the Mo content may be 0.180% or less, 0.150% or less, 0.120% or less, 0.100% or less, or 0.080% 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 0.005% or more, 0.010% or more, or 0.020% or more.
  • the V content is preferably 0.100% or less.
  • the V content may be 0.080% or less, 0.060% or less, or 0.040% or less.
  • the As content may be 0%, but in order to obtain such an effect, the As content is preferably 0.001% or more.
  • the As content may be 0.005% or more, 0.008% or more, or 0.010% or more.
  • the As content is preferably 0.100% or less.
  • the As content may be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.
  • Zr is an element capable of controlling the morphology of sulfides.
  • the Zr content may be 0%, but in order to obtain such an effect, the Zr content is preferably 0.001% or more.
  • the Zr content may be 0.005% or more or 0.010% or more.
  • the Zr content is preferably 0.100% or less.
  • the Zr content may be 0.050% or less, 0.030% or less, or 0.020% or less.
  • Ca is an element that can control the form of sulfides.
  • the Ca content may be 0%, but in order to obtain such an effect, the Ca content is preferably 0.0001% or more.
  • the Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the Ca content is preferably 0.0050% or less.
  • the Ca content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.
  • Mg is an element that can control the morphology of sulfides.
  • the Mg content may be 0%, but in order to obtain such an effect, the Mg content is preferably 0.001% or more, and may be 0.005% or more or 0.008% or more.
  • the Mg content is preferably 0.100% or less.
  • the Mg content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.
  • Bi is an element effective in improving corrosion resistance.
  • the Bi content may be 0%, but in order to obtain such an effect, the Bi content is preferably 0.001% or more.
  • the Bi content may be 0.002% or more or 0.003% or more.
  • the Bi content is preferably 0.020% or less.
  • the Bi content may be 0.010% or less, 0.008% or less, or 0.005% 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 0.03% or more or 0.05% or more.
  • the Co content is preferably 0.20% or less.
  • the Co content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% 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 0.03% or more or 0.05% or more.
  • excessive W content may reduce weldability. Therefore, the W content is preferably 0.20% or less.
  • the W content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% 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 0.03% or more or 0.05% or more.
  • the Zn content is preferably 0.20% or less.
  • the Zn content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.
  • REM 0 to 0.1000%
  • REM rare earth metal
  • 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 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • REM is a collective term for 17 elements, namely, scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.
  • 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 coils.
  • the chemical composition of the hot-rolled coil according to the embodiment of the present invention may be measured by a general analytical method.
  • the chemical composition of the hot-rolled coil 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 effective Ti amount corresponds to the amount of solid solution Ti just before aging treatment, that is, the value obtained by subtracting the amount of Ti fixed by N from the total amount of Ti in the hot-rolled coil.
  • TiN has a very low solubility, and therefore TiN once precipitated cannot be re-dissolved at a normal solution treatment temperature. Therefore, the amount of Ti effective for aging precipitation of precipitates such as Ti carbides is the value obtained by subtracting the amount that can be fixed as TiN from the total amount of Ti, and is calculated by the following formula.
  • Effective Ti content (%) [Ti] - 48/14 [N]
  • [Ti] and [N] are the contents (mass%) of each element in the hot rolled coil.
  • the effective Ti amount may be 0.075% or more, 0.080% or more, 0.085% or more, or 0.090% or more.
  • the upper limit is not particularly limited, but for example, the effective Ti amount may be 0.150% or less, 0.145% or less, 0.140% or less, or 0.135% or less.
  • the strength improvement effect due to fine grain strengthening can be fully exhibited, and in combination with the strength improvement effect due to precipitation strengthening based on precipitates such as Ti carbides, which will be described in detail later, it is possible to reliably achieve high tensile strength, for example, tensile strength of 780 MPa or more, in the finally obtained hot-rolled coil. If the average grain size of the crystal grains is less than 5.0 ⁇ m or more than 8.0 ⁇ m, the strength improvement effect due to such fine grain strengthening cannot be fully obtained, and the desired high strength cannot be achieved in the hot-rolled coil.
  • the average grain size of the crystal grains may be, for example, 5.5 ⁇ m or more, 6.0 ⁇ m or more, or 6.5 ⁇ m or more. Similarly, the average grain size of the crystal grains may be, for example, 7.5 ⁇ m or less, or 7.0 ⁇ m or less.
  • the average grain size of crystal grains surrounded by grain boundaries with an orientation difference of 15° or more is measured by electron backscattered diffraction (EBSD). More specifically, a sample is first taken from the center of the longitudinal direction and the center of the width direction of the hot-rolled coil so that the observation surface is a cross section of the plate thickness parallel to the rolling direction and perpendicular to the plate surface.
  • EBSD electron backscattered diffraction
  • an area of 200 ⁇ m in the rolling direction of the sample and 100 ⁇ m in the normal direction of the rolling surface is analyzed by EBSD analysis at a measurement interval of 0.2 ⁇ m to obtain crystal orientation information.
  • 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 region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a crystal grain, the circle equivalent diameter of the crystal grain is analyzed, and the average value is calculated and determined as the average grain size.
  • the grain size of the crystal grain defined as above can be determined using the area average value calculated by the grain size (diameter) in the function installed in the software "OIM Analysis (registered trademark) Version 7.0.1" attached to the EBSD analysis device.
  • the average grain size of the precipitates is controlled within a range of 3.0 to 9.5 nm in the central portion in the longitudinal direction.
  • the strength improvement effect by precipitation strengthening can be fully exerted, and in combination with the strength improvement effect by fine grain strengthening described above, it is possible to reliably achieve a high tensile strength, for example, a tensile strength of 780 MPa or more, in the finally obtained hot-rolled coil.
  • the precipitates may contain Ti carbides or may be Ti carbides.
  • the Ti carbides are not particularly limited, but may be, for example, TiC, or may be a composite carbide containing Ti and an element other than Ti, such as Nb. If the grain size of the precipitates is small, the precipitates cannot act sufficiently as an obstacle to dislocation motion, and therefore the strength improvement effect by precipitation strengthening cannot be fully obtained. On the other hand, if the grain size of the precipitates is too large, the desired precipitation strengthening may not be obtained in the same way.
  • the average grain size of the precipitates may be, for example, 4.0 nm or more, 5.0 nm or more, or 6.0 nm or more.
  • the average grain size of the crystal grains may be, for example, 9.0 nm or less, 8.0 nm or less, or 7.0 nm or less.
  • the difference between the maximum and minimum grain sizes of the precipitates in the width direction is 15.0% or less of the average grain size of the precipitates
  • the difference between the maximum and minimum grain size of the precipitate in the width direction of the longitudinal center portion is controlled to 15.0% or less of the average grain size of the precipitate.
  • the desired precipitation state of the precipitate cannot be reliably obtained even in the central portion due to the influence of the cooling.
  • the longitudinal front end, central portion, and tail end portions of the hot-rolled coil are appropriately cooled, if the cooling in the width direction is not performed uniformly, the precipitation state of the precipitate in the longitudinal direction is also affected due to the temperature deviation in the width direction.
  • the manufacturing method of the hot-rolled coil it is necessary to appropriately cool the hot-rolled coil in the longitudinal and width directions, so that the variation in grain size of the precipitates in the width direction of the longitudinal center of the hot-rolled coil can be controlled within a predetermined range, and more specifically, the difference between the maximum and minimum grain sizes of the precipitates in the width direction of the longitudinal center of the hot-rolled coil can be controlled to 15.0% or less of the average grain size of the precipitates. As a result, it becomes possible to significantly suppress or reduce the strength variation in the hot-rolled coil in the width direction.
  • the difference between the maximum and minimum grain size of the precipitate in the width direction at the longitudinal center of the hot-rolled coil is smaller than the average grain size of the precipitate. Therefore, it is preferable that the difference between the maximum and minimum grain size of the precipitate in the width direction at the longitudinal center of the hot-rolled coil is 12.0% or less, 10.0% or less, or 8.0% or less of the average grain size of the precipitate.
  • the lower limit is not particularly limited, and for example, the difference between the maximum and minimum grain size of the precipitate in the width direction at the longitudinal center of the hot-rolled coil may be 2.0% or more, 3.0% or more, or 5.0% or more of the average grain size of the precipitate.
  • the average grain size of the precipitates in the longitudinal center of the hot-rolled coil is determined as follows. First, when the total width of the hot-rolled coil is W, samples are taken by a replica method at each of the positions of 1/10W, 3/10W, 5/10W, 7/10W, and 9/10W from the end of the width direction in the longitudinal center of the hot-rolled coil. Next, 50 to 100 precipitates are observed in the taken samples using a transmission electron microscope (TEM, for example, "JEM-2100" manufactured by JEOL can be used), and the grain size of each precipitate is calculated as a circle equivalent diameter.
  • TEM transmission electron microscope
  • the average value of all the calculated circle equivalent diameters is determined as the grain size of the precipitates at each width direction position, and the arithmetic average of the five grain sizes obtained is determined as the average grain size of the precipitates in the longitudinal center. Finally, the difference between the maximum and minimum values of the five grain sizes obtained is calculated, and the calculated value is divided by the average grain size of the precipitates to determine the ratio of the difference between the maximum and minimum grain sizes of the precipitates in the width direction to the average grain size.
  • the constituent elements of the precipitates can be identified by EDS analysis.
  • the difference between the maximum and minimum grain sizes of precipitates in the longitudinal direction is 15.0% or less of the average grain size of precipitates
  • the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise central portion is controlled to 15.0% or less of the average grain size of the precipitates in the longitudinal direction.
  • the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise center of the hot-rolled coil is smaller than the average grain size of the precipitates in the longitudinal direction. Therefore, it is preferable that the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise center of the hot-rolled coil is 12.0% or less, 10.0% or less, or 8.0% or less of the average grain size of the precipitates in the longitudinal direction.
  • the lower limit is not particularly limited, but for example, the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise center of the hot-rolled coil may be 1.0% or more, 2.0% or more, or 3.0% or more of the average grain size of the precipitates in the longitudinal direction.
  • the average grain size of the precipitates in the longitudinal direction of the widthwise center of the hot-rolled coil is determined as follows. First, when the total length of the hot-rolled coil is L, samples are taken by the replica method at each of the positions of 1/10L, 5/10L, and 9/10L from the longitudinal end of the widthwise center of the hot-rolled coil.
  • the grain size of each precipitate is calculated as a circle equivalent diameter
  • the average value of all the calculated circle equivalent diameters is determined as the grain size of the precipitates at each longitudinal position
  • the arithmetic average of the three grain sizes obtained is determined as the average grain size of the precipitates in the longitudinal direction of the widthwise center.
  • the difference between the maximum and minimum values of the obtained three grain sizes is calculated, and the calculated value is divided by the average grain size of the precipitates in the longitudinal direction to determine the ratio of the difference between the maximum and minimum grain sizes of the precipitates in the longitudinal direction to the average grain size of the precipitates in the longitudinal direction.
  • the constituent elements of the precipitates can be identified by EDS analysis.
  • the metal structure of the hot rolled coil according to the embodiment of the present invention is not particularly limited, but may contain ferrite in an area percentage of 50% or more, for example.
  • the present invention aims to provide a hot rolled coil having high strength and reduced strength variation, and achieves the object by having a predetermined chemical composition, utilizing fine grain strengthening and precipitation strengthening, and appropriately controlling the grain size of precipitates that contribute to the precipitation strengthening in the width direction of the central part in the longitudinal direction of the hot rolled coil and in the longitudinal direction of the central part in the width direction. Therefore, it is clear that other features related to the metal structure are not essential technical features for achieving the object of the present invention.
  • the metal structure is constituted by a soft ferrite single phase structure, it is possible to reliably achieve a tensile strength of, for example, 780 MPa or more by satisfying the requirements related to the chemical composition, crystal grains, and precipitates described above.
  • the area ratio of ferrite may be, for example, 55% or more, 60% or more, 70% or more, 80% or more, or 90% or more.
  • the upper limit is not particularly limited, but the area ratio of ferrite may be, for example, 100% or less, or 95% or less.
  • the remaining structure basically includes structures harder than ferrite, such as martensite, bainite, pearlite, and retained austenite. Therefore, from the viewpoint of providing a high-strength hot-rolled coil, it is clear that the specific structure of the remaining structure is not limited.
  • the ferrite area ratio is determined as follows. First, a sample is taken having a cross section through the plate thickness parallel to the rolling direction of the hot-rolled coil and perpendicular to the plate surface, and this cross section is used as the observation surface. Next, a 100 ⁇ m x 100 ⁇ m region is observed within the range of 1/8 to 3/8 of the plate thickness, centered at the 1/4 position, in an electron channeling contrast image taken with an FE-SEM (field emission scanning electron microscope) to determine the area ratio. More specifically, within the above region, the part that appears with a uniform contrast is identified as ferrite, and the area ratio can be calculated by image analysis using the image analysis software Image J.
  • the hot rolled coil according to the embodiment of the present invention has the above-described chemical composition and metal structure, and thus can achieve high tensile strength, for example, 780 MPa or more.
  • the tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more.
  • the grain size of the precipitates can be appropriately controlled in the width direction of the longitudinal center part of the hot rolled coil and in the longitudinal direction of the width direction of the central part of the width direction, thereby significantly suppressing or reducing the strength variation in the longitudinal direction and width direction of the hot rolled coil.
  • the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot rolled coil may be 980 MPa or less, 950 MPa or less, or 900 MPa or less.
  • the tensile strength is determined as follows. First, a tensile test piece No. 5 of JIS Z2241:2011 is taken from the longitudinal center part and the width direction center part of the hot rolled coil, with the test direction being parallel to the rolling direction. Next, a tensile test in accordance with JIS Z2241:2011 is carried out using the tensile test specimen to determine the tensile strength of the hot-rolled coil according to the embodiment of the present invention.
  • the hot rolled coil according to the embodiment of the present invention may have any overall width W.
  • the overall width W may be 700 mm or more, 800 mm or more, 900 mm or more, or 1000 mm or more.
  • the overall width may be 2500 mm or less, 2200 mm or less, 2000 mm or less, 1800 mm or less, 1600 mm or less, 1500 mm or less, 1400 mm or less, or 1300 mm or less.
  • the hot rolled coil according to the embodiment of the present invention generally has a sheet thickness of 1.0 to 6.0 mm, although not particularly limited thereto.
  • the sheet 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, 4.0 mm or less, or 3.0 mm or less.
  • a method for producing a hot rolled coil according to an embodiment of the present invention includes the steps of: a hot rolling process comprising heating a slab having the chemical composition described above in relation to the hot rolled coil to a temperature of 1230-1260°C and subjecting it to rough rolling and finish rolling, wherein the rough rolling exit temperature is 1070-1140°C, the finish rolling entry temperature (F0) is 980-1050°C, the finish rolling exit temperature (FT) is 850-920°C, and the finish rolling total reduction is 85-95%; a cooling step in which the finish-rolled steel plate is primarily cooled in a temperature range from the finish-rolling exit temperature (FT) to a temperature T1 in a range of 650 to 720°C at an average cooling rate of 60 to 100°C/s, and then secondarily cooled in a temperature range from the temperature T1 to a coiling temperature CTf at a position 1/10L of the total coil length L in the longitudinal direction at an average cooling rate of 5 to 10°C/s, wherein the top
  • a slab having the chemical composition described above in relation to the hot-rolled coil is heated.
  • the slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method.
  • the slab used contains a relatively large amount of alloying elements, particularly Ti. For this reason, it is necessary to dissolve the alloying elements in the slab, and in particular, it is necessary to sufficiently dissolve Ti. If Ti is not sufficiently dissolved during slab heating, it becomes difficult to improve the strength of the steel by precipitation strengthening by finely precipitating Ti as carbide (TiC) or the like in the steel during the coiling process.
  • the heating temperature of the slab needs to be 1230°C or higher.
  • the heating temperature of the slab is set to 1260°C or lower.
  • the heated slab is subjected to rough rolling before finish rolling for plate thickness adjustment and the like.
  • the delivery temperature of the rough rolling is set to 1070 to 1140 ° C, preferably 1100 to 1140 ° C. If the delivery temperature of the rough rolling is less than 1070 ° C, it is difficult to obtain an delivery temperature of 850 ° C or more in the finish rolling following the rough rolling. In addition, if the delivery temperature of the rough rolling exceeds 1140 ° C, the crystal grains may become coarse, and the toughness of the obtained hot rolled coil may decrease.
  • the rough-rolled slab is then subjected to finish rolling.
  • the slab used contains a relatively large amount of alloy elements, it is necessary to increase the rolling load during hot rolling. For this reason, hot rolling is performed under high temperature and pressure, specifically, the entry temperature (F0) of the finish rolling is 980 to 1050 ° C, the exit temperature (FT) of the finish rolling is 850 to 920 ° C, and the total reduction rate of the finish rolling is 85 to 95%.
  • the exit temperature (FT) of the finish rolling is important in terms of controlling the metal structure of the steel sheet.
  • the exit temperature (FT) of the finish rolling is low, the metal structure may become non-uniform, the formability may decrease, and/or the average grain size of the crystal grains surrounded by grain boundaries with an orientation difference of 15 ° or more in the center of the longitudinal direction may become less than 5.0 ⁇ m, and the strength may decrease. For this reason, the exit temperature (FT) of the finish rolling is set to 850 ° C or more. On the other hand, if the exit temperature (FT) of the finish rolling exceeds 920°C, the austenite grains become coarse, and it becomes impossible to control the average grain size of the crystal grains obtained by the subsequent cooling, more specifically, the crystal grains surrounded by grain boundaries having an orientation difference of 15° or more, to 8.0 ⁇ m or less.
  • the finish-rolled steel sheet is first cooled at an average cooling rate of 60 to 100 ° C./s in a temperature range from the finish-rolling exit temperature (FT) to a temperature T1 in the range of 650 to 720 ° C.
  • FT finish-rolling exit temperature
  • T1 temperature in the range of 650 to 720 ° C.
  • the average cooling rate of the primary cooling is less than 60 ° C./s, it may not be possible to control the average grain size of crystal grains surrounded by grain boundaries with an orientation difference of 15 ° or more within a desired range.
  • the average cooling rate of the primary cooling is more than 100 ° C./s, it becomes difficult to uniformly cool the steel sheet in the width direction due to the high cooling rate, and uneven cooling (temperature deviation) occurs in the width direction.
  • the average cooling rate of the primary cooling is set to 60 to 100°C/s, and preferably 65 to 85°C/s.
  • the primary cooling in addition to controlling the average cooling rate, it is extremely important to cool the steel plate evenly on its upper and lower surfaces.
  • Such cooling is performed so that the upper and lower cooling ratio of the upper surface to the lower surface of the steel plate is 0.8 to 1.2, more specifically, the amount of cooling water sprayed on the upper surface of the steel plate is 0.8 to 1.2 times the amount of cooling water sprayed on the lower surface of the steel plate.
  • the variation in the grain size of precipitates such as TiC in the width direction of the finally obtained hot-rolled coil cannot be sufficiently suppressed, i.e., the difference between the maximum and minimum grain sizes of precipitates such as TiC in the width direction cannot be controlled to 15.0% or less of the average grain size of the precipitates.
  • the strength variation in the longitudinal direction and/or width direction of the hot-rolled coil cannot be sufficiently suppressed or reduced.
  • the above upper and lower cooling ratio does not mean the ratio of the amount of cooling water on the entire upper surface to the amount of cooling water on the entire lower surface in the FT to T1°C section. More specifically, in this manufacturing method, the FT to T1°C section is divided into sections every 10 m, and the upper and lower cooling ratios are calculated for each section from the amount of cooling water on the upper surface and the amount of cooling water on the lower surface, and the upper and lower cooling ratios of each section calculated in this way are all controlled within the range of 0.8 to 1.2.
  • each section has multiple cooling water nozzles arranged above and below the steel plate along the direction of travel of the steel plate, and by appropriately spraying these cooling water nozzles based on on/off control, it is relatively easy to control the upper/lower cooling ratio of each section within the range of 0.8 to 1.2.
  • the average cooling rate of the secondary cooling is less than 5 ° C./s, the crystal grains surrounded by grain boundaries with an orientation difference of 15° or more become coarse, and it becomes impossible to control the average grain size to 8.0 ⁇ m or less. In this case, the strength improvement effect due to fine grain strengthening cannot be sufficiently obtained, and therefore the desired high strength cannot be reliably achieved in the finally obtained hot rolled coil.
  • the average cooling rate of the secondary cooling exceeds 10°C/s, it may not be possible to properly generate crystal grains surrounded by grain boundaries with an orientation difference of 15° or more, or it may not be possible to control the average grain size of the crystal grains to 5.0 ⁇ m or more. In this case as well, the strength improvement effect of fine grain strengthening cannot be sufficiently obtained.
  • the average cooling rate of the secondary cooling exceeds 10°C/s, the occurrence of strength variations in the longitudinal direction and/or width direction of the hot rolled coil may become significant due to excessive generation of hard structures.
  • the X/10L position (X is a natural number from 1 to 9) in the total length L of the hot-rolled coil in the longitudinal direction means a position in the hot-rolled coil that is a distance "X/10L" away from the leading end in the longitudinal direction (rolling direction) toward the tail end in the longitudinal direction.
  • the "1/10L position" is a position that is a distance "100 m" away from the leading end in the longitudinal direction toward the tail end in the longitudinal direction.
  • the secondary cooling in addition to controlling the average cooling rate, it is extremely important to cool the steel plate evenly on its upper and lower surfaces.
  • this cooling is performed so that the upper and lower cooling ratio of the upper surface to the lower surface of the steel plate is 0.8 to 1.2, and more specifically, the amount of cooling water sprayed on the upper surface of the steel plate is 0.8 to 1.2 times the amount of cooling water sprayed on the lower surface of the steel plate.
  • the variation in the grain size of precipitates such as TiC in the width direction of the finally obtained hot-rolled coil cannot be sufficiently suppressed, i.e., the difference between the maximum and minimum grain sizes of precipitates such as TiC in the width direction cannot be controlled to 15.0% or less of the average grain size of the precipitates.
  • the strength variation in the longitudinal direction and/or width direction of the hot-rolled coil cannot be sufficiently suppressed or reduced.
  • the above upper and lower cooling ratio does not mean the ratio of the amount of cooling water on the entire upper surface to the amount of cooling water on the entire lower surface in the section from T1 to CTf°C. More specifically, in this manufacturing method, the section from T1 to CTf°C is divided into sections of 10 m each, and the upper and lower cooling ratios are calculated for each of these sections from the amount of cooling water on the upper surface and the amount of cooling water on the lower surface, and the upper and lower cooling ratios of each section calculated in this manner are all controlled within the range of 0.8 to 1.2.
  • each section has multiple cooling water nozzles arranged above and below the steel plate along the direction of travel of the steel plate, and by appropriately spraying these cooling water nozzles based on on/off control, it is relatively easy to control the upper/lower cooling ratio of each section within the range of 0.8 to 1.2.
  • the precipitation state of the precipitates is greatly affected by the cooling history after coiling.
  • the tip and tail of the hot-rolled coil in the longitudinal direction correspond to the innermost and outermost parts of the hot-rolled coil, respectively, and are therefore exposed to the atmosphere.
  • the tip and tail of the hot-rolled coil generally cool quickly, and the precipitation of the precipitates may not progress sufficiently. Therefore, the precipitation state is in a sub-aged state, and the strength tends to decrease. In this case, the desired strength may not be obtained, or the precipitation state of the precipitates may differ in the longitudinal direction, resulting in strength variations.
  • the central part of the hot-rolled coil in the longitudinal direction is not directly exposed to the atmosphere, so it is difficult to cool, and is maintained at a relatively high temperature. Therefore, in some cases, the precipitates may coarsen, and the peak aging at which a high effect of precipitation strengthening is obtained may be exceeded, resulting in an over-aged state, and the strength may also decrease. In this case, the desired strength may not be obtained, or strength variations may occur. In addition, if cooling in the width direction after coiling is not uniform, the temperature deviation in the width direction will affect the state of precipitation in the longitudinal direction. As a result, the final hot-rolled coil may not have the desired strength or may have significant variations in strength.
  • a heat retention treatment is performed on the edge portion in the width direction of the steel sheet after coiling, which is directly exposed to the atmosphere, so that the temperature deviation in the width direction of the hot-rolled coil after coiling can be reduced. If the edge portion is not heat-retained, the precipitation state of the precipitates in the longitudinal and width directions will also be affected due to the temperature deviation in the width direction. As a result, the average grain size of the precipitates in the longitudinal center portion and the grain size variation of the precipitates in the longitudinal center portion may not be controlled within the desired range.
  • Such heat retention treatment of the edge portion can be performed by any appropriate means known to those skilled in the art.
  • the heat retention treatment of the edge portion can be performed by arranging multiple hot-rolled coils adjacent to each other to prevent the edge portion from being cooled by the atmosphere.
  • "arranging multiple hot-rolled coils adjacent to each other” includes arranging the end faces (edge portions) of the hot-rolled coils in the width direction so that they face each other.
  • the diameters of multiple hot-rolled coils are the same, it is preferable to arrange them so that the central axes of each hot-rolled coil overlap, that is, to arrange them side by side on the same axis.
  • the distance between the end faces (edges) is preferably 200 to 800 mm, more preferably 200 to 600 mm, and even more preferably 200 to 500 mm.
  • the hot-rolled coil is also appropriately cooled in the longitudinal direction.
  • the coiling temperature CTm (°C) at a position 5/10L of the total length L of the coil in the longitudinal direction is controlled to satisfy the following formula 1. 550 ⁇ CTm ⁇ 620 ... Formula 1 If CTm is less than 550 ° C, the precipitates in the central part are in an underaged state, whereas if CTm is more than 620 ° C, the precipitates in the central part are in an overaged state.
  • the strength improvement effect due to precipitation strengthening may not be sufficiently obtained, and/or the strength variation in the longitudinal and width directions of the hot-rolled coil may not be sufficiently suppressed or reduced.
  • CTm satisfies formula 1
  • CTf does not satisfy formula 2
  • CTt does not satisfy formula 3
  • the precipitation state of the precipitates in the longitudinal center part is greatly affected. In this case, it may not be possible to control the average grain size of the precipitates in the longitudinal center part and the grain size variation in the width direction of the longitudinal center part within the desired range.
  • the coiling temperature CTf (°C) at 1/10L of the longitudinal coil total length L is controlled to a temperature range 15 to 30°C higher than CTm so as to satisfy the following formula 2.
  • Formula 2 Taking into account cooling by air, it is possible to maintain the precipitation state of the precipitates at the tip portion in the same precipitation state as that at the center portion by controlling CTf to a temperature range 15 to 30°C higher than CTm as shown in formula 2.
  • the precipitation state of the precipitates at the tip portion may be in an underaged state, while if CTf is more than CTm + 30°C, the precipitation state of the precipitates at the tip portion may be in an overaged state. In either case, the strength improvement effect due to precipitation strengthening may not be sufficiently obtained, and/or the strength variation in the longitudinal and transverse directions of the hot-rolled coil may not be sufficiently suppressed or reduced.
  • the tail end of the hot rolled coil in the longitudinal direction corresponds to the outermost peripheral portion of the hot rolled coil, and is therefore more easily cooled than the front end corresponding to the innermost peripheral portion. Therefore, the coiling temperature CTt (°C) at the 9/10L position with respect to the total length L of the coil in the longitudinal direction is controlled to a temperature range 30 to 50°C higher than CTm so as to satisfy the following formula 3.
  • CTm+30 ⁇ CTt ⁇ CTm+50 ... Formula 3 Taking into account cooling by air, it is possible to maintain the precipitation state of the precipitates in the tail end portion in the same precipitation state as that in the central portion by controlling CTm to a temperature range 30 to 50°C higher than CTm as shown in Equation 3.
  • the precipitation state of the precipitates in the tail end portion may be in an underaged state, whereas if CTt is more than CTm+50°C, the precipitation state of the precipitates in the tail end portion may be in an overaged state. In either case, the strength improvement effect by precipitation strengthening may not be sufficiently obtained, and/or the strength variation in the longitudinal and transverse directions of the hot-rolled coil may not be sufficiently suppressed or reduced.
  • the coiling temperature control based on the above formulas 1 to 3 can be performed by any appropriate means, and is not particularly limited. For example, it can be performed relatively easily by appropriately controlling the amount of cooling water in the secondary cooling.
  • it has been common to coil the steel sheet after hot rolling within a predetermined temperature range, no operation has been performed to change the coiling temperature control range at the tip, center, and tail end in the longitudinal direction of the hot rolled coil. Therefore, the inventors have now revealed for the first time that the strength variation due to precipitation strengthening can be significantly suppressed or reduced by making the cooling history of the tip, center, and tail end in the longitudinal direction of the hot rolled coil appropriate, and further by performing heat retention treatment in the width direction of the hot rolled coil.
  • the hot-rolled coil manufactured by the above manufacturing method can reliably achieve high tensile strength, for example tensile strength of 780 MPa or more, in the finally obtained hot-rolled coil based on a combination of fine grain strengthening by controlling the average grain size of crystal grains surrounded by grain boundaries with an orientation difference of 15° or more in the longitudinal center to within the range of 5.0 to 8.0 ⁇ m, and precipitation strengthening by controlling the average grain size of precipitates to within the range of 3.0 to 9.5 nm.
  • the difference between the maximum and minimum values of the grain size of the precipitates in the width direction of the longitudinal center of the hot-rolled coil and in the longitudinal direction of the width direction center can be controlled to 15.0% or less of the average grain size of the precipitates, and as a result, the strength variation in the longitudinal direction and width direction of the hot-rolled coil can be significantly suppressed or reduced. Therefore, the hot-rolled coil manufactured by the above manufacturing method can achieve a significantly suppressed or reduced strength variation despite its high strength. This reduces the risk of forming defects during press working of the steel sheet, and also significantly improves productivity. Therefore, the hot-rolled coil is of course particularly useful in the automotive field, but can also be used very effectively in other fields.
  • molten steel was cast by continuous casting to form slabs with various chemical compositions shown in Table 1. These slabs were heated under the conditions shown in Table 2, and then hot-rolled. Hot rolling was performed by rough rolling and finish rolling, and the rough rolling exit temperature and the finish rolling entry temperature (F0), exit temperature (FT), and total rolling reduction were as shown in Table 2.
  • the finish-rolled steel sheet was subjected to primary cooling under the conditions shown in Table 2 in a temperature range from the finish rolling exit temperature (FT) to a temperature T1 in the range of 650-720°C, and then secondary cooling in a temperature range from temperature T1 to a coiling temperature CTf at 1/10L of the total coil length L in the longitudinal direction.
  • the FT to T1°C section and the T1 to CTf°C section were each divided into 10m sections, and the top and bottom cooling ratios were calculated for each section from the amount of cooling water on the top surface and the amount of cooling water on the bottom surface. Cooling was performed so that the top and bottom cooling ratios of each section calculated in this way were controlled within a predetermined range.
  • the top and bottom cooling ratios in the primary and secondary cooling in Table 2 indicate the top and bottom cooling ratios of each section in the primary and secondary cooling that have the largest absolute difference from a cooling ratio of 1.
  • the secondary cooled steel sheet was coiled, and the coiling temperatures CTf (°C), CTm (°C), and CTt (°C) at the 1/10L, 5/10L, and 9/10L positions relative to the total length L of the coil in the longitudinal direction were as shown in Table 2.
  • the heat retention treatment of the edge portion after coiling was performed by arranging multiple hot rolled coils adjacent to each other. The distance between the edges of each adjacently arranged hot rolled coil was 300 mm.
  • the notation "adjacent" in Table 2 indicates that heat retention treatment was performed after coiling.
  • the notation "single” in Table 2 means that the hot-rolled coil was air-cooled alone, that is, the edge portion was not subjected to heat retention treatment after coiling.
  • the obtained hot-rolled coil had a sheet thickness of about 2.3 to 4.0 mm, a total width W of about 800 to 1500 mm, and a total length L of about 500 to 1200 m.
  • the properties of the obtained hot-rolled coil were measured and evaluated using the following methods.
  • tensile test pieces No. 5 of JIS Z2241:2011 were taken from the 1/10L, 5/10L, and 9/10L positions with respect to the total length L of the coil in the longitudinal direction in the width direction center of the hot-rolled coil, with the test direction being parallel to the rolling direction.
  • these tensile test pieces were used to perform a tensile test in accordance with JIS Z2241:2011 to obtain three tensile strength values, and then the difference between the maximum and minimum values was calculated, and the obtained value was determined as the value of the strength variation in the longitudinal direction.
  • the value of this strength variation was 15.0 MPa or less, the longitudinal strength variation was evaluated as passing, and when it exceeded 15.0 MPa, the longitudinal strength variation was evaluated as failing.
  • Hot-rolled coils with a tensile strength of 780 MPa or more and acceptable strength variations in both the longitudinal and transverse directions were evaluated as having high strength and reduced strength variations. The results are shown in Table 3.
  • Comparative Example 18 the winding temperatures CTf and CTt were low, so the variation in particle size of the precipitates in the width direction of the longitudinal center and the variation in particle size of the precipitates in the longitudinal direction of the width direction center could not be controlled within the desired range. As a result, the strength variation in the longitudinal direction became significant.
  • Comparative Example 19 the average cooling rate of the primary cooling was high, and the upper and lower cooling ratio of the primary cooling was not appropriate, so that the variation in the grain size of the precipitates in the width direction of the longitudinal center part could not be controlled within the desired range due to the occurrence of cooling unevenness. As a result, the strength variation in the width direction became significant.
  • Comparative Example 20 the Nb content was low, so the crystal grains were not refined sufficiently, and the strength improvement effect by fine grain strengthening could not be fully obtained. As a result, the desired tensile strength could not be achieved.
  • Comparative Example 21 it is considered that the austenite grains were coarsened because the exit temperature (FT) of the finish rolling was high. As a result, the average grain size of the crystal grains could not be sufficiently refined even by the subsequent cooling, and the desired tensile strength could not be achieved.
  • the coiling temperatures CTf and CTt were high, so that the variation in the grain size of the precipitates in the longitudinal direction of the width center part could not be controlled within the desired range. As a result, the strength variation in the longitudinal direction became significant.
  • Comparative Example 23 the coiling temperature CTm was low, so the average grain size of the precipitates in the longitudinal center was small, and the desired tensile strength could not be achieved.
  • Comparative Example 24 the exit temperature (FT) of the finishing rolling was low, so the average grain size of the crystal grains in the longitudinal center was small, and the desired tensile strength could not be achieved.
  • the average cooling rate of the primary cooling was low, so the average grain size of the crystal grains in the longitudinal center could not be controlled within the desired range, and the desired tensile strength could not be achieved.
  • the average cooling rate of the primary cooling was high, so the variation in grain size of the precipitates in the width direction of the longitudinal center could not be controlled within the desired range due to the occurrence of cooling unevenness. As a result, the strength variation in the width direction became significant.
  • Comparative Examples 27 and 28 the upper and lower cooling ratios of the primary cooling were not appropriate, so the variation in grain size of the precipitates in the width direction of the longitudinal center could not be controlled within the desired range due to the occurrence of cooling unevenness. As a result, the strength variation in the width direction became significant.
  • Comparative Example 29 because the coiling temperature CTm was high, the precipitates in the center of the longitudinal direction became coarse and the average grain size became large, and the desired tensile strength could not be achieved.
  • a tensile strength of 780 MPa or more could be achieved based on a combination of fine grain strengthening by controlling the average grain size of crystal grains surrounded by grain boundaries with an orientation difference of 15° or more in the longitudinal center to within the range of 5.0 to 8.0 ⁇ m, and precipitation strengthening by controlling the average grain size of precipitates to within the range of 3.0 to 9.5 nm.
  • the difference between the maximum and minimum grain sizes of precipitates in the width direction of the longitudinal center of the hot-rolled coil and in the longitudinal direction of the width direction center of the hot-rolled coil could be controlled to 15.0% or less of the average grain size of the precipitates ("Width direction variation of precipitate grain size" and "Longitudinal direction variation of precipitate grain size” in Table 3), and as a result, the strength variation in the longitudinal and width directions of the hot-rolled coil could be significantly suppressed or reduced.
  • the area ratio of ferrite was 90% or more in all hot-rolled coils according to the invention examples.

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Abstract

The present invention provides a hot-rolled coil characterized by having prescribed chemical composition, and by having a metal structure in which, at a center part in the length direction, when a crystal grain is defined as a region surrounded by a grain boundary having an orientation difference of not less than 15°, the average particle size of crystal grains is 5.0-8.0 μm, the average particle size of deposited material is 3.0-9.5 nm, the difference between the maximum value and minimum value of particle size of the deposited material in the width direction is not more than 15.0% of the average particle size of the deposited material, and at a center part in the width direction, the difference between the maximum value and the minimum value of the particle size of the deposited material in the length direction is not more than 15.0% of the average particle size of the deposited material in the length direction.

Description

熱延コイルHot rolled coil
　本発明は、熱延コイルに関する。 The present invention relates to hot-rolled coils.
　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及び成形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、加工性等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 and formability of the steel plates generally decrease. For this reason, when developing high-strength steel plates, it is important to increase strength while maintaining a certain level of workability, etc.
　これに関連して、例えば、特許文献１では、所定の組成と、面積率で９５％以上のフェライト相を有し、該フェライト相中に、微細炭化物が１．０×１０22個／ｍ3以上、セメンタイト粒子が１０個／１００００μｍ2以上析出し、前記微細炭化物の平均粒径が１０ｎｍ以下であり、板厚中央位置の硬さＨＶ1/2tと板厚１／４位置の硬さＨＶ1/4tまたは板厚３／４位置の硬さＨＶ3/4tとの差ΔＨＶ0.025が２０ＨＶ以下である組織を有することを特徴とする高強度熱延鋼板が記載されている。また、特許文献１では、上記の構成によれば、引張強さ：７８０ＭＰａ以上の高強度を有し、延性、穴拡げ性等の加工性に優れた高強度熱延鋼板を容易に製造でき、産業上格段の効果を奏すると記載されている。さらに、特許文献１では、Ｔｉは１０ｎｍ以下の微細な炭化物（Ｔｉ炭化物）の形成に寄与し、析出強化により所望の高強度を確保するとともに、所望の穴拡げ性を確保するために、Ｔｉ含有量を０．０７０～０．２２０％の範囲とすることが教示されている。 In this regard, for example, Patent Document 1 describes a high-strength hot-rolled steel sheet having a predetermined composition and a ferrite phase with an area ratio of 95% or more, in which fine carbides are precipitated in the ferrite phase at 1.0 x 1022 particles/ m3 or more, cementite particles are precipitated at 10 particles/10,000 μm2 or more, the average particle size of the fine carbides is 10 nm or less, and the difference ΔHV0.025 between the hardness HV1 /2t at the center position of the sheet thickness and the hardness HV1 / 4t at the 1/4 position of the sheet thickness or the hardness HV3/4t at the 3/4 position of the sheet thickness is 20 HV or less. Patent Document 1 also describes that according to the above configuration, a high-strength hot-rolled steel sheet having a high strength of tensile strength: 780 MPa or more and excellent workability such as ductility and hole expandability can be easily manufactured, which is an industrially significant effect. Furthermore, in Patent Document 1, Ti contributes to the formation of fine carbides (Ti carbides) having a size of 10 nm or less, and the desired high strength is ensured by precipitation strengthening. In order to ensure the desired hole expandability, the Ti content is set in the range of 0.070 to 0.220%.
　特許文献２では、所定の化学組成を有する鋼板であって、その平均結晶粒径が８μｍ以下であり、方位差が１５°以上の大角結晶粒界へのＣの偏析量が４～１５ａｔｏｍｓ／ｎｍ2であり、上記大角結晶粒界上の球相当径３ｎｍ以上のＴｉＣ析出物の個数が０．０１個／ｎｍ2未満であり、結晶粒内の球相当径０．８ｎｍ以上２ｎｍ以下のＴｉＣ析出物の個数密度が８×１０16個／ｃｍ3以上である結晶粒の比率が１０～７０％であることを特徴とする高強度熱延鋼板が記載されている。また、特許文献２では、上記の構成によれば、成形性および低温靭性に優れた引張強さが７４０ＭＰａ以上の高強度熱延鋼板を提供でき、産業上の貢献が極めて顕著であると記載されている。さらに、特許文献２では、結晶粒内のＴｉＣ析出物の球相当径を０．８ｎｍ以上２ｎｍ以下とすることで、析出強化を効率良く付与することができ、強度の上昇に有効であると教示されている。 Patent Document 2 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, an average grain size of 8 μm or less, a segregation amount of C to high-angle grain boundaries with an orientation difference of 15° or more of 4 to 15 atoms/nm 2 , the number of TiC precipitates with a sphere-equivalent diameter of 3 nm or more on the high-angle grain boundaries is less than 0.01/nm 2 , and the ratio of grains having a number density of TiC precipitates with a sphere-equivalent diameter of 0.8 nm to 2 nm or less in the grains of 8×10 16 /cm 3 or more is 10 to 70%. Patent Document 2 also describes that the above configuration can provide a high-strength hot-rolled steel sheet with a tensile strength of 740 MPa or more that is excellent in formability and low-temperature toughness, and that this is an extremely significant contribution to industry. Patent Document 2 also teaches that by setting the sphere-equivalent diameter of TiC precipitates in the grains to 0.8 nm to 2 nm, precipitation strengthening can be efficiently imparted, which is effective in increasing strength.
　特許文献３では、重量％で、Ｃ：０．０３～０．１５％、Ｍｎ≧０．２％、Ｎ≦０．０１％、Ｔｉ：０．０５～０．３５％を含み、かつＭｏ≦０．６％、Ｗ≦１．５％から選ばれる１種以上を含み、ＭｏおよびＷがそれぞれ単独で含まれる場合には、Ｍｏ≧０．１％、Ｗ≧０．２％であり、Ｅｘ．Ｃ＝Ｃ－｛Ｔｉ－Ｎ×（４８／１４）－Ｓ×（４８／３２）｝×（１２／４８）－Ｍｏ×（１２／９６）－Ｗ×（１２／１８４）の式で示すＥｘ．Ｃが０．０１５％以下であり、かつ、Ｍｎ≦１．７－３０×Ｅｘ．Ｃを満たし、実質的にフェライト組織に、Ｔｉと、ＭｏおよびＷのうち１種以上とを含む１０ｎｍ未満の析出物が分散してなることを特徴とする引張強度が５５０ＭＰａ以上の強度安定性に優れた高成形性高張力鋼板が記載されている。 In Patent Document 3, the alloy contains, by weight, C: 0.03-0.15%, Mn≧0.2%, N≦0.01%, Ti: 0.05-0.35%, and one or more selected from Mo≦0.6% and W≦1.5%, and when Mo and W are each contained alone, Mo≧0.1% and W≧0.2%, and Ex. C, as shown by the formula Ex. C=C-{Ti-N×(48/14)-S×(48/32)}×(12/48)-Mo×(12/96)-W×(12/184), is 0.015% or less, and Mn≦1.7-30×Ex. The paper describes a highly formable, high-tensile steel sheet with excellent strength stability and a tensile strength of 550 MPa or more, characterized in that the steel satisfies the above C requirements and has a ferritic structure in which precipitates smaller than 10 nm containing Ti and one or more of Mo and W are dispersed.
特開２０１５－０６３７４８号公報JP 2015-063748 A 特開２０１５－２１８３５２号公報JP 2015-218352 A 特開２００３－３２１７３５号公報JP 2003-321735 A
　特許文献１及び２では、加工性等を改善しつつ、７００ＭＰａを超える高強度を達成するために、上記のとおりＴｉ炭化物による析出強化の利用が教示されている。一方で、析出強化を利用した鋼材の高強度化においては、例えば、熱延コイルの長手方向や幅方向において析出物の析出状態が異なることに起因して強度のばらつきが生じることがある。 Patent Documents 1 and 2 teach the use of precipitation strengthening by Ti carbide as described above in order to achieve high strength exceeding 700 MPa while improving workability, etc. On the other hand, when using precipitation strengthening to increase the strength of steel materials, for example, variations in strength may occur due to differences in the precipitation state of precipitates in the longitudinal and transverse directions of the hot-rolled coil.
　これに関連して、特許文献３では、フェライト単相組織を、Ｔｉと、ＭｏおよびＷの１種以上とを含む微細析出物で強化した鋼において、Ｔｉ、Ｍｏ、Ｗと結合しないＣであるＥｘ．Ｃが０．０１５％以下で、かつ、Ｍｎを０．２≦Ｍｎ１．７－３０×Ｅｘ．Ｃとすることによりコイル内の長手方向の材質変動、特に強度変動が低減されると教示されている。しかしながら、特許文献３では、上記のように、主に鋼板の化学組成の観点から強度変動の低減について検討されているものの、上記微細析出物の鋼板中における析出状態を適切なものとする観点からは必ずしも十分な検討はなされていない。したがって、特許文献３に記載の発明においては、強度ばらつきの抑制に関して依然として改善の余地があった。 In this regard, Patent Document 3 teaches that in a steel in which a single-phase ferritic structure is strengthened with fine precipitates containing Ti and one or more of Mo and W, the material variation in the longitudinal direction within the coil, particularly the strength variation, can be reduced by setting Ex. C, which is C that does not bond with Ti, Mo, or W, to 0.015% or less and Mn to 0.2≦Mn1.7-30×Ex. C. However, as described above, Patent Document 3 mainly considers the reduction of strength variation from the perspective of the chemical composition of the steel sheet, but does not necessarily fully consider the precipitation state of the fine precipitates in the steel sheet as being appropriate. Therefore, the invention described in Patent Document 3 still has room for improvement in terms of suppressing strength variation.
　本発明は、このような実情に鑑みてなされたものであり、その目的とするところは、新規な構成により、高強度でかつ強度ばらつきが低減された熱延コイルを提供することにある。 The present invention was made in consideration of these circumstances, and its purpose is to provide a hot-rolled coil with high strength and reduced strength variation through a new configuration.
　本発明者らは、上記目的を達成するために検討を行った結果、細粒強化と析出強化を利用することで高強度化を達成するとともに、当該析出強化に寄与する析出物の粒径を熱延コイルの長手方向中央部の幅方向及び幅方向中央部の長手方向において適切に制御することで、熱延コイルの長手方向及び幅方向における強度ばらつきを顕著に抑制又は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。 As a result of investigations conducted by the inventors to achieve the above-mentioned objective, they discovered that it is possible to achieve high strength by utilizing fine grain strengthening and precipitation strengthening, and that it is possible to significantly suppress or reduce the strength variation in the longitudinal and lateral directions of the hot-rolled coil by appropriately controlling the grain size of the precipitates that contribute to the precipitation strengthening in the width direction of the longitudinal center of the hot-rolled coil and in the longitudinal direction of the width direction of the hot-rolled coil, thereby 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：０．０５０～０．１００％、
　Ｓｉ：０．０１～０．３０％、
　Ｍｎ：１．３０～２．１０％、
　Ｔｉ：０．０８０～０．１５０％、
　Ｎｂ：０．０２０～０．０５０％、
　Ａｌ：０．００１～０．０５０％、
　Ｐ：０．１００％以下、
　Ｓ：０．０５０％以下、
　Ｎ：０．００５０％以下、
　Ｏ：０．００５０％以下、
　Ｂ：０～０．００５０％、
　Ｃｕ：０～０．２０％、
　Ｎｉ：０～０．２０％、
　Ｓｎ：０～０．１０％、
　Ｃｒ：０～０．４０％、
　Ｍｏ：０～０．２００％、
　Ｖ：０～０．１００％、
　Ａｓ：０～０．１００％、
　Ｚｒ：０～０．１００％、
　Ｃａ：０～０．００５０％、
　Ｍｇ：０～０．１００％、
　Ｂｉ：０～０．０２０％、
　Ｃｏ：０～０．２０％、
　Ｗ：０～０．２０％、
　Ｚｎ：０～０．２０％、
　ＲＥＭ：０～０．１０００％、並びに
　残部：Ｆｅ及び不純物からなる化学組成を有し、
　長手方向の中央部において、
　方位差が１５°以上の粒界によって囲まれた領域を結晶粒と定義した場合に、前記結晶粒の平均粒径が５．０～８．０μｍであり、
　析出物の平均粒径が３．０～９．５ｎｍであり、
　幅方向における前記析出物の粒径の最大値と最小値の差が前記析出物の平均粒径の１５．０％以下であり、
　幅方向の中央部において、長手方向における前記析出物の粒径の最大値と最小値の差が長手方向における前記析出物の平均粒径の１５．０％以下である金属組織を有することを特徴とする、熱延コイル。
　（２）前記化学組成が、質量％で、
　Ｂ：０．０００１～０．００５０％、
　Ｃｕ：０．０１～０．２０％、
　Ｎｉ：０．０１～０．２０％、
　Ｓｎ：０．０１～０．１０％、
　Ｃｒ：０．０１～０．４０％、
　Ｍｏ：０．００１～０．２００％、
　Ｖ：０．００１～０．１００％、
　Ａｓ：０．００１～０．１００％、
　Ｚｒ：０．００１～０．１００％、
　Ｃａ：０．０００１～０．００５０％、
　Ｍｇ：０．００１～０．１００％、
　Ｂｉ：０．００１～０．０２０％、
　Ｃｏ：０．０１～０．２０％、
　Ｗ：０．０１～０．２０％、
　Ｚｎ：０．０１～０．２０％、及び
　ＲＥＭ：０．０００１～０．１０００％
のうち少なくとも１種を含むことを特徴とする、上記（１）に記載の熱延コイル。
　（３）０．０７０％以上の有効Ｔｉ量を有することを特徴とする、上記（１）又は（２）に記載の熱延コイル。 The present invention, which has achieved the above object, is as follows.
(1) In mass%,
C: 0.050 to 0.100%,
Si: 0.01 to 0.30%,
Mn: 1.30 to 2.10%,
Ti: 0.080 to 0.150%,
Nb: 0.020 to 0.050%,
Al: 0.001 to 0.050%,
P: 0.100% or less,
S: 0.050% or less,
N: 0.0050% or less,
O: 0.0050% or less,
B: 0 to 0.0050%,
Cu: 0 to 0.20%,
Ni: 0 to 0.20%,
Sn: 0 to 0.10%,
Cr: 0 to 0.40%,
Mo: 0 to 0.200%,
V: 0 to 0.100%,
As: 0 to 0.100%,
Zr: 0 to 0.100%,
Ca: 0 to 0.0050%,
Mg: 0 to 0.100%,
Bi: 0 to 0.020%,
Co: 0 to 0.20%,
W: 0 to 0.20%,
Zn: 0 to 0.20%,
REM: 0 to 0.1000%, and the balance: Fe and impurities;
At the center in the longitudinal direction,
When a region surrounded by a grain boundary having an orientation difference of 15° or more is defined as a crystal grain, the average grain size of the crystal grain is 5.0 to 8.0 μm,
The average grain size of the precipitates is 3.0 to 9.5 nm;
a difference between a maximum value and a minimum value of a grain size of the precipitate in the width direction is 15.0% or less of an average grain size of the precipitate,
A hot-rolled coil, characterized in that it has a metal structure in a widthwise central portion, in which the difference between the maximum and minimum values of grain size of the precipitates in the longitudinal direction is 15.0% or less of the average grain size of the precipitates in the longitudinal direction.
(2) The chemical composition is, in mass%,
B: 0.0001 to 0.0050%,
Cu: 0.01 to 0.20%,
Ni: 0.01 to 0.20%,
Sn: 0.01 to 0.10%,
Cr: 0.01 to 0.40%,
Mo: 0.001 to 0.200%,
V: 0.001 to 0.100%,
As: 0.001 to 0.100%,
Zr: 0.001 to 0.100%,
Ca: 0.0001 to 0.0050%,
Mg: 0.001 to 0.100%,
Bi: 0.001 to 0.020%,
Co: 0.01 to 0.20%,
W: 0.01 to 0.20%,
Zn: 0.01 to 0.20%, and REM: 0.0001 to 0.1000%
The hot-rolled coil according to the above (1), characterized in that it contains at least one of the following:
(3) The hot-rolled coil according to (1) or (2) above, characterized in that it has an effective Ti content of 0.070% or more.
　本発明によれば、高強度でかつ強度ばらつきが低減された熱延コイルを提供することができる。 The present invention makes it possible to provide hot-rolled coils that are high in strength and have reduced strength variation.
巻き取られた状態の熱延コイルを示す模式図である。FIG. 2 is a schematic diagram showing a hot-rolled coil in a wound state.
＜熱延コイル＞
　本発明の実施形態に係る熱延コイルは、質量％で、
　Ｃ：０．０５０～０．１００％、
　Ｓｉ：０．０１～０．３０％、
　Ｍｎ：１．３０～２．１０％、
　Ｔｉ：０．０８０～０．１５０％、
　Ｎｂ：０．０２０～０．０５０％、
　Ａｌ：０．００１～０．０５０％、
　Ｐ：０．１００％以下、
　Ｓ：０．０５０％以下、
　Ｎ：０．００５０％以下、
　Ｏ：０．００５０％以下、
　Ｂ：０～０．００５０％、
　Ｃｕ：０～０．２０％、
　Ｎｉ：０～０．２０％、
　Ｓｎ：０～０．１０％、
　Ｃｒ：０～０．４０％、
　Ｍｏ：０～０．２００％、
　Ｖ：０～０．１００％、
　Ａｓ：０～０．１００％、
　Ｚｒ：０～０．１００％、
　Ｃａ：０～０．００５０％、
　Ｍｇ：０～０．１００％、
　Ｂｉ：０～０．０２０％、
　Ｃｏ：０～０．２０％、
　Ｗ：０～０．２０％、
　Ｚｎ：０～０．２０％、
　ＲＥＭ：０～０．１０００％、並びに
　残部：Ｆｅ及び不純物からなる化学組成を有し、
　長手方向の中央部において、
　方位差が１５°以上の粒界によって囲まれた領域を結晶粒と定義した場合に、前記結晶粒の平均粒径が５．０～８．０μｍであり、
　析出物の平均粒径が３．０～９．５ｎｍであり、
　幅方向における前記析出物の粒径の最大値と最小値の差が前記析出物の平均粒径の１５．０％以下であり、
　幅方向の中央部において、長手方向における前記析出物の粒径の最大値と最小値の差が長手方向における前記析出物の平均粒径の１５．０％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 coil>
The hot rolled coil according to the embodiment of the present invention has, in mass%,
C: 0.050 to 0.100%,
Si: 0.01 to 0.30%,
Mn: 1.30 to 2.10%,
Ti: 0.080 to 0.150%,
Nb: 0.020 to 0.050%,
Al: 0.001 to 0.050%,
P: 0.100% or less,
S: 0.050% or less,
N: 0.0050% or less,
O: 0.0050% or less,
B: 0 to 0.0050%,
Cu: 0 to 0.20%,
Ni: 0 to 0.20%,
Sn: 0 to 0.10%,
Cr: 0 to 0.40%,
Mo: 0 to 0.200%,
V: 0 to 0.100%,
As: 0 to 0.100%,
Zr: 0 to 0.100%,
Ca: 0 to 0.0050%,
Mg: 0 to 0.100%,
Bi: 0 to 0.020%,
Co: 0 to 0.20%,
W: 0 to 0.20%,
Zn: 0 to 0.20%,
REM: 0 to 0.1000%, and the balance: Fe and impurities;
At the center in the longitudinal direction,
When a region surrounded by a grain boundary having an orientation difference of 15° or more is defined as a crystal grain, the average grain size of the crystal grain is 5.0 to 8.0 μm,
The average grain size of the precipitates is 3.0 to 9.5 nm;
a difference between a maximum value and a minimum value of a grain size of the precipitate in the width direction is 15.0% or less of an average grain size of the precipitate,
The steel sheet is characterized in that it has a metal structure in which, at the center in the width direction, the difference between the maximum and minimum grain sizes of the precipitates in the longitudinal direction is 15.0% or less of the average grain size of the precipitates in the longitudinal direction.
　先に述べたとおり、析出強化を利用した鋼材の高強度化においては、例えば、熱延コイルの長手方向や幅方向において析出物の析出状態が異なることに起因して強度のばらつきが生じることがある。これは、巻き取り後の冷却速度が熱延コイル内で異なることにより、熱延コイルの長手方向及び／又は幅方向において析出物の析出状態が変化するためと考えられる。熱延コイルの長手方向における先端部及び尾端部は、それぞれ熱延コイルの最内周部及び最外周部に対応し、それゆえ大気にさらされる。このため、一般的に、熱延コイルの先端部及び尾端部は冷却が速く、析出物の析出が十分に進行しない場合がある。したがって、析出状態としては亜時効状態となり、強度が低下しやすい傾向にある。熱延コイルの長手方向における先端部及び尾端部で強度の低下が生じると、それらを切断して所望の強度を有する範囲のみを製品として利用することとなるため、歩留まり落ちとなり、生産性が低下してしまう。また、熱延コイルがその長手方向における先端部及び尾端部で所望の強度を有する場合であっても、析出物の析出状態が長手方向において異なることで強度のばらつきが生じると、例えば、プレス加工の際に割れなどの成形不良が発生しやすくなるという問題がある。一方で、熱延コイルの長手方向における中央部は大気に直接さらされていないために冷えにくく、比較的高温の状態で保持される。したがって、場合によっては析出物が粗大化し、析出強化の高い効果が得られるピーク時効を過ぎて過時効状態となり、同様に強度が低下することがある。この場合も同様に、所望の強度が得られなかったり、強度のばらつきが生じたりして、生産性の低下やプレス加工時の成形不良を引き起こすことがある。 As mentioned above, when strengthening steel materials using precipitation strengthening, for example, strength variations may occur due to differences in the precipitation state of precipitates in the longitudinal and/or transverse directions of the hot-rolled coil. This is thought to be because the cooling rate after coiling is different within the hot-rolled coil, which changes the precipitation state of precipitates in the longitudinal and/or transverse directions of the hot-rolled coil. The front and rear ends of the hot-rolled coil in the longitudinal direction correspond to the innermost and outermost parts of the hot-rolled coil, respectively, and are therefore exposed to the atmosphere. For this reason, the front and rear ends of the hot-rolled coil generally cool quickly, and the precipitation of precipitates may not progress sufficiently. Therefore, the precipitation state is in a sub-aged state, and the strength tends to decrease easily. If the strength of the front and rear ends of the hot-rolled coil in the longitudinal direction decreases, they are cut and only the range with the desired strength is used as a product, resulting in a drop in yield and a decrease in productivity. Furthermore, even if the hot-rolled coil has the desired strength at its leading and trailing ends in the longitudinal direction, if the precipitation state of the precipitates differs in the longitudinal direction, causing variations in strength, there is a problem that, for example, forming defects such as cracks are likely to occur during press processing. On the other hand, the central part of the hot-rolled coil in the longitudinal direction is not directly exposed to the atmosphere, so it is difficult to cool and is maintained at a relatively high temperature. Therefore, in some cases, the precipitates may become coarse, and the peak aging at which a high effect of precipitation strengthening is obtained may be exceeded, resulting in an overaging state, and similarly a decrease in strength. In this case as well, the desired strength may not be obtained or variations in strength may occur, causing a decrease in productivity and forming defects during press processing.
　加えて、熱延コイルの長手方向及び幅方向における析出物の析出状態は、相互に密接に関係していると考えられる。このため、例えば、巻き取り後の熱延コイルの長手方向における中央部のみを適切に冷却したとしても、先端部及び／又は尾端部を適切に冷却していない場合には、それらの影響を受けて中央部においても析出物の所望の析出状態を確実に得ることができなくなる。同様に、例えば、巻き取り後の熱延コイルの長手方向における先端部、中央部及び尾端部で適切な冷却を行ったとしても、幅方向における冷却、より具体的には熱間圧延後から巻き取り前までの幅方向における冷却や、巻き取り後の幅方向における冷却が均一に行われない場合には、幅方向の温度偏差に起因して長手方向における析出物の析出状態も影響を受けてしまう。その結果として、同様に最終的に得られる熱延コイルにおいて所望の強度が得られなかったり、強度のばらつきが顕著となってしまったりすることがある。このように析出物の析出状態は温度履歴によって熱延コイルの全長（熱延コイルの圧延方向における全体長さ）及び全幅（熱延コイルの幅方向における全体長さ）で変化するため、当該析出物による析出強化を有効に活用して所望の強度を達成しつつ、熱延コイルの長手方向及び幅方向における強度のばらつきを抑制又は低減することは一般に非常に困難である。 In addition, the precipitation state of the precipitates in the longitudinal direction and the width direction of the hot-rolled coil is considered to be closely related to each other. For this reason, for example, even if only the central part in the longitudinal direction of the hot-rolled coil after coiling is appropriately cooled, if the front end and/or the tail end are not appropriately cooled, the desired precipitation state of the precipitates cannot be reliably obtained even in the central part due to the influence of the above. Similarly, even if the front end, central part and tail end of the hot-rolled coil in the longitudinal direction after coiling are appropriately cooled, for example, if the cooling in the width direction, more specifically, the cooling in the width direction from after hot rolling to before coiling, or the cooling in the width direction after coiling is not performed uniformly, the precipitation state of the precipitates in the longitudinal direction will also be affected due to the temperature deviation in the width direction. As a result, the desired strength may not be obtained in the hot-rolled coil obtained in the end, or the strength may vary significantly. As such, the precipitation state of the precipitates changes over the entire length (total length of the hot-rolled coil in the rolling direction) and width (total length of the hot-rolled coil in the width direction) of the hot-rolled coil depending on the temperature history. Therefore, it is generally very difficult to effectively utilize the precipitation strengthening caused by the precipitates to achieve the desired strength while suppressing or reducing the variation in strength in the longitudinal and width directions of the hot-rolled coil.
　ここで、本明細書において熱延コイルに関連して長手方向という場合には、「長手方向」とは、図１に示すとおり「圧延方向」を意味する。同様に、本明細書において熱延コイルに関連して幅方向という場合には、「幅方向」とは、図１に示すとおり「圧延方向及び板厚方向に直交する方向」を意味する。 Here, in this specification, when referring to the longitudinal direction in relation to a hot-rolled coil, the "longitudinal direction" means the "rolling direction" as shown in Figure 1. Similarly, in this specification, when referring to the width direction in relation to a hot-rolled coil, the "width direction" means the "direction perpendicular to the rolling direction and the plate thickness direction" as shown in Figure 1.
　そこで、本発明者らは、熱延コイルの化学組成を適切なものとすることに加えて、特に当該熱延コイルの金属組織に着目して検討を行った。より詳しく説明すると、まず、本発明者らは、Ｔｉ炭化物等の析出物による析出強化に加えて、Ｎｂ等の添加による細粒強化を利用することで所望の高強度を達成することができることを見出した。より具体的には、本発明者らは、熱延コイルの長手方向中央部において、Ｔｉ炭化物等の析出物の平均粒径を３．０～９．５ｎｍの範囲内に制御することで析出強化による強度向上効果を十分に発揮させるとともに、Ｎｂ等の添加に起因して結晶粒の平均粒径を５．０～８．０μｍの範囲内に制御することで細粒強化による強度向上効果を付加し、その結果として高強度、例えば引張強さが７８０ＭＰａ以上の高強度を確実に達成することができることを見出した。加えて、本発明者らは、上記のとおり、熱延コイルの長手方向及び幅方向における析出物の析出状態は相互に密接に関係していると考えられることから、特に熱延コイルの長手方向中央部の幅方向及び幅方向中央部の長手方向における析出物の析出状態に着目してさらに検討を行った。その結果、本発明者らは、熱延コイルの製造方法について後で詳しく説明されるように、熱間圧延後の冷却工程における冷却処理、並びにその後の巻取工程における冷却処理を適切なものとすることで、熱延コイルの長手方向中央部の幅方向及び幅方向中央部の長手方向における析出物の粒径のばらつきを所定の範囲内に制御することができることを見出した。より詳しくは、本発明者らは、熱間圧延後の冷却工程及び巻取工程における冷却処理を適切なものとすることで、熱延コイルの長手方向中央部の幅方向及び幅方向中央部の長手方向における析出物の粒径の最大値と最小値の差を当該析出物の平均粒径の１５．０％以下に制御することができ、その結果として熱延コイルの長手方向及び幅方向における強度ばらつきを顕著に抑制又は低減することができることを見出した。 The inventors therefore conducted a study focusing on the metal structure of the hot-rolled coil in addition to making the chemical composition of the hot-rolled coil appropriate. To explain in more detail, first, the inventors discovered that the desired high strength can be achieved by utilizing fine grain strengthening by adding Nb and the like in addition to precipitation strengthening by precipitates such as Ti carbides. More specifically, the inventors discovered that the strength improving effect by precipitation strengthening can be fully exerted by controlling the average grain size of precipitates such as Ti carbides in the longitudinal center portion of the hot-rolled coil to within a range of 3.0 to 9.5 nm, and that the strength improving effect by fine grain strengthening can be added by controlling the average grain size of crystal grains due to the addition of Nb and the like to within a range of 5.0 to 8.0 μm, and as a result, high strength, for example, a high strength of tensile strength of 780 MPa or more can be reliably achieved. In addition, since the precipitation states of precipitates in the longitudinal direction and the width direction of the hot-rolled coil are considered to be closely related to each other as described above, the inventors conducted further studies focusing particularly on the precipitation states of precipitates in the width direction of the longitudinal center part of the hot-rolled coil and in the longitudinal direction of the width direction central part. As a result, the inventors found that, as will be described in detail later in the manufacturing method of the hot-rolled coil, the variation in the grain size of precipitates in the width direction of the longitudinal center part of the hot-rolled coil and in the longitudinal direction of the width direction central part can be controlled within a predetermined range by appropriate cooling treatment in the cooling step after hot rolling and the cooling treatment in the subsequent coiling step. More specifically, the inventors discovered that by appropriately performing the cooling process after hot rolling and the cooling treatment in the coiling process, it is possible to control the difference between the maximum and minimum grain sizes of the precipitates in the width direction of the longitudinal center of the hot rolled coil and in the longitudinal direction of the width direction of the central part to 15.0% or less of the average grain size of the precipitates, and as a result, it is possible to significantly suppress or reduce the strength variation in the longitudinal and width directions of the hot rolled coil.
　一般的に、析出物の粒径はナノオーダーであり、その観察には透過型電子顕微鏡（ＴＥＭ）や三次元アトムプローブ等の高精度な測定装置が必要である。したがって、熱延コイルの全長及び全幅にわたって強度ばらつきを低減するために、例えば、熱延コイルの全長及び全幅にわたる析出物の粒径を分析し、それを製造条件にフィードバックすることは多大な時間とコストを要し必ずしも現実的ではない。したがって、熱間圧延後の冷却工程及び巻取工程における冷却処理を適切なものとすることで、上記のように熱延コイルの長手方向中央部の幅方向及び幅方向中央部の長手方向における析出物の粒径のばらつきを所定の範囲内に制御することができ、それによって熱延コイルの長手方向及び幅方向における強度ばらつきを顕著に抑制又は低減することができるという事実は、極めて意外であり、また驚くべきことである。また、本発明の実施形態に係る熱延コイルによれば、上記のとおり熱延コイルの長手方向及び幅方向における強度ばらつきが顕著に抑制又は低減されているため、プレス加工時に成形不良が発生するリスクを低減することができ、生産性も顕著に向上させることが可能となる。したがって、本発明の実施形態に係る熱延コイルは、自動車分野の使用において特に有用であることは当然ながら、他の分野においても非常に有効に使用することが可能である。本明細書において「熱延コイル」とは、図１に示されるように完全に巻き取られたコイル状のものには必ずしも限定されず、例えば、部分的に又は完全に巻き戻されていてもよく、少なくとも部分的に板状の形状（すなわち熱延鋼板）を含む場合も包含するものである。 Generally, the grain size of the precipitates is on the order of nanometers, and observation of the precipitates requires a high-precision measuring device such as a transmission electron microscope (TEM) or a three-dimensional atom probe. Therefore, in order to reduce the strength variation over the entire length and width of the hot-rolled coil, for example, analyzing the grain size of the precipitates over the entire length and width of the hot-rolled coil and feeding it back to the manufacturing conditions requires a lot of time and cost, and is not necessarily realistic. Therefore, by appropriately performing the cooling process in the cooling process and the winding process after hot rolling, the grain size variation of the precipitates in the width direction of the central part of the longitudinal direction of the hot-rolled coil and in the longitudinal direction of the central part of the width direction can be controlled within a predetermined range as described above, and the strength variation in the longitudinal direction and width direction of the hot-rolled coil can be significantly suppressed or reduced. This is extremely unexpected and surprising. In addition, according to the hot-rolled coil according to the embodiment of the present invention, the strength variation in the longitudinal direction and width direction of the hot-rolled coil is significantly suppressed or reduced as described above, so that the risk of forming defects occurring during press processing can be reduced, and productivity can be significantly improved. Therefore, the hot-rolled coil according to the embodiment of the present invention is particularly useful in the automotive field, but can also be used very effectively in other fields. In this specification, the term "hot-rolled coil" is not necessarily limited to a completely wound coil as shown in FIG. 1, but may be partially or completely unwound, and may also include a case where the coil has at least a partially plate-like shape (i.e., a hot-rolled steel sheet).
　以下、本発明の実施形態に係る熱延コイル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 coil 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：０．０５０～０．１００％］
　Ｃは、鋼板の強度を高めるの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、Ｃ含有量は０．０５０％以上とする。Ｃ含有量は０．０５５％以上、０．０６０％以上、０．０６５％以上又は０．０７０％以上であってもよい。一方で、Ｃ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、Ｃ含有量は０．１００％以下とする。Ｃ含有量は０．０９５％以下、０．０９０％以下、０．０８５％以下又は０．０８０％以下であってもよい。 [C: 0.050 to 0.100%]
C is an element effective in increasing the strength of the steel plate. In order to fully obtain such an effect, the C content is set to 0.050% or more. The C content may be 0.055% or more, 0.060% or more, 0.065% or more, or 0.070% or more. On the other hand, if C is contained excessively, the weldability may be reduced. Therefore, the C content is set to 0.100% or less. The C content may be 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.
［Ｓｉ：０．０１～０．３０％］
　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０．０１％以上とする。Ｓｉ含有量は０．０３％以上、０．０５％以上、０．０８％以上、０．１２％以上又は０．１５％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０．３０％以下とする。Ｓｉ含有量は０．２８％以下、０．２５％以下、０．２２％以下又は０．２０％以下であってもよい。 [Si: 0.01 to 0.30%]
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 0.01% or more. The Si content may be 0.03% or more, 0.05% or more, 0.08% or more, 0.12% or more, or 0.15% 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 0.30% or less. The Si content may be 0.28% or less, 0.25% or less, 0.22% or less, or 0.20% or less.
［Ｍｎ：１．３０～２．１０％］
　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１．３０％以上とする。Ｍｎ含有量は１．４０％以上、１．５０％以上、１．６０％以上又は１．７０％以上であってもよい。一方で、Ｍｎを過度に含有すると、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、Ｍｎ含有量は２．１０％以下とする。Ｍｎ含有量は２．００％以下、１．９０％以下又は１．８０％以下であってもよい。 [Mn: 1.30 to 2.10%]
Mn is an element that is effective in increasing strength as an element for hardenability and solid solution strengthening. In order to fully obtain these effects, the Mn content is set to 1.30% or more. The Mn content may be 1.40% or more, 1.50% or more, 1.60% or more, or 1.70% or more. On the other hand, if Mn is contained excessively, a large amount of MnS may be generated, which may reduce toughness. Therefore, the Mn content is set to 2.10% or less. The Mn content may be 2.00% or less, 1.90% or less, or 1.80% or less.
［Ｔｉ：０．０８０～０．１５０％］
　Ｔｉは、ＴｉＣ等のＴｉ炭化物として鋼中に微細に析出し、析出強化により強度の向上に寄与する元素である。このような効果を十分に得るために、Ｔｉ含有量は０．０８０％以上とする。Ｔｉ含有量は０．０９０％以上、０．０９５％以上、０．１００％以上、０．１０５％以上又は０．１１０％以上であってもよい。一方で、Ｔｉを過度に含有すると、析出物が粗大となり、析出強化による強度向上効果を十分に発揮することができない場合がある。したがって、Ｔｉ含有量は０．１５０％以下とする。Ｔｉ含有量は０．１４０％以下、０．１３５％以下、０．１３０％以下、０．１２５％以下又は０．１２０％以下であってもよい。 [Ti: 0.080 to 0.150%]
Ti is an element that precipitates finely in steel as Ti carbides such as TiC, and contributes to improving strength by precipitation strengthening. In order to fully obtain such effects, the Ti content is set to 0.080% or more. The Ti content may be 0.090% or more, 0.095% or more, 0.100% or more, 0.105% or more, or 0.110% or more. On the other hand, if Ti is excessively contained, the precipitates become coarse, and the strength improving effect by precipitation strengthening may not be fully exhibited. Therefore, the Ti content is set to 0.150% or less. The Ti content may be 0.140% or less, 0.135% or less, 0.130% or less, 0.125% or less, or 0.120% or less.
［Ｎｂ：０．０２０～０．０５０％］
　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結晶粒を微細化し、細粒強化により鋼板の高強度化に寄与する元素である。このような効果を十分に得るために、Ｎｂ含有量は０．０２０％以上とする。Ｎｂ含有量は０．０２５％以上、０．０２８％以上、０．０３０％以上又は０．０３２％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靭性を低下させる場合がある。したがって、Ｎｂ含有量は０．０５０％以下とする。Ｎｂ含有量は０．０４５％以下、０．０４２％以下、０．０４０％以下又は０．０３８％以下であってもよい。 [Nb: 0.020 to 0.050%]
Nb is an element that forms carbides, nitrides and/or carbonitrides in steel to refine crystal grains through a pinning effect, and contributes to high strength of steel plate through fine grain strengthening. In order to fully obtain such effects, the Nb content is set to 0.020% or more. The Nb content may be 0.025% or more, 0.028% or more, 0.030% or more, or 0.032% or more. On the other hand, excessive Nb content may generate coarse carbides in the steel, reducing the toughness of the steel plate. Therefore, the Nb content is set to 0.050% or less. The Nb content may be 0.045% or less, 0.042% or less, 0.040% or less, or 0.038% or less.
［Ａｌ：０．００１～０．０５０％］
　Ａｌは、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、Ａｌ含有量は０．００１％以上とする。Ａｌ含有量は０．０１０％以上、０．０２０％以上又は０．０３０％以上であってもよい。一方で、Ａｌを過度に含有すると、粗大な酸化物が形成し、靭性を低下させる場合がある。したがって、Ａｌ含有量は０．０５０％以下とする。Ａｌ含有量は０．０４５％以下又は０．０４０％以下であってもよい。 [Al: 0.001 to 0.050%]
Al is an element that acts as a deoxidizer. In order to fully obtain such an effect, the Al content is set to 0.001% or more. The Al content may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if Al is contained excessively, coarse oxides may be formed, which may reduce toughness. Therefore, the Al content is set to 0.050% or less. The Al content may be 0.045% or less, or 0.040% or less.
［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０．０８０％以下、０．０５０％以下、０．０３０％以下又は０．０２０％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０．０００１％以上、０．０００５％以上又は０．００１％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 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% 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 0.0001% or more, 0.0005% or more, or 0.001% or more.
［Ｓ：０．０５０％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０．０５０％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０．０００１％以上、０．０００５％以上又は０．００１％以上であってもよい。 [S: 0.050% 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 0.050% 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 cost. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.
［Ｎ：０．００５０％以下］
　Ｎは、過度に含有すると粗大な窒化物を形成し、靭性を低下させる場合がある。また、Ｎは、鋼中のＴｉと結合して窒化チタン（ＴｉＮ）を形成することにより、Ｔｉ炭化物等の析出物を形成し得る有効Ｔｉ量を減少させ、析出強化による強度向上効果を低下させる場合がある。したがって、Ｎ含有量は低いほど好ましく、０．００５０％以下とする。Ｎ含有量は０．００４５％以下、０．００４０％以下又は０．００３５％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、０．０００５％以上又は０．００１０％以上であってもよい。 [N: 0.0050% or less]
If N is contained excessively, it may form coarse nitrides and reduce toughness. In addition, N may combine with Ti in the steel to form titanium nitride (TiN), thereby reducing the effective Ti amount capable of forming precipitates such as Ti carbides, and may reduce the strength improvement effect by precipitation strengthening. Therefore, the lower the N content, the more preferable it is, and it is set to 0.0050% or less. The N content may be 0.0045% or less, 0.0040% or less, or 0.0035% 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 cost. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
［Ｏ：０．００５０％以下］
　Ｏは、過度に含有すると粗大な介在物が形成し、靭性を低下させる場合がある。したがって、Ｏ含有量は０．００５０％以下とする。Ｏ含有量は０．００４０％以下、０．００３５％以下又は０．００３０％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、Ｏ含有量を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、０．０００５％以上又は０．００１０％以上であってもよい。 [O: 0.0050% or less]
When O is contained excessively, coarse inclusions may be formed, which may reduce toughness. Therefore, the O content is set to 0.0050% or less. The O content may be 0.0040% or less, 0.0035% or less, or 0.0030% 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, 0.0005% or more, or 0.0010% or more.
　本発明の実施形態に係る熱延コイルの基本化学組成は上記のとおりである。さらに、当該熱延コイルは、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 coil according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled coil may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.
［Ｂ：０～０．００５０％］
　Ｂ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。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を過度に含有すると、靭性及び／又は溶接性が低下する場合がある。したがって、Ｂ含有量は０．００５０％以下であることが好ましい。Ｂ含有量は０．００３０％以下、０．００１５％以下、０．００１２％以下又は０．０００８％以下であってもよい。 [B: 0 to 0.0050%]
B is an element that enhances the hardenability of steel and contributes to improving strength. 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, if B is contained excessively, toughness and/or weldability may decrease. Therefore, the B content is preferably 0.0050% or less. The B content may be 0.0030% or less, 0.0015% or less, 0.0012% or less, or 0.0008% or less.
［Ｃｕ：０～０．２０％］
　Ｃｕは、強度及び／又は耐食性の向上に寄与する元素である。Ｃｕ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、Ｃｕ含有量は０．０１％以上であることが好ましい。Ｃｕ含有量は０．０３％以上又は０．０５％以上であってもよい。一方で、Ｃｕを過度に含有すると、靭性や溶接性の劣化を招く場合がある。したがって、Ｃｕ含有量は０．２０％以下であることが好ましい。Ｃｕ含有量は０．１８％以下、０．１５％以下、０．１２％以下、０．１０％以下、０．０８％以下又は０．０６％以下であってもよい。 [Cu: 0 to 0.20%]
Cu is an element that contributes to improving strength and/or corrosion resistance. The Cu content may be 0%, but in order to obtain these effects, the Cu content is preferably 0.01% or more. The Cu content may be 0.03% or more or 0.05% or more. On the other hand, excessive Cu content may cause deterioration of toughness and weldability. Therefore, the Cu content is preferably 0.20% or less. The Cu content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.
［Ｎｉ：０～０．２０％］
　Ｎｉは、鋼の焼入れ性を高め、強度及び／又は耐食性の向上に寄与する元素である。Ｎｉ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、Ｎｉ含有量は０．０１％以上であることが好ましい。Ｎｉ含有量は０．０３％以上又は０．０５％以上であってもよい。一方で、Ｎｉ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、Ｎｉ含有量は０．２０％以下であることが好ましい。Ｎｉ含有量は０．１８％以下、０．１５％以下、０．１２％以下、０．１０％以下、０．０８％以下又は０．０６％以下であってもよい。 [Ni: 0 to 0.20%]
Ni is an element that enhances the hardenability of steel and contributes to improving strength and/or corrosion resistance. The Ni content may be 0%, but in order to obtain these effects, the Ni content is preferably 0.01% or more. The Ni content may be 0.03% or more or 0.05% or more. On the other hand, even if Ni is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the Ni content is preferably 0.20% or less. The Ni content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.
［Ｓｎ：０～０．１０％］
　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０．１０％以下であることが好ましい。Ｓｎ含有量は０．０８％以下、０．０６％以下又は０．０４％以下であってもよい。 [Sn: 0 to 0.10%]
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. On the other hand, excessive Sn content may cause a decrease in toughness. Therefore, the Sn content is preferably 0.10% or less. The Sn content may be 0.08% or less, 0.06% or less, or 0.04% or less.
［Ｃｒ：０～０．４０％］
　Ｃｒは、鋼の焼入れ性を高め、強度及び／又は耐食性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、これらの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０．０５％以上又は０．１０％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。したがって、Ｃｒ含有量は０．４０％以下であることが好ましい。Ｃｒ含有量は０．３０％以下、０．２０％以下、０．１５％以下又は０．１２％以下であってもよい。 [Cr: 0 to 0.40%]
Cr is an element that enhances the hardenability of steel and contributes to improving strength and/or corrosion resistance. The Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.01% or more. The Cr content may be 0.05% or more or 0.10% or more. On the other hand, even if Cr is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the Cr content is preferably 0.40% or less. The Cr content may be 0.30% or less, 0.20% or less, 0.15% or less, or 0.12% or less.
［Ｍｏ：０～０．２００％］
　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り、耐食性の向上にも寄与する元素である。Ｍｏ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、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、０．０３０％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０．２００％以下であることが好ましい。Ｍｏ含有量は０．１８０％以下、０．１５０％以下、０．１２０％以下、０．１００％以下又は０．０８０％以下であってもよい。 [Mo: 0 to 0.200%]
Mo is an element that enhances the hardenability of steel, contributes to improving strength, and also contributes to improving corrosion resistance. The Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if Mo is contained excessively, the deformation resistance during hot working may increase, and the equipment load may become large. Therefore, the Mo content is preferably 0.200% or less. The Mo content may be 0.180% or less, 0.150% or less, 0.120% or less, 0.100% or less, or 0.080% or less.
［Ｖ：０～０．１００％］
　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０．００５％以上、０．０１０％以上又は０．０２０％以上であってもよい。一方で、Ｖを過度に含有すると、多量の析出物が生成して靭性を低下させる場合がある。したがって、Ｖ含有量は０．１００％以下であることが好ましい。Ｖ含有量は０．０８０％以下、０．０６０％以下又は０．０４０％以下であってもよい。 [V: 0 to 0.100%]
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 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if V is contained excessively, a large amount of precipitates may be generated, which may reduce toughness. Therefore, the V content is preferably 0.100% or less. The V content may be 0.080% or less, 0.060% or less, or 0.040% or less.
［Ａｓ：０～０．１００％］
　Ａｓは、耐食性の向上に有効な元素である。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、Ａｓ含有量は０．００１％以上であることが好ましい。Ａｓ含有量は０．００５％以上、０．００８％以上又は０．０１０％以上であってもよい。一方で、Ａｓ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、Ａｓ含有量は０．１００％以下であることが好ましい。Ａｓ含有量は０．０８０％以下、０．０６０％以下、０．０４０％以下又は０．０２０％以下であってもよい。 [As: 0 to 0.100%]
As is an element effective in improving corrosion resistance. The As content may be 0%, but in order to obtain such an effect, the As content is preferably 0.001% or more. The As content may be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, even if As is excessively contained, the effect is saturated and the manufacturing cost increases. Therefore, the As content is preferably 0.100% or less. The As content may be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.
［Ｚｒ：０～０．１００％］
　Ｚｒは、硫化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０．００１％以上であることが好ましい。Ｚｒ含有量は０．００５％以上又は０．０１０％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、製造コストの上昇を招く。したがって、Ｚｒ含有量は０．１００％以下であることが好ましい。Ｚｒ含有量は０．０５０％以下、０．０３０％以下又は０．０２０％以下であってもよい。 [Zr: 0 to 0.100%]
Zr is an element capable of controlling the morphology of sulfides. The Zr content may be 0%, but in order to obtain such an effect, the Zr content is preferably 0.001% or more. The Zr content may be 0.005% or more or 0.010% or more. On the other hand, even if Zr is excessively contained, the effect is saturated and the manufacturing cost increases. Therefore, the Zr content is preferably 0.100% or less. The Zr content may be 0.050% or less, 0.030% or less, or 0.020% or less.
［Ｃａ：０～０．００５０％］
　Ｃａは、硫化物の形態を制御することができる元素である。Ｃａ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、Ｃａ含有量は０．０００１％以上であることが好ましい。Ｃａ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、Ｃａ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、Ｃａ含有量は０．００５０％以下であることが好ましい。Ｃａ含有量は０．００４０％以下、０．００３０％以下又は０．００２０％以下であってもよい。 [Ca: 0 to 0.0050%]
Ca is an element that can control the form of sulfides. The Ca content may be 0%, but in order to obtain such an effect, the Ca content is preferably 0.0001% or more. The Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if Ca is excessively contained, the effect is saturated and the manufacturing cost increases. Therefore, the Ca content is preferably 0.0050% or less. The Ca content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.
［Ｍｇ：０～０．１００％］
　Ｍｇは、硫化物の形態を制御することができる元素である。Ｍｇ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、Ｍｇ含有量は０．００１％以上であることが好ましく、０．００５％以上又は０．００８％以上であってもよい。一方で、Ｍｇ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、Ｍｇ含有量は０．１００％以下であることが好ましい。Ｍｇ含有量は０．０５０％以下、０．０３０％以下、０．０２０％以下又は０．０１０％以下であってもよい。 [Mg: 0 to 0.100%]
Mg is an element that can control the morphology of sulfides. The Mg content may be 0%, but in order to obtain such an effect, the Mg content is preferably 0.001% or more, and may be 0.005% or more or 0.008% or more. On the other hand, even if Mg is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the Mg content is preferably 0.100% or less. The Mg content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.
［Ｂｉ：０～０．０２０％］
　Ｂｉは、耐食性の向上に有効な元素である。Ｂｉ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、Ｂｉ含有量は０．００１％以上であることが好ましい。Ｂｉ含有量は０．００２％以上又は０．００３％以上であってもよい。一方で、Ｂｉ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、Ｂｉ含有量は０．０２０％以下であることが好ましい。Ｂｉ含有量は０．０１０％以下、０．００８％以下又は０．００５％以下であってもよい。 [Bi: 0 to 0.020%]
Bi is an element effective in improving corrosion resistance. The Bi content may be 0%, but in order to obtain such an effect, the Bi content is preferably 0.001% or more. The Bi content may be 0.002% or more or 0.003% or more. On the other hand, even if Bi is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the Bi content is preferably 0.020% or less. The Bi content may be 0.010% or less, 0.008% or less, or 0.005% or less.
［Ｃｏ：０～０．２０％］
　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０．０３％以上又は０．０５％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０．２０％以下であることが好ましい。Ｃｏ含有量は０．１８％以下、０．１５％以下、０．１２％以下、０．１０％以下又は０．０８％以下であってもよい。 [Co: 0 to 0.20%]
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 0.03% or more or 0.05% 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 0.20% or less. The Co content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.
［Ｗ：０～０．２０％］
　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０．０３％以上又は０．０５％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０．２０％以下であることが好ましい。Ｗ含有量は０．１８％以下、０．１５％以下、０．１２％以下、０．１０％以下又は０．０８％以下であってもよい。 [W: 0 to 0.20%]
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 0.03% or more or 0.05% or more. On the other hand, excessive W content may reduce weldability. Therefore, the W content is preferably 0.20% or less. The W content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.
［Ｚｎ：０～０．２０％］
　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０．０３％以上又は０．０５％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０．２０％以下であることが好ましい。Ｚｎ含有量は０．１８％以下、０．１５％以下、０．１２％以下、０．１０％以下又は０．０８％以下であってもよい。 [Zn: 0 to 0.20%]
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 0.03% or more or 0.05% 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 0.20% or less. The Zn content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.
［ＲＥＭ：０～０．１０００％］
　ＲＥＭ（希土類金属）は、硫化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、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０．０１００％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 (rare earth metal) is an element that can control the morphology of sulfides. 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 REM is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the REM content is preferably 0.1000% or less. The REM content may be 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, namely, scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.
　本発明の実施形態に係る熱延コイルにおいて、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、熱延コイル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 coil 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 coils.
　本発明の実施形態に係る熱延コイルの化学組成は、一般的な分析方法によって測定すればよい。例えば、当該熱延コイルの化学組成は、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 coil 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 coil 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.
［有効Ｔｉ量］
　本発明の実施形態に係る熱延コイルにおいては、析出強化による強度向上効果を十分に発揮させる観点から、有効Ｔｉ量を０．０７０％以上とすることが好ましい。有効Ｔｉ量は、時効処理直前における固溶Ｔｉ量に相当し、すなわち、熱延コイル中の全Ｔｉ量からＮによって固定されるＴｉ量を差し引いた値に相当する。より詳しく説明すると、ＴｉＮは溶解度が非常に小さく、それゆえ一度析出したＴｉＮは、通常の溶体化処理温度では再固溶させることができない。このため、Ｔｉ炭化物等の析出物を時効析出するのに有効なＴｉ量は、全Ｔｉ量からＴｉＮとして固定され得る量を差し引いた値となり、下記式によって算出される。
　　　有効Ｔｉ量（％）＝［Ｔｉ］－４８／１４［Ｎ］
　ここで、［Ｔｉ］及び［Ｎ］は、熱延コイル中の各元素の含有量（質量％）である。有効Ｔｉ量を０．０７０％以上とすることで、析出強化による所望の強度向上効果を得るのに十分な析出物を生成させることができる。例えば、有効Ｔｉ量は０．０７５％以上、０．０８０％以上、０．０８５％以上又は０．０９０％以上であってもよい。上限は特に限定されないが、例えば、有効Ｔｉ量は０．１５０％以下、０．１４５％以下、０．１４０％以下又は０．１３５％以下であってもよい。 [Effective Ti content]
In the hot-rolled coil according to the embodiment of the present invention, it is preferable to set the effective Ti amount to 0.070% or more from the viewpoint of fully exerting the strength improvement effect by precipitation strengthening. The effective Ti amount corresponds to the amount of solid solution Ti just before aging treatment, that is, the value obtained by subtracting the amount of Ti fixed by N from the total amount of Ti in the hot-rolled coil. To explain in more detail, TiN has a very low solubility, and therefore TiN once precipitated cannot be re-dissolved at a normal solution treatment temperature. Therefore, the amount of Ti effective for aging precipitation of precipitates such as Ti carbides is the value obtained by subtracting the amount that can be fixed as TiN from the total amount of Ti, and is calculated by the following formula.
Effective Ti content (%) = [Ti] - 48/14 [N]
Here, [Ti] and [N] are the contents (mass%) of each element in the hot rolled coil. By setting the effective Ti amount to 0.070% or more, it is possible to generate precipitates sufficient to obtain the desired strength improvement effect by precipitation strengthening. For example, the effective Ti amount may be 0.075% or more, 0.080% or more, 0.085% or more, or 0.090% or more. The upper limit is not particularly limited, but for example, the effective Ti amount may be 0.150% or less, 0.145% or less, 0.140% or less, or 0.135% or less.
［金属組織］
［方位差が１５°以上の粒界によって囲まれた結晶粒の平均粒径：５．０～８．０μｍ］
　本発明の実施形態に係る熱延コイルでは、長手方向の中央部において、方位差が１５°以上の粒界によって囲まれた領域を結晶粒と定義した場合に、当該結晶粒の平均粒径は５．０～８．０μｍの範囲内に制御される。当該結晶粒の平均粒径をＮｂの炭化物、窒化物及び／又は炭窒化物に基づくピン止め効果等によって５．０～８．０μｍの範囲内に制御することで細粒強化による強度向上効果を十分に発揮させることができ、後で詳しく説明するＴｉ炭化物等の析出物に基づく析出強化による強度向上効果と組み合わせて、最終的に得られる熱延コイルにおいて高い引張強さ、例えば７８０ＭＰａ以上の引張強さを確実に達成することが可能となる。上記結晶粒の平均粒径が５．０μｍ未満であるか又は８．０μｍ超であると、このような細粒強化による強度向上効果を十分に得ることができず、熱延コイルにおいて所望の高強度を達成することができない。上記結晶粒の平均粒径は、例えば５．５μｍ以上、６．０μｍ以上又は６．５μｍ以上であってもよい。同様に、上記結晶粒の平均粒径は、例えば７．５μｍ以下又は７．０μｍ以下であってもよい。 [Metal structure]
[Average grain size of crystal grains surrounded by grain boundaries with misorientation of 15° or more: 5.0 to 8.0 μm]
In the hot-rolled coil according to the embodiment of the present invention, when the region surrounded by grain boundaries with an orientation difference of 15° or more in the longitudinal center is defined as a crystal grain, the average grain size of the crystal grain is controlled within the range of 5.0 to 8.0 μm. By controlling the average grain size of the crystal grain within the range of 5.0 to 8.0 μm by the pinning effect based on Nb carbides, nitrides and/or carbonitrides, the strength improvement effect due to fine grain strengthening can be fully exhibited, and in combination with the strength improvement effect due to precipitation strengthening based on precipitates such as Ti carbides, which will be described in detail later, it is possible to reliably achieve high tensile strength, for example, tensile strength of 780 MPa or more, in the finally obtained hot-rolled coil. If the average grain size of the crystal grains is less than 5.0 μm or more than 8.0 μm, the strength improvement effect due to such fine grain strengthening cannot be fully obtained, and the desired high strength cannot be achieved in the hot-rolled coil. The average grain size of the crystal grains may be, for example, 5.5 μm or more, 6.0 μm or more, or 6.5 μm or more. Similarly, the average grain size of the crystal grains may be, for example, 7.5 μm or less, or 7.0 μm or less.
［方位差が１５°以上の粒界によって囲まれた結晶粒の平均粒径の測定］
　方位差が１５°以上の粒界によって囲まれた結晶粒の平均粒径は、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、熱延コイルの長手方向中央部かつ幅方向中央部から、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試料を採取する。次いで、試料表面から板厚の１／４深さ位置で、試料の圧延方向に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、方位差が１５°以上の粒界によって囲まれた領域を結晶粒と定義し、当該結晶粒の円相当直径を解析し、それらの平均値を求め、平均粒径として決定する。上記のように定義した結晶粒の粒径は、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」に搭載された機能でＧｒａｉｎ　Ｓｉｚｅ（ｄｉａｍｅｔｅｒ）により算出されるＡｒｅａ　ａｖｅｒａｇｅの値を利用して決定することができる。 [Measurement of the average grain size of crystal grains surrounded by grain boundaries with a misorientation of 15° or more]
The average grain size of crystal grains surrounded by grain boundaries with an orientation difference of 15° or more is measured by electron backscattered diffraction (EBSD). More specifically, a sample is first taken from the center of the longitudinal direction and the center of the width direction of the hot-rolled coil so that the observation surface is a cross section of the plate thickness parallel to the rolling direction and perpendicular to the plate surface. Next, at a depth position of 1/4 of the plate thickness from the surface of the sample, an area of 200 μm in the rolling direction of the sample and 100 μm in the normal direction of 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, the region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a crystal grain, the circle equivalent diameter of the crystal grain is analyzed, and the average value is calculated and determined as the average grain size. The grain size of the crystal grain defined as above can be determined using the area average value calculated by the grain size (diameter) in the function installed in the software "OIM Analysis (registered trademark) Version 7.0.1" attached to the EBSD analysis device.
［析出物の平均粒径：３．０～９．５ｎｍ］
　本発明の実施形態に係る熱延コイルでは、長手方向の中央部において、析出物の平均粒径は３．０～９．５ｎｍの範囲内に制御される。析出物の平均粒径を３．０～９．５ｎｍの範囲内に制御することで析出強化による強度向上効果を十分に発揮させることができ、先に説明した細粒強化による強度向上効果と組み合わせて、最終的に得られる熱延コイルにおいて高い引張強さ、例えば７８０ＭＰａ以上の引張強さを確実に達成することが可能となる。析出物は、Ｔｉ炭化物を含むか又はＴｉ炭化物であってもよい。Ｔｉ炭化物としては、特に限定されないが、例えば、ＴｉＣであってもよいし、又はＴｉとＴｉ以外の他の元素、例えばＮｂとを含む複合炭化物であってもよい。析出物の粒径が小さいと、当該析出物が転位運動の障害物として十分に作用することができず、それゆえ析出強化による強度向上効果を十分に得ることができない。一方で、析出物の粒径が大きすぎても、同様に所望の析出強化を得ることができない場合がある。 [Average particle size of precipitates: 3.0 to 9.5 nm]
In the hot-rolled coil according to the embodiment of the present invention, the average grain size of the precipitates is controlled within a range of 3.0 to 9.5 nm in the central portion in the longitudinal direction. By controlling the average grain size of the precipitates within a range of 3.0 to 9.5 nm, the strength improvement effect by precipitation strengthening can be fully exerted, and in combination with the strength improvement effect by fine grain strengthening described above, it is possible to reliably achieve a high tensile strength, for example, a tensile strength of 780 MPa or more, in the finally obtained hot-rolled coil. The precipitates may contain Ti carbides or may be Ti carbides. The Ti carbides are not particularly limited, but may be, for example, TiC, or may be a composite carbide containing Ti and an element other than Ti, such as Nb. If the grain size of the precipitates is small, the precipitates cannot act sufficiently as an obstacle to dislocation motion, and therefore the strength improvement effect by precipitation strengthening cannot be fully obtained. On the other hand, if the grain size of the precipitates is too large, the desired precipitation strengthening may not be obtained in the same way.
　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、これは、析出物が粗大となることで転位運動との関係で強化機構が変化し、例えば転位線が析出物を横切って通過するのではなく、粗大な析出物の周りに転位線のループを残して通過するようになり、析出強化量としては小さくなってしまうためと考えられる。加えて、析出物の粗大化に伴い、当該析出物の個数密度も大きく低下してしまうため、析出強化によって強度を十分に高めることができなくなる。したがって、析出強化によって熱延コイルの強度を効果的に高めるためには、析出物の平均粒径は３．０～９．５ｎｍの範囲に制御することが重要となる。析出物の平均粒径は、例えば４．０ｎｍ以上、５．０ｎｍ以上又は６．０ｎｍ以上であってもよい。同様に、上記結晶粒の平均粒径は、例えば９．０ｎｍ以下、８．０ｎｍ以下又は７．０ｎｍ以下であってもよい。 Without intending to be bound by any particular theory, this is believed to be because the strengthening mechanism changes in relation to dislocation motion as the precipitates become coarse, and for example, dislocation lines do not pass across the precipitates but pass around the coarse precipitates, leaving a loop of dislocation lines, resulting in a small amount of precipitation strengthening. In addition, as the precipitates become coarse, the number density of the precipitates also decreases significantly, making it impossible to sufficiently increase the strength by precipitation strengthening. Therefore, in order to effectively increase the strength of the hot-rolled coil by precipitation strengthening, it is important to control the average grain size of the precipitates to a range of 3.0 to 9.5 nm. The average grain size of the precipitates may be, for example, 4.0 nm or more, 5.0 nm or more, or 6.0 nm or more. Similarly, the average grain size of the crystal grains may be, for example, 9.0 nm or less, 8.0 nm or less, or 7.0 nm or less.
［幅方向における析出物の粒径の最大値と最小値の差が析出物の平均粒径の１５．０％以下］
　本発明の実施形態に係る熱延コイルでは、長手方向中央部の幅方向における析出物の粒径の最大値と最小値の差が当該析出物の平均粒径の１５．０％以下に制御される。長手方向中央部の幅方向における析出物の粒径の最大値と最小値の差を当該析出物の平均粒径の１５．０％以下に制御することで、熱延コイルの幅方向における強度ばらつきを低減することが可能となる。先に述べたとおり、熱延コイルの長手方向及び幅方向における析出物の析出状態は、相互に密接に関係していると考えられる。このため、例えば、熱延コイルの長手方向における中央部のみを適切に冷却したとしても、先端部及び／又は尾端部を適切に冷却していない場合には、それらの影響を受けて中央部においても析出物の所望の析出状態を確実に得ることができなくなる。同様に、例えば、熱延コイルの長手方向における先端部、中央部及び尾端部で適切な冷却を行ったとしても、幅方向における冷却が均一に行われない場合には、幅方向の温度偏差に起因して長手方向における析出物の析出状態も影響を受けてしまう。したがって、熱延コイルの製造方法について後で詳しく説明されるように、長手方向及び幅方向の冷却を適切に行う必要があり、それによって熱延コイルの長手方向中央部の幅方向における析出物の粒径のばらつきを所定の範囲内に制御することができ、より具体的には熱延コイルの長手方向中央部の幅方向における析出物の粒径の最大値と最小値の差を当該析出物の平均粒径の１５．０％以下に制御することができる。その結果として、熱延コイルの幅方向における強度ばらつきを顕著に抑制又は低減することが可能となる。 [The difference between the maximum and minimum grain sizes of the precipitates in the width direction is 15.0% or less of the average grain size of the precipitates]
In the hot-rolled coil according to the embodiment of the present invention, the difference between the maximum and minimum grain size of the precipitate in the width direction of the longitudinal center portion is controlled to 15.0% or less of the average grain size of the precipitate. By controlling the difference between the maximum and minimum grain size of the precipitate in the width direction of the longitudinal center portion to 15.0% or less of the average grain size of the precipitate, it is possible to reduce the strength variation in the width direction of the hot-rolled coil. As described above, the precipitation states of the precipitate in the longitudinal direction and the width direction of the hot-rolled coil are considered to be closely related to each other. For this reason, for example, even if only the longitudinal center portion of the hot-rolled coil is appropriately cooled, if the front end and/or the tail end portion are not appropriately cooled, the desired precipitation state of the precipitate cannot be reliably obtained even in the central portion due to the influence of the cooling. Similarly, for example, even if the longitudinal front end, central portion, and tail end portions of the hot-rolled coil are appropriately cooled, if the cooling in the width direction is not performed uniformly, the precipitation state of the precipitate in the longitudinal direction is also affected due to the temperature deviation in the width direction. Therefore, as will be described later in detail about the manufacturing method of the hot-rolled coil, it is necessary to appropriately cool the hot-rolled coil in the longitudinal and width directions, so that the variation in grain size of the precipitates in the width direction of the longitudinal center of the hot-rolled coil can be controlled within a predetermined range, and more specifically, the difference between the maximum and minimum grain sizes of the precipitates in the width direction of the longitudinal center of the hot-rolled coil can be controlled to 15.0% or less of the average grain size of the precipitates. As a result, it becomes possible to significantly suppress or reduce the strength variation in the hot-rolled coil in the width direction.
　強度ばらつきを抑制又は低減する観点からは、熱延コイルの長手方向中央部において、幅方向における析出物の粒径の最大値と最小値の差は、当該析出物の平均粒径に対して小さいほど好ましい。したがって、熱延コイルの長手方向中央部において、幅方向における析出物の粒径の最大値と最小値の差は、当該析出物の平均粒径の１２．０％以下、１０．０％以下又は８．０％以下であることが好ましい。一方、下限は特に限定されないが、例えば、熱延コイルの長手方向中央部において、幅方向における析出物の粒径の最大値と最小値の差は、当該析出物の平均粒径の２．０％以上、３．０％以上又は５．０％以上であってもよい。 From the viewpoint of suppressing or reducing the strength variation, it is preferable that the difference between the maximum and minimum grain size of the precipitate in the width direction at the longitudinal center of the hot-rolled coil is smaller than the average grain size of the precipitate. Therefore, it is preferable that the difference between the maximum and minimum grain size of the precipitate in the width direction at the longitudinal center of the hot-rolled coil is 12.0% or less, 10.0% or less, or 8.0% or less of the average grain size of the precipitate. On the other hand, the lower limit is not particularly limited, and for example, the difference between the maximum and minimum grain size of the precipitate in the width direction at the longitudinal center of the hot-rolled coil may be 2.0% or more, 3.0% or more, or 5.0% or more of the average grain size of the precipitate.
［長手方向中央部の析出物の平均粒径、並びに当該平均粒径に対する幅方向における析出物の粒径の最大値と最小値の差の割合の決定］
　熱延コイルの長手方向中央部の析出物の平均粒径等は、以下のようにして決定される。まず、熱延コイルの全幅をＷとした場合に、当該熱延コイルの長手方向中央部における幅方向の端部から１／１０Ｗ位置、３／１０Ｗ位置、５／１０Ｗ位置、７／１０Ｗ位置、及び９／１０Ｗ位置の各位置においてレプリカ法によりサンプルを採取する。次いで、採取したサンプルを透過型電子顕微鏡（ＴＥＭ、例えば、ＪＥＯＬ製の「ＪＥＭ－２１００」を用いることができる。）を用いて５０～１００個の析出物を観察し、個々の析出物の粒径を円相当直径として算出し、算出された全ての円相当直径の平均値を各幅方向位置における析出物の粒径として決定し、得られた５つの粒径の算術平均を長手方向中央部の析出物の平均粒径として決定する。最後に、得られた５つの粒径のうち最大値と最小値の差を算出し、算出された値を析出物の平均粒径によって割り算することにより、当該平均粒径に対する幅方向における析出物の粒径の最大値と最小値の差の割合を決定する。析出物の構成元素はＥＤＳ分析によって同定することが可能である。 [Determination of the average grain size of precipitates at the center in the longitudinal direction and the ratio of the difference between the maximum and minimum grain sizes of precipitates in the width direction to the average grain size]
The average grain size of the precipitates in the longitudinal center of the hot-rolled coil is determined as follows. First, when the total width of the hot-rolled coil is W, samples are taken by a replica method at each of the positions of 1/10W, 3/10W, 5/10W, 7/10W, and 9/10W from the end of the width direction in the longitudinal center of the hot-rolled coil. Next, 50 to 100 precipitates are observed in the taken samples using a transmission electron microscope (TEM, for example, "JEM-2100" manufactured by JEOL can be used), and the grain size of each precipitate is calculated as a circle equivalent diameter. The average value of all the calculated circle equivalent diameters is determined as the grain size of the precipitates at each width direction position, and the arithmetic average of the five grain sizes obtained is determined as the average grain size of the precipitates in the longitudinal center. Finally, the difference between the maximum and minimum values of the five grain sizes obtained is calculated, and the calculated value is divided by the average grain size of the precipitates to determine the ratio of the difference between the maximum and minimum grain sizes of the precipitates in the width direction to the average grain size. The constituent elements of the precipitates can be identified by EDS analysis.
［長手方向における析出物の粒径の最大値と最小値の差が析出物の平均粒径の１５．０％以下］
　本発明の実施形態に係る熱延コイルでは、幅方向中央部の長手方向における析出物の粒径の最大値と最小値の差が長手方向における析出物の平均粒径の１５．０％以下に制御される。幅方向中央部の長手方向における析出物の粒径の最大値と最小値の差を長手方向における析出物の平均粒径の１５．０％以下に制御することで、熱延コイルの長手方向における強度ばらつきを低減することが可能となる。これに関連して、熱延コイルの製造方法について後で詳しく説明されるように、巻き取り後の熱延コイルの長手方向における先端部、中央部及び尾端部で適切な冷却を行うことが特に重要であり、それによって熱延コイルの幅方向中央部の長手方向における析出物の粒径の最大値と最小値の差を長手方向における析出物の平均粒径の１５．０％以下に制御することが可能となる。 [The difference between the maximum and minimum grain sizes of precipitates in the longitudinal direction is 15.0% or less of the average grain size of precipitates]
In the hot rolled coil according to the embodiment of the present invention, the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise central portion is controlled to 15.0% or less of the average grain size of the precipitates in the longitudinal direction. By controlling the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise central portion to 15.0% or less of the average grain size of the precipitates in the longitudinal direction, it is possible to reduce the strength variation in the longitudinal direction of the hot rolled coil. In this regard, as will be described in detail later with respect to the manufacturing method of the hot rolled coil, it is particularly important to perform appropriate cooling at the front end, central portion, and tail end portion in the longitudinal direction of the hot rolled coil after coiling, and thereby it is possible to control the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise central portion of the hot rolled coil to 15.0% or less of the average grain size of the precipitates in the longitudinal direction.
　強度ばらつきを抑制又は低減する観点からは、熱延コイルの幅方向中央部において、長手方向における析出物の粒径の最大値と最小値の差は、長手方向における析出物の平均粒径に対して小さいほど好ましい。したがって、熱延コイルの幅方向中央部において、長手方向における析出物の粒径の最大値と最小値の差は、長手方向における析出物の平均粒径の１２．０％以下、１０．０％以下又は８．０％以下であることが好ましい。一方、下限は特に限定されないが、例えば、熱延コイルの幅方向中央部において、長手方向における析出物の粒径の最大値と最小値の差は、長手方向における析出物の平均粒径の１．０％以上、２．０％以上又は３．０％以上であってもよい。 From the viewpoint of suppressing or reducing the strength variation, it is preferable that the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise center of the hot-rolled coil is smaller than the average grain size of the precipitates in the longitudinal direction. Therefore, it is preferable that the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise center of the hot-rolled coil is 12.0% or less, 10.0% or less, or 8.0% or less of the average grain size of the precipitates in the longitudinal direction. On the other hand, the lower limit is not particularly limited, but for example, the difference between the maximum and minimum values of the grain size of the precipitates in the longitudinal direction at the widthwise center of the hot-rolled coil may be 1.0% or more, 2.0% or more, or 3.0% or more of the average grain size of the precipitates in the longitudinal direction.
［幅方向中央部の長手方向における析出物の平均粒径、並びに当該平均粒径に対する長手方向における析出物の粒径の最大値と最小値の差の割合の決定］
　熱延コイルの幅方向中央部の長手方向における析出物の平均粒径等は、以下のようにして決定される。まず、熱延コイルの全長をＬとした場合に、当該熱延コイルの幅方向中央部における長手方向の端部から１／１０Ｌ位置、５／１０Ｌ位置、及び９／１０Ｌ位置の各位置においてレプリカ法によりサンプルを採取する。次いで、採取したサンプルを透過型電子顕微鏡（ＴＥＭ）を用いて５０～１００個の析出物を観察し、個々の析出物の粒径を円相当直径として算出し、算出された全ての円相当直径の平均値を各長手方向位置における析出物の粒径として決定し、得られた３つの粒径の算術平均を幅方向中央部の長手方向における析出物の平均粒径として決定する。最後に、得られた３つの粒径のうち最大値と最小値の差を算出し、算出された値を長手方向における析出物の平均粒径によって割り算することにより、長手方向における析出物の平均粒径に対する長手方向における析出物の粒径の最大値と最小値の差の割合を決定する。析出物の構成元素はＥＤＳ分析によって同定することが可能である。 [Determination of the average grain size of precipitates in the longitudinal direction at the center of the width direction, and the ratio of the difference between the maximum and minimum grain sizes of precipitates in the longitudinal direction to the average grain size]
The average grain size of the precipitates in the longitudinal direction of the widthwise center of the hot-rolled coil is determined as follows. First, when the total length of the hot-rolled coil is L, samples are taken by the replica method at each of the positions of 1/10L, 5/10L, and 9/10L from the longitudinal end of the widthwise center of the hot-rolled coil. Next, 50 to 100 precipitates are observed in the taken samples using a transmission electron microscope (TEM), the grain size of each precipitate is calculated as a circle equivalent diameter, the average value of all the calculated circle equivalent diameters is determined as the grain size of the precipitates at each longitudinal position, and the arithmetic average of the three grain sizes obtained is determined as the average grain size of the precipitates in the longitudinal direction of the widthwise center. Finally, the difference between the maximum and minimum values of the obtained three grain sizes is calculated, and the calculated value is divided by the average grain size of the precipitates in the longitudinal direction to determine the ratio of the difference between the maximum and minimum grain sizes of the precipitates in the longitudinal direction to the average grain size of the precipitates in the longitudinal direction. The constituent elements of the precipitates can be identified by EDS analysis.
［フェライト：５０％以上］
　本発明の実施形態に係る熱延コイルの金属組織は、特に限定されないが、例えば、面積％で、フェライトを５０％以上含むものであってよい。本発明は、上記のとおり、高強度でかつ強度ばらつきが低減された熱延コイルを提供することを目的とするものであって、所定の化学組成を有するとともに、細粒強化と析出強化を利用しかつ当該析出強化に寄与する析出物の粒径を熱延コイルの長手方向中央部の幅方向及び幅方向中央部の長手方向において適切に制御することによって当該目的を達成するものである。したがって、金属組織に関する他の特徴は、本発明の目的を達成する上で必須の技術的特徴でないことは明らかである。実際、本発明の実施形態に係る熱延コイルでは、金属組織を軟質なフェライト単相組織によって構成した場合においても、先に説明した化学組成、結晶粒及び析出物に関する要件を満たすことで、例えば７８０ＭＰａ以上の引張強さを確実に達成することが可能である。フェライトの面積率は、例えば５５％以上、６０％以上、７０％以上、８０％以上又は９０％以上であってもよい。上限は特に限定されないが、例えば、フェライトの面積率は１００％であってもよく、９５％以下であってもよい。フェライト以外の残部組織が存在する場合には、残部組織は、基本的にフェライトよりも硬質な組織、例えばマルテンサイト、ベイナイト、パーライト及び残留オーステナイトを含むことになる。このため、高強度の熱延コイルを提供するという観点からは、残部組織の具体的な組織が限定されないことは明らかである。 [Ferrite: 50% or more]
The metal structure of the hot rolled coil according to the embodiment of the present invention is not particularly limited, but may contain ferrite in an area percentage of 50% or more, for example. As described above, the present invention aims to provide a hot rolled coil having high strength and reduced strength variation, and achieves the object by having a predetermined chemical composition, utilizing fine grain strengthening and precipitation strengthening, and appropriately controlling the grain size of precipitates that contribute to the precipitation strengthening in the width direction of the central part in the longitudinal direction of the hot rolled coil and in the longitudinal direction of the central part in the width direction. Therefore, it is clear that other features related to the metal structure are not essential technical features for achieving the object of the present invention. In fact, in the hot rolled coil according to the embodiment of the present invention, even if the metal structure is constituted by a soft ferrite single phase structure, it is possible to reliably achieve a tensile strength of, for example, 780 MPa or more by satisfying the requirements related to the chemical composition, crystal grains, and precipitates described above. The area ratio of ferrite may be, for example, 55% or more, 60% or more, 70% or more, 80% or more, or 90% or more. The upper limit is not particularly limited, but the area ratio of ferrite may be, for example, 100% or less, or 95% or less. When a remaining structure other than ferrite is present, the remaining structure basically includes structures harder than ferrite, such as martensite, bainite, pearlite, and retained austenite. Therefore, from the viewpoint of providing a high-strength hot-rolled coil, it is clear that the specific structure of the remaining structure is not limited.
　フェライトの面積率は、以下のようにして決定される。まず、熱延コイルの圧延方向に平行かつ板面に垂直な方向の板厚断面を有する試料を採取し、当該断面を観察面とする。次いで、ＦＥ－ＳＥＭ（電界放射型走査型電子顕微鏡）による電子チャンネリングコントラスト像において、この観察面のうち、板厚１／４位置を中心とする板厚１／８～３／８の範囲内で１００μｍ×１００μｍの領域を観察することにより求める。より具体的には、上記領域内において、均一なコントラストで写る部分をフェライトとして特定し、その面積率を画像解析ソフトウェアＩｍａｇｅ　Ｊを用いて画像解析により算出することができる。 The ferrite area ratio is determined as follows. First, a sample is taken having a cross section through the plate thickness parallel to the rolling direction of the hot-rolled coil and perpendicular to the plate surface, and this cross section is used as the observation surface. Next, a 100 μm x 100 μm region is observed within the range of 1/8 to 3/8 of the plate thickness, centered at the 1/4 position, in an electron channeling contrast image taken with an FE-SEM (field emission scanning electron microscope) to determine the area ratio. More specifically, within the above region, the part that appears with a uniform contrast is identified as ferrite, and the area ratio can be calculated by image analysis using the image analysis software Image J.
［引張強さ］
　本発明の実施形態に係る熱延コイルでは、上で説明した化学組成及び金属組織を有することで、高い引張強さ、例えば７８０ＭＰａ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８００ＭＰａ以上、８２０ＭＰａ以上又は８４０ＭＰａ以上である。本発明の実施形態に係る熱延コイルによれば、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、析出物の粒径を熱延コイルの長手方向中央部の幅方向及び幅方向中央部の長手方向において適切に制御することで、熱延コイルの長手方向及び幅方向における強度ばらつきを顕著に抑制又は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、熱延コイルの引張強さは９８０ＭＰａ以下、９５０ＭＰａ以下又は９００ＭＰａ以下であってもよい。引張強さは、以下のようにして決定される。まず、熱延コイルの長手方向中央部かつ幅方向中央部から、圧延方向に平行な方向を試験方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を採取する。次いで、当該引張試験片を用いてＪＩＳ　Ｚ２２４１：２０１１に準拠した引張試験を行うことにより本発明の実施形態に係る熱延コイルの引張強さが決定される。 [Tensile strength]
The hot rolled coil according to the embodiment of the present invention has the above-described chemical composition and metal structure, and thus can achieve high tensile strength, for example, 780 MPa or more. The tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more. According to the hot rolled coil according to the embodiment of the present invention, despite having such a very high tensile strength, the grain size of the precipitates can be appropriately controlled in the width direction of the longitudinal center part of the hot rolled coil and in the longitudinal direction of the width direction of the central part of the width direction, thereby significantly suppressing or reducing the strength variation in the longitudinal direction and width direction of the hot rolled coil. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot rolled coil may be 980 MPa or less, 950 MPa or less, or 900 MPa or less. The tensile strength is determined as follows. First, a tensile test piece No. 5 of JIS Z2241:2011 is taken from the longitudinal center part and the width direction center part of the hot rolled coil, with the test direction being parallel to the rolling direction. Next, a tensile test in accordance with JIS Z2241:2011 is carried out using the tensile test specimen to determine the tensile strength of the hot-rolled coil according to the embodiment of the present invention.
［全幅Ｗ］
　本発明の実施形態に係る熱延コイルは、任意の全幅Ｗを有することができる。特に限定されないが、例えば、全幅Ｗは７００ｍｍ以上、８００ｍｍ以上、９００ｍｍ以上又は１０００ｍｍ以上であってもよい。上限は特に限定されないが、例えば、全幅は２５００ｍｍ以下、２２００ｍｍ以下、２０００ｍｍ以下、１８００ｍｍ以下、１６００ｍｍ以下、１５００ｍｍ以下、１４００ｍｍ以下又は１３００ｍｍ以下であってもよい。 [Total width W]
The hot rolled coil according to the embodiment of the present invention may have any overall width W. Although not particularly limited, for example, the overall width W may be 700 mm or more, 800 mm or more, 900 mm or more, or 1000 mm or more. Although there is no particular upper limit, for example, the overall width may be 2500 mm or less, 2200 mm or less, 2000 mm or less, 1800 mm or less, 1600 mm or less, 1500 mm or less, 1400 mm or less, or 1300 mm or less.
［板厚］
　本発明の実施形態に係る熱延コイルは、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、４．０ｍｍ以下若しくは３．０ｍｍ以下であってもよい。 [Thickness]
The hot rolled coil according to the embodiment of the present invention generally has a sheet thickness of 1.0 to 6.0 mm, although not particularly limited thereto. For example, the sheet 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, 4.0 mm or less, or 3.0 mm or less.
＜熱延コイルの製造方法＞
　次に、本発明の実施形態に係る熱延コイル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延コイル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、当該熱延コイル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 coil>
Next, a preferred method for manufacturing a hot-rolled coil according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a hot-rolled coil according to an embodiment of the present invention, and is not intended to limit the hot-rolled coil to one manufactured by the manufacturing method described below.
　本発明の実施形態に係る熱延コイルの製造方法は、
　熱延コイルに関連して上で説明した化学組成を有するスラブを１２３０～１２６０℃の温度に加熱して粗圧延及び仕上げ圧延することを含む熱間圧延工程であって、前記粗圧延の出側温度が１０７０～１１４０℃であり、前記仕上げ圧延の入側温度（Ｆ０）が９８０～１０５０℃であり、前記仕上げ圧延の出側温度（ＦＴ）が８５０～９２０℃であり、仕上げ圧延の総圧下率が８５～９５％である熱間圧延工程、
　仕上げ圧延された鋼板を、前記仕上げ圧延の出側温度（ＦＴ）から６５０～７２０℃の範囲内の温度Ｔ１までの温度域を６０～１００℃／ｓの平均冷却速度で１次冷却し、次いで温度Ｔ１から長手方向のコイル全長Ｌに対して１／１０Ｌ位置の巻取温度ＣＴｆまでの温度域を５～１０℃／ｓの平均冷却速度で２次冷却する冷却工程であって、前記１次冷却における鋼板の下面に対する上面の上下冷却比が０．８～１．２であり、前記２次冷却における鋼板の下面に対する上面の上下冷却比が０．８～１．２である冷却工程、並びに
　２次冷却された鋼板を巻き取り、次いで巻き取られた鋼板の幅方向におけるエッジ部に保熱処理を施す巻取工程であって、長手方向のコイル全長Ｌに対してそれぞれ１／１０Ｌ位置、５／１０Ｌ位置、及び９／１０Ｌ位置の巻取温度ＣＴｆ（℃）、ＣＴｍ（℃）及びＣＴｔ（℃）が下記式１～３を満足する巻取工程
を含むことを特徴としている。
　５５０≦ＣＴｍ≦６２０　　　　　　　　　・・・式１
　ＣＴｍ＋１５≦ＣＴｆ≦ＣＴｍ＋３０　　　・・・式２
　ＣＴｍ＋３０≦ＣＴｔ≦ＣＴｍ＋５０　　　・・・式３
　以下、各工程について詳しく説明する。 A method for producing a hot rolled coil according to an embodiment of the present invention includes the steps of:
a hot rolling process comprising heating a slab having the chemical composition described above in relation to the hot rolled coil to a temperature of 1230-1260°C and subjecting it to rough rolling and finish rolling, wherein the rough rolling exit temperature is 1070-1140°C, the finish rolling entry temperature (F0) is 980-1050°C, the finish rolling exit temperature (FT) is 850-920°C, and the finish rolling total reduction is 85-95%;
a cooling step in which the finish-rolled steel plate is primarily cooled in a temperature range from the finish-rolling exit temperature (FT) to a temperature T1 in a range of 650 to 720°C at an average cooling rate of 60 to 100°C/s, and then secondarily cooled in a temperature range from the temperature T1 to a coiling temperature CTf at a position 1/10L of the total coil length L in the longitudinal direction at an average cooling rate of 5 to 10°C/s, wherein the top and bottom cooling ratio of the top surface to the bottom surface of the steel plate in the primary cooling is 0.8 to 1.2, and the top and bottom cooling ratio of the top surface to the bottom surface of the steel plate in the secondary cooling is 0.8 to 1.2; This coiling process is characterized in that it includes winding the secondarily cooled steel sheet and then subjecting an edge portion in the width direction of the wound steel sheet to a heat retention treatment, and the coiling temperatures CTf (°C), CTm (°C) and CTt (°C) at 1/10L position, 5/10L position and 9/10L position with respect to the total length L of the coil in the longitudinal direction respectively satisfy the following formulas 1 to 3.
550≦CTm≦620 ... Formula 1
CTm+15≦CTf≦CTm+30 ... Formula 2
CTm+30≦CTt≦CTm+50 ... Formula 3
Each step will be described in detail below.
［熱間圧延工程］
［スラブの加熱］
　まず、熱延コイル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。使用されるスラブは合金元素を比較的多く含有し、とりわけＴｉを含有している。このため、合金元素をスラブ中に固溶させる必要があり、特にＴｉを十分に溶体化させる必要がある。スラブ加熱時にＴｉが十分に溶体化しないと、巻取工程においてＴｉを炭化物（ＴｉＣ）等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。したがって、Ｔｉを十分に溶体化させるため、スラブの加熱温度は１２３０℃以上とする必要がある。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。したがって、スラブの加熱温度は１２６０℃以下とする。 [Hot rolling process]
[Heating the slab]
First, a slab having the chemical composition described above in relation to the hot-rolled coil is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method. The slab used contains a relatively large amount of alloying elements, particularly Ti. For this reason, it is necessary to dissolve the alloying elements in the slab, and in particular, it is necessary to sufficiently dissolve Ti. If Ti is not sufficiently dissolved during slab heating, it becomes difficult to improve the strength of the steel by precipitation strengthening by finely precipitating Ti as carbide (TiC) or the like in the steel during the coiling process. Therefore, in order to sufficiently dissolve Ti, the heating temperature of the slab needs to be 1230°C or higher. On the other hand, if the heating temperature of the slab is higher than 1260°C, the yield decreases due to scale-off. Therefore, the heating temperature of the slab is set to 1260°C or lower.
［粗圧延］
　本方法では、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される。粗圧延は、所望のシートバー寸法を確保するとともに、仕上げ圧延における８５０℃以上の温度域での総圧下率を所望の範囲内に調整できるようにするため、粗圧延の出側温度を１０７０～１１４０℃とし、好ましくは１１００～１１４０℃である。粗圧延の出側温度が１０７０℃未満であると、粗圧延に続く仕上げ圧延で８５０℃以上の出側温度を得ることが困難となる。また、粗圧延の出側温度が１１４０℃を超えると、結晶粒が粗大化して、得られる熱延コイルの靭性が低下する場合がある。 [Rough rolling]
In this method, the heated slab is subjected to rough rolling before finish rolling for plate thickness adjustment and the like. In rough rolling, in order to ensure the desired sheet bar dimensions and to adjust the total rolling reduction in the temperature range of 850 ° C or more in the finish rolling to within a desired range, the delivery temperature of the rough rolling is set to 1070 to 1140 ° C, preferably 1100 to 1140 ° C. If the delivery temperature of the rough rolling is less than 1070 ° C, it is difficult to obtain an delivery temperature of 850 ° C or more in the finish rolling following the rough rolling. In addition, if the delivery temperature of the rough rolling exceeds 1140 ° C, the crystal grains may become coarse, and the toughness of the obtained hot rolled coil may decrease.
［仕上げ圧延］
　粗圧延されたスラブは、次に仕上げ圧延を施される。上記のように、使用されるスラブは合金元素を比較的多く含有しているため、熱間圧延の際に圧延荷重を大きくする必要がある。このため、熱間圧延は高温及び高圧下で行われ、具体的には仕上げ圧延の入側温度（Ｆ０）は９８０～１０５０℃、仕上げ圧延の出側温度（ＦＴ）は８５０～９２０℃、仕上げ圧延の総圧下率は８５～９５％とする。特に仕上げ圧延の出側温度（ＦＴ）は、鋼板の金属組織の制御の点で重要である。より詳しくは、仕上げ圧延の出側温度（ＦＴ）が低いと、金属組織が不均一となり、成形性が低下する場合があるか、及び／又は長手方向の中央部において方位差が１５°以上の粒界によって囲まれた結晶粒の平均粒径が５．０μｍ未満となり、強度が低下する場合がある。このため、仕上げ圧延の出側温度（ＦＴ）は８５０℃以上とする。一方で、仕上げ圧延の出側温度（ＦＴ）が９２０℃超であると、オーステナイト粒が粗大化してしまい、その後の冷却によって得られる結晶粒、より具体的には方位差が１５°以上の粒界によって囲まれた結晶粒の平均粒径を８．０μｍ以下に制御することができなくなる。 [Finish rolling]
The rough-rolled slab is then subjected to finish rolling. As described above, since the slab used contains a relatively large amount of alloy elements, it is necessary to increase the rolling load during hot rolling. For this reason, hot rolling is performed under high temperature and pressure, specifically, the entry temperature (F0) of the finish rolling is 980 to 1050 ° C, the exit temperature (FT) of the finish rolling is 850 to 920 ° C, and the total reduction rate of the finish rolling is 85 to 95%. In particular, the exit temperature (FT) of the finish rolling is important in terms of controlling the metal structure of the steel sheet. More specifically, if the exit temperature (FT) of the finish rolling is low, the metal structure may become non-uniform, the formability may decrease, and/or the average grain size of the crystal grains surrounded by grain boundaries with an orientation difference of 15 ° or more in the center of the longitudinal direction may become less than 5.0 μm, and the strength may decrease. For this reason, the exit temperature (FT) of the finish rolling is set to 850 ° C or more. On the other hand, if the exit temperature (FT) of the finish rolling exceeds 920°C, the austenite grains become coarse, and it becomes impossible to control the average grain size of the crystal grains obtained by the subsequent cooling, more specifically, the crystal grains surrounded by grain boundaries having an orientation difference of 15° or more, to 8.0 μm or less.
［冷却工程］
［１次冷却］
　仕上げ圧延された鋼板は、次の冷却工程において、まず、仕上げ圧延の出側温度（ＦＴ）から６５０～７２０℃の範囲内の温度Ｔ１までの温度域を６０～１００℃／ｓの平均冷却速度で１次冷却される。この温度域を６０～１００℃／ｓの平均冷却速度で１次冷却することで、結晶粒の粗大化を抑制しつつ、最終的に得られる金属組織を幅方向において均一なものとすることができる。これに関連して、その後の巻取工程において析出するＴｉＣ等の析出物の幅方向における粒径のばらつきを顕著に抑制することが可能となる。１次冷却の平均冷却速度が６０℃／ｓ未満であると、方位差が１５°以上の粒界によって囲まれた結晶粒の平均粒径を所望の範囲内に制御することができない場合がある。一方で、１次冷却の平均冷却速度が１００℃／ｓ超であると、冷却速度が速いために鋼板を幅方向で均一に冷却することが難しくなり、幅方向において冷却むら（温度偏差）が生じてしまう。この場合には、最終的に得られる熱延コイルにおいて幅方向でＴｉＣ等の析出物の粒径のばらつきを十分に抑制することができず、より具体的には幅方向におけるＴｉＣ等の析出物の粒径の最大値と最小値の差を当該析出物の平均粒径の１５．０％以下に制御することができなくなる。したがって、１次冷却の平均冷却速度は６０～１００℃／ｓとし、好ましくは６５～８５℃／ｓである。 [Cooling process]
[Primary cooling]
In the next cooling step, the finish-rolled steel sheet is first cooled at an average cooling rate of 60 to 100 ° C./s in a temperature range from the finish-rolling exit temperature (FT) to a temperature T1 in the range of 650 to 720 ° C. By performing the primary cooling at an average cooling rate of 60 to 100 ° C./s in this temperature range, the grain size of the final metal structure can be made uniform in the width direction while suppressing the coarsening of the crystal grains. In relation to this, it is possible to significantly suppress the variation in grain size in the width direction of precipitates such as TiC precipitated in the subsequent coiling step. If the average cooling rate of the primary cooling is less than 60 ° C./s, it may not be possible to control the average grain size of crystal grains surrounded by grain boundaries with an orientation difference of 15 ° or more within a desired range. On the other hand, if the average cooling rate of the primary cooling is more than 100 ° C./s, it becomes difficult to uniformly cool the steel sheet in the width direction due to the high cooling rate, and uneven cooling (temperature deviation) occurs in the width direction. In this case, it is not possible to sufficiently suppress the variation in the grain size of precipitates such as TiC in the width direction in the finally obtained hot rolled coil, and more specifically, it is not possible to control the difference between the maximum and minimum grain sizes of precipitates such as TiC in the width direction to 15.0% or less of the average grain size of the precipitates. Therefore, the average cooling rate of the primary cooling is set to 60 to 100°C/s, and preferably 65 to 85°C/s.
　１次冷却においては、平均冷却速度の制御に加えて、鋼板をその上面と下面で均等に冷却することが極めて重要である。このような冷却は、鋼板の下面に対する上面の上下冷却比が０．８～１．２となるように行われ、より具体的には鋼板の上面に噴射される冷却水の量が、鋼板の下面に噴射される冷却水の量に対して０．８～１．２倍になるように行われる。このように鋼板の上下面における冷却を均等に行うことで冷却むらの発生を顕著に抑制又は低減することが可能となる。これに関連して、その後の巻取工程において析出するＴｉＣ等の析出物の幅方向における粒径のばらつきを顕著に抑制することができ、その結果として最終的に得られる熱延コイルの長手方向及び幅方向における強度ばらつきを顕著に抑制又は低減することが可能となる。上下冷却比が０．８未満であるか又は１．２超であると、幅方向における冷却むらすなわち温度偏差の発生に起因して、その後の巻取工程においても幅方向に均一な冷却を実施することが困難となる。この場合には、最終的に得られる熱延コイルにおいて幅方向でＴｉＣ等の析出物の粒径のばらつきを十分に抑制することができず、すなわち幅方向におけるＴｉＣ等の析出物の粒径の最大値と最小値の差を当該析出物の平均粒径の１５．０％以下に制御することができなくなる。その結果として、熱延コイルの長手方向及び／又は幅方向における強度ばらつきを十分に抑制又は低減することができなくなる。 In the primary cooling, in addition to controlling the average cooling rate, it is extremely important to cool the steel plate evenly on its upper and lower surfaces. Such cooling is performed so that the upper and lower cooling ratio of the upper surface to the lower surface of the steel plate is 0.8 to 1.2, more specifically, the amount of cooling water sprayed on the upper surface of the steel plate is 0.8 to 1.2 times the amount of cooling water sprayed on the lower surface of the steel plate. By cooling the upper and lower surfaces of the steel plate evenly in this way, it is possible to significantly suppress or reduce the occurrence of cooling unevenness. In relation to this, it is possible to significantly suppress the variation in grain size in the width direction of precipitates such as TiC precipitated in the subsequent coiling process, and as a result, it is possible to significantly suppress or reduce the strength variation in the longitudinal and width directions of the finally obtained hot rolled coil. If the upper and lower cooling ratio is less than 0.8 or exceeds 1.2, it becomes difficult to perform uniform cooling in the width direction in the subsequent coiling process due to the occurrence of cooling unevenness in the width direction, i.e., temperature deviation. In this case, the variation in the grain size of precipitates such as TiC in the width direction of the finally obtained hot-rolled coil cannot be sufficiently suppressed, i.e., the difference between the maximum and minimum grain sizes of precipitates such as TiC in the width direction cannot be controlled to 15.0% or less of the average grain size of the precipitates. As a result, the strength variation in the longitudinal direction and/or width direction of the hot-rolled coil cannot be sufficiently suppressed or reduced.
　ここで、上記の上下冷却比とは、ＦＴ～Ｔ１℃の区間の上面全体の冷却水量と下面全体の冷却水量の比を意味するものではない。より具体的には、本製造方法では、ＦＴ～Ｔ１℃の区間を１０ｍごとのセクションに分け、これらのセクションごとに上面の冷却水量と下面の冷却水量から上下冷却比を計算し、このようにして計算された各セクションの上下冷却比が全て０．８～１．２の範囲内に制御される。分割されたセクションごとではなく区間全体の上下冷却比の制御では、例えば局所的な過冷などに起因する冷却むらの発生を十分に抑制することは非常に困難である。しかしながら、このようなセクションごとの上下冷却比の制御を実現することで、局所的な過冷などを低減して冷却むらの発生を確実に抑制することが可能となる。また、このようなセクションごとの上下冷却比の制御は、任意の適切な手段によって行うことができる。特に限定されないが、例えば、各セクションには鋼板の上側と下側で鋼板の進行方向に沿って複数の冷却水ノズルが配置されているため、これらの冷却水ノズルをオンオフ制御に基づいて適切に噴射することにより、各セクションの上下冷却比を０．８～１．２の範囲内に比較的容易に制御することが可能である。 Here, the above upper and lower cooling ratio does not mean the ratio of the amount of cooling water on the entire upper surface to the amount of cooling water on the entire lower surface in the FT to T1°C section. More specifically, in this manufacturing method, the FT to T1°C section is divided into sections every 10 m, and the upper and lower cooling ratios are calculated for each section from the amount of cooling water on the upper surface and the amount of cooling water on the lower surface, and the upper and lower cooling ratios of each section calculated in this way are all controlled within the range of 0.8 to 1.2. When controlling the upper and lower cooling ratios of the entire section rather than for each divided section, it is very difficult to sufficiently suppress the occurrence of cooling unevenness caused by, for example, local overcooling. However, by realizing such control of the upper and lower cooling ratios for each section, it is possible to reduce local overcooling and reliably suppress the occurrence of cooling unevenness. In addition, such control of the upper and lower cooling ratios for each section can be performed by any appropriate means. Although not particularly limited, for example, each section has multiple cooling water nozzles arranged above and below the steel plate along the direction of travel of the steel plate, and by appropriately spraying these cooling water nozzles based on on/off control, it is relatively easy to control the upper/lower cooling ratio of each section within the range of 0.8 to 1.2.
［２次冷却］
　１次冷却された鋼板は、次いで温度Ｔ１から長手方向のコイル全長Ｌにおける先端から１／１０Ｌ位置の巻取温度ＣＴｆまでの温度域を５～１０℃／ｓの平均冷却速度で２次冷却される。温度Ｔ１から巻取温度ＣＴｆまでの温度域を５～１０℃／ｓの平均冷却速度で２次冷却することで、方位差が１５°以上の粒界によって囲まれた結晶粒を適切に生成するとともに、その平均粒径を所望の範囲内に制御することができる。２次冷却の平均冷却速度が５℃／ｓ未満であると、方位差が１５°以上の粒界によって囲まれた結晶粒が粗大化し、その平均粒径を８．０μｍ以下に制御することができなくなる。この場合には、細粒強化による強度向上効果を十分に得られず、それゆえ最終的に得られる熱延コイルにおいて所望の高強度を確実に達成することができなくなる。一方で、２次冷却の平均冷却速度が１０℃／ｓ超であると、方位差が１５°以上の粒界によって囲まれた結晶粒を適切に生成できないか、又は当該結晶粒の平均粒径を５．０μｍ以上に制御することができない場合がある。この場合も同様に、細粒強化による強度向上効果を十分に得られなくなる。加えて、２次冷却の平均冷却速度が１０℃／ｓ超であると、硬質組織の過度な生成に起因して熱延コイルの長手方向及び／又は幅方向における強度ばらつきの発生が顕著となってしまう場合がある。 [Secondary cooling]
The steel sheet that has been primarily cooled is then secondarily cooled at an average cooling rate of 5 to 10 ° C./s in a temperature range from temperature T1 to the coiling temperature CTf at 1/10L from the tip of the entire coil length L in the longitudinal direction. By performing secondary cooling at an average cooling rate of 5 to 10 ° C./s in a temperature range from temperature T1 to coiling temperature CTf, crystal grains surrounded by grain boundaries with an orientation difference of 15° or more can be appropriately generated and the average grain size can be controlled within a desired range. If the average cooling rate of the secondary cooling is less than 5 ° C./s, the crystal grains surrounded by grain boundaries with an orientation difference of 15° or more become coarse, and it becomes impossible to control the average grain size to 8.0 μm or less. In this case, the strength improvement effect due to fine grain strengthening cannot be sufficiently obtained, and therefore the desired high strength cannot be reliably achieved in the finally obtained hot rolled coil. On the other hand, if the average cooling rate of the secondary cooling exceeds 10°C/s, it may not be possible to properly generate crystal grains surrounded by grain boundaries with an orientation difference of 15° or more, or it may not be possible to control the average grain size of the crystal grains to 5.0 μm or more. In this case as well, the strength improvement effect of fine grain strengthening cannot be sufficiently obtained. In addition, if the average cooling rate of the secondary cooling exceeds 10°C/s, the occurrence of strength variations in the longitudinal direction and/or width direction of the hot rolled coil may become significant due to excessive generation of hard structures.
　なお、本明細書において、長手方向の熱延コイルの全長ＬにおけるＸ／１０Ｌ位置（Ｘは１～９の自然数）とは、熱延コイルにおいて、長手方向（圧延方向）における先端から、長手方向の尾端に向かって距離「Ｘ／１０Ｌ」だけ離れた位置を意味する。たとえば、熱延コイルの長手方向における全長Ｌが１０００ｍであった場合に、「１／１０Ｌ位置」とは、長手方向における先端から、長手方向の尾端に向かって距離「１００ｍ」だけ離れた位置である。 In this specification, the X/10L position (X is a natural number from 1 to 9) in the total length L of the hot-rolled coil in the longitudinal direction means a position in the hot-rolled coil that is a distance "X/10L" away from the leading end in the longitudinal direction (rolling direction) toward the tail end in the longitudinal direction. For example, if the total length L of the hot-rolled coil in the longitudinal direction is 1000 m, the "1/10L position" is a position that is a distance "100 m" away from the leading end in the longitudinal direction toward the tail end in the longitudinal direction.
　２次冷却においても、１次冷却の場合と同様に、平均冷却速度の制御に加えて、鋼板をその上面と下面で均等に冷却することが極めて重要である。このような冷却は、１次冷却の場合と同様に、鋼板の下面に対する上面の上下冷却比が０．８～１．２となるように行われ、より具体的には鋼板の上面に噴射される冷却水の量が、鋼板の下面に噴射される冷却水の量に対して０．８～１．２倍になるように行われる。このように鋼板の上下面における冷却を均等に行うことで冷却むらの発生を顕著に抑制又は低減することが可能となる。これに関連して、その後の巻取工程において析出するＴｉＣ等の析出物の幅方向における粒径のばらつきを顕著に抑制することができ、その結果として最終的に得られる熱延コイルの長手方向及び幅方向における強度ばらつきを顕著に抑制又は低減することが可能となる。上下冷却比が０．８未満であるか又は１．２超であると、幅方向における冷却むらすなわち温度偏差の発生に起因して、その後の巻取工程においても幅方向に均一な冷却を実施することが困難となる。この場合には、最終的に得られる熱延コイルにおいて幅方向でＴｉＣ等の析出物の粒径のばらつきを十分に抑制することができず、すなわち幅方向におけるＴｉＣ等の析出物の粒径の最大値と最小値の差を当該析出物の平均粒径の１５．０％以下に制御することができなくなる。その結果として、熱延コイルの長手方向及び／又は幅方向における強度ばらつきを十分に抑制又は低減することができなくなる。 In the secondary cooling, as in the primary cooling, in addition to controlling the average cooling rate, it is extremely important to cool the steel plate evenly on its upper and lower surfaces. As in the primary cooling, this cooling is performed so that the upper and lower cooling ratio of the upper surface to the lower surface of the steel plate is 0.8 to 1.2, and more specifically, the amount of cooling water sprayed on the upper surface of the steel plate is 0.8 to 1.2 times the amount of cooling water sprayed on the lower surface of the steel plate. By uniformly cooling the upper and lower surfaces of the steel plate in this way, it is possible to significantly suppress or reduce the occurrence of cooling unevenness. In relation to this, it is possible to significantly suppress the variation in grain size in the width direction of precipitates such as TiC precipitated in the subsequent coiling process, and as a result, it is possible to significantly suppress or reduce the strength variation in the longitudinal and width directions of the finally obtained hot rolled coil. If the upper and lower cooling ratio is less than 0.8 or exceeds 1.2, it becomes difficult to perform uniform cooling in the width direction in the subsequent coiling process due to the occurrence of cooling unevenness in the width direction, i.e., temperature deviation. In this case, the variation in the grain size of precipitates such as TiC in the width direction of the finally obtained hot-rolled coil cannot be sufficiently suppressed, i.e., the difference between the maximum and minimum grain sizes of precipitates such as TiC in the width direction cannot be controlled to 15.0% or less of the average grain size of the precipitates. As a result, the strength variation in the longitudinal direction and/or width direction of the hot-rolled coil cannot be sufficiently suppressed or reduced.
　ここで、上記の上下冷却比とは、Ｔ１～ＣＴｆ℃の区間の上面全体の冷却水量と下面全体の冷却水量の比を意味するものではない。より具体的には、本製造方法では、Ｔ１～ＣＴｆ℃の区間を１０ｍごとのセクションに分け、これらのセクションごとに上面の冷却水量と下面の冷却水量から上下冷却比を計算し、このようにして計算された各セクションの上下冷却比が全て０．８～１．２の範囲内に制御される。分割されたセクションごとではなく区間全体の上下冷却比の制御では、例えば局所的な過冷などに起因する冷却むらの発生を十分に抑制することは非常に困難である。しかしながら、このようなセクションごとの上下冷却比の制御を実現することで、局所的な過冷などを低減して冷却むらの発生を確実に抑制することが可能となる。また、このようなセクションごとの上下冷却比の制御は、任意の適切な手段によって行うことができる。特に限定されないが、１次冷却の場合と同様に、例えば、各セクションには鋼板の上側と下側で鋼板の進行方向に沿って複数の冷却水ノズルが配置されているため、これらの冷却水ノズルをオンオフ制御に基づいて適切に噴射することにより、各セクションの上下冷却比を０．８～１．２の範囲内に比較的容易に制御することが可能である。 Here, the above upper and lower cooling ratio does not mean the ratio of the amount of cooling water on the entire upper surface to the amount of cooling water on the entire lower surface in the section from T1 to CTf°C. More specifically, in this manufacturing method, the section from T1 to CTf°C is divided into sections of 10 m each, and the upper and lower cooling ratios are calculated for each of these sections from the amount of cooling water on the upper surface and the amount of cooling water on the lower surface, and the upper and lower cooling ratios of each section calculated in this manner are all controlled within the range of 0.8 to 1.2. When controlling the upper and lower cooling ratios of the entire section rather than for each divided section, it is very difficult to sufficiently suppress the occurrence of cooling unevenness caused by, for example, localized overcooling. However, by realizing such control of the upper and lower cooling ratios for each section, it is possible to reduce localized overcooling and reliably suppress the occurrence of cooling unevenness. Moreover, such control of the upper and lower cooling ratios for each section can be performed by any appropriate means. Although not particularly limited, as in the case of primary cooling, for example, each section has multiple cooling water nozzles arranged above and below the steel plate along the direction of travel of the steel plate, and by appropriately spraying these cooling water nozzles based on on/off control, it is relatively easy to control the upper/lower cooling ratio of each section within the range of 0.8 to 1.2.
［巻取工程］
　２次冷却された鋼板は、最後に巻取工程において巻き取られ、次いで巻き取られた鋼板の幅方向におけるエッジ部に保熱処理が施される。加えて、巻取工程では、長手方向のコイル全長Ｌに対してそれぞれ１／１０Ｌ位置、５／１０Ｌ位置、及び９／１０Ｌ位置の巻取温度ＣＴｆ（℃）、ＣＴｍ（℃）及びＣＴｔ（℃）が下記式１～３を満足する必要がある。
　５５０≦ＣＴｍ≦６２０　　　　　　　　　・・・式１
　ＣＴｍ＋１５≦ＣＴｆ≦ＣＴｍ＋３０　　　・・・式２
　ＣＴｍ＋３０≦ＣＴｔ≦ＣＴｍ＋５０　　　・・・式３ [Winding process]
The secondarily cooled steel sheet is finally wound in a winding process, and then a heat retention treatment is performed on the edge portion in the width direction of the wound steel sheet. In addition, in the winding process, the winding temperatures CTf (°C), CTm (°C), and CTt (°C) at the 1/10L position, 5/10L position, and 9/10L position with respect to the total coil length L in the longitudinal direction, respectively, need to satisfy the following formulas 1 to 3.
550≦CTm≦620 ... Formula 1
CTm+15≦CTf≦CTm+30 ... Formula 2
CTm+30≦CTt≦CTm+50 ... Formula 3
　析出物の析出状態は、巻き取り後の冷却履歴によって大きく影響を受ける。例えば、熱延コイルの長手方向における先端部及び尾端部は、それぞれ熱延コイルの最内周部及び最外周部に対応し、それゆえ大気にさらされる。このため、一般的に、熱延コイルの先端部及び尾端部は冷却が速く、析出物の析出が十分に進行しない場合がある。したがって、析出状態としては亜時効状態となり、強度が低下しやすい傾向にある。この場合には、所望の強度が得られなかったり、析出物の析出状態が長手方向において異なることで強度のばらつきが生じたりすることがある。一方で、熱延コイルの長手方向における中央部は大気に直接さらされていないために冷えにくく、比較的高温の状態で保持される。したがって、場合によっては析出物が粗大化し、析出強化の高い効果が得られるピーク時効を過ぎて過時効状態となり、同様に強度が低下することがある。この場合も同様に、所望の強度が得られなかったり、強度のばらつきが生じたりすることがある。加えて、巻き取り後の幅方向における冷却が均一に行われない場合には、幅方向の温度偏差に起因して長手方向における析出物の析出状態も影響を受けてしまう。その結果として、同様に最終的に得られる熱延コイルにおいて所望の強度が得られなかったり、強度のばらつきが顕著となってしまったりすることがある。 The precipitation state of the precipitates is greatly affected by the cooling history after coiling. For example, the tip and tail of the hot-rolled coil in the longitudinal direction correspond to the innermost and outermost parts of the hot-rolled coil, respectively, and are therefore exposed to the atmosphere. For this reason, the tip and tail of the hot-rolled coil generally cool quickly, and the precipitation of the precipitates may not progress sufficiently. Therefore, the precipitation state is in a sub-aged state, and the strength tends to decrease. In this case, the desired strength may not be obtained, or the precipitation state of the precipitates may differ in the longitudinal direction, resulting in strength variations. On the other hand, the central part of the hot-rolled coil in the longitudinal direction is not directly exposed to the atmosphere, so it is difficult to cool, and is maintained at a relatively high temperature. Therefore, in some cases, the precipitates may coarsen, and the peak aging at which a high effect of precipitation strengthening is obtained may be exceeded, resulting in an over-aged state, and the strength may also decrease. In this case, the desired strength may not be obtained, or strength variations may occur. In addition, if cooling in the width direction after coiling is not uniform, the temperature deviation in the width direction will affect the state of precipitation in the longitudinal direction. As a result, the final hot-rolled coil may not have the desired strength or may have significant variations in strength.
　そこで、本製造方法では、まず、大気に直接さらされる巻き取り後の鋼板の幅方向におけるエッジ部に保熱処理を施すことで、巻き取り後の熱延コイルの幅方向における温度偏差を低減することができる。エッジ部に保熱処理を施さない場合には、幅方向の温度偏差に起因して長手方向及び幅方向における析出物の析出状態も影響を受けてしまう。その結果として、長手方向中央部における析出物の平均粒径や、長手方向中央部における析出物の粒径のばらつきを所望の範囲内に制御することができない場合がある。このようなエッジ部の保熱処理は、当業者に公知の任意の適切な手段によって実施することができる。特に限定されないが、例えば、エッジ部の保熱処理は、複数の熱延コイルを隣接して配置することで、大気によるエッジ部の冷却を防ぐようにして実施することが可能である。ここで、「複数の熱延コイルを隣接して配置すること」は、熱延コイルの幅方向における端面（エッジ部）同士が向かい合うように配置することを含む。また、複数の熱延コイルの直径が同等である場合、各熱延コイルの中心軸が重なるように、すなわち、同軸上に並べて配置することが好ましい。また、複数の熱延コイルを隣接して配置する際、上記端面（エッジ部）間の距離が２００～８００ｍｍであることが好ましく、２００～６００ｍｍであることがより好ましく、２００～５００ｍｍであることが更に好ましい。 Therefore, in this manufacturing method, first, a heat retention treatment is performed on the edge portion in the width direction of the steel sheet after coiling, which is directly exposed to the atmosphere, so that the temperature deviation in the width direction of the hot-rolled coil after coiling can be reduced. If the edge portion is not heat-retained, the precipitation state of the precipitates in the longitudinal and width directions will also be affected due to the temperature deviation in the width direction. As a result, the average grain size of the precipitates in the longitudinal center portion and the grain size variation of the precipitates in the longitudinal center portion may not be controlled within the desired range. Such heat retention treatment of the edge portion can be performed by any appropriate means known to those skilled in the art. Although not particularly limited, for example, the heat retention treatment of the edge portion can be performed by arranging multiple hot-rolled coils adjacent to each other to prevent the edge portion from being cooled by the atmosphere. Here, "arranging multiple hot-rolled coils adjacent to each other" includes arranging the end faces (edge portions) of the hot-rolled coils in the width direction so that they face each other. In addition, when the diameters of multiple hot-rolled coils are the same, it is preferable to arrange them so that the central axes of each hot-rolled coil overlap, that is, to arrange them side by side on the same axis. In addition, when multiple hot-rolled coils are arranged adjacent to each other, the distance between the end faces (edges) is preferably 200 to 800 mm, more preferably 200 to 600 mm, and even more preferably 200 to 500 mm.
　加えて、本製造方法では、熱延コイルの長手方向においても適切な冷却が施される。具体的には、熱延コイルの長手方向の中央部における析出物の析出状態を適切なものとするために、長手方向のコイル全長Ｌに対して５／１０Ｌ位置の巻取温度ＣＴｍ（℃）が下記式１を満足するように制御される。
　５５０≦ＣＴｍ≦６２０　　　　　　　　　・・・式１
　ＣＴｍが５５０℃未満であると、中央部における析出物の析出状態が亜時効状態となり、一方で、ＣＴｍが６２０℃超であると、中央部における析出物の析出状態が過時効状態となる。いずれの場合も、析出強化による強度向上効果を十分に得ることができないか、及び／又は熱延コイルの長手方向及び幅方向における強度ばらつきを十分に抑制若しくは低減することができない場合がある。また、ＣＴｍが式１を満足する場合であっても、ＣＴｆが式２を満たさないか及び／又はＣＴｔが式３を満たさない場合には、長手方向中央部における析出物の析出状態が大きく影響を受ける。この場合には、長手方向中央部における析出物の平均粒径や、長手方向中央部の幅方向における析出物の粒径のばらつきを所望の範囲内に制御することができないことがある。 In addition, in the present manufacturing method, the hot-rolled coil is also appropriately cooled in the longitudinal direction. Specifically, in order to make the precipitation state of the precipitates in the longitudinal center of the hot-rolled coil appropriate, the coiling temperature CTm (°C) at a position 5/10L of the total length L of the coil in the longitudinal direction is controlled to satisfy the following formula 1.
550≦CTm≦620 ... Formula 1
If CTm is less than 550 ° C, the precipitates in the central part are in an underaged state, whereas if CTm is more than 620 ° C, the precipitates in the central part are in an overaged state. In either case, the strength improvement effect due to precipitation strengthening may not be sufficiently obtained, and/or the strength variation in the longitudinal and width directions of the hot-rolled coil may not be sufficiently suppressed or reduced. Even if CTm satisfies formula 1, if CTf does not satisfy formula 2 and/or CTt does not satisfy formula 3, the precipitation state of the precipitates in the longitudinal center part is greatly affected. In this case, it may not be possible to control the average grain size of the precipitates in the longitudinal center part and the grain size variation in the width direction of the longitudinal center part within the desired range.
　熱延コイルの長手方向の先端部は、上記のとおり熱延コイルの最内周部に対応するため、大気にさらされて比較的冷却されやすい。そこで、長手方向のコイル全長Ｌに対して１／１０Ｌ位置の巻取温度ＣＴｆ（℃）は、下記式２を満足するようにＣＴｍよりも１５～３０℃高い温度範囲に制御される。
　ＣＴｍ＋１５≦ＣＴｆ≦ＣＴｍ＋３０　　　・・・式２
　大気による冷却を考慮して、ＣＴｆを式２に示すようにＣＴｍよりも１５～３０℃高い温度範囲に制御することで、先端部における析出物の析出状態を中央部と同様の析出状態に維持することが可能となる。ＣＴｆがＣＴｍ＋１５℃未満であると、先端部における析出物の析出状態が亜時効状態となり、一方で、ＣＴｆがＣＴｍ＋３０℃超であると、先端部における析出物の析出状態が過時効状態となる場合がある。いずれの場合も、析出強化による強度向上効果を十分に得ることができないか、及び／又は熱延コイルの長手方向及び幅方向における強度ばらつきを十分に抑制若しくは低減することができない場合がある。 Since the longitudinal tip of the hot-rolled coil corresponds to the innermost circumferential portion of the hot-rolled coil as described above, it is exposed to the atmosphere and is relatively easily cooled. Therefore, the coiling temperature CTf (°C) at 1/10L of the longitudinal coil total length L is controlled to a temperature range 15 to 30°C higher than CTm so as to satisfy the following formula 2.
CTm+15≦CTf≦CTm+30 ... Formula 2
Taking into account cooling by air, it is possible to maintain the precipitation state of the precipitates at the tip portion in the same precipitation state as that at the center portion by controlling CTf to a temperature range 15 to 30°C higher than CTm as shown in formula 2. If CTf is less than CTm + 15°C, the precipitation state of the precipitates at the tip portion may be in an underaged state, while if CTf is more than CTm + 30°C, the precipitation state of the precipitates at the tip portion may be in an overaged state. In either case, the strength improvement effect due to precipitation strengthening may not be sufficiently obtained, and/or the strength variation in the longitudinal and transverse directions of the hot-rolled coil may not be sufficiently suppressed or reduced.
　熱延コイルの長手方向の尾端部は、上記のとおり熱延コイルの最外周部に対応するため、最内周部に対応する先端部に比べてさらに冷却されやすい。そこで、長手方向のコイル全長Ｌに対して９／１０Ｌ位置の巻取温度ＣＴｔ（℃）は、下記式３を満足するようにＣＴｍよりも３０～５０℃高い温度範囲に制御される。
　ＣＴｍ＋３０≦ＣＴｔ≦ＣＴｍ＋５０　　　・・・式３
　大気による冷却を考慮して、ＣＴｍを式３に示すようにＣＴｍよりも３０～５０℃高い温度範囲に制御することで、尾端部における析出物の析出状態を中央部と同様の析出状態に維持することが可能となる。ＣＴｔがＣＴｍ＋３０℃未満であると、尾端部における析出物の析出状態が亜時効状態となり、一方で、ＣＴｔがＣＴｍ＋５０℃超であると、尾端部における析出物の析出状態が過時効状態となる場合がある。いずれの場合も、析出強化による強度向上効果を十分に得ることができないか、及び／又は熱延コイルの長手方向及び幅方向における強度ばらつきを十分に抑制若しくは低減することができない場合がある。 As described above, the tail end of the hot rolled coil in the longitudinal direction corresponds to the outermost peripheral portion of the hot rolled coil, and is therefore more easily cooled than the front end corresponding to the innermost peripheral portion. Therefore, the coiling temperature CTt (°C) at the 9/10L position with respect to the total length L of the coil in the longitudinal direction is controlled to a temperature range 30 to 50°C higher than CTm so as to satisfy the following formula 3.
CTm+30≦CTt≦CTm+50 ... Formula 3
Taking into account cooling by air, it is possible to maintain the precipitation state of the precipitates in the tail end portion in the same precipitation state as that in the central portion by controlling CTm to a temperature range 30 to 50°C higher than CTm as shown in Equation 3. If CTt is less than CTm+30°C, the precipitation state of the precipitates in the tail end portion may be in an underaged state, whereas if CTt is more than CTm+50°C, the precipitation state of the precipitates in the tail end portion may be in an overaged state. In either case, the strength improvement effect by precipitation strengthening may not be sufficiently obtained, and/or the strength variation in the longitudinal and transverse directions of the hot-rolled coil may not be sufficiently suppressed or reduced.
　上記式１～３に基づく巻取温度の制御は、任意の適切な手段によって実施することができ特に限定されないが、例えば、上記２次冷却における冷却水量を適切に制御することで比較的容易に実施することができる。従来、熱間圧延後の鋼板を所定の温度範囲内で巻き取ることは一般的に行われているものの、熱延コイルの長手方向における先端部、中央部及び尾端部で巻取温度の制御範囲を変更するというような操作は行われていない。したがって、熱延コイルの長手方向における先端部、中央部及び尾端部の冷却履歴を適切なものとし、さらには熱延コイルの幅方向における保熱処理を行うことで、析出強化に起因する強度ばらつきを顕著に抑制又は低減することができるという事実は、今回、本発明者らによって初めて明らかにされたことである。 The coiling temperature control based on the above formulas 1 to 3 can be performed by any appropriate means, and is not particularly limited. For example, it can be performed relatively easily by appropriately controlling the amount of cooling water in the secondary cooling. Conventionally, although it has been common to coil the steel sheet after hot rolling within a predetermined temperature range, no operation has been performed to change the coiling temperature control range at the tip, center, and tail end in the longitudinal direction of the hot rolled coil. Therefore, the inventors have now revealed for the first time that the strength variation due to precipitation strengthening can be significantly suppressed or reduced by making the cooling history of the tip, center, and tail end in the longitudinal direction of the hot rolled coil appropriate, and further by performing heat retention treatment in the width direction of the hot rolled coil.
　上記の製造方法によって製造された熱延コイルによれば、長手方向の中央部において、方位差が１５°以上の粒界によって囲まれた結晶粒の平均粒径を５．０～８．０μｍの範囲内に制御することによる細粒強化と、析出物の平均粒径を３．０～９．５ｎｍの範囲内に制御することによる析出強化との組み合わせに基づき、最終的に得られる熱延コイルにおいて高い引張強さ、例えば７８０ＭＰａ以上の引張強さを確実に達成することが可能となる。加えて、熱延コイルの長手方向中央部の幅方向及び幅方向中央部の長手方向における析出物の粒径の最大値と最小値の差を当該析出物の平均粒径の１５．０％以下に制御することができ、その結果として熱延コイルの長手方向及び幅方向における強度ばらつきを顕著に抑制又は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延コイルは、高強度であるにもかかわらず、顕著に抑制又は低減された強度ばらつきを達成することができる。このため、鋼板のプレス加工時に成形不良が生じるリスクを低減することができ、生産性も顕著に向上させることが可能となる。したがって、当該熱延コイルは、自動車分野の使用において特に有用であることは当然ながら、他の分野においても非常に有効に使用することが可能である。 The hot-rolled coil manufactured by the above manufacturing method can reliably achieve high tensile strength, for example tensile strength of 780 MPa or more, in the finally obtained hot-rolled coil based on a combination of fine grain strengthening by controlling the average grain size of crystal grains surrounded by grain boundaries with an orientation difference of 15° or more in the longitudinal center to within the range of 5.0 to 8.0 μm, and precipitation strengthening by controlling the average grain size of precipitates to within the range of 3.0 to 9.5 nm. In addition, the difference between the maximum and minimum values of the grain size of the precipitates in the width direction of the longitudinal center of the hot-rolled coil and in the longitudinal direction of the width direction center can be controlled to 15.0% or less of the average grain size of the precipitates, and as a result, the strength variation in the longitudinal direction and width direction of the hot-rolled coil can be significantly suppressed or reduced. Therefore, the hot-rolled coil manufactured by the above manufacturing method can achieve a significantly suppressed or reduced strength variation despite its high strength. This reduces the risk of forming defects during press working of the steel sheet, and also significantly improves productivity. Therefore, the hot-rolled coil is of course particularly useful in the automotive field, but can also be used very effectively in other fields.
　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.
　まず、溶鋼を連続鋳造法にて鋳造して表１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２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、粗圧延の出側温度並びに仕上げ圧延の入側温度（Ｆ０）、出側温度（ＦＴ）及び総圧下率は表２に示すとおりであった。次に、仕上げ圧延された鋼板を、表２に示す条件下で、まず仕上げ圧延の出側温度（ＦＴ）から６５０～７２０℃の範囲内の温度Ｔ１までの温度域において１次冷却を施し、次いで温度Ｔ１から長手方向のコイル全長Ｌに対して１／１０Ｌ位置の巻取温度ＣＴｆまでの温度域において２次冷却を施した。 First, molten steel was cast by continuous casting to form slabs with various chemical compositions shown in Table 1. These slabs were heated under the conditions shown in Table 2, and then hot-rolled. Hot rolling was performed by rough rolling and finish rolling, and the rough rolling exit temperature and the finish rolling entry temperature (F0), exit temperature (FT), and total rolling reduction were as shown in Table 2. Next, the finish-rolled steel sheet was subjected to primary cooling under the conditions shown in Table 2 in a temperature range from the finish rolling exit temperature (FT) to a temperature T1 in the range of 650-720°C, and then secondary cooling in a temperature range from temperature T1 to a coiling temperature CTf at 1/10L of the total coil length L in the longitudinal direction.
　１次冷却及び２次冷却においては、それぞれＦＴ～Ｔ１℃の区間とＴ１～ＣＴｆ℃の区間とを１０ｍごとのセクションに分け、これらのセクションごとに上面の冷却水量と下面の冷却水量から上下冷却比を計算し、このようにして計算される各セクションの上下冷却比が所定の範囲内に制御されるようにして冷却を実施した。表２の１次冷却及び２次冷却における上下冷却比は、１次冷却及び２次冷却における各セクションの上下冷却比のうち冷却比１からの差の絶対値が最も大きいものを示している。最後に、２次冷却された鋼板を巻き取り、長手方向のコイル全長Ｌに対してそれぞれ１／１０Ｌ位置、５／１０Ｌ位置、及び９／１０Ｌ位置の巻取温度ＣＴｆ（℃）、ＣＴｍ（℃）及びＣＴｔ（℃）は表２に示すとおりであった。また、巻き取り後のエッジ部の保熱処理は、複数の熱延コイルを隣接して配置することにより行った。隣接して配置した各熱延コイルのエッジ部間の距離は３００ｍｍとした。表２中の「隣接」の表記は、巻き取り後に保熱処理を施したことを示している。一方で、表２中の「単独」の表記は、熱延コイルが単独で空冷されたことを意味し、すなわち巻き取り後のエッジ部に保熱処理を施さなかったことを示している。得られた熱延コイルは、約２．３～４．０ｍｍの板厚、約８００～１５００ｍｍの全幅Ｗ、及び約５００～１２００ｍの全長Ｌを有するものであった。 In the primary and secondary cooling, the FT to T1°C section and the T1 to CTf°C section were each divided into 10m sections, and the top and bottom cooling ratios were calculated for each section from the amount of cooling water on the top surface and the amount of cooling water on the bottom surface. Cooling was performed so that the top and bottom cooling ratios of each section calculated in this way were controlled within a predetermined range. The top and bottom cooling ratios in the primary and secondary cooling in Table 2 indicate the top and bottom cooling ratios of each section in the primary and secondary cooling that have the largest absolute difference from a cooling ratio of 1. Finally, the secondary cooled steel sheet was coiled, and the coiling temperatures CTf (°C), CTm (°C), and CTt (°C) at the 1/10L, 5/10L, and 9/10L positions relative to the total length L of the coil in the longitudinal direction were as shown in Table 2. In addition, the heat retention treatment of the edge portion after coiling was performed by arranging multiple hot rolled coils adjacent to each other. The distance between the edges of each adjacently arranged hot rolled coil was 300 mm. The notation "adjacent" in Table 2 indicates that heat retention treatment was performed after coiling. On the other hand, the notation "single" in Table 2 means that the hot-rolled coil was air-cooled alone, that is, the edge portion was not subjected to heat retention treatment after coiling. The obtained hot-rolled coil had a sheet thickness of about 2.3 to 4.0 mm, a total width W of about 800 to 1500 mm, and a total length L of about 500 to 1200 m.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　得られた熱延コイル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 obtained hot-rolled coil were measured and evaluated using the following methods.
［熱延コイルの引張強さ］
　まず、熱延コイルの長手方向中央部かつ幅方向中央部から、圧延方向に平行な方向を試験方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を採取した。次いで、当該引張試験片を用いてＪＩＳ　Ｚ２２４１：２０１１に準拠した引張試験を行うことにより熱延コイルの引張強さを決定した。 [Tensile strength of hot rolled coil]
First, a tensile test piece No. 5 of JIS Z2241:2011 was taken from the center of the hot-rolled coil in the longitudinal direction and the center of the width direction, with the test direction being parallel to the rolling direction. Next, the tensile test piece was used to perform a tensile test in accordance with JIS Z2241:2011 to determine the tensile strength of the hot-rolled coil.
［長手方向の強度ばらつき］
　まず、熱延コイルの幅方向中央部において長手方向のコイル全長Ｌに対して１／１０Ｌ、５／１０Ｌ、及び９／１０Ｌ位置から、圧延方向に平行な方向を試験方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を採取した。次いで、これらの引張試験片を用いてＪＩＳ　Ｚ２２４１：２０１１に準拠した引張試験を行うことにより３つの引張強さの値を得、次いでそれらの最大値と最小値の差を算出し、得られた値を長手方向の強度ばらつきの値として決定した。この強度ばらつきの値が１５．０ＭＰａ以下の場合を長手方向の強度ばらつきが合格、１５．０ＭＰａ超の場合を長手方向の強度ばらつきが不合格として評価した。 [Strength variation in the longitudinal direction]
First, tensile test pieces No. 5 of JIS Z2241:2011 were taken from the 1/10L, 5/10L, and 9/10L positions with respect to the total length L of the coil in the longitudinal direction in the width direction center of the hot-rolled coil, with the test direction being parallel to the rolling direction. Next, these tensile test pieces were used to perform a tensile test in accordance with JIS Z2241:2011 to obtain three tensile strength values, and then the difference between the maximum and minimum values was calculated, and the obtained value was determined as the value of the strength variation in the longitudinal direction. When the value of this strength variation was 15.0 MPa or less, the longitudinal strength variation was evaluated as passing, and when it exceeded 15.0 MPa, the longitudinal strength variation was evaluated as failing.
［幅方向の強度ばらつき］
　まず、熱延コイルの長手方向のコイル全長Ｌに対して１／１０Ｌ位置における幅方向の端部から１／１０Ｗ位置、３／１０Ｗ位置、５／１０Ｗ位置、７／１０Ｗ位置、及び９／１０Ｗ位置の各位置において、圧延方向に平行な方向を試験方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を採取した。次いで、これらの引張試験片を用いてＪＩＳ　Ｚ２２４１：２０１１に準拠した引張試験を行うことにより５つの引張強さの値を得、次いでそれらの最大値と最小値の差を算出し、得られた値を幅方向の強度ばらつきの値として決定した。熱延コイルの長手方向のコイル全長Ｌに対して５／１０Ｌ及び９／１０Ｌ位置についても同様に引張試験を行い、各位置において幅方向の強度ばらつきの値を決定した。長手方向の１／１０Ｌ、５／１０Ｌ及び９／１０Ｌ位置における全ての強度ばらつきの値が１５．０ＭＰａ以下の場合を幅方向の強度ばらつきが合格、１５．０ＭＰａ超の場合を幅方向の強度ばらつきが不合格として評価した。 [Strength variation in the width direction]
First, tensile test pieces No. 5 of JIS Z2241:2011 were taken from the end of the width direction at the 1/10L position with respect to the total length L of the hot rolled coil in the longitudinal direction at the 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position, with the test direction being parallel to the rolling direction. Next, tensile tests were performed using these tensile test pieces in accordance with JIS Z2241:2011 to obtain five tensile strength values, and the difference between the maximum and minimum values was calculated, and the obtained value was determined as the value of the strength variation in the width direction. Similarly, tensile tests were performed at the 5/10L and 9/10L positions with respect to the total length L of the hot rolled coil in the longitudinal direction, and the value of the strength variation in the width direction was determined at each position. When all strength variation values at the 1/10L, 5/10L and 9/10L positions in the longitudinal direction were 15.0 MPa or less, the widthwise strength variation was evaluated as pass, and when they exceeded 15.0 MPa, the widthwise strength variation was evaluated as fail.
　熱延コイルの引張強さが７８０ＭＰａ以上であり、長手方向と幅方向の両方の強度ばらつきが合格である場合を、高強度でかつ強度ばらつきが低減された熱延コイルとして評価した。その結果を表３に示す。 Hot-rolled coils with a tensile strength of 780 MPa or more and acceptable strength variations in both the longitudinal and transverse directions were evaluated as having high strength and reduced strength variations. The results are shown in Table 3.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　表１～３を参照すると、比較例１６は、巻取温度ＣＴｆ及びＣＴｔが低く、巻き取り後の保熱処理を行わなかったために、長手方向中央部における析出物の平均粒径、長手方向中央部の幅方向における析出物の粒径のばらつき、並びに幅方向中央部の長手方向における析出物の粒径のばらつきを所望の範囲内に制御することができなかった。その結果として長手方向及び幅方向の強度ばらつきが顕著となった。比較例１７は、巻き取り後の保熱処理を行わなかったために、長手方向中央部における析出物の平均粒径及び長手方向中央部の幅方向における析出物の粒径のばらつきを所望の範囲内に制御することができなかった。その結果として幅方向の強度ばらつきが顕著となった。比較例１８は、巻取温度ＣＴｆ及びＣＴｔが低かったために、長手方向中央部の幅方向における析出物の粒径のばらつき及び幅方向中央部の長手方向における析出物の粒径のばらつきを所望の範囲内に制御することができなかった。その結果として長手方向の強度ばらつきが顕著となった。比較例１９は、１次冷却の平均冷却速度が高く、また１次冷却の上下冷却比が適切でなかったために、冷却むらの発生に起因して長手方向中央部の幅方向における析出物の粒径のばらつきを所望の範囲内に制御することができなかった。その結果として幅方向の強度ばらつきが顕著となった。比較例２０は、Ｎｂ含有量が低かったために、結晶粒の微細化が不十分であり、細粒強化による強度向上効果を十分に得ることができなかった。その結果として所望の引張強さを達成することができなかった。比較例２１は、仕上げ圧延の出側温度（ＦＴ）が高かったためにオーステナイト粒が粗大化してしまったと考えられる。結果として、その後の冷却によっても結晶粒の平均粒径を十分に微細化することができず、所望の引張強さを達成することができなかった。比較例２２は、巻取温度ＣＴｆ及びＣＴｔが高かったために、幅方向中央部の長手方向における析出物の粒径のばらつきを所望の範囲内に制御することができなかった。その結果として長手方向の強度ばらつきが顕著となった。比較例２３は、巻取温度ＣＴｍが低かったために、長手方向中央部における析出物の平均粒径が小さくなり、所望の引張強さを達成することができなかった。 Referring to Tables 1 to 3, in Comparative Example 16, the winding temperatures CTf and CTt were low, and no heat retention treatment was performed after winding, so the average particle size of the precipitates in the longitudinal center, the variation in particle size of the precipitates in the width direction of the longitudinal center, and the variation in particle size of the precipitates in the longitudinal direction of the width direction center could not be controlled within the desired range. As a result, the strength variation in the longitudinal direction and the width direction became significant. In Comparative Example 17, the heat retention treatment was not performed after winding, so the average particle size of the precipitates in the longitudinal center and the variation in particle size of the precipitates in the width direction of the longitudinal center could not be controlled within the desired range. As a result, the strength variation in the width direction became significant. In Comparative Example 18, the winding temperatures CTf and CTt were low, so the variation in particle size of the precipitates in the width direction of the longitudinal center and the variation in particle size of the precipitates in the longitudinal direction of the width direction center could not be controlled within the desired range. As a result, the strength variation in the longitudinal direction became significant. In Comparative Example 19, the average cooling rate of the primary cooling was high, and the upper and lower cooling ratio of the primary cooling was not appropriate, so that the variation in the grain size of the precipitates in the width direction of the longitudinal center part could not be controlled within the desired range due to the occurrence of cooling unevenness. As a result, the strength variation in the width direction became significant. In Comparative Example 20, the Nb content was low, so the crystal grains were not refined sufficiently, and the strength improvement effect by fine grain strengthening could not be fully obtained. As a result, the desired tensile strength could not be achieved. In Comparative Example 21, it is considered that the austenite grains were coarsened because the exit temperature (FT) of the finish rolling was high. As a result, the average grain size of the crystal grains could not be sufficiently refined even by the subsequent cooling, and the desired tensile strength could not be achieved. In Comparative Example 22, the coiling temperatures CTf and CTt were high, so that the variation in the grain size of the precipitates in the longitudinal direction of the width center part could not be controlled within the desired range. As a result, the strength variation in the longitudinal direction became significant. In Comparative Example 23, the coiling temperature CTm was low, so the average grain size of the precipitates in the longitudinal center was small, and the desired tensile strength could not be achieved.
　比較例２４は、仕上げ圧延の出側温度（ＦＴ）が低かったために、長手方向中央部における結晶粒の平均粒径が小さくなり、所望の引張強さを達成することができなかった。比較例２５は、１次冷却の平均冷却速度が低かったために、長手方向中央部における結晶粒の平均粒径を所望の範囲内に制御することができず、同様に所望の引張強さを達成することができなかった。比較例２６は、１次冷却の平均冷却速度が高かったために、冷却むらの発生に起因して長手方向中央部の幅方向における析出物の粒径のばらつきを所望の範囲内に制御することができなかった。その結果として幅方向の強度ばらつきが顕著となった。比較例２７及び２８は、１次冷却の上下冷却比が適切でなかったために、冷却むらの発生に起因して長手方向中央部の幅方向における析出物の粒径のばらつきを所望の範囲内に制御することができなかった。その結果として幅方向の強度ばらつきが顕著となった。比較例２９は、巻取温度ＣＴｍが高かったために、長手方向中央部における析出物が粗大化して平均粒径が大きくなり、所望の引張強さを達成することができなかった。 In Comparative Example 24, the exit temperature (FT) of the finishing rolling was low, so the average grain size of the crystal grains in the longitudinal center was small, and the desired tensile strength could not be achieved. In Comparative Example 25, the average cooling rate of the primary cooling was low, so the average grain size of the crystal grains in the longitudinal center could not be controlled within the desired range, and the desired tensile strength could not be achieved. In Comparative Example 26, the average cooling rate of the primary cooling was high, so the variation in grain size of the precipitates in the width direction of the longitudinal center could not be controlled within the desired range due to the occurrence of cooling unevenness. As a result, the strength variation in the width direction became significant. In Comparative Examples 27 and 28, the upper and lower cooling ratios of the primary cooling were not appropriate, so the variation in grain size of the precipitates in the width direction of the longitudinal center could not be controlled within the desired range due to the occurrence of cooling unevenness. As a result, the strength variation in the width direction became significant. In Comparative Example 29, because the coiling temperature CTm was high, the precipitates in the center of the longitudinal direction became coarse and the average grain size became large, and the desired tensile strength could not be achieved.
　これとは対照的に、全ての発明例に係る熱延コイルにおいて、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、長手方向の中央部において、方位差が１５°以上の粒界によって囲まれた結晶粒の平均粒径を５．０～８．０μｍの範囲内に制御することによる細粒強化と、析出物の平均粒径を３．０～９．５ｎｍの範囲内に制御することによる析出強化との組み合わせに基づき、７８０ＭＰａ以上の引張強さを達成することができた。加えて、熱延コイルの長手方向中央部の幅方向及び幅方向中央部の長手方向における析出物の粒径の最大値と最小値の差を当該析出物の平均粒径の１５．０％以下に制御することができ（表３中の「析出物粒径の幅方向ばらつき」及び「析出物粒径の長手方向ばらつき」）、その結果として熱延コイルの長手方向及び幅方向における強度ばらつきを顕著に抑制又は低減することができた。また、得られた熱延コイルの金属組織を分析したところ、全ての発明例に係る熱延コイルにおいて、フェライトの面積率は９０％以上であった。 In contrast, in all of the hot-rolled coils according to the examples of the invention, by having a predetermined chemical composition and further appropriately controlling each condition in the manufacturing method, a tensile strength of 780 MPa or more could be achieved based on a combination of fine grain strengthening by controlling the average grain size of crystal grains surrounded by grain boundaries with an orientation difference of 15° or more in the longitudinal center to within the range of 5.0 to 8.0 μm, and precipitation strengthening by controlling the average grain size of precipitates to within the range of 3.0 to 9.5 nm. In addition, the difference between the maximum and minimum grain sizes of precipitates in the width direction of the longitudinal center of the hot-rolled coil and in the longitudinal direction of the width direction center of the hot-rolled coil could be controlled to 15.0% or less of the average grain size of the precipitates ("Width direction variation of precipitate grain size" and "Longitudinal direction variation of precipitate grain size" in Table 3), and as a result, the strength variation in the longitudinal and width directions of the hot-rolled coil could be significantly suppressed or reduced. In addition, when the metal structure of the obtained hot-rolled coil was analyzed, the area ratio of ferrite was 90% or more in all hot-rolled coils according to the invention examples.

Claims (3)

  1. 　質量％で、
    　Ｃ：０．０５０～０．１００％、
    　Ｓｉ：０．０１～０．３０％、
    　Ｍｎ：１．３０～２．１０％、
    　Ｔｉ：０．０８０～０．１５０％、
    　Ｎｂ：０．０２０～０．０５０％、
    　Ａｌ：０．００１～０．０５０％、
    　Ｐ：０．１００％以下、
    　Ｓ：０．０５０％以下、
    　Ｎ：０．００５０％以下、
    　Ｏ：０．００５０％以下、
    　Ｂ：０～０．００５０％、
    　Ｃｕ：０～０．２０％、
    　Ｎｉ：０～０．２０％、
    　Ｓｎ：０～０．１０％、
    　Ｃｒ：０～０．４０％、
    　Ｍｏ：０～０．２００％、
    　Ｖ：０～０．１００％、
    　Ａｓ：０～０．１００％、
    　Ｚｒ：０～０．１００％、
    　Ｃａ：０～０．００５０％、
    　Ｍｇ：０～０．１００％、
    　Ｂｉ：０～０．０２０％、
    　Ｃｏ：０～０．２０％、
    　Ｗ：０～０．２０％、
    　Ｚｎ：０～０．２０％、
    　ＲＥＭ：０～０．１０００％、並びに
    　残部：Ｆｅ及び不純物からなる化学組成を有し、
    　長手方向の中央部において、
    　方位差が１５°以上の粒界によって囲まれた領域を結晶粒と定義した場合に、前記結晶粒の平均粒径が５．０～８．０μｍであり、
    　析出物の平均粒径が３．０～９．５ｎｍであり、
    　幅方向における前記析出物の粒径の最大値と最小値の差が前記析出物の平均粒径の１５．０％以下であり、
    　幅方向の中央部において、長手方向における前記析出物の粒径の最大値と最小値の差が長手方向における前記析出物の平均粒径の１５．０％以下である金属組織を有することを特徴とする、熱延コイル。     In mass percent,
    C: 0.050 to 0.100%,
    Si: 0.01 to 0.30%,
    Mn: 1.30 to 2.10%,
    Ti: 0.080 to 0.150%,
    Nb: 0.020 to 0.050%,
    Al: 0.001 to 0.050%,
    P: 0.100% or less,
    S: 0.050% or less,
    N: 0.0050% or less,
    O: 0.0050% or less,
    B: 0 to 0.0050%,
    Cu: 0 to 0.20%,
    Ni: 0 to 0.20%,
    Sn: 0 to 0.10%,
    Cr: 0 to 0.40%,
    Mo: 0 to 0.200%,
    V: 0 to 0.100%,
    As: 0 to 0.100%,
    Zr: 0 to 0.100%,
    Ca: 0 to 0.0050%,
    Mg: 0 to 0.100%,
    Bi: 0 to 0.020%,
    Co: 0 to 0.20%,
    W: 0 to 0.20%,
    Zn: 0 to 0.20%,
    REM: 0 to 0.1000%, and the balance: Fe and impurities;
    At the center in the longitudinal direction,
    When a region surrounded by a grain boundary having an orientation difference of 15° or more is defined as a crystal grain, the average grain size of the crystal grain is 5.0 to 8.0 μm,
    The average grain size of the precipitates is 3.0 to 9.5 nm;
    a difference between a maximum value and a minimum value of a grain size of the precipitate in the width direction is 15.0% or less of an average grain size of the precipitate,
    A hot-rolled coil, characterized in that it has a metal structure in a widthwise central portion, in which the difference between the maximum and minimum values of grain size of the precipitates in the longitudinal direction is 15.0% or less of the average grain size of the precipitates in the longitudinal direction.
  2. 　前記化学組成が、質量％で、
    　Ｂ：０．０００１～０．００５０％、
    　Ｃｕ：０．０１～０．２０％、
    　Ｎｉ：０．０１～０．２０％、
    　Ｓｎ：０．０１～０．１０％、
    　Ｃｒ：０．０１～０．４０％、
    　Ｍｏ：０．００１～０．２００％、
    　Ｖ：０．００１～０．１００％、
    　Ａｓ：０．００１～０．１００％、
    　Ｚｒ：０．００１～０．１００％、
    　Ｃａ：０．０００１～０．００５０％、
    　Ｍｇ：０．００１～０．１００％、
    　Ｂｉ：０．００１～０．０２０％、
    　Ｃｏ：０．０１～０．２０％、
    　Ｗ：０．０１～０．２０％、
    　Ｚｎ：０．０１～０．２０％、及び
    　ＲＥＭ：０．０００１～０．１０００％
    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に記載の熱延コイル。     The chemical composition, in mass%,
    B: 0.0001 to 0.0050%,
    Cu: 0.01 to 0.20%,
    Ni: 0.01 to 0.20%,
    Sn: 0.01 to 0.10%,
    Cr: 0.01 to 0.40%,
    Mo: 0.001 to 0.200%,
    V: 0.001 to 0.100%,
    As: 0.001 to 0.100%,
    Zr: 0.001 to 0.100%,
    Ca: 0.0001 to 0.0050%,
    Mg: 0.001 to 0.100%,
    Bi: 0.001 to 0.020%,
    Co: 0.01 to 0.20%,
    W: 0.01 to 0.20%,
    Zn: 0.01 to 0.20%, and REM: 0.0001 to 0.1000%
    The hot rolled coil according to claim 1 , characterized in that it contains at least one of the following:
  3. 　０．０７０％以上の有効Ｔｉ量を有することを特徴とする、請求項１又は２に記載の熱延コイル。 The hot-rolled coil according to claim 1 or 2, characterized in that it has an effective Ti content of 0.070% or more.
PCT/JP2023/039477 2022-11-02 2023-11-01 Hot-rolled coil WO2024096073A1 (en)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2007231409A (en) * 2006-03-03 2007-09-13 Nippon Steel Corp Hot rolled coil and its production method
WO2013047702A1 (en) * 2011-09-27 2013-04-04 新日鐵住金株式会社 Hot coil for line pipe and manufacturing method therefor
JP2020525652A (en) * 2017-07-06 2020-08-27 ポスコＰｏｓｃｏ Ultra-high-strength hot-rolled steel sheet with excellent surface quality with little material variation and method for producing the same

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2007231409A (en) * 2006-03-03 2007-09-13 Nippon Steel Corp Hot rolled coil and its production method
WO2013047702A1 (en) * 2011-09-27 2013-04-04 新日鐵住金株式会社 Hot coil for line pipe and manufacturing method therefor
JP2020525652A (en) * 2017-07-06 2020-08-27 ポスコＰｏｓｃｏ Ultra-high-strength hot-rolled steel sheet with excellent surface quality with little material variation and method for producing the same

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