WO2024203777A1 - 鋼板、部材およびそれらの製造方法 - Google Patents

鋼板、部材およびそれらの製造方法 Download PDF

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WO2024203777A1
WO2024203777A1 PCT/JP2024/011161 JP2024011161W WO2024203777A1 WO 2024203777 A1 WO2024203777 A1 WO 2024203777A1 JP 2024011161 W JP2024011161 W JP 2024011161W WO 2024203777 A1 WO2024203777 A1 WO 2024203777A1
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steel sheet
temperature
cooling
content
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English (en)
French (fr)
Japanese (ja)
Inventor
三周 知場
英之 木村
真一 古谷
洋一郎 松井
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JFE Steel Corp
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JFE Steel Corp
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Priority to KR1020257031748A priority Critical patent/KR20250150145A/ko
Priority to JP2024547724A priority patent/JP7704308B2/ja
Priority to CN202480021126.9A priority patent/CN120958159A/zh
Priority to EP24779896.0A priority patent/EP4667608A1/en
Publication of WO2024203777A1 publication Critical patent/WO2024203777A1/ja
Priority to MX2025011404A priority patent/MX2025011404A/es
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys 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
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to steel sheets, components, and their manufacturing methods that are suitable for use in press-molded products with complex shapes that are used in automobiles, home appliances, etc. through press molding processes and that have excellent chemical conversion treatability.
  • TRIP steel in which residual austenite (residual ⁇ ) is dispersed in the microstructure of the steel sheet, has been developed as a technology to improve the ductility of steel sheet.
  • a large amount of Si is added to TRIP steel in order to form residual ⁇ in the microstructure.
  • Patent Document 1 discloses that austempering (carbon distribution associated with bainite transformation) is performed by holding steel containing 0.04-0.12% C, 0.8-2.5% Si, and 0.5-2.0% Mn at 300-500°C for 10-900 seconds after annealing, thereby generating 2-10% residual ⁇ , resulting in a steel sheet with high ductility of TS ⁇ El ⁇ 21000MPa ⁇ % and high stretch flange formability of 70% or more.
  • Patent Document 2 discloses a method of improving the chemical conversion treatability by adding Ni to prevent Si from concentrating on the steel sheet surface.
  • Patent Document 3 discloses a method for improving chemical conversion treatability by appropriately controlling the content of Mn, which concentrates on the surface together with Si, so that the Si/Mn ratio is 0.40 or less, thereby forming a Mn-Si composite oxide on the surface.
  • Patent Document 4 discloses a method for improving chemical conversion treatability by directly removing Si-based oxides by pickling or brushing after annealing.
  • Patent Documents 2 and 4 are effective as methods for improving the chemical conversion treatability of steels with a high Si content, but there has been a demand for the establishment of other techniques that adjust the alloy elements to be contained, annealing conditions, etc. Furthermore, the inventors' investigations revealed that even in the method disclosed in Patent Document 3, good chemical conversion treatability is not necessarily ensured. Thus, there has been a demand for the establishment of a new technology for producing high-strength steel sheet that has excellent ductility and chemical conversion treatability, as well as hole expandability.
  • the present invention was made in consideration of the above circumstances, and its purpose is to provide steel plates, components, and manufacturing methods thereof that have excellent ductility, hole expansion properties, and chemical conversion treatability, and have a tensile strength of 780 MPa or more.
  • tensile strength refers to the tensile strength (TS) obtained in accordance with JIS Z2241 (2011).
  • excellent ductility means that the total elongation EL obtained in accordance with JIS Z2241 (2011) satisfies any one of the following (A) to (C).
  • the hole expansion ratio ⁇ (%) may be less than 45%.
  • Excellent chemical conversion treatability means that after degreasing (treatment temperature: 40°C, treatment time: 120 seconds, spray degreasing, degreaser: FC-E2011 manufactured by Nippon Parkerizing Co., Ltd.), surface conditioning (pH 9.5, treatment temperature: room temperature, treatment time: 20 seconds, surface conditioner: PL-X manufactured by Nippon Parkerizing Co., Ltd.), and then chemical conversion treatment using a zinc phosphate chemical conversion treatment solution (chemical conversion treatment solution temperature: 35°C, treatment time: 120 seconds, chemical conversion treatment solution: Palbond PB-L3065 manufactured by Nippon Parkerizing Co., Ltd.), the area where the base steel is exposed is less than 10% of the total area.
  • the present inventors have conducted extensive research into the steel components, heat treatment conditions, and microstructures that affect the ductility and chemical conversion treatability of various thin steel sheets having a tensile strength of 780 MPa or more.
  • the following components, in mass % were found to be C: 0.05-0.25%, Si: 0.30-1.50%, Mn: 1.5-4.5%, P: 0.005-0.050%, S: 0.01% or less, sol.
  • the composition contains Al: less than 1.0% and N: less than 0.015%, satisfies the following formula (1), with the balance consisting of iron and unavoidable impurities, the area ratio of polygonal ferrite is 10% to 80%, the total area ratio of upper bainite, tempered martensite, and lower bainite is 10% to 70%, the volume ratio of retained austenite (residual ⁇ ) is 3% to 15%, the area ratio of quenched martensite is 15% or less (including 0%), and the steel structure consists of the remaining structure.
  • the area ratio of polygonal ferrite is 10% to 80%
  • the total area ratio of upper bainite, tempered martensite, and lower bainite is 10% to 70%
  • the volume ratio of retained austenite (residual ⁇ ) is 3% to 15%
  • the area ratio of quenched martensite is 15% or less (including 0%)
  • the steel structure consists of the remaining structure.
  • the present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] In mass%, C: 0.05-0.25%, Si: 0.30-1.50%, Mn: 1.5-4.5%, P: 0.005-0.050%, S: 0.01% or less, sol.
  • the component composition further includes, in mass%, Ti: 0.1% or less, B: 0.001% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, REM: 0.0050% or less,
  • a method for producing a steel sheet comprising the steps of: subjecting a steel slab having the composition according to [1] or [2] to hot rolling, pickling and cold rolling; and then annealing the resulting cold-rolled steel sheet;
  • the annealing is A soaking temperature holding step of heating the cold-rolled steel sheet to a soaking temperature of A c1 point +20 ° C. or more and A c3 point or less and Tc or more calculated by formula (3) in a furnace atmosphere having a dew point of ⁇ 40 ° C. or less, and holding the soaking temperature for 30 to 500 s;
  • a first cooling step of cooling the temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550 ° C.
  • t is the holding time (s) at the soaking temperature
  • Tdp is the dew point (° C.).
  • a method for producing a steel sheet comprising: subjecting a steel slab having the composition according to [1] or [2] to hot rolling, pickling and cold rolling; and then annealing the resulting cold-rolled steel sheet;
  • the annealing is A soaking temperature holding step of heating the cold-rolled steel sheet to a soaking temperature of A c1 point +20 ° C. or more and A c3 point or less and Tc or more calculated by formula (3) in a furnace atmosphere having a dew point of ⁇ 40 ° C. or less, and holding the soaking temperature for 30 to 500 s;
  • t is the holding time (s) at the soaking temperature
  • Tdp is the dew point (° C.).
  • the present invention it is possible to obtain a steel plate and a member which have a high strength, tensile strength TS of 780 MPa or more, and which also have excellent ductility, hole expandability and chemical conversion treatability.
  • the steel sheet of the present invention When the steel sheet of the present invention is applied to the frame members of an automobile body, it can be used to manufacture difficult-to-form parts of complex shapes by cold pressing, which can greatly contribute to reducing the weight of the automobile body. It is possible to reduce material costs because there is no need to use expensive alloying elements or to improve chemical conversion treatability through post-treatment after annealing.
  • FIG. 1 is a graph for explaining the maximum P concentration [Pm] of the present invention.
  • the steel plate of the present invention contains, by mass%, C: 0.05 to 0.25%, Si: 0.30 to 1.50%, Mn: 1.5 to 4.5%, P: 0.005 to 0.050%, S: 0.01% or less, sol. Al: less than 1.0%, and N: less than 0.015%, and satisfies the following formula (1), with the balance being iron and unavoidable impurities.
  • the steel plate has a composition in which the area ratio of polygonal ferrite is 10% or more and 80% or less, and the total area ratio of upper bainite, tempered martensite, and lower bainite is 10% or more and 70% or less.
  • the steel sheet has a high strength of tensile strength TS of 780 MPa or more, and is excellent in ductility, hole expandability, and chemical conversion treatability.
  • the steel sheet of the present invention will be explained below in the order of chemical composition and steel structure. First, the reasons for limiting the chemical composition of the present invention will be explained. In the following explanation, all percentages indicating the steel composition are mass percentages unless otherwise specified.
  • C is contained from the viewpoint of securing a predetermined strength by transformation strengthening and improving ductility by securing a predetermined amount of retained austenite (hereinafter also referred to as retained ⁇ ). If the amount is less than this, these effects cannot be sufficiently ensured.
  • the upper limit of the C content is set at 0.25% due to concerns about hole expandability, which is important in press formability, and weldability, which is important in spot welding or laser welding when assembling the material into a car body after forming into an automobile component. Let us assume that. For this reason, the C content is set to 0.05 to 0.25%.
  • the C content is preferably 0.08% or more, and more preferably 0.10% or more. , preferably 0.22% or less, and more preferably 0.20% or less.
  • Si is contained from the viewpoint of strengthening ferrite to increase strength, and from the viewpoint of suppressing the formation of carbides in martensite and bainite, securing a predetermined amount of retained ⁇ , and improving ductility. If it is less than 30%, these effects cannot be sufficiently ensured. On the other hand, if the Si content exceeds 1.50%, good chemical conversion treatability cannot be ensured even with the production method specified in the present invention. For this reason, the Si content is set to 0.30 to 1.50%.
  • the Si content is preferably 0.35% or more, and more preferably 0.40% or more. is 1.20% or less, more preferably 1.00% or less.
  • Mn improves the hardenability of steel sheets and promotes high strength through transformation strengthening, and like Si, it suppresses the formation of carbides in bainite and promotes the formation of retained austenite that contributes to ductility, thereby improving ductility.
  • the Mn content must be 1.5% or more.
  • the bainite transformation is significantly delayed, and it may not be possible to secure a predetermined amount of retained austenite, resulting in a decrease in ductility.
  • the Mn content is set to 1.5 to 4.5%.
  • the Mn content is preferably 1.8% or more, and more preferably 2.0% or more. It is preferably 3.5% or less, and more preferably 3.0% or less.
  • P is an element that strengthens steel.
  • a P-enriched surface portion is formed on the surface of the steel sheet after annealing, thereby improving the chemical conversion treatability.
  • the P content is set to 0.005% or more.
  • a high P content deteriorates spot weldability, and from this viewpoint, the P content is set to 0.050% or less. Therefore, the P content is set to 0.005 to 0.050%.
  • the P content is preferably 0.007% or more, and more preferably 0.009% or more.
  • the content is preferably 0.035% or less, and more preferably 0.020% or less.
  • S has the effect of improving the scale peeling property during hot rolling and the effect of suppressing nitriding during annealing, but it is an element that has a negative effect on spot weldability, bendability, and hole expandability.
  • the S content is at least 0.01% or less, and preferably 0.0050% or less.
  • it is not necessary to include S it is very costly to reduce the S content to less than 0.0001%, so it is preferable to set the S content to 0.0001% or more from the viewpoint of manufacturing costs.
  • the amount is more preferably 0.0005% or more, and further preferably 0.0010% or more.
  • ⁇ Sol. Al Less than 1.0%> Al is contained for the purpose of deoxidation or obtaining residual ⁇ .
  • the sol. Al content is preferably 0.005% or more.
  • the sol. Al content is 1.0% or more, the amount of coarse Al-based inclusions increases, and the stretch flange formability (hole expandability) decreases.
  • Al is an element that deteriorates the chemical conversion treatability of the steel sheet, and when the sol. Al content is 1.0% or more, good chemical conversion treatability cannot be ensured even in the present invention. For this reason, the sol. Al content is less than 1.0%.
  • the sol. Al content is preferably 0.80% or less, more preferably 0.06% or less.
  • N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces stretch flange formability (hole expandability), so its content must be limited. Therefore, the N content is less than 0.015%.
  • the N content is preferably 0.010% or less, and more preferably 0.006% or less.
  • the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs.
  • the N content is more preferably 0.0005% or more, and further preferably 0.001% or more.
  • [Si] is the Si content (% by mass)
  • [Mn] is the Mn content (% by mass).
  • [Si]/[Mn] (Si/Mn ratio) determines the component ratio of Si and Mn in the surface oxide formed during annealing. If [Si]/[Mn] exceeds 0.35, good chemical conversion treatability cannot be ensured. Therefore, [Si]/[Mn] is set to 0.35 or less.
  • [Si]/[Mn] is preferably 0.
  • the ratio [Si]/[Mn] is preferably 0.10 or more, more preferably 0.15 or more, although there is no particular lower limit. be.
  • composition of the steel plate in the present invention contains the above-mentioned elemental elements as the basic components, with the remainder being iron (Fe) and unavoidable impurities. It is preferable that the composition of the steel plate in the present invention has a composition in which the remainder is Fe and unavoidable impurities.
  • composition of the steel sheet of the present invention may contain, in addition to the above-mentioned components, one or more elements selected from the following as optional elements (optional elements).
  • optional elements optional elements.
  • Ti has the effect of fixing N in steel as TiN, improving hot ductility, and of improving the hardenability of B. It also has the effect of refining the structure by precipitating TiC.
  • the Ti content is preferably 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. The Ti content is More preferably, it is 0.010% or more. On the other hand, if the Ti content exceeds 0.1%, it leads to an increase in the rolling load and a decrease in ductility due to an increase in the amount of precipitation strengthening, so when Ti is contained, the Ti content is set to 0.1% or less.
  • the Ti content is preferably 0.05% or less, and more preferably 0.03% or less.
  • B is an element that improves the hardenability of steel and has the advantage of easily forming tempered martensite and/or bainite with a specified area ratio. Therefore, the B content is preferably 0.0005% or more. . On the other hand, if the B content exceeds 0.001%, the oxides are concentrated during annealing, which promotes the coarsening of the oxides and deteriorates the chemical conversion treatability.
  • the B content is set to 0.001% or less, and the B content is preferably set to less than 0.0010%.
  • Cu improves corrosion resistance in the environment in which the automobile is used.
  • the corrosion product of Cu covers the surface of the steel sheet, and has the effect of suppressing hydrogen penetration into the steel sheet.
  • Cu is an element that is mixed in when scrap is used as a raw material, and by allowing the inclusion of Cu, recycled materials can be used as raw materials, and manufacturing costs can be reduced. From this viewpoint, it is preferable to contain Cu at 0.005% or more, and furthermore, from the viewpoint of improving delayed fracture resistance, it is more preferable to contain Cu at 0.05% or more.
  • the Cu content is more preferably 0.10% or more. More preferably, the Cu content is 0.25% or more, and even more preferably, 0.50% or more. However, if the Cu content is too high, it will cause surface defects, so if Cu is contained, the Cu content is set to 1% or less.
  • Ni like Cu, is an element that improves corrosion resistance. Ni also has the effect of suppressing the occurrence of surface defects that tend to occur when Cu is contained. For this reason, it is desirable to contain Ni at 0.01% or more.
  • the Ni content is more preferably 0.04% or more, and even more preferably 0.06% or more.
  • the Ni content is set to 1% or less.
  • the Ni content is 0.5% or less, and more preferably, 0.3% or less.
  • ⁇ Cr 1% or less> Cr can be added to improve the hardenability of the steel and to suppress the formation of carbides in martensite and upper/lower bainite.
  • the Cr content is preferably 0.01% or more.
  • the Cr content is more preferably 0.03% or more, and further preferably 0.06% or more.
  • an excessive Cr content deteriorates the pitting corrosion resistance, so when Cr is contained, the Cr content is set to 1% or less, preferably 0.3% or less, and more preferably 0.1% or less.
  • Mo can be added because of its effect of improving the hardenability of steel and its effect of suppressing the formation of carbides in martensite and upper/lower bainite.
  • the Mo content should be 0.01
  • the Mo content is preferably 0.03% or more, and more preferably 0.06% or more.
  • the Mo content is more preferably 0.1% or more. and even more preferably, equal to or greater than 0.2%.
  • Mo significantly deteriorates the chemical conversion treatability of the cold-rolled steel sheet, so if Mo is contained, the Mo content is set to 0.5% or less.
  • V 0.5% or less> V is added because of its effects of improving the hardenability of steel, suppressing the formation of carbides in martensite and upper/lower bainite, refining the structure, and precipitating carbides to improve delayed fracture resistance.
  • the V content is preferably 0.003% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. % or more. Even more preferably, the V content is 0.020% or more, and even more preferably, 0.050% or more. However, if a large amount of V is contained, castability is significantly deteriorated, so if V is contained, the V content is set to 0.5% or less. Preferably, the V content is set to 0.3% or less.
  • the V content is preferably 0.2% or less, more preferably 0.1% or less.
  • Nb can be added because of its effects of refining the steel structure to increase strength, promoting bainite transformation through grain refinement, improving bendability, and enhancing delayed fracture resistance.
  • the Nb content is preferably 0.010% or more.
  • the Nb content is preferably 0.015% or more, and more preferably 0.020% or more.
  • the Nb content is set to 0.1%.
  • the Nb content is 0.08% or less, and more preferably 0.05% or less.
  • Mg fixes O as MgO and contributes to improving formability such as bendability. Therefore, the Mg content is preferably 0.0002% or more.
  • the Mg content is preferably 0.0010 % or more, and more preferably 0.0015% or more.
  • the Mg content is set to 0.0050% or less.
  • the Mg content is set to 0.0040% or less. .
  • Sn suppresses oxidation and nitridation in the surface layer of the steel sheet, and suppresses the resulting reduction in the content of C and B in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing the strength, and From this viewpoint, the Sn content is preferably 0.003% or more, more preferably 0.010% or more, and further preferably 0.015% or more.
  • the Sn content is preferably 0.020% or more, and more preferably 0.030% or more.
  • Sn content if the Sn content exceeds 0.1%, castability is deteriorated.
  • Sn segregates at the prior ⁇ grain boundaries, and the delayed fracture resistance is deteriorated. Therefore, when Sn is contained, Sn The content shall be 0.1% or less.
  • Sb suppresses oxidation and nitridation in the surface layer of the steel sheet, and suppresses the resulting reduction in the content of C and B in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing the strength, and From this viewpoint, the Sb content is preferably 0.002% or more, more preferably 0.004% or more, and further preferably 0.006% or more. % or more. More preferably, the Sb content is 0.008% or more, and even more preferably, 0.010% or more. The Sb content is preferably 0.015% or more, More preferably, it is 0.030% or more. On the other hand, if the Sb content exceeds 0.1%, castability is deteriorated, and the Sb segregates at the prior ⁇ grain boundaries, deteriorating the delayed fracture resistance. shall be 0.1% or less.
  • REM is an element that suppresses the adverse effect of sulfides on stretch flangeability by making the shape of sulfides spheroidal, thereby improving stretch flangeability.
  • the REM content is preferably 0.0005% or more.
  • the REM content is more preferably 0.0010% or more, and further preferably 0.0020% or more.
  • the REM content exceeds 0.0050%, the effect of improving the stretch flangeability is saturated, so when REM is contained, the REM content is set to 0.0050% or less.
  • REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
  • the REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs.
  • the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention. Therefore, when the optional elements are contained in amounts less than the lower limit, the optional elements are considered to be contained as unavoidable impurities.
  • the steel plate of the present invention has a tensile strength (TS) of 780 MPa or more.
  • TS tensile strength
  • ductility is ensured by ensuring a total elongation EL of 16.0% or more when TS is 780 MPa or more and less than 980 MPa, EL of 14.0% or more when TS is 980 MPa or more and less than 1180 MPa, and EL of 12.0% or more when TS is 1180 MPa or more.
  • hole expansion is ensured by ensuring a hole expansion ratio ⁇ of 30% or more. This significantly improves the stability of press forming.
  • a steel sheet having a tensile strength of 780 MPa or more is considered to be a high-strength steel sheet.
  • a steel sheet having a total elongation EL of 16.0% or more when TS is 780 MPa or more and less than 980 MPa, 14.0% or more when TS is 980 MPa or more and less than 1180 MPa, and 12.0% or more when TS is 1180 MPa or more is considered to be a steel sheet having excellent ductility.
  • the hole expansion ratio ⁇ (%) may be less than 45%.
  • the area ratio of polygonal ferrite is set to 10% or more, and in order to obtain even higher ductility, it is preferably set to 20% or more.
  • the area ratio of polygonal ferrite is set to 80% or less, preferably 75% or less, and more preferably 70% or less.
  • the total area ratio of upper bainite, tempered martensite and lower bainite is set to 10% or more, and in order to obtain even higher strength, it is preferably set to 15% or more.
  • the area ratio is set to 70% or less, more preferably 65% or less, and further preferably 60% or less.
  • volume fraction of retained austenite (retained ⁇ ): 3% or more and 15% or less> If the volume fraction of the retained austenite is less than 3%, it may not be possible to ensure the desired ductility. From the viewpoint of ductility, the volume fraction of the retained austenite is set to 3% or more, and preferably 5% or more. On the other hand, if the volume fraction of the retained austenite exceeds 15%, the stretch flangeability (hole expandability) decreases, so the volume fraction of the retained austenite is set to 15% or less, and preferably 13% or less.
  • ⁇ Area ratio of quenched martensite 15% or less (including 0%)> Since the hard quenched martensite structure reduces ⁇ , its area ratio needs to be suppressed. In order to obtain a practically necessary ⁇ , the area ratio of the quenched martensite is set to 15% or less. In order to obtain ⁇ more stably, the area ratio of the quenched martensite is preferably 12% or less, more preferably 10% or less. The area ratio of the quenched martensite may be 0% or 3% or more.
  • the steel structure is preferably made up of a remaining structure other than the above.
  • the area ratio of the remaining structure is preferably 5% or less.
  • the remaining structure may be unrecrystallized ferrite, carbide, or pearlite. These structures may be determined by SEM observation as described later.
  • [Pm] is preferably 0.030 mass% or more, more preferably 0.035 mass% or more. Although there is no particular upper limit, [Pm] is preferably 0.100 mass% or less, more preferably 0.090 mass% or less. [Pm]/[P] is preferably 1.7 or more, more preferably 1.9 or more. Although there is no particular upper limit, [Pm]/[P] is preferably 10.0 or less, more preferably 9.0 or less.
  • the area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite were measured by cutting out a cross section of the sheet thickness parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. At the 1/4 thickness position, an area of 25 ⁇ m ⁇ 20 ⁇ m was observed in 10 fields of view at 5,000 times magnification using an SEM, and the photographed structure was quantified by image analysis.
  • Polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. It is the area that appears the blackest under SEM.
  • Upper bainite is a ferrite structure with the formation of carbides or retained austenite inside that appear white under SEM.
  • the region of ferrite with an aspect ratio of ⁇ 2.0 is classified as polygonal ferrite, and the region with an aspect ratio of >2.0 is classified as upper bainite, and the area ratio is calculated.
  • the aspect ratio is calculated by determining the major axis length a at which the particle length is the longest, and the minor axis length b at the particle length when it crosses the particle the longest in the direction perpendicular to the major axis length a, and a/b is the aspect ratio.
  • Tempered martensite and lower bainite are regions that are accompanied by a lath-shaped substructure and carbide precipitation inside when viewed under an SEM.
  • Hardened martensite fresh martensite
  • the remaining structure is a structure containing at least one of non-recrystallized ferrite, carbide, and pearlite, and can be confirmed by SEM as black contrast ferrite containing deformed structure introduced by rolling, and white contrast carbide and pearlite.
  • Carbide is a structure with a particle size of 1 ⁇ m or less, and pearlite is a lamellar (layer) structure, so it can be distinguished.
  • the volume fraction of retained austenite is determined by chemically polishing the surface layer at 1/4 thickness and then performing X-ray diffraction.
  • a Co-K ⁇ source is used for the incident X-rays, and the volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.
  • the volume fraction of retained austenite determined by X-ray diffraction can be taken as the area fraction of retained austenite.
  • the surface enrichment amount of the P surface enriched part on the steel sheet surface is analyzed by sputtering in the depth direction (sheet thickness direction) using a GDS (manufactured by Shimadzu Corporation) under the conditions of Ar gas pressure: 600 Pa, high frequency output: 35 W, measurement time interval: 0.1 s, and measurement time: 150 s.
  • the surface enrichment amount of P is then measured, and the maximum P concentration [Pm] within 1 ⁇ m in the sheet thickness direction from the steel sheet surface is obtained using a calibration curve obtained in advance.
  • the highest P intensity value during the measurement time of 150 seconds is converted into mass % using a calibration curve and is defined as the maximum concentration ([Pm]).
  • the method of converting to mass% involves using a standard material having a known amount of P, measuring the data under the same conditions, determining the correlation between the intensity of the P element obtained by GDS and the amount of P, and converting the measured P intensity of the example into a concentration.
  • Pi is the P content (mass%) in the steel sheet.
  • the manufacturing method of the steel sheet according to the first embodiment of the present invention is a manufacturing method of a steel sheet in which a steel slab having the above-mentioned composition is subjected to hot rolling, pickling and cold rolling, and then the obtained cold-rolled steel sheet is annealed.
  • the annealing is performed in a furnace atmosphere having a dew point of ⁇ 40° C. or less, at a temperature of A c1 point +20° C. or more.
  • Tc (°C) 663-1.2 x exp (20/t) x Tdp...Formula (3)
  • t represents the holding time at the above-mentioned soaking temperature (soaking holding time) (s)
  • Tdp represents the dew point (° C.).
  • the method of hot rolling a steel slab includes a method of rolling the slab after heating, a method of directly rolling the slab after continuous casting without heating it, and a method of rolling the slab after continuous casting by applying a short-term heat treatment.
  • Hot rolling may be performed according to a conventional method, for example, the slab heating temperature may be 1100°C or higher.
  • the slab heating temperature may be 1300°C or lower.
  • the soaking temperature may be 20 min or higher.
  • the soaking temperature may be 300 min or lower.
  • the finish rolling temperature may be A r3 transformation point or higher.
  • the finish rolling temperature may be A r3 transformation point + 200°C or lower.
  • the coiling temperature may be 400°C or higher.
  • the coiling temperature may be 720°C or lower.
  • the coiling temperature is preferably controlled from the viewpoint of suppressing thickness fluctuation and stably ensuring high strength. Specifically, the coiling temperature is preferably 430°C or higher. The winding temperature is preferably 530° C. or less.
  • the Ar3 transformation point can be calculated from the components of the steel sheet and the following empirical formula (A).
  • a r3 point (°C) 910-310 ⁇ [C]-80 ⁇ [Mn]-20 ⁇ [Cu]-15 ⁇ [Cr]-55 ⁇ [Ni]-80 ⁇ [Mo]...Formula (A) (In the above formula, [M] is the content (mass%) of element M in the steel slab, and the value of an element that is not contained is zero (0).)
  • the pickling may be carried out in a conventional manner.
  • Cold rolling may be performed according to a conventional method, and the rolling ratio (cumulative rolling ratio) may be 30% or more.
  • the rolling ratio (cumulative rolling ratio) may be 85% or less.
  • the rolling ratio is preferably controlled from the viewpoint of stably securing high strength and reducing anisotropy. Specifically, the rolling ratio is preferably 35% or more.
  • softening annealing treatment can be performed at 450 to 730°C in a CAL (continuous annealing line) or BAF (box annealing furnace).
  • a cold-rolled steel sheet manufactured according to a conventional method is annealed under the following conditions.
  • the annealing equipment is not particularly limited, but it is preferable to perform the annealing in a continuous annealing line (CAL) from the viewpoints of productivity and ensuring the desired heating rate and cooling rate.
  • CAL continuous annealing line
  • the dew point affects the formation of oxides on the steel sheet surface during annealing, and if the dew point exceeds -40°C, the amount of oxides formed on the steel sheet surface increases excessively, deteriorating the chemical conversion treatability, and the technology of the present disclosure cannot satisfy the chemical conversion treatability. For this reason, the dew point is set to -40°C or lower. Although there is no particular lower limit, the dew point is preferably ⁇ 70° C. or higher, and more preferably ⁇ 60° C. or higher.
  • the steel sheet obtained in the present invention contains a soft ferrite structure, which improves ductility, and therefore the soaking temperature is set to be in the range of A c1 point +20° C. or more and A c3 point or less, at which ferrite is formed.
  • Tc is calculated from the dew point and the soaking holding time in formula (3).
  • Tc (°C) 663-1.2 x exp (20/t) x Tdp...Formula (3)
  • t is the holding time (s) at the soaking temperature
  • Tdp is the dew point (° C.). If the soaking temperature is less than Tc (°C), the specified amount of P concentration on the surface cannot be ensured, and the chemical conversion treatability deteriorates. Therefore, in a furnace atmosphere with a dew point of -40°C or less, the soaking temperature is set to be A c1 point +20°C or more and A c3 point or less, and to be Tc (°C) or more.
  • the time for holding at the soaking temperature (soaking holding time) is less than 30 seconds, formation of austenite at the soaking temperature may not be sufficient, resulting in an increase in polygonal ferrite, and thus making it impossible to obtain the desired total area ratio of upper bainite, tempered martensite, and lower bainite, making it impossible to obtain the desired strength, or making it impossible to obtain a sufficient amount of retained austenite, making it impossible to ensure the desired ductility.
  • the time for holding at the above-mentioned soaking temperature exceeds 500 seconds, the structure will become significantly coarse, and the desired strength may not be ensured.
  • the time for which the steel sheet is held at the annealing temperature is set to 30 to 500 seconds.
  • the time for which the material is held at the soaking temperature is preferably 60 seconds or more, more preferably 100 seconds or more.
  • the time for which the material is held at the soaking temperature (soaking time) is preferably 400 seconds or less, more preferably 300 seconds or less.
  • a c1 and A c3 obtained from the empirical formulas (4) and (5) below may be used.
  • a c1 723+22 ⁇ [C]-18 ⁇ [Si]+17 ⁇ [Cr]+4.5 ⁇ [Mo]+16 ⁇ [V] ...Formula (4)
  • a c3 910-203 ⁇ ([C]) 1/2 +44.7 ⁇ [Si]-30 ⁇ [Mn]+700 ⁇ [P]+400 ⁇ [sol.
  • [M] is the mass % of each element.
  • First cooling step cooling to the first cooling stop temperature at a first average cooling rate of 2 to 50 ° C./s in the temperature range from the soaking temperature to the first cooling stop temperature of 350 to 550 ° C.
  • the material After being held at a soaking temperature that is equal to or higher than A c1 point + 20° C. and equal to or lower than A c3 point and is equal to or higher than Tc (after the soaking temperature holding step), the material is cooled in a temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550° C. at a first average cooling rate: 2 to 50° C./s.
  • the first average cooling rate is set to 2° C./s or more, and preferably 5° C./s or more.
  • the first average cooling rate is set to 50° C./s or less.
  • the first average cooling rate is preferably 40° C./s or less, and more preferably less than 30° C./s.
  • the first average cooling rate is "(soaking temperature (°C)-first cooling stop temperature (°C))/cooling time (seconds) from the soaking temperature to the first cooling stop temperature.”
  • the holding time exceeds 60 seconds, the concentration of C from bainite to the blocky untransformed ⁇ progresses, leading to an increase in the amount of remaining blocky quenched martensite structure, and there is a concern of a decrease in ⁇ . Therefore, the holding time is set to 10 seconds or more and 60 seconds or less. This holding time is preferably 20 seconds or more. Moreover, this holding time is preferably 50 seconds or less.
  • the second cooling step (1) can be omitted, in which case the material is heated to a soaking temperature of A c1 +20° C. or more and A c3 or less and Tc or more, and is held at the soaking temperature for 30 to 500 s, followed by the treatment in the second cooling step (2).
  • a manufacturing method in which the second cooling step (1) is omitted will be described in the second embodiment described later.
  • the second average cooling rate in the temperature range from the residence end temperature to the second cooling stop temperature of 200° C. or more and 420° C. or less is set to 2° C./s or more.
  • the second average cooling rate is preferably 5° C./s or more, and more preferably 8° C./s or more. If the cooling rate in this temperature range is too high, the plate shape deteriorates, so the cooling rate in this temperature range (second average cooling rate) is set to 50° C./s or less, preferably 40° C./s or less.
  • the second cooling stop temperature is set to 420°C or less.
  • the second cooling stop temperature is preferably 400°C or less.
  • the second cooling stop temperature is set to 200°C or higher.
  • the second cooling stop temperature is preferably 220°C or higher.
  • the second average cooling rate is "retention end temperature (°C)-second cooling stop temperature (°C)/cooling time (seconds) from the retention end temperature to the second cooling stop temperature".
  • the holding at the second cooling stop temperature is performed from the viewpoint of adjusting the strength by tempering the formed martensite and promoting the concentration of C in the residual ⁇ . If the holding time is less than 60s, the tempering is insufficient and high-strength martensite is not formed. In addition, since the concentration of C in the residual ⁇ is suppressed, the desired strength and ductility may not be ensured. On the other hand, if the holding time at the second cooling stop temperature exceeds 3000 s, the martensite is excessively tempered, and the desired strength may not be ensured.
  • the holding time at the second cooling stop temperature is set to 60 seconds or more and 3000 seconds or less.
  • the holding time at the second cooling stop temperature is preferably 100 seconds or more, and more preferably 150 seconds or more.
  • the holding time at the stop temperature is preferably 2500 seconds or less, more preferably 2000 seconds or less.
  • the manufacturing method of the steel sheet according to the second embodiment of the present invention is a manufacturing method of a steel sheet in which a steel slab having the above-mentioned composition is subjected to hot rolling, pickling and cold rolling, and then the obtained cold-rolled steel sheet is annealed.
  • the annealing is a manufacturing method of a steel sheet in which the cold-rolled steel sheet is heated to a soaking temperature that is A c1 point +20 ° C. or more and A c3 point or less, and is equal to or higher than Tc calculated by formula (3) in a furnace atmosphere having a dew point of -40 ° C.
  • Tc (°C) 663-1.2 x exp (20/t) x Tdp...Formula (3)
  • t is the holding time (s) at the above-mentioned soaking temperature
  • Tdp is the dew point (° C.).
  • the treatments in the soaking steps of hot rolling, pickling, cold rolling, and annealing can be performed under the same conditions as in the first embodiment.
  • the first cooling step in the annealing of the first embodiment can be omitted.
  • the cooling step in the annealing corresponds to the second cooling step in the annealing in the first embodiment.
  • the retention treatment in the second cooling step in the first embodiment (retention for 10 to 60 s in a temperature range of 350 to 550° C.) can be omitted.
  • the isothermal holding step in the annealing of the second embodiment can be substantially the same as the isothermal holding step in the annealing of the first embodiment, except that the second cooling stop temperature is the cooling stop temperature.
  • the cooling step in the annealing will be mainly described below.
  • the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature of 200° C. or more and 420° C. or less is set to 2° C./s or more.
  • the average cooling rate is preferably 5° C./s or more, and more preferably 8° C./s or more. If the cooling rate in this temperature range is too high, the plate shape will deteriorate, so the cooling rate (average cooling rate) in this temperature range is set to 50° C./s or less, preferably 40° C./s or less. If the cooling stop temperature exceeds 420°C, the formation of carbides occurs significantly, the retained ⁇ cannot be secured, and the ductility deteriorates. For this reason, the cooling stop temperature is set to 420°C or lower.
  • the cooling stop temperature is set to 200°C or higher.
  • the average cooling rate is "soaking temperature (° C.) ⁇ cooling stop temperature (° C.)/cooling time (seconds) from the soaking temperature to the cooling stop temperature.”
  • the steel sheet of the present invention obtained as described above preferably has a thickness of 0.5 mm or more, and more preferably has a thickness of 3.0 mm or less.
  • the member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining processes.
  • the manufacturing method of the member of the present invention also includes a step of subjecting the steel plate of the present invention to at least one of forming and joining processes to form the member.
  • the steel plate of the present invention has a tensile strength of 780 MPa or more, and has excellent ductility, hole expandability, and chemical conversion treatability. Therefore, the member obtained using the steel plate of the present invention also has a tensile strength of 780 MPa or more, and has excellent ductility, hole expandability, and chemical conversion treatability. Furthermore, the use of the member of the present invention makes it possible to reduce the weight. Therefore, the member of the present invention can be suitably used, for example, in vehicle body frame parts.
  • general processing methods such as pressing can be used without restrictions.
  • general welding methods such as spot welding and arc welding, riveting, crimping, etc. can be used without restrictions.
  • Example 1 A slab having the chemical composition shown in Table 1 produced by continuous casting was heated to 1200°C, with a soaking time of 200 min, a finish rolling temperature of 860°C or higher, and a coiling temperature of 550°C. After the hot rolling process, the slab was cold rolled at a rolling ratio of 50% to produce a cold-rolled steel sheet having a thickness of 1.4 mm. The cold-rolled steel sheet was treated under the annealing conditions shown in Table 2 to produce the steel sheet of the present invention and the steel sheet of the comparative example.
  • the steel structure was measured by the following method. The measurement results are shown in Table 3.
  • the area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite) were measured by cutting out a cross section of the plate thickness parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. At the 1/4 thickness position, an area of 25 ⁇ m ⁇ 20 ⁇ m was observed in 10 fields of view at 5,000 times magnification using an SEM, and the photographed structure was quantified by image analysis.
  • Polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. It is the area that appears the blackest under SEM.
  • Upper bainite is a ferrite structure with the formation of carbides or retained austenite inside that appear white under SEM.
  • the area of ferrite with an aspect ratio of ⁇ 2.0 was classified as polygonal ferrite, and the area of ferrite with an aspect ratio of >2.0 was classified as upper bainite, and the area ratio was calculated.
  • the aspect ratio was calculated by determining the major axis length a at which the particle length is the longest, and the minor axis length b at the particle length when it crosses the particle the longest in the direction perpendicular to the major axis length a, and a/b was defined as the aspect ratio.
  • Tempered martensite and lower bainite are regions that are accompanied by a lath-shaped substructure and carbide precipitation inside when viewed under an SEM.
  • Hardened martensite fresh martensite
  • the remaining structure is a structure containing at least one of non-recrystallized ferrite, carbide, and pearlite, and non-recrystallized ferrite can be confirmed by SEM as ferrite with black contrast containing deformed structure introduced by rolling.
  • Carbide and pearlite are structures that can be confirmed by white contrast.
  • Carbide is a structure with a particle size of 1 ⁇ m or less
  • pearlite is a lamellar (layer) structure, so it can be distinguished.
  • the volume fraction of retained austenite was determined by chemically polishing the surface layer at 1/4 thickness and then performing X-ray diffraction. A Co-K ⁇ source was used for the incident X-rays, and the volume fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.
  • a sputtering analysis was performed in the depth direction to measure the amount of P in the surface-enriched areas of the steel sheet surface using a GDS (manufactured by Shimadzu Corporation) under conditions of Ar gas pressure: 600 Pa, high frequency output: 35 W, measurement time interval: 0.1 s, and measurement time: 150 s, and the maximum P concentration near the surface layer (within 1 ⁇ m in the sheet thickness direction from the steel sheet surface) was measured.
  • a calibration curve for P was obtained using standard materials with various P contents from 0.005 to 0.020 mass%.
  • the annealed steel sheet was degreased and surface-conditioned, and then chemically treated using a zinc phosphate chemical conversion treatment solution.
  • the chemical conversion treatment was performed as follows: degreasing step: treatment temperature; 40°C, treatment time; 120 seconds, spray degreasing, surface conditioning step: pH 9.5, treatment temperature; room temperature, treatment time; 20 seconds, chemical conversion treatment step: temperature of chemical conversion treatment solution; 35°C, treatment time; 120 seconds.
  • degreaser FC-E2011
  • surface conditioning agent PL-X
  • chemical conversion treatment solution Palbond PB-L3065, manufactured by Nihon Parkerizing Co., Ltd.
  • the surface chemical conversion structure was observed by SEM observation at a magnification of 1000 times in five fields (areas of 50,000 ⁇ m2 or more ), and the area where the base steel was exposed was evaluated as ⁇ when it was less than 10% of the total area, and x when it was 10% or more. The results are shown in Table 3.
  • Example 2 A slab produced by continuous casting having the composition shown in Table 1 was heated to 1200°C, and subjected to a hot rolling process in which the soaking time was 200 min, the finish rolling temperature was 860°C or higher, and the coiling temperature was 550°C.
  • the cold-rolled steel sheet having a thickness of 1.4 mm was produced by cold rolling at a rolling ratio of 50%, and was treated under the annealing conditions shown in Table 4 to produce the steel sheet of the present invention and the steel sheet of the comparative example. The same evaluation as in Example 1 was carried out. The results are shown in Table 5.
  • the components obtained by forming and joining the steel plate of the present invention have excellent strength, ductility, hole expandability and chemical treatability, just like the steel plate of the present invention, because the steel plate of the present invention has excellent strength, ductility, hole expandability and chemical treatability.

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