Classifications

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  • 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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
  • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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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
  • 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")
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/04—Hardening by cooling below 0 degrees Celsius
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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
  • 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
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  • 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
  • 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
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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
  • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
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  • 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
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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  • 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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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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
• • 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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • 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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
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  • 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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
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  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; Strips
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  • 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
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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
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
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  • C22C18/00—Alloys based on zinc

Definitions

• the present invention relates to plated steel sheets, components, and methods for manufacturing the same.
• Patent Document 1 discloses a high-strength steel sheet having excellent workability and a manufacturing method thereof.
• Patent Document 2 discloses a high-strength cold-rolled steel sheet and a manufacturing method thereof.
• Patent Documents 1 and 2 all have a tensile strength TS of less than 1,180 MPa, and delayed fracture resistance is not taken into consideration.
• the present invention was made in consideration of these circumstances, and aims to provide plated steel sheets, components, and methods for manufacturing the same that have high strength with a tensile strength TS of 1180 MPa or more and excellent delayed fracture resistance and plating properties.
• high strength means that the tensile strength TS measured in accordance with JIS Z2241 (2011) is 1180 MPa or more.
• excellent resistance to delayed fracture means that the test piece is subjected to a constant load test with a tensile stress of 1,800 MPa on the surface layer, and no cracks are observed 100 hours after immersion in a hydrochloric acid solution with a pH of 3.
• excellent plating properties refers to the absence of unplated areas when visually inspected.
• martensite or bainite be the main phase. Also, it is effective to include 0.0015 mass% or more of B to improve delayed fracture resistance.
• B has the effect of strengthening the grain boundaries that serve as fracture paths by segregating at the prior austenite grain boundaries.
• oxides of Si and Mn containing B are concentrated on the surface during annealing, and galvanic properties are reduced. In this case, it was found that the effect on galvanic properties is small if the B content is less than 0.0015 mass%. Therefore, a method of improving delayed fracture resistance by setting the B content to less than 0.0015% was studied.
• the inventors have studied a means for non-uniformly segregating B on prior austenite grain boundaries and increasing the B concentration on some grain boundaries. As a result, they have found that B can be non-uniformly segregated by undergoing the process of B segregating on grain boundaries twice and nucleating new austenite grains the second time. When a cold-rolled sheet is annealed in the austenite region, B segregates on the austenite grain boundaries, but at this point, the diffusion of B is insufficient and solute B is present, so that the segregated B is non-uniform but insufficiently concentrated.
• an austenite reverse transformation structure is formed.
• austenite exists after the first annealing and cooling, it serves as a nucleus to form an austenite structure with the same crystal orientation as that of the first annealing.
• martensite and bainite contain a large amount of dislocations, and B dissolved in them diffuses rapidly to the austenite grain boundaries through the dislocations during the second annealing, so that B segregates uniformly.
• the grain boundary concentration of B becomes low, so it is necessary to segregate B thickly only at some grain boundaries and thinly at other boundaries.
• austenite with the same crystal orientation as that in the first annealing is generated.
• this structure is cooled, ferrite is generated from the grain boundaries with low and non-uniform B concentration, and if the area ratio of ferrite is 3.0% or less, a high strength of 1180 MPa or more can be ensured.
• B content is less than 0.0015%, B is concentrated at the old austenite grain boundaries sandwiched on both sides by martensite, and delayed fracture resistance can be improved.
• [3] A member made using the plated steel sheet according to [1] or [2] above.
• [4] A hot rolling process in which a steel slab having the composition according to [1] or [2] is hot rolled to obtain a hot rolled sheet;
• the present invention can provide plated steel sheets, components, and methods for manufacturing the same that have a tensile strength TS of 1180 MPa or more, high strength, and excellent delayed fracture resistance and plating properties.
• the plated steel sheet according to the present embodiment contains, by mass%, C: 0.10% or more and 0.30% or less, Si: more than 1.20% and 2.00% or less, Mn: 2.5% or more and 4.0% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.01% or less, Ti: 0.100% or less, Nb: 0.002% or more and 0.050% or less, and B: 0.0008% or more and less than 0.0015%, and satisfies the following formula (1), with the balance being Fe and unavoidable impurities.
• the total area ratio of martensite and bainite is 95.0% or more, and the area ratio of retained austenite is 4.0% or more.
• the present invention relates to a steel sheet having a B concentration of 0.05% or more by mass, a B area ratio of 0.2% to 3.0%, a prior austenite grain size of 10 ⁇ m or less, a prior austenite grain boundary between adjacent martensite particles having a B concentration of 0.05% or more by mass, a variation in the B concentration of the prior austenite grain boundary between adjacent martensite particles within the same grain boundary of less than 0.010% by mass, and a variation in the B concentration of the prior austenite grain boundary between martensite and ferrite within the same grain boundary of 0.010% by mass, and a plating layer formed on at least one surface of the steel sheet. ([%N]/14)/([%Ti]/47.9) ⁇ 1.0...Formula (1) In formula (1), [%N] and [%Ti] respectively represent the N and Ti contents (mass%) in the steel.
• C 0.10% or more and 0.30% or less C has the effect of strengthening the martensite-bainite structure. If the C content is less than 0.10%, the area ratio of martensite and bainite decreases, and a tensile strength TS (hereinafter sometimes simply referred to as TS) of 1180 MPa or more cannot be obtained. Therefore, the C content is set to 0.10% or more. The C content is preferably set to 0.11% or more. On the other hand, if the C content exceeds 0.30%, a carboboride of B and iron is formed during annealing, and a sufficient amount of B cannot be segregated on the grain boundaries. Therefore, the C content is set to 0.30% or less. The C content is preferably 0.28% or less.
• Si more than 1.20% and not more than 2.00% Si is an element effective for solid solution strengthening, and also has the effect of suppressing carbide precipitation that deteriorates delayed fracture resistance, so a content of more than 1.20% is required. If it is 1.20% or less, carbide precipitation occurs, the B concentration at the grain boundary decreases, and the delayed fracture resistance decreases. Therefore, the Si content is made to be more than 1.20%. The Si content is preferably made to be 1.30% or more. On the other hand, since Si is a ferrite stabilizing element and increases the transformation point, if the Si content exceeds 2.00%, the annealing temperature becomes high and the prior austenite grain size cannot be made 10 ⁇ m or less. Therefore, the Si content is set to 2.00% or less. The Si content is preferably set to 1.80% or less.
• Mn 2.5% or more and 4.0% or less Mn is effective for improving hardenability. If the Mn content is less than 2.5%, the area ratio of martensite and bainite decreases, resulting in a decrease in strength. Therefore, the Mn content is set to 2.5% or more. The Mn content is preferably set to 2.8% or more. On the other hand, if the Mn content exceeds 4.0%, the segregated portion becomes excessively hard and the delayed fracture resistance property is deteriorated. Therefore, the Mn content is set to 4.0% or less, and preferably, the Mn content is set to 3.5% or less.
• P 0.050% or less P segregates at prior austenite grain boundaries and reduces delayed fracture resistance, so the P content is set to 0.050% or less.
• the P content is preferably set to 0.025% or less.
• S 0.020% or less S segregates at prior austenite grain boundaries and reduces delayed fracture resistance, so the S content is set to 0.020% or less.
• the S content is preferably set to 0.018% or less.
• the S content is more preferably set to 0.0040% or less, and further preferably set to 0.0020% or less. There is no particular lower limit for the S content, but since a content of less than 0.0001% increases the production costs, the S content is preferably 0.0001% or more.
• Al 0.10% or less
• Al is an element that acts as a deoxidizer, and in order to obtain such an effect, the Al content is preferably 0.005% or more.
• the Al content is set to 0.10% or less.
• the Al content is preferably set to 0.05% or less.
• N 0.01% or less N forms nitrides with Nb and B, reducing the effects of adding Nb and B. Therefore, the N content is set to 0.01% or less.
• the N content is preferably set to 0.006% or less. There is no particular lower limit, but from the viewpoint of manufacturing costs, the N content is preferably set to 0.0001% or more.
• Ti 0.100% or less Ti fixes N in steel as TiN, inhibits the formation of BN and NbN, improves the effect of adding Nb and B, and has the effect of improving delayed fracture resistance.
• the Ti content is preferably 0.005% or more.
• the Ti content is set to 0.100% or less.
• the Ti content is preferably set to 0.050% or less.
• Nb 0.002% or more and 0.050% or less Nb precipitates as a solid solution or fine carbides, and suppresses the growth of austenite grains during annealing. It can also refine the crystal grain size to complicate the fracture path and improve delayed fracture resistance. To obtain such effects, the Nb content is set to 0.002% or more. The Nb content is preferably set to 0.005% or more. On the other hand, if the Nb content exceeds 0.050%, not only the effect is saturated, but also coarse Nb carbides are precipitated, and the delayed fracture resistance property is deteriorated. Therefore, the Nb content is set to 0.050% or less. Moreover, the Nb content is preferably 0.040% or less.
• B 0.0008% or more and less than 0.0015% B has the effect of segregating at prior austenite grain boundaries to increase grain boundary strength and improve delayed fracture resistance.
• the B content is set to 0.0008% or more.
• the B content is preferably set to 0.0009% or more.
• the B content is set to less than 0.0015%.
• the B content is preferably set to 0.0014% or less.
• the remainder other than the above-mentioned components is Fe and unavoidable impurities. Note that for the optional components described below, if the content is below the lower limit, the effect of the present invention is not impaired, so if these optional elements are contained below the lower limit, they are treated as unavoidable impurities.
• the steel sheet of the plated steel sheet according to this embodiment may further contain, in mass %, at least one element selected from V: 0.100% or less, Mo: 0.500% or less, Cr: 1.00% or less, Cu: 1.00% or less, Ni: 0.50% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.200% or less, W: 0.400% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Co: 0.020% or less, REM: 0.0200% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less.
• V 0.100% or less V has the effect of forming fine carbides and increasing strength. If the V content exceeds 0.100%, coarse V carbides precipitate, and delayed fracture resistance may decrease. Therefore, when V is contained, the V content is set to 0.100% or less.
• the V content is preferably 0.080% or less, and more preferably 0.060% or less.
• the lower limit of the V content is not particularly limited and may be 0.000%, but since V has the effect of forming fine carbides and increasing strength, it is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.010% or more.
• Mo 0.500% or less Mo has the effect of improving hardenability and increasing the area ratio of bainite and martensite. If the Mo content exceeds 0.500%, the effect is saturated. Therefore, if Mo is contained, the Mo content is set to 0.500% or less.
• the Mo content is preferably 0.200% or less, and more preferably 0.150% or less.
• the lower limit of the Mo content is not particularly limited and may be 0.000%, but since Mo has the effect of improving hardenability and increasing the area ratio of bainite and martensite, it is preferably 0.010% or more, more preferably 0.020% or more, and further preferably 0.030% or more.
• Cr 1.00% or less Cr has the effect of improving hardenability and increasing the area ratio of bainite and martensite. If the Cr content exceeds 1.00%, the effect is saturated. Therefore, if Cr is contained, the Cr content is set to 1.00% or less.
• the Cr content is preferably 0.300% or less, and more preferably 0.250% or less.
• the lower limit of the Cr content is not particularly limited and may be 0.000%, but since it has the effect of improving hardenability and increasing the area ratio of bainite and martensite, it is preferably 0.01% or more, more preferably 0.015% or more, and further preferably 0.030% or more.
• Cu 1.00% or less
• Cu has the effect of increasing strength by solid solution.
• Cu also has the effect of improving delayed fracture resistance. If the Cu content exceeds 1.00%, grain boundary cracking is likely to occur. Therefore, when Cu is contained, the Cu content is set to 1.00% or less.
• the Cu content is preferably 0.60% or less, and more preferably 0.30% or less.
• the lower limit of the Cu content is not particularly limited and may be 0.00%, but since it has the effect of increasing strength by solid solution, it is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.05% or more.
• Ni 0.50% or less
• Ni has the effect of improving hardenability, but if the Ni content exceeds 0.50%, the effect is saturated. Therefore, when Ni is contained, the Ni content is set to 0.50% or less.
• the Ni content is preferably 0.20% or less, and more preferably 0.15% or less.
• the lower limit of the Ni content is not particularly limited and may be 0.00%, but since Ni has an effect of improving hardenability, it is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.03% or more.
• Sb 0.200% or less
• Sb has the effect of suppressing surface oxidation, nitridation, and decarburization of the steel sheet, but if the Sb content exceeds 0.200%, the effect is saturated. Therefore, if Sb is contained, the Sb content is set to 0.200% or less.
• the Sb content is preferably 0.050% or less, and more preferably 0.020% or less.
• the lower limit of the Sb content is not particularly limited and may be 0.000%, but since Sb has the effect of suppressing surface oxidation, nitridation, and decarburization of the steel sheet, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more.
• Sn 0.200% or less Like Sb, Sn has the effect of suppressing surface oxidation, nitridation, and decarburization of the steel sheet. If the Sn content exceeds 0.200%, the effect is saturated. Therefore, if Sn is contained, the Sn content is set to 0.200% or less.
• the Sn content is preferably 0.050% or less, and more preferably 0.020% or less.
• the lower limit of the Sn content is not particularly limited and may be 0.000%, but since it has the effect of suppressing surface oxidation, nitridation, and decarburization of the steel sheet, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more.
• Ta 0.200% or less Ta has the effect of forming fine carbides and increasing strength. If the Ta content exceeds 0.200%, coarse Ta carbides precipitate, and delayed fracture resistance may decrease. Therefore, when Ta is contained, the Ta content is set to 0.200% or less.
• the Ta content is preferably 0.100% or less, and more preferably 0.070% or less.
• the lower limit of the Ta content is not particularly limited and may be 0.000%, but since it has the effect of forming fine carbides and increasing strength, it is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.010% or more.
• W 0.400% or less W has the effect of forming fine carbides and increasing strength. If the W content exceeds 0.400%, coarse W carbides may precipitate, and the delayed fracture resistance may decrease. Therefore, when W is contained, the W content is set to 0.400% or less.
• the W content is preferably 0.300% or less, and more preferably 0.250% or less.
• the lower limit of the W content is not particularly limited and may be 0.000%, but since it has the effect of forming fine carbides and increasing strength, it is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.010% or more.
• Zr 0.0200% or less
• Zr has the effect of making the shape of inclusions spherical and suppressing stress concentration, thereby improving toughness. If the Zr content exceeds 0.0200%, a large amount of inclusions may be formed, resulting in a decrease in toughness. Therefore, when Zr is contained, the Zr content is set to 0.0200% or less.
• the Zr content is preferably 0.0150% or less, and more preferably 0.0100% or less.
• the lower limit of the Zr content is not particularly limited and may be 0.0000%, but since it has the effect of making the shape of inclusions spherical, suppressing stress concentration, and improving toughness, it is preferably 0.0001% or more.
• the Zr content is more preferably 0.0010% or more, and further preferably 0.0020% or more.
• Ca 0.0200% or less Ca can be used as a deoxidizer. If the Ca content exceeds 0.0200%, a large amount of Ca-based inclusions is generated, and the delayed fracture resistance may deteriorate. Therefore, when Ca is contained, the Ca content is set to 0.0200% or less.
• the Ca content is preferably 0.0100% or less, and more preferably 0.0080% or less.
• the lower limit of the Ca content is not particularly limited and may be 0.0000%, but since it can be used as a deoxidizer, it is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.
• Mg 0.0200% or less Mg can be used as a deoxidizer. If the Mg content exceeds 0.0200%, a large amount of Mg-based inclusions are generated, and the delayed fracture resistance may deteriorate. Therefore, if Mg is contained, the Mg content is set to 0.0200% or less.
• the Mg content is preferably 0.0100% or less, and more preferably 0.0080% or less.
• the lower limit of the Mg content is not particularly limited and may be 0.0000%, but since Mg can be used as a deoxidizer, it is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.
• Co 0.020% or less Co has the effect of increasing strength by solid solution strengthening. If the Co content exceeds 0.020%, the effect is saturated. Therefore, if Co is contained, the Co content is set to 0.020% or less.
• the Co content is preferably 0.015% or less, and more preferably 0.010% or less.
• the lower limit of the Co content is not particularly limited and may be 0.000%, but since Co has the effect of increasing strength by solid solution strengthening, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more.
• REM 0.0200% or less REM has the effect of making the shape of inclusions spherical and suppressing stress concentration, thereby improving toughness. If the REM content exceeds 0.0200%, a large amount of inclusions may be formed, resulting in a decrease in toughness. Therefore, when REM is contained, the REM content is set to 0.0200% or less.
• the REM content is preferably 0.0100% or less, and more preferably 0.0050% or less.
• the lower limit of the REM content is not particularly limited and may be 0.0000%, but since it has the effect of making the shape of inclusions spherical, suppressing stress concentration, and improving toughness, it is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.
• REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoid elements with atomic numbers from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
• the REM concentration is the total content of one or more elements selected from the above REM.
• the REM preferably contains La, Ce, and Nd.
• Te 0.020% or less Te has the effect of making the shape of inclusions spherical and suppressing stress concentration, thereby improving toughness. If the Te content exceeds 0.020%, a large amount of inclusions may be formed, and toughness may decrease. Therefore, when Te is contained, the Te content is set to 0.020% or less.
• the Te content is preferably 0.015% or less, and more preferably 0.010% or less.
• the lower limit of the Te content is not particularly limited and may be 0.000%, but since it has the effect of making the shape of inclusions spherical, suppressing stress concentration, and improving toughness, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.004% or more.
• Hf 0.10% or less Hf has the effect of making the shape of inclusions spherical and suppressing stress concentration, thereby improving toughness. If the Hf content exceeds 0.10%, a large amount of inclusions is formed and the toughness decreases. Therefore, when Hf is contained, the Hf content is set to 0.10% or less.
• the Hf content is preferably 0.08% or less, and more preferably 0.05% or less.
• the lower limit of the Hf content is not particularly limited and may be 0.00%, but since Hf has the effect of making the shape of inclusions spherical, suppressing stress concentration, and improving toughness, it is preferable to make it 0.01% or more.
• Bi 0.200% or less Bi has the effect of reducing segregation and improving bendability. If the Bi content exceeds 0.200%, a large amount of inclusions may be formed, resulting in a decrease in bendability. Therefore, when Bi is contained, the Bi content is set to 0.200% or less.
• the Bi content is preferably 0.100% or less, more preferably 0.050% or less.
• the Bi content is further preferably 0.010% or less, and even more preferably 0.005% or less.
• the lower limit of the Bi content is not particularly limited and may be 0.000%, but since Bi has the effect of reducing segregation and improving bendability, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more.
• Sum of area ratio of martensite and bainite 95.0% or more Both martensite and bainite are hard phases, and are necessary to achieve a TS of 1180 MPa or more. Therefore, the sum of the area ratios of martensite and bainite is 95.0% or more. The sum of the area ratios of martensite and bainite is preferably 96.0% or more. Since ferrite is generated from grain boundaries with non-uniform B concentration, the upper limit of the sum of the area ratios of martensite and bainite is 99.8%.
• Area ratio of retained austenite 4.8% or less
• Retained austenite may be contained as a residual structure other than martensite and bainite. Therefore, the area ratio of retained austenite is set to 4.8% or less. More preferably, it is 4.7% or less, and even more preferably, it is 4.5% or less.
• the area ratio of retained austenite is preferably 4% or less. More preferably, it is 3.8% or less, and even more preferably, it is 3.7% or less, and even more preferably, it is 3.5% or less.
• the area ratio of retained austenite may be 0% or may exceed 0%.
• Ferrite area ratio 0.2% or more and 3.0% or less Ferrite is formed from austenite grain boundaries with non-uniform B concentration. If the area ratio of ferrite is 0.2% or more, the B concentration of the prior austenite grain boundaries where ferrite is not formed is sufficiently high, and the delayed fracture resistance is good. Therefore, the area ratio of ferrite is set to 0.2% or more.
• the area ratio of ferrite is preferably set to 0.3% or more, and more preferably set to 0.5% or more. On the other hand, if the area ratio of ferrite exceeds 3.0%, the strength decreases. Therefore, the area ratio of ferrite is set to 3% or less.
• the area ratio of ferrite is preferably set to 2.5% or less, and more preferably set to 2.0% or less.
• the area ratio of each structure is measured as follows.
• the area ratio of the retained austenite is determined by chemically polishing the rolled surface of a test piece taken from each steel plate up to the t/4 position of the steel plate thickness, measuring the X-ray diffraction intensity and diffraction peak position of the polished surface with an X-ray diffraction (XRD) device to calculate the volume ratio, and this number is the area ratio of the retained austenite.
• XRD X-ray diffraction
• SEM images of the observation surface are taken at a magnification of 2000 times with a field of view of 57.1 ⁇ m ⁇ 42.9 ⁇ m in three fields of view.
• the obtained SEM images are analyzed by image analysis to determine the total area ratio of martensite, bainite, and retained austenite, as well as the area ratio of structures other than martensite, bainite, and retained austenite (ferrite).
• the area ratios of martensite and bainite are calculated by subtracting the area ratio of retained austenite obtained by XRD from the area ratios of martensite, bainite, and retained austenite obtained by image analysis.
• the average value of the three fields of view is taken as the area ratio of the structure.
• Prior austenite grain size 10 ⁇ m or less
• the prior austenite grain size is set to 10 ⁇ m or less.
• Prior austenite is preferably 9 ⁇ m or less.
• the prior austenite grain size is preferably 1 ⁇ m or more.
• the prior austenite grain size is more preferably 2 ⁇ m or more, and even more preferably 3 ⁇ m or more.
• the grain size of the prior austenite grains is measured as follows. After polishing the plate thickness cross section parallel to the rolling direction of each steel plate, it is etched with picral to prepare the observation surface. On the observation surface, the microstructure at the plate thickness position t/4 is photographed with an SEM at a magnification of 2000 times with a field of view of 57.1 ⁇ m x 42.9 ⁇ m, and three fields of view are obtained to obtain SEM images. The grain size of each prior austenite grain is determined from the obtained structure image by image analysis, and the average value of the three fields of view is taken as the grain size of the prior austenite grains (average crystal grain size).
• B concentration in prior austenite grain boundaries between adjacent martensites 0.05% or more by mass B segregates to the prior austenite grain boundaries to strengthen the grain boundaries and improve delayed fracture resistance. If the B concentration in the prior austenite grain boundaries is 0.05% or more by mass, the above effect can be obtained. Therefore, the B concentration in the prior austenite grain boundaries (prior austenite grain boundaries sandwiched on both sides by martensite structures) between adjacent martensites is set to 0.05% or more by mass.
• the B concentration in the prior austenite grain boundaries is preferably 0.07% or more by mass, and more preferably 0.10% or more by mass.
• the B concentration is preferably less than 6%, and more preferably 2% or less by mass.
• Variation in B concentration of prior austenite grain boundaries between adjacent martensites within the same grain boundary less than 0.010% by mass
• the variation in B concentration of prior austenite grain boundaries is set to less than 0.010% by mass.
• the variation is preferably 0.009% or less by mass, and more preferably 0.008% or less by mass. Although the smaller the variation, the better, from the viewpoint of production technology, the variation should be 0.001% or more. Furthermore, the grain boundary that contributes to the improvement of delayed fracture resistance due to B segregation is a grain boundary where both sides are martensite.
• Variation in B concentration of the old austenite grain boundary between martensite and ferrite within the same grain boundary 0.010% or more by mass
• the B concentration of the old austenite grain boundary sandwiched between martensite on both sides can be made 0.05% by mass or more. If the variation in the B concentration of the old austenite grain boundary between martensite and ferrite within the same grain boundary is less than 0.010%, ferrite will not be formed. Therefore, the variation in the B concentration of the old austenite grain boundary (old austenite grain boundary sandwiched between martensite structure and ferrite) within the same grain boundary between martensite and ferrite is set to 0.010% or more by mass.
• the variation is preferably 0.012% or more by mass, and more preferably 0.014% or more by mass.
• the upper limit is not particularly limited, but the variation is preferably 0.200% or less in mass%, and more preferably 0.100% or less in mass%.
• the B concentration and variation of the prior austenite grain boundary are measured as follows.
• a needle-shaped sample is prepared from a region including the prior austenite grain boundary by the SEM-FIB (Focused Ion Beam) method.
• the obtained needle-shaped sample is subjected to 3DAP analysis using a 3DAP device (LEAP4000XSi, manufactured by AMETEK).
• the measurement is performed in laser mode.
• the sample temperature is 80K or less.
• the B concentration of the prior austenite grain boundary is obtained from the number of B ions and the number of other ions detected from the prior austenite grain boundary.
• the B concentration is the average value of two samples.
• the prior austenite grain size is very large compared to the area sampled by the SEM-FIB method, so the grain boundaries targeted by one sampled sample are all the same grain boundaries.
• the prior austenite grain size is about 9 ⁇ m, while the sampled sample area is about 0.1 ⁇ m in diameter. Therefore, the variation in the obtained B concentration is the variation within the same grain boundary.
• a plated steel sheet having a tensile strength of 1180 MPa or more.
• the tensile strength of the plated steel sheet is preferably 1250 MPa or more.
• the above-mentioned plated steel sheet has a plating layer formed on at least one side of the steel sheet.
• the plating layer is preferably any one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrolytic galvanized layer.
• the composition of the plating layer is not particularly limited and may be a known composition.
• the composition of the hot-dip galvanized layer is not particularly limited and may be any common one.
• the coating layer contains Fe: 20 mass% or less, Al: 0.001 mass% to 1.0 mass% and further contains one or more selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 mass% to 3.5 mass%, with the balance being Zn and unavoidable impurities.
• the Fe content in the plating layer is less than 7 mass%, and when the plating layer is an alloyed hot-dip galvanized layer, in one example, the Fe content in the plating layer is 7 mass% or more and 15 mass% or less, more preferably 8 mass% or more and 13 mass% or less.
• the coating weight is not particularly limited, it is preferable that the coating weight per one side of the plated steel sheet is 20 to 80 g/m 2.
• the plating layer is formed on both the front and back sides of the steel sheet (high-strength cold-rolled steel sheet).
• the method for producing a plated steel sheet according to this embodiment includes a hot rolling process in which a steel slab having the above-described component composition is hot-rolled to obtain a hot-rolled sheet, a pickling process in which the hot-rolled sheet is pickled, a cold rolling process in which the hot-rolled sheet after the pickling process is cold-rolled to obtain a cold-rolled sheet, a first annealing process in which the cold-rolled sheet is heated to a first heating temperature of Ac3 point or more, a cooling process in which cooling is started from the first heating temperature for the cold-rolled sheet after the first annealing process, and cooled to a cooling stop temperature of 100°C or more and less than the Ms point at an average cooling rate of 50°C/s or more, a reheating process in which, after the cooling process, the cold-rolled sheet is heated to a reheating temperature of 300°C or more and 400°C or less, and held at the reheating temperature for
• a steel slab having the above-mentioned composition is produced.
• an example of manufacturing conditions for a steel slab before the hot rolling process will be described.
• the steel material is melted to obtain molten steel having the above-mentioned composition.
• the melting method is not particularly limited, and any of the known melting methods such as converter melting and electric furnace melting are suitable.
• the obtained molten steel is solidified to produce a steel slab.
• the method for producing a steel slab from molten steel is not particularly limited, and a continuous casting method, an ingot casting method, a thin slab casting method, or the like can be used.
• the steel slab may be cooled once and then reheated before being subjected to hot rolling, or the cast steel slab may be continuously hot rolled without being cooled to room temperature.
• the slab heating temperature is preferably 1100°C or higher, and preferably 1300°C or lower, taking into consideration the rolling load and the generation of scale.
• the slab heating method is not particularly limited, and for example, it can be heated in a heating furnace according to a conventional method.
• the above-mentioned steel slab is hot-rolled to obtain a hot-rolled sheet.
• the hot rolling may be performed according to a conventional method.
• the cooling after hot rolling and it is cooled to the coiling temperature.
• the hot-rolled sheet is wound into a coil.
• the coiling temperature is preferably 400°C or higher. If the coiling temperature is 400°C or higher, the strength of the hot-rolled sheet does not increase and the coiling becomes easy. It is more preferable that the coiling temperature is 550°C or higher.
• the coiling temperature is preferably 750°C or lower. Note that the hot-rolled sheet may be heat-treated for the purpose of softening before pickling.
• the hot-rolled sheet is pickled in a pickling process.
• the scale of the hot-rolled sheet wound around the coil can be removed.
• the method for removing the scale is not particularly limited, but in order to completely remove the scale, it is preferable to perform pickling while uncoiling the hot-rolled coil.
• the pickling method is not particularly limited, and may be a conventional method.
• the hot-rolled sheet is cold-rolled in the cold rolling process to obtain a cold-rolled sheet.
• the hot-rolled sheet from which the scale has been removed is appropriately washed and then cold-rolled to obtain a cold-rolled sheet.
• the method of cold rolling is not particularly limited and may be a conventional method.
• First annealing step heating to a first heating temperature of Ac3 point or higher
• the cold-rolled sheet is heated to a first heating temperature of Ac3 point or higher, and annealed in the austenite single phase region. If the first heating temperature is lower than Ac3 point, ferrite is generated. This ferrite has reduced dislocations by annealing, and there are no dislocations that serve as a diffusion path for B (boron) during the second annealing, making it difficult to uniformly segregate boron.
• B boron
• the austenite grain size will become larger than 10 ⁇ m. Therefore, the first heating temperature is set to Ac3 point or higher.
• the first heating temperature is preferably Ac3 point + 10 ° C. or higher, more preferably Ac3 point + 15 ° C. or higher. Since the austenite structure formed in the second annealing has the same crystal structure as the austenite structure formed in the first annealing, the first heating temperature is preferably 980° C. or less so that the austenite grain size in the first annealing is 10 ⁇ m or less. The first heating temperature is more preferably 950° C. or less.
• the Ac3 point is calculated using the following formula.
• the cooling stop temperature of the partial quenching is less than 100°C, martensite transformation occurs before C distribution occurs, and a sufficient amount of residual austenite cannot be obtained before the second annealing. If the amount of residual austenite is insufficient, austenite with a different orientation from that in the first annealing is generated, and it is difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the cooling stop temperature in the cooling step is set to 100°C or higher.
• the cooling stop temperature is preferably 120°C or higher, more preferably 150°C or higher.
• the cooling stop temperature in the cooling step is set to be lower than the Ms point, preferably Ms point -10°C or lower, more preferably Ms point -20°C or lower.
• the Ms point is calculated using the following formula.
• Ms (°C) 499-308 ⁇ [C]-10.8 ⁇ [Si]-32.4 ⁇ [Mn]-16.2 ⁇ [Ni]-27 ⁇ [Cr]-10.8 ⁇ [Mo] (In the above formula, [M] is the content (mass%) of element M in the steel sheet, and the value of an element that is not contained is 0 (zero).)
• the average cooling rate is set to 50° C./s or more.
• the average cooling rate is preferably set to 60° C./s or more, and more preferably set to 70° C./s or more.
• the upper limit of the average cooling rate is not particularly limited, but if the cooling rate is too high, it becomes difficult to control the first cooling stop temperature, so the average cooling rate is preferably 1000° C./s or less, and more preferably 200° C./s or less.
• the average cooling rate (°C/s) in the cooling step is "(first heating temperature (°C))-(cooling stop temperature (°C))/(cooling time (seconds) from the first heating temperature (°C) to the cooling stop temperature (°C))".
• the reheating temperature is set to 300° C. or higher.
• the reheating temperature is preferably 310° C. or higher, and more preferably 320° C. or higher.
• the reheating temperature exceeds 400°C, the untransformed austenite decomposes into cementite, and no residual austenite is formed before the second annealing.
• the reheating temperature is set to 400°C or less.
• the reheating temperature is preferably 390°C or less, and more preferably 380°C or less.
• the holding time at the reheating temperature is set to 60 seconds or more, preferably 80 seconds or more, and more preferably 100 seconds or more.
• the holding time at the reheating temperature is preferably 900 seconds or less, and more preferably 600 seconds or less.
• the sub-zero treatment transforms the blocky retained austenite to obtain martensite with a high C concentration. From this martensite, new austenite with a different orientation from the first treatment is nucleated, resulting in grain boundaries with a non-uniform B concentration. The film-like retained austenite remains even after this treatment, and the subsequent annealing forms an austenite structure with the same crystal orientation as the first austenite, with the grain boundaries being enriched and uniform in B.
• the specific conditions for the sub-zero treatment are not particularly limited, it is preferable to immerse the cold-rolled sheet after the reheating step in liquid nitrogen ( ⁇ 196° C.) for 30 minutes or more. It is also preferable to immerse the cold-rolled sheet in liquid nitrogen for 3 hours or less.
• the second heating temperature is set to the Ac3 point or higher.
• the second heating temperature is preferably the Ac3 point + 10°C or higher, more preferably the Ac3 point + 15°C or higher.
• the second heating temperature is preferably 980° C. or less, and more preferably 950° C. or less.
• a plating step is performed in which at least one side of the steel sheet is plated to obtain a plated steel sheet.
• a heat treatment may be performed on the plated steel sheet (high-strength plated steel sheet) to alloy the plating layer of the plated steel sheet to obtain an alloyed plated steel sheet.
• the production conditions other than those mentioned above can be the same as those in the ordinary methods.
• the plated steel sheet according to this embodiment obtained as described above preferably has a thickness of 0.5 mm or more. Also, it is preferable that the thickness is 2.0 mm or less.
• a member can be provided that is at least partially made of the above-mentioned plated steel sheet.
• the above-mentioned plated steel sheet can be formed into a desired shape by press working to form an automobile part.
• the automobile part may contain a steel sheet other than the plated steel sheet according to this embodiment as a material.
• a plated steel sheet having a TS of 1180 MPa or more and having excellent delayed fracture resistance and platability can be provided, so that a member having a TS of 1180 MPa or more and having both excellent delayed fracture resistance and platability can be provided.
• the steel sheet according to this embodiment can be suitably used as an automobile part that contributes to weight reduction of the vehicle body.
• the plated steel sheet according to this embodiment can be suitably used in general members used as automobile parts, particularly as frame structural parts or reinforcing parts.
• the method for producing the above-mentioned component includes a step of subjecting the above-mentioned plated steel sheet to at least one of forming and joining to form the component.
• the forming process may be performed by a general processing method such as pressing, without any restrictions.
• the joining process may be performed by a general welding method such as spot welding or arc welding, riveting, or crimping, without any restrictions.
• the obtained slab was reheated and hot-rolled, and then wound to obtain a hot-rolled coil (hot-rolled sheet).
• the hot-rolled coil was unwound while being pickled, and cold-rolled to obtain a cold-rolled sheet.
• the thickness of the hot-rolled sheet was 3.0 mm, and the thickness of the cold-rolled sheet was 1.2 mm.
• Annealing (first annealing step, cooling step, reheating step, subzero treatment step, second annealing step) was performed under the conditions shown in Table 2 in a continuous hot-dip galvanizing line to obtain a plated steel sheet (hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA)).
• the hot-dip galvanized steel sheet was immersed in a 460 ° C. plating bath, and the plating amount per side was 35 g / m 2 .
• the galvannealed steel sheet was produced by adjusting the coating weight to 45 g/ m2 per side and then subjecting it to an alloying treatment at 520°C for 40 s.
• the subzero treatment was performed on all steel sheets except for steel sheet No. 6, which was immersed in liquid nitrogen ( ⁇ 196° C.) for 60 minutes.
• the obtained steel sheets were evaluated according to the methods described above for the total area ratio of martensite and bainite, the area ratio of retained austenite, the area ratio of ferrite, the prior austenite grain size, the B concentration of the prior austenite grain boundaries between adjacent martensite, and the variation in the B concentration of the prior austenite grain boundaries within the same grain boundary (the variation in the B concentration of the prior austenite grain boundaries between adjacent martensite within the same grain boundary, and the variation in the B concentration of the prior austenite grain boundaries between martensite and ferrite within the same grain boundary).
• the tensile strength and delayed fracture resistance were evaluated according to the methods described below. The results are shown in Table 3.
• platability evaluation Regarding platability, each plated steel sheet was visually observed for the presence or absence of unplated areas, and those without unplated areas were deemed to have good platability. In the table, steel sheets that were deemed to have good platability are indicated as "excellent,” and steel sheets that were deemed to have poor platability are indicated as “poor.”
• the examples of the present invention have a tensile strength TS of 1180 MPa or more, and are excellent in delayed fracture resistance and platability.
• the comparative examples are inferior in one or more of tensile strength TS, delayed fracture resistance, and platability.
• the steel plates of the examples of the present invention were used to produce members obtained by forming, members obtained by joining, and members obtained by further forming and joining, and because the steel plates of the examples of the present invention have high strength and excellent delayed fracture resistance and platability, they have the same high strength and excellent delayed fracture resistance and platability as the steel plates of the examples of the present invention.

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