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WO2024095534A1 - Tôle en acier laminé à chaud - Google Patents

Tôle en acier laminé à chaud Download PDF

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Publication number
WO2024095534A1
WO2024095534A1 PCT/JP2023/024349 JP2023024349W WO2024095534A1 WO 2024095534 A1 WO2024095534 A1 WO 2024095534A1 JP 2023024349 W JP2023024349 W JP 2023024349W WO 2024095534 A1 WO2024095534 A1 WO 2024095534A1
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Prior art keywords
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ferrite
content
hot
strength
Prior art date
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PCT/JP2023/024349
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English (en)
Japanese (ja)
Inventor
敬士 島田
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
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Nippon Steel Corp
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Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Priority to EP23885307.1A priority Critical patent/EP4613894A1/fr
Priority to KR1020257013816A priority patent/KR20250067186A/ko
Priority to CN202380076086.3A priority patent/CN120129764A/zh
Priority to JP2024554258A priority patent/JPWO2024095534A1/ja
Publication of WO2024095534A1 publication Critical patent/WO2024095534A1/fr
Priority to MX2025004682A priority patent/MX2025004682A/es
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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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
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    • 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
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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    • 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
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    • 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
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to hot-rolled steel sheets.
  • Patent Document 1 describes a high-strength thin steel sheet characterized in that the steel structure is composed of a ferrite phase and a martensite phase, carbonitrides are precipitated in the ferrite phase by interphase interface precipitation, and the interphase interface precipitation plane spacing in a region of 40% or more of the ferrite phase is 20 nm or more and 60 nm or less.
  • Patent Document 1 also teaches that precipitation strengthening by precipitates in the ferrite phase can ensure sufficient strength, and in addition, the mixed structure with the martensite phase makes it possible to obtain the high ductility specific to the mixed structure while ensuring high fatigue properties.
  • high-strength steel plate is manufactured by hot rolling a cast slab, and it is known that there may be cases where the hot rolling causes anisotropy in strength between the strength in the rolling direction (L direction) and the strength in the width direction (C direction) perpendicular to the rolling direction. When the anisotropy in strength increases, it generally causes a problem because the workability of the steel plate decreases.
  • the present invention was made in consideration of the above, and its purpose is to provide a hot-rolled steel sheet with a new structure that, despite its high strength, has improved hole expansion properties and reduced strength anisotropy.
  • the inventors conducted research, focusing particularly on the microstructure of hot-rolled steel sheet.
  • the inventors discovered that by forming the microstructure of a hot-rolled steel sheet having a specified chemical composition from a three-phase structure containing ferrite, bainite, and martensite in specific ratios, it is possible to achieve a high strength of tensile strength of 780 MPa or more while improving the anisotropy of strength, and further that by having TiC precipitates with a suitable diameter present in the ferrite at a specified number density, the ferrite can be precipitation strengthened, thereby reducing the hardness difference in the three-phase structure and improving hole expansion property, and thus completed the present invention.
  • the present invention which has achieved the above object, is as follows. (1) In mass%, C: 0.010 to 0.100%, Si: 0.01 to 0.10%, Mn: 0.50 to 3.00%, Ti: 0.050 to 0.200%, Nb: 0.010 to 0.020%, Al: 0.100 to 1.000%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Ni: 0 to 2.000%, Mo: 0 to 1.000%, Cr: 0 to 2.000%, B: 0 to 0.0100%, Co: 0 to 2.000%, V: 0 to 1.000%, Cu: 0 to 2.000%, W: 0 to 1.0000%, Ta: 0 to 1.0000%, Sn: 0 to 1.0000%, Sb: 0 to 1.0000%, As: 0 to 0.0100%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Zr: 0 to 0.0
  • the chemical composition is, in mass%, Ni: 0.001 to 2.000%, Mo: 0.001 to 1.000%, Cr: 0.001 to 2.000%, B: 0.0001 to 0.0100%, Co: 0.001 to 2.000%, V: 0.001 to 1.000%, Cu: 0.001 to 2.000%, W: 0.0001 to 1.0000%, Ta: 0.0001 to 1.0000%, Sn: 0.0001 to 1.0000%, Sb: 0.0001 to 1.0000%, As: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
  • the hot-rolled steel sheet according to the above (1) characterized in that it contains at least one of the following:
  • the present invention makes it possible to provide a hot-rolled steel sheet that has high strength, but also has improved hole expansion properties and reduced strength anisotropy.
  • the hot-rolled steel sheet according to the embodiment of the present invention has, in mass%, C: 0.010 to 0.100%, Si: 0.01 to 0.10%, Mn: 0.50 to 3.00%, Ti: 0.050 to 0.200%, Nb: 0.010 to 0.020%, Al: 0.100 to 1.000%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Ni: 0 to 2.000%, Mo: 0 to 1.000%, Cr: 0 to 2.000%, B: 0 to 0.0100%, Co: 0 to 2.000%, V: 0 to 1.000%, Cu: 0 to 2.000%, W: 0 to 1.0000%, Ta: 0 to 1.0000%, Sn: 0 to 1.0000%, Sb: 0 to 1.0000%, As: 0 to 0.0100%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Mg: 0 to
  • the properties such as hole expandability decrease with increasing strength of the steel sheet, and that there may be cases where the strength in the rolling direction (L direction) and the width direction (C direction) perpendicular thereto are anisotropic in strength in relation to the hot rolling during steel sheet production.
  • anisotropy of strength due to the anisotropic microstructure obtained by hot rolling during steel sheet production, the tensile strength tends to differ between the rolling direction (L direction) and the width direction (C direction) perpendicular thereto, and generally, the hot rolled steel sheet tends to exhibit anisotropy in strength such that the tensile strength in the L direction is lower than the tensile strength in the C direction.
  • the inventors have found that by forming a microstructure of a hot-rolled steel sheet having a predetermined chemical composition from a three-phase structure containing ferrite, bainite and martensite in a specific ratio, more specifically, a three-phase structure consisting of ferrite: 60-80%, bainite: 15-30%, and martensite: 3-10%, in terms of area ratio, a high strength of tensile strength of 780 MPa or more can be achieved, while the anisotropy of strength can be significantly reduced compared to the case of DP steel (combined phase steel) mainly composed of soft ferrite and hard martensite.
  • DP steel combined phase steel
  • the inventors have found that in the case of a three-phase structure consisting of ferrite, bainite and martensite, the hardness difference between the phases is relatively large, and the hole expandability may be reduced due to such hardness difference, so that the improvement of hole expandability from the viewpoint of reducing the hardness difference in the three-phase structure was examined.
  • the inventors found that the hardness difference in the three-phase structure can be reduced by precipitation strengthening the softest ferrite in the three-phase structure, more specifically, by having TiC precipitates with a diameter of 1.0 to 5.0 nm exist in the ferrite at a number density of 1.0 x 1016 to 100.0 x 1016 pieces/ cm3 .
  • the inventors found that, despite the microstructure being constituted by a three-phase structure in which the hardness difference is relatively likely to become large in order to reduce the anisotropy of strength, the hardness of ferrite can be increased by precipitation strengthening using TiC precipitates of a specific diameter and number density, thereby simultaneously achieving a reduction in the anisotropy of strength and an improvement in hole expansibility.
  • Refining the ferrite grains that constitute the main phase of the three-phase structure not only contributes to improving the strength of the hot-rolled steel sheet as a whole, but also contributes to reducing the hardness difference of the three-phase structure consisting of ferrite, bainite and martensite.
  • the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent workability, and is therefore particularly useful in the automotive field, where both properties are required to be achieved.
  • C is an element effective in increasing the strength of the steel sheet.
  • C forms carbides and/or carbonitrides with Ti and Nb in ferrite, and contributes to precipitation strengthening of ferrite based on the formed precipitates and refinement of ferrite grains due to the pinning effect of the precipitates.
  • the C content is set to 0.010% or more.
  • the C content may be 0.012% or more, 0.015% or more, 0.018% or more, 0.020% or more, or 0.022% or more.
  • the C content is set to 0.100% or less.
  • the C content may be 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.
  • Si is an effective element for increasing strength as a solid solution strengthening element.
  • the Si content is set to 0.01% or more.
  • the Si content may be 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more.
  • excessive Si content may cause surface quality defects called Si scale.
  • Si scale may increase the surface roughness of the hot-rolled steel sheet, and in addition, excessive Si content may increase the amount of ferrite, making it difficult to obtain the desired three-phase structure. Due to these factors, the anisotropy of strength in the tensile strength in the L direction and C direction may become significant. Therefore, the Si content is set to 0.10% or less.
  • the Si content may be 0.09% or less, 0.08% or less, 0.07% or less, or 0.06% or less.
  • Mn is an element that is effective in increasing strength as a hardenability and solid solution strengthening element.
  • the Mn content is set to 0.50% or more.
  • the Mn content may be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more.
  • the Mn content is set to 3.00% or less.
  • the Mn content may be 2.70% or less, 2.50% or less, 2.20% or less, or 2.00% or less.
  • Ti has the effect of forming TiC precipitates, which are carbides, in ferrite and increasing the hardness of the ferrite through precipitation strengthening.
  • the Ti content is set to 0.050% or more.
  • the Ti content may be 0.060% or more, 0.080% or more, 0.100% or more, or 0.120% or more.
  • Ti is contained excessively, the TiC precipitates become coarse, and the desired precipitation strengthening in the ferrite may not be obtained.
  • the TiC precipitates become coarse, the number density of the TiC precipitates also decreases, so in this case, the hardness of the ferrite cannot be sufficiently increased by precipitation strengthening. Therefore, the Ti content is set to 0.200% or less.
  • the Ti content may be 0.190% or less, 0.180% or less, 0.160% or less, or 0.140% or less.
  • Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to the refinement of the structure by the pinning effect.
  • the pinning effect suppresses the coarsening of austenite grains, promotes ferrite transformation, and refines ferrite grains. Refining ferrite grains not only increases the strength of the steel sheet, but also contributes to reducing the hardness difference of the three-phase structure. If the Nb content is low, these effects may not be fully obtained. Therefore, the Nb content is 0.010% or more.
  • the Nb content may be 0.012% or more, 0.013% or more, or 0.015% or more.
  • the Nb content is 0.020% or less.
  • the Nb content may be 0.018% or less or 0.016% or less.
  • Al 0.100 to 1.000%
  • Al is an element that acts as a deoxidizer. If the Al content is low, such effects may not be sufficiently obtained and/or the desired three-phase structure may not be obtained. Therefore, the Al content is set to 0.100% or more.
  • the Al content may be 0.120% or more, 0.150% or more, or 0.020% or more.
  • Al content is set to 1.000% or less.
  • the Al content may be 0.800% or less, 0.600% or less, or 0.400% or less.
  • the P content is set to 0.1000% or less.
  • the P content may be 0.0800% or less, 0.0500% or less, 0.0300% or less, or 0.0250% 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, 0.0010% or more, or 0.0050% or more.
  • the Si content is set to 0.0100% or less.
  • the S content may be 0.0080% or less, 0.0060% or less, or 0.0050% 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, or 0.0005% or more.
  • N 0.0100% or less
  • the N content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.
  • the lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.
  • O is an element that is mixed in during the manufacturing process. If O is contained excessively, coarse inclusions may be formed, which may reduce the toughness of the steel plate. Therefore, the O content is set to 0.0100% or less.
  • the O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • the lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long time for refining, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.
  • the 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.
  • the hot-rolled steel sheet may contain at least one of Ni: 0-2.000%, Mo: 0-1.000%, Cr: 0-2.000%, B: 0-0.0100%, Co: 0-2.000%, V: 0-1.000%, Cu: 0-2.000%, W: 0-1.0000%, and Ta: 0-1.0000%.
  • the hot-rolled steel sheet may contain at least one of Sn: 0-1.0000%, Sb: 0-1.0000%, and As: 0-0.0100%.
  • the hot-rolled steel sheet may also contain at least one of Mg: 0-0.0100%, Ca: 0-0.0100%, Zr: 0-0.0100%, and Hf: 0-0.0100%.
  • the hot-rolled steel sheet may also contain Bi: 0-0.0100%.
  • the hot-rolled steel sheet may also contain REM: 0-0.0100%.
  • 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 0.001% or more, and may be 0.010% or more, 0.030% or more, or 0.050% or more.
  • the Ni content is preferably 2.000% or less, and may be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.150% or less, or 0.100% 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, 0.020% 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.
  • 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 0.001% or more, and may be 0.010% or more, 0.030% or more, or 0.050% or more.
  • the Cr content is preferably 2.000% or less, and may be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.150% or less, or 0.100% or less.
  • 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 0.0100% or less.
  • the B content may be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.
  • 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, 0.020% or more, or 0.050% or more.
  • the Co content is preferably 2.000% or less.
  • the Co content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.200% or less, or 0.100% 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 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.080% 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 0.001% or more.
  • the Cu content may be 0.010% or more, 0.050% or more, or 0.100% or more.
  • the Cu content is preferably 2.000% or less.
  • the Cu content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.150% or less, or 0.100% 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 0.0001% or more.
  • the W content may be 0.0010% or more, 0.0020% or more, or 0.0050% or more.
  • excessive W content may reduce weldability. Therefore, the W content is preferably 1.0000% or less.
  • the W content may be 0.8000% or less, 0.5000% or less, 0.2000% or less, 0.1000% or less, or 0.0500% or less.
  • Ta is an element effective for controlling the morphology of carbides and improving the strength of steel sheet.
  • the Ta content may be 0%, but in order to obtain these effects, the Ta content is preferably 0.0001% or more.
  • the Ta content may be 0.0010% or more, 0.0020% or more, or 0.0050% or less.
  • the Ta content is preferably 1.0000% or less.
  • the Ta content may be 0.8000% or less, 0.5000% or less, 0.2000% or less, 0.1000% or less, or 0.0500% 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 Sn and Sb contents are preferably 0.0001% or more, and may be 0.0010% or more, 0.0020% or more, or 0.0050% or more.
  • excessive Sn and Sb content may cause a decrease in toughness. Therefore, the Sn and Sb contents are preferably 1.0000% or less, and may be 0.8000% or less, 0.5000% or less, 0.3000% or less, 0.1000% or less, or 0.0500% or less.
  • the As content may be 0%, but in order to obtain such an effect, the As content is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if As is contained excessively, the effect is saturated, and containing more As than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the As content is preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • Mg, Ca, Zr and Hf are elements capable of controlling the morphology of sulfides.
  • the contents of Mg, Ca, Zr and Hf may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the contents of Mg, Ca, Zr and Hf 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 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the Bi content is preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • REM is an element 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, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the REM content is preferably 0.0100% or less, and 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 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 microstructure of the hot-rolled steel sheet according to the embodiment of the present invention is composed of ferrite: 60 to 80%, bainite: 15 to 30%, and martensite: 3 to 10% in terms of area ratio.
  • the area ratio of ferrite is set to 60% or more, and may be, for example, 62% or more, 65% or more, or 68% or more.
  • the area ratio of ferrite is set to 80% or less, and may be, for example, 78% or less, 75% or less, or 72% or less.
  • the area ratios of the hard phases bainite and martensite are high.
  • the area ratio of bainite may be 18% or more, 20% or more, or 22% or more.
  • the area ratio of martensite may be more than 3%, 3.1% or more, 3.2% or more, 3.3% or more, 3.5% or more, 3.8% or more, 4% or more, 4.5% or more, 5% or more, or 6% or more.
  • the area ratios of bainite and martensite are low. From this viewpoint, for example, the area ratio of bainite may be 28% or less, 26% or less, or 24% or less. Similarly, the area ratio of martensite may be 9% or less, 8% or less, or 7% or less.
  • the microstructure of the hot-rolled steel sheet according to the embodiment of the present invention is composed of ferrite, bainite, and martensite, and does not contain or does not substantially contain other structures (remaining structures).
  • “Substantially does not contain” means that the area ratio of the remaining structures other than ferrite, bainite, and martensite is 3% or less. Therefore, the area ratio of the remaining structures is 0 to 3%, and may be, for example, 0 to 1.5%, 0 to 1%, or 0 to 0.5%. In other words, the total area ratio of ferrite, bainite, and martensite is 97 to 100%, and may be, for example, 98.5 to 100%, 99 to 100%, or 99.5 to 100%. When a remaining structure is present, the remaining structure is, for example, pearlite.
  • 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 3000 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 a point count method.
  • a total of 100 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 bainite is surrounded by grain boundaries with an orientation difference of 15° or more.
  • martensite containing a large amount of dissolved carbon has a smaller corrosion loss during etching than other structures, and its height is relatively higher than other structures in the observation field after etching. Therefore, it appears whiter than other structures, and martensite 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%. It is not necessary to specifically identify the remaining structure, but when the remaining structure contains pearlite or the like, the 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 hardness difference between each phase in the three-phase structure composed of ferrite, bainite, and martensite can be reduced, and as a result, the hole expandability of the hot-rolled steel sheet can be significantly improved.
  • the diameter of the TiC precipitate is smaller than 1.0 nm, the TiC precipitate cannot act sufficiently as an obstacle to dislocation motion, and therefore the effect of improving the hardness of ferrite by precipitation strengthening cannot be fully obtained.
  • the diameter of the TiC precipitates is too large, it may not be possible to obtain the desired precipitation strengthening in ferrite.
  • the strengthening mechanism changes in relation to dislocation motion as the TiC precipitates become coarse, and for example, the dislocation line does not pass across the TiC precipitates, but passes leaving a loop of dislocation line around the coarse TiC precipitates, resulting in a small amount of precipitation strengthening.
  • the number density of the TiC precipitates also decreases significantly, making it impossible to sufficiently increase the hardness of ferrite by precipitation strengthening.
  • the higher the number density the more preferable it is, for example, 2.0 ⁇ 10 16 /cm 3 or more, 5.0 ⁇ 10 16 /cm 3 or more, 10.0 ⁇ 10 16 /cm 3 or more, or 20.0 ⁇ 10 16 /cm 3 or more.
  • the number density is set to 100.0 ⁇ 10 16 /cm 3 or less, and may be, for example, 80.0 ⁇ 10 16 /cm 3 or less or 50.0 ⁇ 10 16 /cm 3 or less.
  • TiC precipitates having a diameter of 1.0 to 5.0 nm are present in ferrite at a number density of 1.0 x 10 to 100.0 x 10 particles/cm. Therefore, as long as the above diameter and number density requirements are satisfied, for example, coarse TiC precipitates may be present in 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 hot rolled steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm, although not particularly limited thereto.
  • the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 5.0 mm or less, or 4.0 mm or less.
  • the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 1470 MPa or less, 1250 MPa or less, 1180 MPa or less, or 1080 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.
  • Total elongation: EL According to the hot-rolled steel sheet having the above chemical composition and microstructure, in addition to high tensile strength, the total elongation can be improved, and more specifically, a total elongation of 16.0% or more can be achieved.
  • the total elongation is preferably 18.0% or more, more preferably 20.0% or more, and most preferably 22.0% or more.
  • the upper limit is not particularly limited, but for example, the total elongation may be 30.0% or less or 25.0% or less.
  • the total elongation is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.
  • the hole expansion ratio may be preferably 55% or more, 60% or more, or 65% 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.
  • This hole expansion test is performed five times, and the average value thereof is determined as the hole expansion ratio ⁇ .
  • ⁇ 100 ⁇ ⁇ (d1 - d0) / d0 ⁇
  • the method for producing a hot-rolled steel sheet includes: a hot rolling process comprising heating a slab having the chemical composition described above in relation to the hot rolled steel sheet to a temperature of 1100-1300°C and then finish rolling, the end temperature of said finish rolling being 900-1000°C;
  • the method is characterized by including an intermediate cooling step in which the finish-rolled steel sheet is cooled to an intermediate air-cooling temperature of 620 to 700°C at an average cooling rate of 10°C/sec or more, and then air-cooled for 5 to 10 seconds at an average cooling rate of 2°C/sec or more and less than 10°C/sec, and a cooling step in which the intermediate-cooled steel sheet is primarily cooled at an average cooling rate of 10 to 20°C/sec for 1 to 3 seconds, and then secondarily cooled to 200°C or less at an average cooling rate of 25°C/sec or more, and then coiled.
  • Each step will be described in detail below.
  • a slab having the chemical composition described above in relation to the hot rolled steel sheet is heated.
  • the slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method.
  • the slab used contains a relatively large amount of alloying elements in order to obtain a high strength steel sheet. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to dissolve the alloying elements in the slab. If the heating temperature is less than 1100°C, the alloying elements may not be sufficiently dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. For this reason, the heating temperature is preferably 1100°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 heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
  • the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
  • the heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling. Since the slab used as described above contains a relatively large amount of alloying elements, it is necessary to increase the rolling load during hot rolling. For this reason, it is preferable to perform hot rolling at a high temperature.
  • the end temperature of the finish rolling is important in terms of controlling the metal structure of the steel sheet. If the end temperature of the finish rolling is low, recrystallization is suppressed and the metal structure becomes non-uniform, and the strength and/or hole expandability may decrease. For this reason, the end temperature of the finish rolling is set to 900°C or higher. On the other hand, if the end temperature of the finish rolling is high, the austenite becomes coarse and the proportion of ferrite becomes small, and the desired three-phase structure cannot be obtained. Therefore, the end temperature of the finish rolling is set to 1000°C or lower.
  • the finish-rolled steel sheet is cooled to an intermediate air-cooling temperature of 620 to 700 ° C. at an average cooling rate of 10 ° C./s or more, and then air-cooled for 5 to 10 seconds at an average cooling rate of 2 ° C./s or more and less than 10 ° C./s.
  • an intermediate air-cooling temperature of 620 to 700 ° C. at an average cooling rate of 10 ° C./s or more, ferrite can be precipitated in a desired ratio and TiC precipitates having a desired diameter can be formed in the ferrite.
  • the intermediate air-cooling temperature is more than 700 ° C.
  • the average cooling rate to the intermediate air-cooling temperature is less than 10 ° C./s, the ferrite transformation proceeds too much, and it becomes impossible to form a three-phase structure containing ferrite, bainite, and martensite in a specific ratio in the finally obtained hot-rolled steel sheet.
  • the intermediate air-cooling temperature is more than 700 ° C., a relatively large number of coarse TiC precipitates are precipitated, and the desired precipitation strengthening in the ferrite may not be obtained.
  • the average cooling rate to the intermediate air-cooling temperature of 620 to 700 ° C. is preferably 20 ° C./sec or more.
  • the upper limit is not particularly limited, but for example, the average cooling rate may be 30 ° C./sec or less.
  • the intermediate air-cooling temperature is less than 600 ° C.
  • ferrite cannot be sufficiently precipitated, and similarly, a three-phase structure containing ferrite, bainite, and martensite in a specific ratio cannot be formed in the finally obtained hot-rolled steel sheet.
  • the intermediate air-cooling temperature is less than 600 ° C.
  • TiC precipitates having a desired diameter cannot be formed in a sufficient number density even by the subsequent air-cooling, and as a result, the effect of improving the hardness of ferrite by precipitation strengthening cannot be sufficiently obtained.
  • the average cooling rate and time in air cooling after cooling to an intermediate air cooling temperature of 620 to 700 ° C are important for precipitating ferrite in a desired ratio and forming TiC precipitates having a desired diameter in the ferrite.
  • the temperature range of ferrite transformation is in a relatively high temperature range of 620 to 700 ° C, it is possible to precipitate ferrite in a desired ratio and to cause the TiC precipitates precipitated in the ferrite to grow in grains. This is advantageous because it is possible to increase the amount of precipitation strengthening of ferrite by growing the grains of TiC precipitates to a certain extent.
  • ferrite is precipitated in a desired ratio
  • TiC is precipitated in the ferrite
  • the TiC precipitates are appropriately grown in grains to finally have a diameter of 1.0 to 5.0 nm and a number density of 1.0 ⁇ 10 16 to 100.0 ⁇ 10 16 / cm 3 .
  • the hardness of ferrite can be increased by precipitation strengthening, and the difference in hardness in the three-phase structure can be reduced to significantly improve the hole expandability.
  • the above air cooling control is a very important operation not only for precipitation strengthening of ferrite by TiC precipitates, but also for precipitating carbides, nitrides and/or carbonitrides of Nb in ferrite to grow the grains, and for fully exerting the pinning effect of such precipitates to refine the ferrite grains and thereby achieve high strength of the hot-rolled steel sheet.
  • the steel sheet after the intermediate cooling is primarily cooled at an average cooling rate of 10 to 20°C/s for 1 to 3 seconds, then secondary cooled at an average cooling rate of 25°C/s or more to 200°C or less, and then coiled.
  • Such two-stage cooling allows bainite to be properly precipitated mainly in the primary cooling at a relatively slow average cooling rate, and similarly allows martensite to be properly precipitated mainly in the secondary cooling at a relatively fast average cooling rate, so that a three-phase structure containing ferrite, bainite, and martensite in specific ratios can be formed in the finally obtained hot-rolled steel sheet.
  • the average cooling rate of the primary cooling is less than 10°C/s, or the average cooling rate is more than 20°C/s, such primary cooling is substantially eliminated and one-stage cooling consisting of only secondary cooling is performed, bainite cannot be properly precipitated, and therefore a three-phase structure consisting of ferrite, bainite, and martensite cannot be formed in the finally obtained hot-rolled steel sheet.
  • the tensile strength in the L direction of the hot rolled steel sheet tends to be lower than the tensile strength in the C direction, i.e., the anisotropy of the tensile strength in the L direction and the C direction becomes significant.
  • the average cooling rate of the primary cooling is 12 to 18° C./sec.
  • the average cooling rate of the secondary cooling is 27°C/sec or more.
  • the average cooling rate may be 50°C/sec or less or 40°C/sec or less.
  • the coiling temperature is, for example, 100°C or more.
  • the microstructure is composed of a three-phase structure consisting of ferrite: 60-80%, bainite: 15-30%, and martensite: 3-10%, in terms of area ratio, so that the tensile strength of the hot-rolled steel sheet can be achieved with a high strength of 780 MPa or more, while the anisotropy of the strength in the L direction and C direction of the tensile strength can be significantly reduced.
  • hot-rolled steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength (TS), total elongation (EL), hole expansion ratio ( ⁇ ), and strength anisotropy of the obtained hot-rolled steel sheets were investigated.
  • molten steel was cast by continuous casting to form slabs having various chemical compositions shown in Table 1, and 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 end temperature of the finish rolling was as shown in Table 2.
  • the finish-rolled steel plate was intermediately cooled under the conditions shown in Table 2, and the intermediately cooled steel plate was primarily cooled for 2 seconds under the conditions shown in Table 2, then secondarily cooled and coiled to obtain a hot-rolled steel plate having a plate thickness of 3.2 mm.
  • the properties of the resulting hot-rolled steel sheets were measured and evaluated using the following methods.
  • Tensile strength (TS) and total elongation (EL) Tensile strength (TS) and total elongation (EL) were measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the rolling direction of the hot-rolled steel sheet (C direction) and conducting a tensile test in accordance with JIS Z 2241: 2011. The tensile strength obtained here is also called C direction TS.
  • Hot-rolled steel sheets with a tensile strength (TS) of 780 MPa or more, a hole expansion ratio ( ⁇ ) of 50% or more, and a passing evaluation of strength anisotropy were evaluated as hot-rolled steel sheets with improved hole expansion and reduced strength anisotropy despite their high strength.
  • TS tensile strength
  • ⁇ hole expansion ratio
  • Table 3 The results are shown in Table 3.
  • Comparative Example 42 the end temperature of the finish rolling was high, which is thought to have caused the austenite to coarsen and the proportion of ferrite to decrease. As a result, the desired three-phase structure was not obtained, and the anisotropy of strength increased.
  • Comparative Example 43 the end temperature of the finish rolling was low, which resulted in an uneven metal structure and a decrease in TS and ⁇ .
  • Comparative Example 44 the intermediate air-cooling temperature was high, which caused the ferrite transformation to proceed too far, and the desired three-phase structure was not obtained, and the anisotropy of strength increased.
  • Comparative Example 45 the intermediate air-cooling temperature was low, which resulted in the inability to sufficiently precipitate ferrite, and similarly the desired three-phase structure was not obtained, and the anisotropy of strength increased.
  • the intermediate air-cooling temperature was low, so TiC precipitates having the desired diameter could not be formed at a sufficient number density even after subsequent air-cooling. As a result, the effect of improving the hardness of ferrite by precipitation strengthening could not be fully obtained, and ⁇ decreased.
  • Comparative Example 46 the intermediate cooling time was long, so that ferrite was excessively precipitated and the ratio of the hard phase was low, and as a result, the desired three-phase structure was not obtained, and the anisotropy of the strength became significant and the desired tensile strength could not be achieved.
  • Comparative Example 47 the intermediate cooling time was short, so that ferrite could not be sufficiently precipitated, and as a result, the desired three-phase structure was not obtained, and the anisotropy of the strength became large.
  • Comparative Example 48 the average cooling rate of the first cooling in the cooling process was fast, so that bainite could not be properly precipitated, and as a result, the desired three-phase structure was not obtained, and the anisotropy of the strength became large.
  • Comparative Examples 52 and 55 the TS was decreased due to the low C and Mn contents, respectively.
  • the C content was high, so the desired three-phase structure was not obtained, and the anisotropy of the strength increased.
  • Comparative Example 54 the Si content was high, so the surface roughness of the hot-rolled steel sheet increased due to Si scale, and the anisotropy of the strength increased due to the failure to obtain the desired three-phase structure.
  • Comparative Example 56 the Mn content was high, so it is considered that the diffusion coefficient of C was decreased and the diameter of the TiC precipitates was reduced. As a result, the effect of improving the hardness of ferrite due to precipitation strengthening based on TiC precipitates could not be fully obtained, and ⁇ decreased.
  • Comparative Example 57 the Ti content was low, so Ti precipitates could not be formed at a sufficient number density, and ⁇ decreased.
  • Comparative Example 58 the Ti content was high, so the number density of the TiC precipitates decreased due to the coarsening of the TiC precipitates, and ⁇ also decreased.
  • Comparative Examples 59 and 61 the Nb and Al contents were low, respectively, so the desired three-phase structure was not obtained, and the strength anisotropy was large.
  • the Nb content was low, so the pinning effect of carbides and the like was not sufficiently obtained, and it is believed that this is related to the fact that ferrite transformation was not promoted.
  • Comparative Example 60 the Nb content was high, so it is believed that coarse carbides and the like were formed in the steel, and the desired three-phase structure was not obtained. As a result, TS was reduced and the strength anisotropy was large.
  • Comparative Example 62 the Al content was high, so it is believed that coarse oxides were formed, and the desired three-phase structure was not obtained. As a result, TS was reduced and the strength anisotropy was large.
  • the area ratio of the microstructure is 60-80% ferrite, 15-30% bainite, and 3-10% martensite, and the anisotropy of the strength is significantly reduced while achieving a high strength of tensile strength of 780 MPa or more.
  • TiC precipitates having a diameter of 1.0-5.0 nm are present in the ferrite at a number density of 1.0 x 10 16 to 100.0 x 10 16 pieces/cm 3 , thereby precipitation strengthening the ferrite, thereby reducing the hardness difference in the three-phase structure and significantly improving the hole expandability.
  • the area ratio of the residual structure was 0% in many of the invention examples, but when the residual structure was present, the residual structure was pearlite.

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Abstract

L'invention concerne une tôle en acier laminé à chaud caractérisée en ce qu'elle a une composition chimique prédéterminée, qui contient, en termes de rapport de surface, de 60 % à 80 % de ferrite ; de 15 % à 30 % de bainite ; et de 3 % à 10 % de martensite ; qui présente une microstructure dans laquelle des précipités TiC ayant un diamètre de 1,0 à 5,0 nm sont présents dans la ferrite à une densité en nombre de 1,0 × 1016 à 100,0 × 1016 précipités/cm3, et dont la résistance à la traction est de 780 MPa ou plus.
PCT/JP2023/024349 2022-11-02 2023-06-30 Tôle en acier laminé à chaud Ceased WO2024095534A1 (fr)

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EP23885307.1A EP4613894A1 (fr) 2022-11-02 2023-06-30 Tôle en acier laminé à chaud
KR1020257013816A KR20250067186A (ko) 2022-11-02 2023-06-30 열간 압연 강판
CN202380076086.3A CN120129764A (zh) 2022-11-02 2023-06-30 热轧钢板
JP2024554258A JPWO2024095534A1 (fr) 2022-11-02 2023-06-30
MX2025004682A MX2025004682A (es) 2022-11-02 2025-04-22 Lamina de acero laminada en caliente

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2011225935A (ja) 2010-04-20 2011-11-10 Nippon Steel Corp 疲労特性と局部延性に優れた高強度薄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
JP2011225935A (ja) 2010-04-20 2011-11-10 Nippon Steel Corp 疲労特性と局部延性に優れた高強度薄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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CN120129764A (zh) 2025-06-10
MX2025004682A (es) 2025-05-02

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