WO2024095533A1 - Feuille d'acier laminée à chaud - Google Patents

Feuille d'acier laminée à chaud Download PDF

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Publication number
WO2024095533A1
WO2024095533A1 PCT/JP2023/024346 JP2023024346W WO2024095533A1 WO 2024095533 A1 WO2024095533 A1 WO 2024095533A1 JP 2023024346 W JP2023024346 W JP 2023024346W WO 2024095533 A1 WO2024095533 A1 WO 2024095533A1
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hot
ferrite
rolled steel
steel sheet
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PCT/JP2023/024346
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English (en)
Japanese (ja)
Inventor
菜月 大住
顕吾 畑
武 豊田
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日本製鉄株式会社
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Publication of WO2024095533A1 publication Critical patent/WO2024095533A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to hot-rolled steel sheets.
  • Patent Document 1 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, a Weg represented by [Ti]-48/14 x [N]-48/32 x [S] of 0.01 to 0.30%, crystal grains in which the misorientation of the grain boundaries between adjacent crystal grains is 15° or more, and the average misorientation within the crystal grains is 0 to 0.5° in terms of area fraction, the total of martensite, tempered martensite, and retained austenite is 2% to 10% in terms of area fraction, and Ti is present as Ti carbides at a mass percentage of 40% or more of Tief, and the mass of the Ti carbides having a circle-equivalent grain size of 7 nm to 20 nm or less accounts for 50% or more of the mass of the total Ti carbides.
  • a Tief represented by [Ti]-48/14 x [N]-48/32 x [S] of 0.01 to 0.30%
  • Patent Document 1 also teaches that grains with an average intragrain orientation difference of 0 to 0.5° have high ductility and are further strengthened by precipitation with Ti carbides, so by ensuring that such grains account for 50% or more of the area, it is possible to improve ductility while maintaining a tensile strength (TS) of 540 MPa or more.
  • TS tensile strength
  • Patent Document 2 describes a high-strength hot-rolled steel sheet having a predetermined composition, a total volume fraction of the ferrite phase and the bainite phase in the entire structure of 95% or more, a volume fraction of the ferrite phase in the entire structure of 50-90%, precipitates of less than 20 nm in size containing 650-1100 ppm Ti precipitated in the ferrite phase, and a microstructure in which the ⁇ Hv of the bainite phase (the difference between the maximum and minimum Vickers hardness values of the bainite phase measured at 1/4 of the sheet thickness position in the sheet thickness cross section along the rolling direction) is 150 or less.
  • Patent Document 2 also teaches that if a microstructure is formed mainly of ferrite and bainite phases, precipitates of less than 20 nm in size containing 650-1100 ppm Ti precipitated in the ferrite phase, and the ⁇ Hv of the bainite phase is set to 150 or less, a TS of 780 MPa or more can be secured, and excellent stretch flangeability (hole expandability) and impact resistance can be achieved at the same time.
  • the present invention aims to provide a hot-rolled steel sheet that has high strength, high hole expansion property and high yield ratio.
  • the inventors conducted research, focusing particularly on the metal structure of hot-rolled steel sheet.
  • the strength of hot-rolled steel sheet can be significantly increased by configuring the metal structure of hot-rolled steel sheet having a specified chemical composition to mainly contain ferrite, bainite, and martensite, controlling the bainite fraction to a relatively high range, and utilizing dislocation strengthening, and further discovered that ferrite can be precipitation strengthened by causing TiC precipitates having a suitable diameter to exist in ferrite at a specified number density, thereby further increasing the strength of the hot-rolled steel sheet while reducing the difference in hardness between ferrite and bainite and increasing the hole expandability and yield ratio, thus completing the present invention.
  • the present invention which has achieved the above object, is as follows. (1) In mass%, C: 0.03 to 0.10%, Si: 0.010 to 0.100%, Mn: 0.50 to 3.00%, Ti: 0.05 to 0.20%, Al: 0.20 to 0.40%, P: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0 to 0.050%, V: 0 to 1.000%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.000%, B: 0 to 0.0100%, Sn: 0 to 1.000%, Sb: 0 to 1.000%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.010%, REM: 0 to 0.0100%, As: 0 to 0.010%, Zr: 0 to 0.010%, Co:
  • the chemical composition is, in mass%, Nb: 0.001 to 0.050%, V: 0.001 to 1.000%, Cr: 0.001 to 2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sn: 0.001 to 1.000%, Sb: 0.001 to 1.000%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.001 to 0.010%, REM: 0.0001 to 0.0100%, As: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Co: 0.001 to 2.000%, Zn: 0.001 to 0.010%, and W: 0.001 to 1.000%
  • the hot-rolled steel sheet according to the above (1) characterized in that it contains at least one of the following: (3) The hot-rolled steel sheet according to (1) or (2) above, characterized in that the
  • the present invention provides hot-rolled steel sheets that have high strength, high hole expansion properties, and high yield ratios.
  • the hot-rolled steel sheet according to the embodiment of the present invention has, in mass%, C: 0.03 to 0.10%, Si: 0.010 to 0.100%, Mn: 0.50 to 3.00%, Ti: 0.05 to 0.20%, Al: 0.20 to 0.40%, P: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0 to 0.050%, V: 0 to 1.000%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.000%, B: 0 to 0.0100%, Sn: 0 to 1.000%, Sb: 0 to 1.000%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.010%, REM: 0 to 0.0100%, As: 0 to 0.010%, Zr:
  • the present inventors conducted research, focusing in particular on the metal structure of the hot-rolled steel plate, in addition to making the chemical composition of the hot-rolled steel plate appropriate. To explain in more detail, first, the present inventors found that by configuring the metal structure of a hot-rolled steel plate having a predetermined chemical composition to mainly contain ferrite, bainite, and martensite, it is possible to improve the hole expandability and yield ratio while maintaining the strength of the hot-rolled steel plate at a relatively high level.
  • the present inventors found that, in order to further increase the strength, the strength of the hot-rolled steel plate can be significantly increased by controlling the fraction of bainite, which is a hard phase in the metal structure, to a relatively high range and utilizing dislocation strengthening. More specifically, the inventors have discovered that in addition to controlling the area ratio of bainite in the metal structure to within the range of 30 to 60%, the strength of the hot-rolled steel sheet can be significantly increased by introducing dislocations into the steel sheet during hot rolling such that the average aspect ratio of the prior austenite grains in the region containing the bainite and martensite is 3.0 or more, as will be described in detail later in relation to the manufacturing method of the hot-rolled steel sheet.
  • the fraction of bainite is controlled to a relatively high range in a three-phase structure mainly composed of ferrite, bainite, and martensite as described above, it is very effective in terms of increasing strength, but the hardness difference between the hard phase bainite and the soft phase ferrite becomes relatively large. If the hardness difference between each phase in the metal structure becomes large, the hole expandability and the yield ratio may decrease. For this reason, the present inventors have studied the improvement of hole expandability and the realization of a high yield ratio from the viewpoint of reducing the hardness difference between each phase in a metal structure including such a three-phase structure.
  • precipitation strengthening of the softest ferrite in the three-phase structure more specifically, by having TiC precipitates with a diameter of 2.0 to 8.0 nm in ferrite exist at a number density of 1.0 x 10 16 pieces/cm 3 or more, not only contributes to improving the strength of the hot-rolled steel sheet as a whole, but also sufficiently reduces the hardness difference between the bainite, which is a hard phase relatively abundant in the metal structure, and the softest ferrite in the three-phase structure.
  • the hot-rolled steel sheet according to the embodiment of the present invention in order to have TiC precipitates with a diameter of 2.0 to 8.0 nm exist in ferrite at a number density of 1.0 x 10 16 pieces/cm 3 or more, in addition to the manufacturing method described in detail later, it is necessary to make the chemical composition of the hot-rolled steel sheet appropriate.
  • Si and Al contained in the steel have the effect of suppressing the precipitation of cementite.
  • the hot-rolled steel sheet according to the embodiment of the present invention can be effectively used in components that require both the contradictory properties of high strength and excellent workability, and further require impact resistance properties, and is therefore particularly useful in the automotive field.
  • C is an element effective in increasing the strength of steel plate.
  • C forms carbides and/or carbonitrides with Ti and Nb in steel, and contributes to precipitation strengthening based on the formed precipitates and to refinement of the structure due to the pinning effect of the precipitates.
  • the C content is set to 0.03% or more.
  • the C content may be 0.04% or more, 0.05% or more, or 0.06% or more.
  • the C content is set to 0.10% or less.
  • the C content may be 0.09% or less, 0.08% or less, or 0.07% or less.
  • Si 0.010 to 0.100%
  • Si is an element effective in increasing strength as a solid solution strengthening element.
  • Si also has the effect of suppressing the precipitation of cementite. Therefore, by containing Si, it is possible to suppress the consumption of C in the steel to form cementite, and thereby it is possible to promote the formation of TiC precipitates during cooling after hot rolling.
  • the Si content is set to 0.010% or more.
  • the Si content may be 0.020% or more, 0.030% or more, or 0.040% or more.
  • a surface quality defect called Si scale may occur. Therefore, the Si content is set to 0.100% or less.
  • the Si content may be 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.
  • Mn is an element that is effective in increasing strength as a hardenability and solid solution strengthening element. In order to fully obtain these effects, the Mn content is set to 0.50% or more. The Mn content may be 0.70% or more, 1.00% or more, 1.20% or more, or 1.50% 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 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.
  • Ti is an element that finely precipitates in steel as carbide (TiC), improves the strength of steel by precipitation strengthening, and increases the hardness of ferrite. Ti also forms carbides to fix C, and is an element that suppresses the formation of cementite, which is harmful to hole expansion. In order to fully obtain these effects, the Ti content is set to 0.05% or more. The Ti content may be 0.08% or more, 0.10% or more, 0.12% or more, or 0.14% or more. On the other hand, if Ti is contained excessively, the carbides become coarse, and the desired precipitation strengthening in ferrite may not be obtained.
  • the Ti content is set to 0.20% or less.
  • the Ti content may be 0.18% or less, 0.17% or less, 0.16% or less, or 0.15% or less.
  • Al 0.20 to 0.40%
  • Al is an element that acts as a deoxidizer for molten steel.
  • Al also has the effect of suppressing the precipitation of cementite. Therefore, by containing Al, it is possible to suppress the consumption of C in the steel to form cementite, and thereby it is possible to promote the formation of TiC precipitates during cooling after hot rolling.
  • the Al content is set to 0.20% or more.
  • the Al content may be 0.22% or more, 0.25% or more, or 0.28% or more.
  • the Al content is set to 0.40% or less.
  • the Al content may be 0.38% or less, 0.35% or less, or 0.32% 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, 0.001% or more, or 0.005% or more.
  • the Si content is set to 0.0100% or less.
  • the S content may be 0.0050% or less, 0.0030% or less, or 0.0020% 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 0.0010% or more.
  • N 0.010% or less
  • the N content is set to 0.010% or less.
  • the N content may be 0.008% or less, 0.005% or less, or 0.003% or less.
  • the lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.
  • O is an element that is mixed in during the manufacturing process. If O is contained excessively, coarse inclusions may be formed, which may reduce the toughness of the steel plate. Therefore, the O content is set to 0.010% or less.
  • the O content may be 0.008% or less, 0.006% or less, or 0.004% or less.
  • the lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long time for refining, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.
  • the basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.
  • Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to refining the structure and thus increasing the strength of the steel sheet by the pinning effect.
  • the Nb content may be 0%, but in order to obtain such an effect, the Nb content is preferably 0.001% or more.
  • the Nb content may be 0.005% or more, 0.010% or more, 0.012% or more, 0.015% or more, or 0.020% or more.
  • the Nb content is preferably 0.050% or less.
  • the Nb content may be 0.040% or less, 0.030% or less, or 0.025% or less.
  • V is an element that contributes to improving strength by precipitation strengthening, etc.
  • the V content may be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more.
  • the V content may be 0.010% or more, 0.030% or more, or 0.050% or more.
  • the V content is preferably 1.000% or less.
  • the V content may be 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.
  • Cr is an element that enhances the hardenability of steel and contributes to improving strength.
  • the Cr content may be 0%, but in order to obtain such an effect, the Cr content is preferably 0.001% or more.
  • the Cr content may be 0.01% or more, 0.03% or more, or 0.05% or more.
  • the Cr content is preferably 2.00% or less.
  • the Cr content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
  • Ni and Cu are elements that contribute to improving strength by precipitation strengthening or solid solution strengthening.
  • the Ni and Cu contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.001% or more, and may be 0.01% or more, 0.03% or more, or 0.05% or more.
  • the Ni and Cu contents are preferably 2.00% or less, and may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
  • Mo is an element that improves the hardenability of steel and contributes to improving strength.
  • the Mo content may be 0%, but in order to obtain such an effect, the Mo content is preferably 0.001% or more.
  • the Mo content may be 0.010% or more, 0.020% or more, or 0.050% or more.
  • the Mo content is preferably 1.000% or less.
  • the Mo content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.
  • [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness.
  • the B content may be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more.
  • the B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more.
  • the B content is preferably 0.0100% or less.
  • the B content may be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.
  • Sn and Sb are elements effective for improving corrosion resistance.
  • the Sn and Sb contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, excessive inclusion of these elements may cause a decrease in toughness. Therefore, the Sn and Sb contents are preferably 1.000% or less, and may be 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.080% or less.
  • Ca, Mg and Hf are elements capable of controlling the morphology of nonmetallic inclusions.
  • the Ca, Mg and Hf contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more or 0.0010% or more.
  • the Ca, Mg and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • 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 0.001% or more or 0.002% or more.
  • the Bi content is preferably 0.010% or less.
  • the Bi content may be 0.005% or less or 0.003% or less.
  • REM is an element capable of controlling the form of nonmetallic inclusions.
  • the REM content may be 0%, but in order to obtain such an effect, the REM content is preferably 0.0001% or more.
  • the REM content may be 0.0005% or more or 0.0010% or more.
  • the REM content is preferably 0.0100% or less.
  • the REM content may be 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 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 0.002% or more or 0.003% or more.
  • the As content is preferably 0.010% or less.
  • the As content may be 0.008% or less or 0.005% or less.
  • Zr is an element capable of controlling the form of nonmetallic inclusions.
  • the Zr content may be 0%, but in order to obtain such an effect, the Zr content is preferably 0.001% or more.
  • the Zr content may be 0.002% or more or 0.003% or more.
  • the Zr content is preferably 0.010% or less.
  • the Zr content may be 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 0.001% or more.
  • the Co content may be 0.010% or more, 0.050% or more, or 0.100% or more.
  • the Co content is preferably 2.000% or less.
  • the Co content may be 1.000% or less, 0.500% or less, 0.300% or less, or 0.200% or less.
  • Zn is an element that may be contained in a steel sheet when scrap or the like is used as a steel raw material. Therefore, the Zn content is preferably 0.010% or less, and may be 0.008% or less or 0.005% or less.
  • the Zn content may be 0%, but reducing the Zn content to less than 0.001% requires a long time for refining, which leads to a decrease in productivity. Therefore, the Zn content may be 0.001% or more, 0.002% or more, or 0.003% or more.
  • 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 0.001% or more.
  • the W content may be 0.010% or more, 0.050% or more, or 0.100% or more.
  • excessive W content may reduce weldability. Therefore, the W content is preferably 1.000% or less.
  • the W content may be 0.800% or less, 0.500% or less, 0.300% or less, or 0.200% or less.
  • the remainder other than the above elements consists of Fe and impurities.
  • Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing hot-rolled steel sheets.
  • the chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention may be measured by a general analytical method.
  • the chemical composition of the hot-rolled steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES).
  • C and S may be measured using the combustion-infrared absorption method
  • N may be measured using the inert gas fusion-thermal conductivity method
  • O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
  • the metal structure of the hot rolled steel sheet according to the embodiment of the present invention includes, in terms of area percentage, ferrite: 30 to 60%, bainite: 30 to 60%, and martensite: 5 to 20%.
  • ferrite 30 to 60%
  • bainite 30 to 60%
  • martensite 5 to 20%.
  • the area ratio of ferrite needs to be 30% or more, and may be, for example, 35% or more, 40% or more, or 45% or more.
  • the area ratio of ferrite is 60% or less, and may be, for example, less than 60%, 59% or less, 58% or less, 55% or less, 52% or less, or 50% or less.
  • the area ratios of the hard phases bainite and martensite are high.
  • the area ratio of bainite may be more than 30%, 31% or more, 32% or more, 35% or more, 40% or more, or 45% or more.
  • the area ratio of martensite may be 8% or more, 10% or more, or 12% or more.
  • the area ratios of bainite and martensite are low. From this viewpoint, for example, the area ratio of bainite may be 58% or less, 55% or less, 52% or less, or 50% or less.
  • the area ratio of martensite may be 18% or less, 16% or less, or 14% or less.
  • the metal structure of the hot-rolled steel sheet according to the embodiment of the present invention includes ferrite, bainite, and martensite, and may include other remaining structures, but the area ratio of the remaining structure is preferably small and may be 0%.
  • the area ratio of the remaining structure is not particularly limited, and may be, for example, 0 to 5%, 0 to 4%, or 0 to 3%.
  • the total area ratio of ferrite, bainite, and martensite may be, for example, 95 to 100%, 96 to 100%, or 97 to 100%.
  • the lower limit of the remaining structure may be 1% or 2%.
  • the remaining structure may include at least one of pearlite and retained austenite or may be at least one of them.
  • the structure observation is performed with a scanning electron microscope. Prior to the observation, the sample for structure observation is polished by wet polishing with emery paper and diamond abrasive grains having an average particle size of 1 ⁇ m, and the observation surface is mirror-finished, and then the structure is etched with a 3% nitric acid alcohol solution. The magnification of the observation is 2000 times, and 10 random images of a 30 ⁇ m x 40 ⁇ m field of view at a position of 1/4 of the plate thickness from the surface are taken. The ratio of the structure is obtained by the point count method.
  • a total of 225 lattice points are set at intervals of 3 ⁇ m vertically and 4 ⁇ m horizontally for the obtained structure image, and the structure present under the lattice points is identified, and the structure ratio contained in the steel material is obtained from the average value of the 10 sheets.
  • Ferrite is a blocky crystal grain that does not contain iron-based carbides with a major axis of 100 nm or more inside.
  • Bainite is a collection of lath-shaped crystal grains, and does not contain iron-based carbides with a major axis of 20 nm or more inside, or contains iron-based carbides with a major axis of 20 nm or more inside, and the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction.
  • the group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by 5° or less.
  • Bainite is counted as one bainite grain when it is surrounded by grain boundaries with an orientation difference of 15° or more.
  • martensite which contains a large amount of dissolved carbon, has a higher brightness and appears whiter than other structures, so it can be distinguished from other structures.
  • the area ratio of the remaining structure is determined by subtracting the total area ratio of ferrite, bainite, and martensite from 100%.
  • pearlite has a unique structure in which cementite is precipitated in a lamellar form, and therefore can be identified by a scanning electron microscope.
  • the volume fraction of the retained austenite can be calculated by X-ray diffraction measurement, and since the volume fraction of the retained austenite is equivalent to the area fraction, this can be regarded as the area fraction of the retained austenite.
  • TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite at a number density of 1.0 x 10 16 pieces/cm 3 or more.
  • TiC precipitates include not only TiC but also composite carbides containing Ti and other elements other than Ti, such as V and Nb.
  • the hardness difference in the metal structure mainly composed of ferrite, bainite and martensite can be reduced.
  • it is possible to significantly increase the hole expandability and yield ratio of the hot-rolled steel sheet and it is possible to achieve, for example, a hole expansion ratio ( ⁇ ) of 60.0% or more and a yield ratio (YR) of 0.70 or more.
  • the precipitation strengthening by TiC precipitates also contributes to improving the strength of the entire hot-rolled steel sheet.
  • the diameter of the TiC precipitates is smaller than 2.0 nm, the TiC precipitates cannot act sufficiently as obstacles to dislocation motion, and therefore the effect of improving the hardness of ferrite by precipitation strengthening cannot be fully obtained. In addition, the effect of improving the strength of the hot-rolled steel sheet may not be fully exhibited. On the other hand, if the diameter of the TiC precipitates is too large, the desired precipitation strengthening in ferrite may not be obtained.
  • TiC precipitates having a diameter of 2.0 to 8.0 nm need to be present in the ferrite at a number density of 1.0 ⁇ 10 16 /cm 3 or more as described above.
  • the higher the number density the more preferable it is, and it may be, for example, 1.2 ⁇ 10 16 /cm 3 or more, 1.5 ⁇ 10 16 /cm 3 or more, 2.0 ⁇ 10 16 /cm 3 or more, 5.0 ⁇ 10 16 /cm 3 or more, or 10.0 ⁇ 10 16 /cm 3 or more.
  • C and Ti which are the supply sources of TiC precipitates, if the number density becomes too high, it may be difficult to control the diameter of the TiC precipitates within the desired range.
  • the number density is not particularly limited as long as the diameter of 2.0 to 8.0 nm is satisfied, but may be, for example, 75.0 x 10 16 pieces/cm 3 or less, 50.0 x 10 16 pieces/cm 3 or less, 30.0 x 10 16 pieces/cm 3 or less, or 20.0 x 10 16 pieces/cm 3 or less.
  • TiC precipitates having a diameter of 2.0 to 8.0 nm may be present in the ferrite at a number density of 1.0 x 10 16 pieces/cm 3 or more, and therefore, as long as the above diameter and number density requirements are satisfied, for example, coarse TiC precipitates may be present in the ferrite.
  • the precipitate is defined as a fine TiC precipitate.
  • the diameter of the fine TiC precipitate is the circle equivalent diameter calculated from the number of Ti atoms constituting the observed fine Ti precipitate and the lattice constant of the fine Ti precipitate, assuming that the fine Ti precipitate is spherical.
  • the method of calculating the diameter (circle equivalent diameter) R of the fine TiC precipitate using the number of Ti atoms of the fine TiC precipitate obtained by the three-dimensional atom probe measurement method is shown below.
  • the ratio of the average nanohardness of ferrite to the average nanohardness of bainite i.e., (average nanohardness of ferrite)/(average nanohardness of bainite) is controlled within the range of 0.75 to 1.20.
  • the ratio of the average nanohardness of ferrite to the average nanohardness of bainite within such a range, the hardness difference between ferrite and bainite in the metal structure can be further reduced.
  • the ratio of the average nanohardness of ferrite to the average nanohardness of bainite may be 0.80 or more, 0.85 or more, or 0.90 or more, and may be 1.15 or less, 1.10 or less, 1.05 or less, or 1.00 or less.
  • the average nanohardness of bainite is not particularly limited, but may be, for example, 3.0 to 5.0 GPa, 3.2 to 4.8 GPa, or 3.5 to 4.5 GPa.
  • the average nanohardness of ferrite is not particularly limited, but may be, for example, 2.5 to 5.0 GPa, 2.8 to 4.8 GPa, or 3.0 to 4.5 GPa.
  • the ratio of the average nanohardness of ferrite to the average nanohardness of bainite is determined as follows. First, a sample is cut out from a hot-rolled steel sheet so that a cross section perpendicular to the surface can be observed. The cross section of the sample is polished to a mirror finish by wet polishing with emery paper and diamond abrasive grains with an average particle size of 1 ⁇ m. The mirror-finished cross section is indented with a test load of 3 gf at a depth position of 1/4 of the plate thickness from the surface using a microhardness tester, and the nanohardness is measured, obtaining a total of 100 or more measured values.
  • the same sample is measured using a scanning electron microscope, and only measurement points with indentations inside the ferrite grains and inside the bainite are extracted with reference to the obtained structure analysis results.
  • the arithmetic average of the nanohardnesses for the 10 or more extracted ferrite crystal grains is determined as the average nanohardness of ferrite
  • the arithmetic average of the nanohardnesses for the 10 or more extracted bainite grains is determined as the average nanohardness of bainite
  • the ratio thereof (average nanohardness of ferrite)/(average nanohardness of bainite) is determined as the ratio of the average nanohardness of ferrite to the average nanohardness of bainite.
  • the average aspect ratio of the prior austenite grains in the region containing bainite and martensite must be 3.0 or more.
  • the strength of the hot-rolled steel sheet can be increased by configuring the metal structure to contain a relatively large amount of bainite, 30 to 60 area %, of the hard phase.
  • the desired high strength cannot be reliably achieved simply by increasing the area ratio of bainite. Therefore, in the hot-rolled steel sheet according to the embodiment of the present invention, in addition to containing a relatively large amount of bainite, dislocation strengthening is utilized to further increase the strength.
  • dislocations can be introduced into the steel sheet while suppressing recrystallization of the metal structure by applying an appropriate reduction during hot rolling.
  • the metal structure in which dislocations are appropriately introduced in this way has a relatively large aspect ratio because recrystallization is suppressed. That is, by appropriately controlling the average aspect ratio of the prior austenite grains in the region containing bainite and martensite, the strength improvement effect obtained by containing a relatively large amount of bainite can be further increased due to dislocation strengthening.
  • the metal structure is configured to contain 30 to 60 area % of bainite, and the average aspect ratio of the prior austenite grains in the region containing the bainite and martensite is controlled to 3.0 or more, so that the strength of the hot-rolled steel sheet can be significantly increased due to a combination of the strength improving effect of bainite and dislocation strengthening.
  • the larger the average aspect ratio the more preferable it is, and it may be, for example, 3.2 or more, 3.5 or more, 3.8 or more, or 4.0 or more.
  • the upper limit of the average aspect ratio is not particularly limited, but for example, the average aspect ratio may be 6.0 or less, 5.5 or less, or 5.0 or less.
  • the average aspect ratio of the prior austenite grains in the region containing bainite and martensite is measured by a scanning electron microscope. Prior to the measurement, the sample for structure observation is first polished by wet polishing with emery paper and diamond abrasives with an average particle size of 1 ⁇ m, and the L-direction cross section (cross section parallel to the rolling direction and the plate thickness direction) is mirror-finished as the observation surface, and then the structure is etched with a picric acid solution.
  • the magnification of the observation is 1000 times, and 10 random photographs are taken of a 60 ⁇ m x 80 ⁇ m field of view at a position of 1/4 of the plate thickness from the surface.
  • the grain size in each of the plate thickness direction and the rolling direction is obtained by a cutting method, with the grain boundaries revealed by corrosion with picric acid as the target.
  • the cutting method five straight lines parallel to the plate thickness direction and the rolling direction of the photographed image are drawn at equal intervals, and the intersections with the grain boundaries are counted. The total length of the five straight lines divided by the number of intersections is taken as the grain size.
  • the grain size in the rolling direction is divided by the grain size in the sheet thickness direction to determine the average aspect ratio of the prior austenite grains in the region containing bainite and martensite.
  • the hot rolled steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm, although not particularly limited thereto.
  • the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 5.0 mm or less, or 4.0 mm or less.
  • the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 1180 MPa or less, 980 MPa or less, 940 MPa or less, 900 MPa or less, or 860 MPa or less.
  • the tensile strength is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.
  • the yield ratio can be increased, and more specifically, a yield ratio of 0.70 or more can be achieved.
  • the yield ratio is preferably 0.75 or more, more preferably 0.80 or more.
  • the upper limit is not particularly limited, but for example, the yield ratio may be 0.90 or less or 0.85 or less.
  • the yield ratio is determined by the following formula based on the tensile strength and 0.2% proof stress measured by taking a JIS No.
  • the hole expansion ratio may be preferably 65.0% or more, more preferably 70.0% or more or 80% or more.
  • the upper limit of the hole expansion ratio is not particularly limited, but for example, the hole expansion ratio may be 120% or less, 110% or less, or 100% or less.
  • the hole expansion ratio is determined as follows.
  • the initial hole is expanded with a conical punch having an apex angle of 60° until a crack penetrating the plate thickness occurs, with the burr facing the die side, and the hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio ⁇ (%) of each test piece is calculated using the following formula.
  • This hole expansion test is carried out three times, and the average value is determined as the hole expansion ratio ⁇ .
  • ⁇ 100 ⁇ ⁇ (d1 - d0) / d0 ⁇
  • the method for producing a hot-rolled steel sheet according to an embodiment of the present invention includes: A hot rolling process comprising heating a slab having the chemical composition described above in relation to the hot rolled steel sheet and then finish rolling the slab, the hot rolling process satisfying the following conditions (a) to (e): (a) The heating temperature of the slab is 1200 to 1300°C; (b) the holding time in the temperature range of 1200 to 1300° C.
  • finish rolling is 1000 to 4000 seconds;
  • finish rolling is performed using a tandem rolling mill consisting of five or more rolling stands, and the total reduction in the rolling passes of the front stages other than the rear three stages is 60 to 90%;
  • rolling reduction in each rolling pass of the latter three stages is 10% or more, and the total rolling reduction in the rolling passes of the latter three stages is 30 to 50%, and
  • end temperature of finish rolling is 900 to 1000°C.
  • the method is characterized by including an intermediate air-cooling step in which the finish-rolled steel sheet is primarily cooled to an intermediate air-cooling temperature of 670 to 750°C at an average cooling rate of 50 to 200°C/s, and then intermediate air-cooled for 3 to 10 seconds, and a cooling step in which the intermediate air-cooled steel sheet is secondarily cooled at an average cooling rate of 50 to 200°C/s, and then coiled at a coiling temperature of 20 to 200°C.
  • an intermediate air-cooling step in which the finish-rolled steel sheet is primarily cooled to an intermediate air-cooling temperature of 670 to 750°C at an average cooling rate of 50 to 200°C/s, and then intermediate air-cooled for 3 to 10 seconds
  • a cooling step in which the intermediate air-cooled steel sheet is secondarily cooled at an average cooling rate of 50 to 200°C/s, and then coiled at a coiling temperature of 20 to 200°C.
  • the heating temperature is preferably 1200°C or higher.
  • the upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or lower from the viewpoint of the capacity and productivity of the heating equipment.
  • the upper limit of the holding time is not particularly limited, but is preferably 4000 seconds or less from the viewpoint of productivity, etc.
  • the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
  • the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
  • Refining the metal structure by such recrystallization is very advantageous in forming a desired metal structure and improving properties such as hole expandability. If the total reduction rate in the rolling passes of the front stages is less than 60%, the desired metal structure containing ferrite, bainite, and martensite in a specific ratio cannot be obtained, and properties such as hole expandability may be reduced. Therefore, the total reduction in the front rolling passes is set to 60% or more, and preferably 70% or more. On the other hand, if the total reduction in the front rolling passes is too high, the rolling load becomes excessive, and the load on the rolling mill increases. For this reason, the total reduction in the front rolling passes is set to 90% or less.
  • the reduction rate in each rolling pass of the last three stages is less than 10%, recrystallization is suppressed in each rolling pass, but dislocations cannot be sufficiently introduced in each rolling pass, and the desired aspect ratio cannot be achieved in the final metal structure.
  • the total reduction rate in the rolling passes of the last three stages is less than 30%, dislocations cannot be sufficiently introduced in at least one rolling pass of the last three stages, and the average aspect ratio of the old austenite grains in the region containing bainite and martensite cannot be achieved in the final metal structure.
  • the reduction rate in each rolling pass of the last three stages is more than 30% or the total reduction rate in the last three stages is more than 50%, recrystallization is promoted and dislocations cannot be sufficiently introduced, and similarly, the average aspect ratio of the old austenite grains in the region containing bainite and martensite cannot be achieved in the final metal structure.
  • the total reduction rate in the last three rolling passes of the finish rolling is preferably controlled within the range of 35 to 50%.
  • the finish rolling end temperature is also important in controlling the metal structure of the steel sheet. If the finish rolling end temperature is low, the metal structure may become non-uniform, and the strength and/or hole expandability may decrease. For this reason, the finish rolling end temperature is set to 900°C or higher. On the other hand, if the finish rolling end temperature is high, recrystallization is promoted in the rolling passes in the rear three stages of the finish rolling, and dislocations cannot be sufficiently introduced. As a result, in the finally obtained metal structure, it is not possible to achieve an average aspect ratio of 3.0 or more of the prior austenite grains in the region including bainite and martensite. Therefore, the finish rolling end temperature is set to 1000°C or lower.
  • the finish-rolled steel sheet is primarily cooled on a run-out table (ROT) to an intermediate cooling temperature of 670 to 750 ° C. at an average cooling rate of 50 to 200 ° C./s, and then intermediate cooling is performed for 3 to 10 seconds.
  • ROT run-out table
  • the intermediate cooling temperature in order to sufficiently precipitation strengthen ferrite by promoting the generation and grain growth of TiC precipitates during intermediate cooling, it is necessary to set the intermediate cooling temperature to a relatively high temperature range, i.e., a temperature range of 670 to 750 ° C.
  • a relatively high temperature range i.e., a temperature range of 670 to 750 ° C.
  • ferrite is generated excessively, and the area ratio of ferrite in the final metal structure exceeds 60%, making it impossible to obtain the desired characteristics. Therefore, in this manufacturing method, the average cooling rate in the primary cooling from the finish rolling to the intermediate air cooling temperature is set to 50 ° C / sec or more, thereby suppressing excessive generation of ferrite and allowing TiC precipitates to be sufficiently precipitated by the next intermediate air cooling at high temperature.
  • the average cooling rate of the primary cooling is set to 200 ° C / sec or less, preferably 160 ° C / sec or less.
  • the intermediate cooling temperature is above 750°C or the intermediate cooling time is more than 10 seconds, excessive ferrite is generated or the TiC precipitates become coarse. If excessive ferrite is generated, the desired metal structure containing ferrite, bainite and martensite in a specific ratio cannot be formed in the final hot-rolled steel sheet. In addition, if the TiC precipitates become coarse, the number density of the TiC precipitates also decreases significantly, and the hardness improvement effect of ferrite due to precipitation strengthening cannot be fully obtained. On the other hand, if the intermediate cooling temperature is below 670°C or the intermediate cooling time is less than 3 seconds, the generation and grain growth of TiC precipitates are suppressed, and the desired diameter and/or number density cannot be obtained.
  • the intermediate cooling temperature is primarily cooled to 670 to 750 ° C., preferably 690 to 750 ° C. at an average cooling rate of 50 to 200 ° C./s, preferably 50 to 160 ° C./s, and then intermediate cooling is performed for 3 to 10 seconds, preferably 4 to 9 seconds, to precipitate ferrite in a desired ratio, generate TiC precipitates in the ferrite, and allow them to grow appropriately, so that TiC precipitates having a diameter of 2.0 to 8.0 nm are finally present at a number density of 1.0 ⁇ 10 16 / cm 3 or more.
  • the intermediate cooling temperature is primarily cooled to 670 to 750 ° C., preferably 690 to 750 ° C. at an average cooling rate of 50 to 200 ° C./s, preferably 50 to 160 ° C./s, and then intermediate cooling is performed for 3 to 10 seconds, preferably 4 to 9 seconds, to precipitate ferrite in a desired ratio, generate TiC precipitates in the ferrite, and allow them
  • the average cooling rate of the second cooling is set to 200° C./sec or less, and preferably 180° C./sec or less.
  • the coiling temperature is set to 20°C or higher.
  • the area ratio of bainite which is a hard layer in the metal structure, is controlled within a relatively high range of 30 to 60%, and the average aspect ratio of the prior austenite grains in the region containing the bainite and martensite is 3.0 or more, and the dislocation strengthening associated therewith can be utilized, and as a result, the strength of the hot-rolled steel sheet can be significantly increased.
  • the hot-rolled steel sheet manufactured by the above-mentioned manufacturing method is particularly useful in the automotive field because it can be effectively used in components that require both the contradictory properties of high strength and excellent workability, and furthermore, impact resistance.
  • hot-rolled steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength (TS), hole expansion ratio ( ⁇ ), and yield ratio (YR) of the obtained hot-rolled steel sheets were investigated.
  • ingots having various chemical compositions shown in Table 1 were produced in a vacuum melting furnace, then reheated to the heating temperature shown in Table 2, and rough rolled to produce rough bars with a thickness of 30 mm.
  • the rough bars were held at a temperature range of 1200 to 1300°C for 3600 seconds, and then finish rolling was performed using a rolling mill consisting of multiple rolling stands, with two or more rolling passes in the front stage and three rolling passes in the rear stage, under the conditions shown in Table 2.
  • the end temperature of the finish rolling was as shown in Table 2.
  • the finish-rolled steel plate was primarily cooled to the intermediate air-cooling temperature under the conditions shown in Table 2, and then intermediate air-cooled.
  • the intermediate air-cooled steel plate was secondarily cooled to the coiling temperature under the conditions shown in Table 2, and then coiled at the coiling temperature, to obtain a hot-rolled steel plate with a thickness of 2.5 mm.
  • the properties of the resulting hot-rolled steel sheets were measured and evaluated using the following methods.
  • TS tensile strength
  • YR yield ratio
  • the tensile strength (TS) was measured by taking a JIS No. 5 test piece having a length of 200 mm and a thickness of 2.5 mm from a direction (C direction) in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the rolling direction of the hot-rolled steel plate, and performing a tensile test in accordance with JIS Z 2241:2011. More specifically, the test was performed at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test piece, and strain was applied until it broke.
  • the burr was set to be on the die side, and the initial hole was pushed out with a conical punch having an apex angle of 60 ° until a crack penetrating the plate thickness occurred, and the hole diameter d1 mm at the time of the crack occurrence was measured, and the hole expansion ratio ⁇ (%) of each test piece was calculated using the following formula.
  • Hot-rolled steel sheets with a tensile strength (TS) of 780 MPa or more, a hole expansion ratio ( ⁇ ) of 60.0% or more, and a yield ratio (YR) of 0.70 or more were evaluated as hot-rolled steel sheets with high strength, high hole expansion property, and high yield ratio.
  • TS tensile strength
  • ⁇ hole expansion ratio
  • YR yield ratio
  • Comparative Example 18 With reference to Tables 1 to 3, in Comparative Example 18, the average cooling rate in the primary cooling to the intermediate air-cooling temperature was high, so that the generation of ferrite was excessively suppressed, and the area ratio of ferrite in the final metal structure was less than 30%. As a result, ⁇ was reduced. In Comparative Example 19, it is considered that the total reduction rate in the rolling passes in the front stages of the finish rolling was low, so that recrystallization was suppressed in the front rolling passes, and the metal structure could not be refined. As a result, the desired metal structure was not obtained, and ⁇ was reduced.
  • Comparative Example 20 it is considered that the total reduction rate in the rolling passes in the rear three stages of the finish rolling was high, so that recrystallization was promoted in the rear rolling passes, and dislocations could not be sufficiently introduced. As a result, the average aspect ratio of the prior austenite grains in the region including bainite and martensite was less than 3.0, and TS was reduced. In Comparative Example 21, it is considered that the coiling temperature was high, so that a relatively large amount of cementite was precipitated, and C in the steel was consumed in the formation of cementite.
  • Comparative Example 23 it is considered that the end temperature of the finish rolling was high, so that recrystallization was promoted in the rolling passes in the latter three stages of the finish rolling, and dislocations could not be sufficiently introduced. As a result, the average aspect ratio of the prior austenite grains in the region including bainite and martensite was less than 3.0, and TS was reduced. In Comparative Example 24, the average cooling rate in the primary cooling to the intermediate air-cooling temperature was slow, so that ferrite was generated excessively, and the area ratio of ferrite in the final metal structure was more than 60%, resulting in a decrease in TS.
  • Comparative Example 25 the average cooling rate in the secondary cooling after the intermediate air cooling was slow, so the area ratio of martensite was less than 5%, and TS was reduced.
  • Comparative Example 26 the intermediate air cooling temperature was low, so the generation and grain growth of TiC precipitates were suppressed, and the desired diameter of the TiC precipitates could not be obtained. As a result, the hardness improvement effect of ferrite due to precipitation strengthening could not be fully obtained, and ⁇ and YR were reduced.
  • Comparative Example 27 the intermediate air cooling time was long, so ferrite was generated excessively, and further the TiC precipitates became coarse, and in connection with this, the number density of the TiC precipitates also decreased. As a result, TS, ⁇ , and YR were reduced.
  • Comparative Example 28 the C content was high, so ⁇ and YR were reduced due to the formation of cementite.
  • the Ti content was low, so the TiC precipitates could not be precipitated at a sufficient number density. As a result, TS, ⁇ , and YR were reduced.
  • Comparative Example 30 since the Si content was low, the precipitation of cementite could not be sufficiently suppressed, and it is considered that the C in the steel was consumed in the formation of cementite.
  • the formation of TiC precipitates was suppressed, and the number density of the TiC precipitates was less than 1.0 ⁇ 10 16 / cm 3 , and the effect of improving the hardness of ferrite by precipitation strengthening and the effect of improving the strength of the hot-rolled steel sheet could not be sufficiently obtained, and the TS and YR were reduced.
  • a hot-rolled steel sheet having a predetermined chemical composition and appropriately controlling each condition in the manufacturing method which contains ferrite: 30-60%, bainite: 30-60%, and martensite: 5-20% in terms of area ratio, TiC precipitates having a diameter of 2.0-8.0 nm are present in the ferrite at a number density of 1.0 x 1016 /cm3 or more, and the average aspect ratio of the prior austenite grains in the region containing bainite and martensite is 3.0 or more, was obtained.

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Abstract

L'invention propose une feuille d'acier laminée à chaud caractérisée en ce qu'elle possède une composition chimique prédéterminée, et possède une structure métallique qui comprend, en % en surface, 30 à 60 % de ferrite, 30 à 60 % de bainite, et 5 à 20 % de martensite et dans laquelle des dépôts TiC possédant un diamètre de 2,0 à 8,0 nm existent à une densité en nombre d'au moins 1,0 × 1016/cm3 dans la ferrite, et le rapport d'aspect moyen des grains d'austénite antérieure dans une région contenant de la bainite et de la martensite est d'au moins 3,0.
PCT/JP2023/024346 2022-11-02 2023-06-30 Feuille d'acier laminée à chaud WO2024095533A1 (fr)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2011052321A (ja) * 2009-08-06 2011-03-17 Jfe Steel Corp 低温靭性に優れた高強度熱延鋼板およびその製造方法
WO2013024860A1 (fr) * 2011-08-17 2013-02-21 株式会社神戸製鋼所 Tôle en acier laminée à chaud hautement résistante
WO2018179388A1 (fr) * 2017-03-31 2018-10-04 新日鐵住金株式会社 Tôle en acier laminée à chaud
WO2021210644A1 (fr) * 2020-04-17 2021-10-21 日本製鉄株式会社 Tôle d'acier laminée à chaud à résistance élevée

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2011052321A (ja) * 2009-08-06 2011-03-17 Jfe Steel Corp 低温靭性に優れた高強度熱延鋼板およびその製造方法
WO2013024860A1 (fr) * 2011-08-17 2013-02-21 株式会社神戸製鋼所 Tôle en acier laminée à chaud hautement résistante
WO2018179388A1 (fr) * 2017-03-31 2018-10-04 新日鐵住金株式会社 Tôle en acier laminée à chaud
WO2021210644A1 (fr) * 2020-04-17 2021-10-21 日本製鉄株式会社 Tôle d'acier laminée à chaud à résistance élevée

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