EP4520848A2 - Acier ayant des propriétés de traitement améliorées pour le formage à température élevée - Google Patents

Acier ayant des propriétés de traitement améliorées pour le formage à température élevée Download PDF

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Publication number
EP4520848A2
EP4520848A2 EP24218807.6A EP24218807A EP4520848A2 EP 4520848 A2 EP4520848 A2 EP 4520848A2 EP 24218807 A EP24218807 A EP 24218807A EP 4520848 A2 EP4520848 A2 EP 4520848A2
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EP
European Patent Office
Prior art keywords
sheet metal
flat steel
temperature
steel product
metal part
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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EP24218807.6A
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German (de)
English (en)
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EP4520848A3 (fr
Inventor
Janko Banik
Dirk Rosenstock
Cássia CASTRO MÜLLER
Thomas Gerber
Maria KÖYER
Sebastian STILLE
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ThyssenKrupp Steel Europe AG
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ThyssenKrupp Steel Europe AG
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Publication of EP4520848A2 publication Critical patent/EP4520848A2/fr
Publication of EP4520848A3 publication Critical patent/EP4520848A3/fr
Withdrawn legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • 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
    • C21D1/26—Methods of annealing
    • C21D1/28—Normalising
    • 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
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • 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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
    • 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
    • C21D8/0236—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
    • C21D8/0252—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 with application of tension
    • 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
    • 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
    • C21D8/0278—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 involving a particular surface 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
    • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
    • 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
    • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0421—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
    • 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
    • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0421—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
    • C21D8/0436—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
    • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0447—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the heat treatment
    • C21D8/0463—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 to produce plates or strips for drawing, e.g. for deep-drawing 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
    • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0478—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 to produce plates or strips for drawing, e.g. for deep-drawing involving a particular surface 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
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C21/00—Alloys based on aluminium
    • C22C21/10—Alloys based on aluminium with zinc as the next major constituent
    • 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
    • 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
    • C23C2/12—Aluminium or alloys based thereon
    • 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
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C23C2/29—Cooling or quenching
    • 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
    • C23C2/50—Controlling or regulating the coating processes
    • C23C2/52—Controlling or regulating the coating processes with means for measuring or sensing
    • C23C2/522—Temperature of the bath
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    • 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
    • C21D2211/004—Dispersions; Precipitations
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the invention relates to a flat steel product for hot forming and a method for producing such a flat steel product. Furthermore, the invention relates to a sheet metal part with improved properties and a method for producing such a sheet metal part from a flat steel product.
  • a “flat steel product” or a “sheet metal product” refers to rolled products, such as steel strips or sheets, from which "sheet metal blanks” (also called blanks) are cut for the production of, for example, car body components.
  • sheet metal blanks also called blanks
  • “Formed sheet metal parts” or “sheet metal components” of the type according to the invention are made from such sheet metal blanks, whereby the terms “formed sheet metal part” and “sheet metal component” are used synonymously here.
  • the microstructure was determined on longitudinal sections etched with 5% Nital (alcoholic nitric acid). The content of retained austenite was determined by X-ray diffraction.
  • the sheet metal part consists of a steel which, in addition to iron and unavoidable impurities, is composed of (in wt. %) 0.10 - 0.30% C, 0.5 - 2.0% Si, 0.5 - 2.4% Mn, 0.01 - 0.2% Al, 0.005 - 1.5% Cr, 0.01 - 0.1% P, and possibly further optional elements, in particular 0.005 - 0.1% Nb.
  • the sheet metal component comprises an anti-corrosive coating containing aluminum.
  • the task was to further develop a flat steel product for hot forming in such a way that, in combination with an aluminum-based anti-corrosive coating, improved processing properties of the hot-formed sheet metal part could be achieved. Furthermore, a process was to be specified for the practical production of such sheet metal parts.
  • a flat steel product for hot forming comprising a steel substrate made of steel which, in addition to iron and unavoidable impurities (in wt.%), consists of C: 0.50 - 0.50%, Si: 0.05 - 0.6%, Mn: 0.5 - 3.0%, Al: 0.10 - 1.0%, Note: 0.001 - 0.2%, T: 0.001 - 0.10% B: 0.0005 - 0.01% P: ⁇ 0.05%, S: ⁇ 0.02%, N: ⁇ 0.02%, Sn: ⁇ 0.05%, Ace: ⁇ 0.01% and optionally one or more of the elements "Cr, Cu, Mo, Ni, V, Ca, W" in the following contents Cr: 0.01 - 1.0%, Cu: 0.01 - 0.2%, Mo: 0.002 - 0.3%, No: 0.01 - 0.5%, V: 0.001 - 0.3%, Approx: 0.0005 - 0.005%, W: 0.001 -1.00%
  • the steel substrate of the flat steel product according to the invention has an aluminum content of at least 0.10 wt.%, particularly preferably at least 0.11 wt.%, in particular at least 0.12 wt.%, preferably at least 0.140 wt.%, in particular at least 0.15 wt.%, preferably at least 0.16 wt.%.
  • the maximum aluminum content is 1.0 wt.%, in particular a maximum of 0.8 wt.%.
  • the aluminum content is at least 0.10 wt.%, particularly preferably at least 0.11 wt.%, in particular at least 0.12 wt.%, preferably at least 0.140 wt.%, in particular at least 0.15 wt.%, preferably at least 0.16 wt.%.
  • the maximum aluminum content in this variant is a maximum of 0.50 wt.%, in particular a maximum of 0.35 wt.%, preferably a maximum of 0.25 wt.%, in particular a maximum of 0.24 wt.%.
  • the aluminum content is at least 0.50 wt.%, preferably at least 0.60 wt.%, preferably at least 0.70 wt.%.
  • the maximum aluminum content in this variant is a maximum of 1.0 wt.%, in particular a maximum of 0.9 wt.%, preferably a maximum of 0.80 wt.%.
  • Al Aluminum
  • Al is known to be added as a deoxidizer in the production of steel. At least 0.01 wt.% Al is required to reliably bind the oxygen contained in the molten steel. In addition, Al can be used to bind undesirable, but unavoidable, N contents due to manufacturing processes. Relatively high aluminum contents have been avoided so far, as the Ac3 temperature also shifts upwards with increasing aluminum content. This has a negative impact on austenitization, which is important for hot forming. However, it has been shown that increased aluminum contents surprisingly lead to positive effects when combined with an aluminum-based corrosion protection coating.
  • iron diffuses from the steel substrate into the liquid corrosion protection coating.
  • iron-aluminide compounds with a higher density are formed via a multi-stage phase transformation (Fe2Al5 ⁇ Fe2Al ⁇ FeAl ⁇ Fe3Al).
  • the formation of such denser phases is associated with higher aluminum consumption than with less dense phases.
  • This locally higher aluminum consumption leads to the formation of pores (vacancies) in the resulting phase.
  • These pores preferably form in the transition region between the steel substrate and the corrosion protection coating, where the proportion of available aluminum is strongly influenced by the aluminum content of the steel substrate. In particular, an accumulation of pores in the form of a band can occur in the transition region.
  • the Al content is too high, especially at contents exceeding 1.0 wt% Al, there is a risk of Al oxides forming on the surface of a product made from steel alloyed according to the invention, which would impair the wetting behavior during hot-dip coating. Furthermore, higher Al contents promote the formation of non-metallic Al-based inclusions, which, as coarse inclusions, negatively impact crash behavior. Therefore, the Al content is preferably selected below the aforementioned upper limits.
  • the bending behavior of the sheet metal component is particularly supported by the niobium content ("Nb") of at least 0.001 wt.% according to the invention.
  • the niobium content is preferably at least 0.005 wt.%, in particular at least 0.010 wt.%, preferably at least 0.015 wt.%, particularly preferably at least 0.020 wt.%, in particular at least 0.024 wt.%, preferably at least 0.025 wt.%.
  • the specified niobium content leads, particularly in the process described below for producing a flat steel product for hot forming with a corrosion protection coating, to a distribution of niobium carbonitrides, which results in a particularly fine hardened microstructure during subsequent hot forming.
  • the coated flat steel product is kept for a certain time in a temperature range between 400 °C and 300 °C. In this temperature range, a certain diffusion rate of carbon still exists in the steel substrate, while the thermodynamic solubility is very low. Thus, carbon diffuses to lattice defects and accumulates there.
  • Lattice defects are caused in particular by dissolved niobium atoms, which, due to their significantly higher atomic volume, expand the atomic lattice and thus enlarge the tetrahedral and octahedral gaps in the atomic lattice, so that the local solubility of C is increased.
  • clusters of C and Nb form in the steel substrate, which then transform into very fine precipitates in the subsequent austenitizing step of hot forming and act as additional austenite nuclei. This results in a refined austenite microstructure with smaller austenite grains and thus also a refined hardening microstructure.
  • the refined ferritic microstructure in the interdiffusion layer helps reduce the tendency for crack initiation under bending loads.
  • the Nb content is a maximum of 0.2 wt.%.
  • the niobium content is preferably a maximum of 0.20 wt.%, in particular a maximum of 0.15 wt.%, preferably a maximum of 0.10 wt.%, in particular a maximum of 0.05 wt.%.
  • Al/Nb ratio of less than or equal to 1.6 wt.% for low manganese contents, for which the following applies: Al / Nb ⁇ 20 . 0 , which corresponds approximately to an atomic ratio of both elements ⁇ 6.
  • the ratio Al/Nb is preferably ⁇ 18.0, in particular ⁇ 16.0, preferably ⁇ 14.0, particularly preferably ⁇ 12.0, in particular ⁇ 10.0, preferably ⁇ 9.0, in particular ⁇ 8.0, preferably ⁇ 7.0.
  • the ratio Al/Nb is preferably ⁇ 28.0, in particular ⁇ 26.0, preferably ⁇ 24.0, particularly preferably ⁇ 22.0, preferably ⁇ 20.0, in particular ⁇ 18.0, in particular ⁇ 16.0, preferably ⁇ 14.0, particularly preferably ⁇ 12.0, in particular ⁇ 10.0, preferably ⁇ 9.0, in particular ⁇ 8.0, preferably ⁇ 7.0.
  • the ratio Al/Nb is ⁇ 18.0, in particular ⁇ 16.0, preferably ⁇ 14.0, particularly preferably ⁇ 12.0, in particular ⁇ 10.0, preferably ⁇ 9.0, in particular ⁇ 8.0, preferably ⁇ 7.0.
  • Carbon is present in the steel substrate of the flat steel product in concentrations of 0.50 - 0.50 wt.%. Such adjusted C contents contribute to the hardenability of the steel by delaying ferrite and bainite formation and stabilizing the residual austenite in the microstructure.
  • the carbon content can be adjusted to 0.45 wt.%, preferably to a maximum of 0.42 wt.%, particularly preferably to 0.40 wt.%, preferably to a maximum of 0.38 wt.%, and in particular to a maximum of 0.35 wt.%.
  • C contents of at least 0.32 wt.%, preferably 0.55 wt.%, in particular at least 0.34 wt.%, preferably at least 0.35 wt.% can be provided.
  • tensile strengths of the sheet metal part of at least 1700 MPa, in particular at least 1800 MPa, after hot press forming can be reliably achieved, subject to the further provisions of the invention.
  • Silicon is used to further increase the hardenability of the flat steel product as well as the strength of the press-hardened product through solid solution strengthening. Silicon also enables the use of ferro-silicon-manganese as an alloying agent, which has a beneficial effect on production costs. A hardening effect is already evident at a Si content of 0.05 wt.%. A significant increase in strength occurs at a Si content of at least 0.15 wt.%, and especially at least 0.20 wt.%. Si contents above 0.6 wt.% have a detrimental effect on the coating behavior, especially in Al-based coatings. Si contents of 0.50 wt.% or less, especially 0.50 wt.% or less, are preferred to improve the surface quality of the coated flat steel product.
  • Manganese acts as a hardening element by significantly retarding the formation of ferrite and bainite. At manganese contents below 0.4 wt. %, significant amounts of ferrite and bainite are formed during press hardening, even at very rapid cooling rates, which should be avoided. Mn contents of at least 0.5 wt. %, preferably at least 0.7 wt. %, in particular at least 0.8 wt. %, preferably at least 0.9 wt. %, in particular at least 1.00 wt. %, preferably at least 1.05 wt. %, and particularly preferably at least 1.10 wt.
  • Mn content of flat steel products according to the invention is limited to a maximum of 3.0 wt. %, preferably a maximum of 2.5 wt. %.
  • Weldability, in particular, is severely limited, which is why the Mn content is preferably limited to a maximum of 1.6 wt%, and in particular to 1.30 wt%, and in particular to a maximum of 1.20 wt%.
  • Manganese contents of less than or equal to 1.6 wt% are also preferred for economic reasons.
  • Titanium is a microalloying element that is added to contribute to grain refinement. At least 0.001 wt.% Ti, in particular at least 0.004 wt.%, and preferably at least 0.010 wt.% Ti, should be added to ensure sufficient availability. Above 0.10 wt.% Ti, cold rollability and recrystallizability deteriorate significantly, which is why higher Ti contents should be avoided. To improve cold rollability, the Ti content can preferably be limited to 0.08 wt.%, in particular to 0.038 wt.%, particularly preferably to 0.020 wt.%, and especially 0.015 wt.%. Titanium also has the effect of binding nitrogen, thus enabling boron to exert its strong ferrite-inhibiting effect. Therefore, in a preferred embodiment, the titanium content is more than 3.42 times the nitrogen content to achieve sufficient nitrogen binding.
  • B Boron
  • a significant effect on hardenability occurs at contents of at least 0.0005 wt.%, preferably at least 0.0007 wt.%, in particular at least 0.0010 wt.%, in particular at least 0.0020 wt.%.
  • the boron content is limited to a maximum of 0.01 wt.%, preferably a maximum of 0.0100 wt.%, preferably a maximum of 0.0050 wt.%, in particular a maximum of 0.0035 wt.%, in particular a maximum of 0.0050 wt.%, preferably a maximum of 0.0025 wt.%.
  • Phosphorus (“P”) and sulfur (“S”) are elements that are introduced into steel as impurities through iron ore and cannot be completely eliminated in the large-scale steelmaking process.
  • the P and S contents should be kept as low as possible, since mechanical properties such as notch impact energy deteriorate with increasing P and S contents.
  • embrittlement of the martensite begins to occur, which is why the P content of a flat steel product according to the invention is limited to a maximum of 0.03 wt.%, in particular a maximum of 0.02 wt.%.
  • the S content of a flat steel product according to the invention is limited to a maximum of 0.02 wt.%, preferably a maximum of 0.0010 wt.%, in particular a maximum of 0.005 wt.%.
  • Nitrogen is also present in small amounts as an impurity in steel due to the steelmaking process.
  • the N content should be kept as low as possible and should not exceed 0.02 wt.%. Nitrogen is particularly harmful to alloys containing boron, as it inhibits the transformation-retarding effect of boron through the formation of boron nitrides. Therefore, the nitrogen content in this case should preferably not exceed 0.010 wt.%, and in particular not exceed 0.007 wt.%.
  • Sn tin
  • As arsenic
  • Sn content is a maximum of 0.05 wt.%, preferably a maximum of 0.02 wt.%.
  • As content is a maximum of 0.01 wt.%, in particular a maximum of 0.005 wt.%.
  • Chromium, copper, molybdenum, nickel, vanadium, calcium and tungsten can optionally be alloyed to the steel of a flat steel product according to the invention, either individually or in combination with one another.
  • the Cr content of the steel or the steel substrate is limited to a maximum of 1.0 wt.%, preferably a maximum of 0.80 wt.%, in particular a maximum of 0.75 wt.%, preferably a maximum of 0.50 wt.%, in particular a maximum of 0.50 wt.%.
  • Vanadium (V) can optionally be added in amounts of 0.001–1.0 wt.%.
  • the vanadium content is preferably a maximum of 0.3 wt.%. For cost reasons, a maximum of 0.2 wt.% vanadium is added.
  • Molybdenum can be optionally added to improve process stability, as it significantly slows ferrite formation. Starting at concentrations of 0.002 wt.%, dynamic molybdenum-carbon clusters form, extending to ultrafine molybdenum carbides at the grain boundaries. which significantly slow the mobility of the grain boundary and thus diffusive phase transformations. Molybdenum also reduces the grain boundary energy, which reduces the nucleation rate of ferrite.
  • the Mo content is preferably at least 0.004 wt.%, in particular at least 0.01 wt.%. Due to the high costs associated with a molybdenum alloy, the content should be at most 0.3 wt.%, in particular at most 0.10 wt.%, and preferably at most 0.08 wt.%.
  • Nickel (Ni) stabilizes the austenitic phase and can optionally be added to the alloy to reduce the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive influence on hot rollability, particularly when the steel contains copper. Copper impairs hot rollability. To counteract the negative influence of copper on hot rollability, 0.01 wt.% nickel can be added to the steel; the Ni content is preferably at least 0.015 wt.%, more preferably at least 0.020 wt.%. For economic reasons, the nickel content should be limited to a maximum of 0.5 wt.%, in particular a maximum of 0.20 wt.%. The Ni content is preferably a maximum of 0.10 wt.%.
  • a flat steel product according to the invention can optionally contain at least 0.0005 wt.% Ca, in particular at least 0.0010 wt.%, preferably at least 0.0020 wt.%.
  • the maximum Ca content is 0.01 wt.%, in particular a maximum of 0.007 wt.%, preferably a maximum of 0.005 wt.%.
  • Tungsten (W) can optionally be added to the alloy in concentrations of 0.001–1.0 wt.% to slow ferrite formation. A positive effect on hardenability is already achieved at W contents of at least 0.001 wt.%. For cost reasons, a maximum of 1.0 wt.% tungsten is added.
  • the sum of the Mn content and the Cr content (“Mn+Cr”) is more than 0.7 wt.%, in particular more than 0.8 wt.%, preferably more than 1.1 wt.%. Below a minimum sum of both elements, their necessary transformation-inhibiting effect is lost. Irrespective of this, the sum of the Mn content and the Cr content is less than 3.5 wt.%, preferably less than 2.5 wt.%, in particular less than 2.0 wt.%, particularly preferably less than 1.5 wt.%. The upper limit values for both elements arise from ensuring coating performance and ensuring adequate welding behavior.
  • the flat steel product preferably comprises a corrosion protection coating to protect the steel substrate from oxidation and corrosion during hot forming and during use of the produced steel component.
  • the flat steel product preferably comprises an aluminum-based anti-corrosive coating.
  • the anti-corrosive coating can be applied to one or both sides of the flat steel product.
  • the two large, opposing surfaces of the flat steel product are referred to as the two sides of the flat steel product.
  • the narrow surfaces are referred to as the edges.
  • Such a corrosion protection coating is preferably produced by hot-dip coating the flat steel product.
  • the flat steel product is passed through a liquid melt consisting of up to 15 wt.% Si, preferably more than 1.0 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, with the remainder being aluminum.
  • the Si content of the melt is 1.0 - 3.5 wt.% or 5-15 wt.%, in particular 7 - 12 wt.%, in particular 8 - 10 wt.%.
  • the optional content of alkali or alkaline earth metals in the melt comprises 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg.
  • the optional content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.
  • the alloy layer lies on the steel substrate and is directly adjacent to it.
  • the alloy layer is essentially formed from aluminum and iron.
  • the remaining elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer.
  • the alloy layer preferably consists of 35-60 wt.% Fe, preferably ⁇ -iron, optional further constituents whose total contents are limited to a maximum of 5.0 wt.%, preferably 2.0%, and the remainder aluminum, with the Al content preferably increasing towards the surface.
  • the optional further constituents include in particular the remaining constituents of the melt (i.e. silicon and optionally alkali or alkaline earth metals, in particular Mg or Ca) and the remaining portions of the steel substrate in addition to iron.
  • the Al base layer lies on top of the alloy layer and directly adjoins it.
  • the composition of the Al base layer preferably corresponds to the composition of the melt of the molten bath. This means that it consists of 0.1–15 wt.% Si, optionally 2–4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, with the remainder being aluminum.
  • the optional content of alkali or alkaline earth metals comprises 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg.
  • the optional content of alkali or alkaline earth metals in the Al base layer can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca.
  • the Si content in the alloy layer is lower than the Si content in the Al base layer.
  • the corrosion protection coating preferably has a thickness of 5 - 60 ⁇ m, in particular 10 - 40 ⁇ m.
  • the coating weight of the corrosion protection coating is in particular 30 ⁇ 360 g m 2 with corrosion protection coatings on both sides or 15 ⁇ 180 g m 2 in the one-sided variant.
  • the coating weight of the corrosion protection coating is preferably 100 ⁇ 200 g m 2 for double-sided coatings or 50 ⁇ 100 g m 2 for one-sided coatings.
  • the coating weight of the corrosion protection coating is particularly preferred 120 ⁇ 180 g m 2 for double-sided coatings or 60 ⁇ 90 g m 2 for one-sided covers.
  • the thickness of the alloy layer is preferably less than 20 ⁇ m, more preferably less than 16 ⁇ m, in particular less than 12 ⁇ m, particularly preferably less than 10 ⁇ m, preferably less than 8 ⁇ m, in particular less than 5 ⁇ m.
  • the thickness of the Al base layer results from the difference between the thicknesses of the anti-corrosive coating and the alloy layer.
  • the thickness of the Al base layer is preferably at least 1 ⁇ m, even with thin anti-corrosive coatings.
  • the flat steel product comprises an oxide layer arranged on the corrosion protection coating.
  • the oxide layer is located in particular on the aluminum base layer and preferably forms the outer edge of the corrosion protection coating.
  • the oxide layer consists in particular of more than 80 wt.% oxides, with the majority of the oxides (i.e., more than 50 wt.% of the oxides) being aluminum oxide.
  • hydroxides and/or magnesium oxide are present in the oxide layer, alone or as a mixture.
  • the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and/or magnesium in metallic form.
  • zinc oxide components are also present in the oxide layer.
  • the oxide layer of the flat steel product has a thickness greater than 50 nm.
  • the thickness of the oxide layer is a maximum of 500 nm.
  • the flat steel product includes a zinc-based corrosion protection coating.
  • the corrosion protection coating can be applied to one or both sides of the flat steel product.
  • the two large, opposing surfaces of the flat steel product are referred to as the two sides.
  • the narrow surfaces are referred to as the edges.
  • Such a zinc-based corrosion protection coating preferably comprises 0.2-6.0 wt.% Al, 0.1-10.0 wt.% Mg, optionally 0.1-40 wt.% manganese or copper, optionally 0.1-10.0 wt.% cerium, optionally at most 0.2 wt.% other elements, unavoidable impurities, and the remainder zinc.
  • the Al content is a maximum of 2.0 wt.%, preferably a maximum of 1.5 wt.%.
  • the Mg content is in particular a maximum of 3.0 wt.%, preferably a maximum of 1.0 wt.%.
  • the corrosion protection coating can be applied by hot-dip coating, by physical vapor deposition, or by electrolytic processes.
  • a further developed flat steel product preferably has a high uniform elongation Ag of at least 10.0%, in particular at least 11.0%, preferably at least 11.5%, in particular at least 12.0%.
  • the yield strength of a specially designed flat steel product exhibits a continuous curve or only a slight degree of variation.
  • continuous curve means that there is no pronounced yield strength.
  • a yield strength with a continuous curve can also be referred to as a proof strength Rp0.2.
  • Particularly good ageing resistance can be achieved with flat steel products for which the difference ⁇ Re is not more than 25 MPa.
  • a specially developed flat steel product has an elongation at break A80 of at least 15%, in particular at least 18%, preferably at least 19%, particularly preferably at least 20%.
  • the flat steel product has fine precipitates in the structure, particularly in the form of niobium carbonitrides and/or titanium carbonitrides.
  • fine precipitates are defined as all precipitates with a diameter of less than 30 nm.
  • the remaining precipitates are referred to as coarse precipitates.
  • the fine precipitates in the structure are rounded precipitates with a diameter of up to 20 nm.
  • the diameter is at least 2 nm.
  • the diameter is a maximum of 15 nm, in particular a maximum of 12 nm.
  • the flat steel product has a largely fine precipitates in its microstructure.
  • largely fine precipitates means that more than 80%, preferably more than 90%, of all precipitates are fine precipitates. This means that more than 80%, preferably more than 90%, of all precipitates have a diameter of less than 30 nm.
  • the density of the fine precipitates is at least 0.018 per 100 nm 2 , preferably at least 0.020 per 100 nm 2 .
  • the fine precipitates result in a particularly fine microstructure with small grain diameters.
  • This fine microstructure makes the material more homogeneous. This results in improved mechanical properties, particularly reduced crack susceptibility, resulting in improved flexural properties and higher elongation at fracture. This also results in improved toughness with more pronounced necking behavior.
  • the precipitates in the flat steel product and the formed sheet metal part are determined using electron-optical and X-ray images (TEM and EDX) based on carbon extraction replicas (known in the technical literature as "carbon extraction replicas").
  • the carbon extraction replicas are created from longitudinal sections (20 x 30 mm).
  • the measurement resolution is between 10,000 and 200,000 times.
  • the precipitates can be divided into coarse and fine precipitates. Fine precipitates are all precipitates with a diameter of less than 30 nm. The remaining precipitates are referred to as coarse precipitates.
  • the proportion of fine precipitates to the total number of precipitates in the measurement field and the total number of fine precipitates in the measurement field are determined by simple counting. For the fine precipitates, the mean diameter is also calculated using computer-assisted image analysis.
  • the flat steel product is, in particular, further developed such that it has regions of different thicknesses.
  • the method described below for producing a shaped sheet metal part is preferably further developed such that such a flat steel product with regions of different thicknesses is used.
  • the shaped sheet metal part explained below is further developed such that it has regions of different thicknesses.
  • Areas of varying thickness have the advantage of allowing specific areas of the final sheet metal part (see below) to be reinforced. This makes it possible to design those sections subject to particular stresses (e.g., during a crash) with increased rigidity, while making other sections thinner to reduce the weight of the component. The result is a weight-optimized component with targeted reinforcements in areas subject to high stress.
  • the method according to the invention for producing a flat steel product for hot forming with a corrosion protection coating comprises the following steps: a) Providing a slab or a thin slab made of steel which, in addition to iron and unavoidable impurities (in % by weight), consists of C: 0.30 - 0.50%, Si: 0.05 - 0.6%, Mn: 0.5 - 3.0%, Al: 0.10-1.0%, Note: 0.001 - 0.2%, T: 0.001 - 0.10% B: 0.0005 - 0.01% P: ⁇ 0.03%, S: ⁇ 0.02%, N: ⁇ 0.02%, Sn: ⁇ 0.03% Ace: ⁇ 0.01% and optionally one or more of the elements "Cr, Cu, Mo, Ni, V, Ca, W" in the following contents Cr: 0.01 - 1.0%, Cu: 0.01 - 0.2%, Mo: 0.002 - 0.3%, No: 0.01 - 0.5% V: 0.001 - 0.3% Approx: 0.0005
  • a semi-finished product composed according to the alloy specified for the flat steel product according to the invention is provided.
  • This can be a slab produced by conventional continuous slab casting or by thin slab casting.
  • step b) the semi-finished product is thoroughly heated at a temperature (T1) of 1100 - 1400 °C. If the semi-finished product has cooled down after casting, it is first reheated to 1100 - 1400 °C for thorough heating.
  • the thorough heating temperature should at least 1100 °C to ensure good formability for the subsequent rolling process.
  • the soaking temperature should not exceed 1400 °C to avoid the presence of molten phases in the semi-finished product.
  • the semi-finished product is pre-rolled into an intermediate product.
  • Thin slabs are not usually subjected to pre-rolling.
  • Thick slabs to be rolled into hot strip can be pre-rolled if required.
  • the temperature of the intermediate product (T2) at the end of pre-rolling should be at least 1000 °C to ensure that the intermediate product retains sufficient heat for the subsequent finish-rolling step.
  • high rolling temperatures can also promote grain growth during the rolling process, which has a detrimental effect on the mechanical properties of the flat steel product.
  • the temperature of the intermediate product at the end of pre-rolling should not exceed 1200 °C.
  • step d) the slab or thin slab, or if step c) has been performed, the intermediate product, is rolled into a hot-rolled flat steel product.
  • step c) the intermediate product is typically finish-rolled immediately after rough rolling. Typically, finish rolling begins no later than 90 seconds after the end of rough rolling.
  • the slab, the thin slab, or, if step c) has been performed, the intermediate product are rolled to a final rolling temperature (T3).
  • the final rolling temperature i.e., the temperature of the finished hot-rolled flat steel product at the end of the hot rolling process, is 750–1000 °C. At finish rolling temperatures below 750 °C, the amount of free vanadium decreases because larger amounts of vanadium carbides are precipitated.
  • the vanadium carbides precipitated during finish rolling are very large. They typically have an average grain size of 30 nm or more and are not dissolved in subsequent annealing processes, such as those performed prior to hot-dip coating.
  • the final rolling temperature is limited to a maximum of 1000 °C to prevent coarsening of the austenite grains. Furthermore, final rolling temperatures of a maximum of 1000 °C are relevant for the process to achieve coiling temperatures (T4) below 700 °C.
  • Hot rolling of the steel flat product can be performed as continuous hot strip rolling or reversing rolling.
  • step e) provides for an optional coiling of the hot-rolled steel flat product.
  • the hot strip is heated to a coiling temperature (T4) within less than 50 seconds after hot rolling. cooled.
  • the cooling medium used can be water, air, or a combination of both.
  • the coiling temperature (T4) should not exceed 700 °C to avoid the formation of large vanadium carbides. In principle, there is no lower limit on the coiling temperature. However, coiling temperatures of at least 500 °C have proven favorable for cold rolling.
  • the coiled hot strip is then cooled to room temperature in air using the conventional method.
  • step f the hot-rolled flat steel product is optionally descaled in a conventional manner by pickling or by another suitable treatment.
  • the scale-cleaned hot-rolled flat steel product can optionally be subjected to cold rolling before annealing in step g), for example, to meet higher thickness tolerance requirements for the flat steel product.
  • the cold rolling degree (KWG) should be at least 30% to inject sufficient deformation energy into the flat steel product for rapid recrystallization.
  • the flat steel product before cold rolling is usually a hot strip with a hot strip thickness of d.
  • the flat steel product after cold rolling is also commonly referred to as cold strip.
  • the cold rolling degree can, in principle, assume very high values of over 90%. However, cold rolling degrees of no more than 80% have proven to be beneficial in preventing strip breakage.
  • the flat steel product undergoes an annealing treatment at annealing temperatures (T5) of 650–900 °C.
  • T5 annealing temperatures
  • the flat steel product is first heated to the annealing temperature within 10 to 120 seconds and then held at the annealing temperature for 30 to 600 seconds.
  • the annealing temperature is at least 650 °C, preferably at least 720 °C. Annealing temperatures above 900 °C are undesirable for economic reasons.
  • step i) the flat steel product is cooled after annealing to an immersion temperature (T6) to prepare it for subsequent coating treatment.
  • the immersion temperature is lower than the annealing temperature and is adjusted to the temperature of the molten bath.
  • the immersion temperature is 600-800 °C, preferably at least 650 °C, more preferably at least 670 °C, and most preferably at most 700 °C.
  • the cooling time of the annealed flat steel product from the annealing temperature T5 to the immersion temperature T6 is preferably 10 - 180 s.
  • the immersion temperature T6 deviates from the temperature of the molten bath T7 by no more than 30 K, in particular no more than 20 K, preferably no more than 10 K.
  • the flat steel product is subjected to a coating treatment in step j).
  • the coating treatment is preferably carried out by continuous hot-dip coating.
  • the coating can be applied to just one side, both sides, or all sides of the flat steel product.
  • the coating treatment is preferably carried out as a hot-dip coating process, in particular as a continuous process.
  • the flat steel product usually comes into contact with the molten bath on all sides, so that it is coated on all sides.
  • the molten bath which contains the alloy to be applied to the flat steel product in liquid form, typically has a temperature (T7) of 660–800 °C, preferably 680–740 °C.
  • Aluminum-based alloys have proven particularly suitable for coating ageing-resistant flat steel products with a corrosion-protective coating.
  • the molten bath contains up to 15 wt.% Si, preferably more than 1.0%, optionally 2-4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0% wt.% alkali or alkaline earth metals, and optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum.
  • the Si content of the melt is 1.0-3.5 wt.% or 7-12 wt.%, in particular 8-10 wt.%.
  • the optional content of alkali or alkaline earth metals in the melt comprises 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg.
  • the optional content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.
  • a first cooling time t mT in the temperature range between 600 °C and 450 °C is more than 5 s, preferably more than 10 s, in particular more than 14 s
  • a second cooling time t nT in the temperature range between 400 °C and 300 °C is more than 4 s, preferably more than 8 s, in particular more than 12 s.
  • the first cooling time t mT can be achieved in the temperature range between 600 °C and 450 °C (medium temperature range mT) by slow, continuous cooling or by holding the product at a temperature for a certain time within this temperature range. Intermediate heating is even possible.
  • the important thing is that the flat steel product remains in the temperature range between 600 °C and 450 °C for at least the cooling time t mT .
  • this temperature range on the one hand, there is a significant diffusion rate of iron into aluminum and, on the other hand, the diffusion of aluminum into steel is inhibited because the temperature is below half the melting temperature of steel. This allows diffusion of iron into the corrosion protection coating without strong diffusion of aluminum into the steel substrate.
  • the diffusion of iron into the corrosion protection coating has several advantages: Firstly, it delays the melting of the corrosion protection coating during austenitizing prior to press hardening. Secondly, it homogenizes the thermal expansion coefficients of the corrosion protection coating and the substrate. This means that the transition area between the substrate and surface thermal expansion coefficients becomes wider, which reduces thermal stresses during reheating.
  • the diffusion of aluminum into the steel substrate would have significant disadvantages: Due to the very high affinity of aluminum to nitrogen, a high aluminum content can lead to nitrogen dissolving from fine precipitates, such as niobium carbonitrides or titanium carbonitrides, and instead, coarse precipitates, such as aluminum nitrides, forming preferentially at the grain boundaries. These would impair crash performance and reduce the bending angle. Furthermore, this destabilizes the fine precipitates (e.g., the niobium-containing Precipitations) in the uppermost substrate area, which are important for many preferred properties.
  • fine precipitates e.g., the niobium-containing Precipitations
  • the iron concentration in the transition boundary layer increases to such an extent that the activity of aluminum in the coating directly at the substrate boundary is further reduced. This then leads to an even further reduced aluminum uptake into the substrate during austenitization before press hardening, with the associated advantages described above.
  • the second cooling time t nT in the temperature range between 400 °C and 300 °C can also be achieved by slow, continuous cooling or by holding at a temperature within this temperature range for a certain period of time. Intermediate heating is even possible. The only important thing is that the flat steel product remains in the temperature range between 400 °C and 300 °C for at least the cooling time t nT .
  • transition carbides very fine iron carbides (so-called transition carbides) are also formed, which in turn dissolve very quickly during austenitizing and lead to additional austenite nuclei and thus an even finer austenite structure and thus also a hardening structure.
  • the coated flat steel product can optionally be subjected to skin passing with a skin passing degree of up to 2% to improve the surface roughness of the flat steel product.
  • the invention further relates to a sheet metal part formed from a flat steel product comprising a steel substrate as described above and a corrosion protection coating.
  • the corrosion protection coating has the advantage of preventing scale formation during austenitization during hot forming. Furthermore, such a corrosion protection coating protects the formed sheet metal part against corrosion.
  • the sheet metal part preferably comprises an aluminum-based corrosion protection coating.
  • the corrosion protection coating of the sheet metal part preferably comprises an alloy layer and an aluminum base layer.
  • the alloy layer is often also referred to as an interdiffusion layer.
  • the thickness of the alloy layer is preferably less than 30 ⁇ m, particularly preferably less than 20 ⁇ m, in particular less than 16 ⁇ m, and particularly preferably less than 12 ⁇ m.
  • the thickness of the Al base layer results from the difference between the thicknesses of the anti-corrosive coating and the alloy layer.
  • the alloy layer lies directly on the steel substrate.
  • the alloy layer of the sheet metal part preferably consists of 35–90 wt.% Fe, 0.1–10 wt.% Si, optionally up to 0.5 wt.% Mg, and optional additional components, the total contents of which are limited to a maximum of 2.0 wt.%, with the remainder being aluminum. Due to the further diffusion of iron into the alloy layer, the Si and Mg contents are correspondingly lower than their respective contents in the melt of the molten bath.
  • the alloy layer preferably has a ferritic structure.
  • the Al base layer of the sheet metal part lies on top of the alloy layer of the steel component and is directly adjacent to it.
  • the Al base layer of the steel component preferably consists of 35-55 wt.% Fe, 0.4-10 wt.% Si, optionally up to 0.5 wt.% Mg, and optionally further Components whose total contents are limited to a maximum of 2.0% by weight, and the remainder being aluminium.
  • the Al base layer can have a homogeneous element distribution, with local element contents varying by no more than 10%.
  • Preferred variants of the Al base layer have silicon-poor phases and silicon-rich phases. Silicon-poor phases are regions whose average Si content is at least 20% lower than the average Si content of the Al base layer. Silicon-rich phases are regions whose average Si content is at least 20% higher than the average Si content of the Al base layer.
  • the silicon-rich phases are arranged within the silicon-poor phase.
  • the silicon-rich phases form at least a 40% continuous layer bordered by silicon-poor regions.
  • the silicon-rich phases are arranged in island-like patterns within the silicon-poor phase.
  • island-shaped means an arrangement in which discrete, unconnected areas are enclosed by another material - i.e., "islands” of a particular material are located within another material.
  • the steel component comprises an oxide layer arranged on the corrosion protection coating.
  • the oxide layer is located in particular on the Al base layer and preferably forms the outer edge of the corrosion protection coating.
  • the oxide layer of the steel component consists, in particular, of more than 80 wt.% oxides, with the majority of the oxides (i.e., more than 50 wt.% of the oxides) being aluminum oxide.
  • the majority of the oxides i.e., more than 50 wt.% of the oxides
  • aluminum oxide in addition to aluminum oxide, hydroxides and/or magnesium oxide are present in the oxide layer, alone or as a mixture.
  • the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and/or magnesium in metallic form.
  • the oxide layer preferably has a thickness of at least 50 nm, in particular of at least 100 nm. Furthermore, the thickness is a maximum of 4 ⁇ m, in particular a maximum of 2 ⁇ m.
  • the sheet metal part includes a zinc-based anti-corrosion coating.
  • Such a zinc-based corrosion protection coating preferably comprises up to 80 wt.% Fe, 0.2 - 6.0 wt.% Al, 0.1 - 10.0 wt.% Mg, optionally 0.1 - 40 wt.% manganese or copper, optionally 0.1 - 10.0 wt.% cerium, optionally at most 0.2 wt.% other elements, unavoidable impurities, and the remainder zinc.
  • the Al content is a maximum of 2.0 wt.%, preferably a maximum of 1.5 wt.%.
  • the Fe content, which results from diffusion, is preferably more than 20 wt.%, in particular more than 30 wt.
  • the Fe content is in particular a maximum of 70 wt.%, in particular a maximum of 60 wt.%
  • the Mg content is in particular a maximum of 3.0 wt.%, preferably a maximum of 1.0 wt.%.
  • the corrosion protection coating can be applied by hot-dip coating, by physical vapor deposition or by electrolytic processes.
  • the steel substrate of the sheet metal part has a microstructure with at least partially more than 80% martensite and/or lower bainite, preferably at least partially more than 90% martensite and/or lower bainite, in particular at least partially more than 95%, particularly preferably at least partially more than 98%.
  • the steel substrate of the sheet metal part has a microstructure with at least partially more than 80% martensite, preferably at least partially more than 90% martensite, in particular at least partially more than 95%, particularly preferably at least partially more than 98%.
  • “partially having” is to be understood as meaning that there are regions of the sheet metal part that have the mentioned microstructure.
  • the high martensite content allows very high tensile strengths and yield points to be achieved.
  • the former austenite grains of the martensite have an average grain diameter of less than 14 ⁇ m, in particular less than 12 ⁇ m, preferably less than 10 ⁇ m.
  • the fine microstructure makes it more homogeneous. This results in an improvement in the mechanical properties, in particular, a lower crack susceptibility and thus improved bending properties and higher elongation at fracture.
  • the sheet metal part has at least partially a yield strength of at least 950 MPa, in particular at least 1100 MPa, in particular at least 1200 MPa, preferably at least 1500 MPa, particularly preferably at least 1400 MPa, in particular at least 1500 MPa.
  • the sheet metal part has at least partially a tensile strength of at least 1000 MPa, in particular at least 1100 MPa, preferably at least 1500 MPa, preferably at least 1400 MPa, in particular at least 1600 MPa, preferably 1700 MPa, particularly preferably 1800 MPa.
  • the sheet metal part has at least partially an elongation at break A80 of at least 3.5%, in particular at least 4%, in particular at least 4.5%, preferably at least 5%, particularly preferably at least 6%.
  • the sheet metal part can at least partially have a bending angle of at least 30°, in particular at least 40°, particularly preferably at least 45°, particularly preferably at least 50°.
  • the bending angle here is understood to be the bending angle corrected for the sheet thickness.
  • partially exhibit means that there are areas of the sheet metal part that exhibit the stated mechanical property. In addition, there may also be areas of the sheet metal part whose mechanical properties are below the limit value. The sheet metal part therefore exhibits the stated mechanical property in sections or regions. This is because different areas of the sheet metal part can undergo different heat treatments. For example, individual areas can be cooled more quickly than others, which means that more martensite, for example, forms in the faster-cooled areas. This means that different mechanical properties also arise in the different areas. The same applies to the Vickers hardness explained below.
  • the sheet metal part has, at least in part, a yield strength ratio (ratio of yield strength to tensile strength) of at least 60% and at most 85%.
  • the yield strength ratio is at least 65%, in particular at least 70%.
  • the sheet metal part has fine precipitations in the structure, in particular in the form of niobium carbonitrides and/or titanium carbonitrides.
  • fine precipitates are defined as all precipitates with a diameter of less than 30 nm.
  • the remaining precipitates are referred to as coarse precipitates.
  • the average diameter of the fine precipitates is a maximum of 11 nm, preferably a maximum of 10 nm, in particular a maximum of 8 nm, preferably a maximum of 6 nm.
  • the sheet metal part has largely fine precipitates in its structure.
  • largely fine precipitates means that more than 80%, preferably more than 90%, of all precipitates are fine precipitates. This means that more than 80%, preferably more than 90%, of all precipitates have a diameter of less than 30 nm.
  • the fine precipitates result in a particularly fine microstructure with small grain diameters.
  • This fine microstructure makes the material more homogeneous. This results in improved mechanical properties, particularly reduced crack susceptibility, resulting in improved flexural properties and higher elongation at fracture. This also results in improved toughness with more pronounced necking behavior.
  • the Vickers hardness is qualitatively the resistance to penetration of a test specimen and thus the resistance to plastic deformation.
  • the characterization by means of Vickers hardness has the The advantage is that the Vickers hardness can also be determined for smaller component sections. This allows for the targeted examination of individual areas of the component where tensile tests are not possible due to their geometry (e.g., curved workpieces or areas with varying sheet thickness).
  • Vickers hardness is determined according to DIN EN ISO 6507 (2018.07).
  • the value "1" refers to the test force in kiloponds (kp), i.e., 1 kp in this case.
  • kp kiloponds
  • the actual mechanical properties of the sheet metal part are determined by first cathodically coating the part with dip paint or subjecting it to an analogous heat treatment.
  • Cathodic dip painting is generally carried out for corresponding components in the automotive industry.
  • the components are first coated in an aqueous solution. This coating is then baked in a heat treatment.
  • the sheet metal parts are heated to 170°C and held at this temperature for 20 minutes.
  • the components are then cooled to room temperature in ambient air.
  • the mechanical properties are to be understood as being present on a component with a cathodic dip coating or on a component that, after forming, was subjected to a heat treatment analogous to a cathodic dip coating.
  • the heat treatment of cathodic dip coating varies slightly. Temperatures of 165°C–180°C and holding times of 12–30 minutes are common. However, the changes in mechanical properties due to these variations (165°C–180°C; 12–30 minutes) are negligible.
  • the sheet metal part comprises a cathodic dip coating.
  • a further developed variant of the sheet metal part is characterized in that the corrosion protection coating is an aluminum-based corrosion protection coating and the sheet metal part comprises an alloy layer and an Al base layer.
  • the Nb content in the alloy layer is greater than 0.010 wt.%, preferably greater than 0.015 wt.%, in particular greater than 0.018 wt.%.
  • the sheet metal part according to the invention is preferably a component for a land vehicle, marine vehicle, or aircraft. It is particularly preferably an automotive part, in particular a body part.
  • the component is preferably a B-pillar, side member, A-pillar, sill, or cross member.
  • a blank which consists of a steel suitably composed in accordance with the above explanations (working step a)), which is then heated in a manner known per se such that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einlg of the blank when inserted into a forming tool intended for hot press forming (working step c)) is at least partially above Ms+100°C, in particular above Ms+300°C.
  • the temperature T Einlg of the blank during insertion at least partially exceeds 600°C.
  • the temperature T Einlg of the blank during insertion is at least partially, in particular completely, in the range 600°C to 850°C, in order to ensure good formability and sufficient hardenability.
  • partially exceeding a temperature means that at least 30%, in particular at least 60%, of the volume of the blank, preferably the entire blank, exceeds a corresponding temperature.
  • at least 30% of the blank has an austenitic microstructure, i.e.
  • the transformation from a ferritic to an austenitic microstructure does not have to be complete when it is placed in the forming tool.
  • up to 70% of the volume of the blank when it is placed in the forming tool can consist of other microstructure components, such as tempered bainite, tempered martensite and/or non- or partially recrystallized ferrite.
  • certain areas of the blank can be kept at a lower temperature than others during heating.
  • the heat can be specifically directed only at certain sections of the blank, or the parts that are to be heated less can be shielded from the heat supply.
  • Maximum strength properties of the obtained sheet metal part can be achieved by ensuring that the temperature reached at least partially in the sheet metal blank is between Ac3 and 1000 °C, preferably between 850 °C and 950 °C.
  • An optimally uniform distribution of properties can be achieved by heating the blank completely in step b).
  • the average heating rate r furnace of the sheet blank during heating in step b) is at least 5 K/s, preferably at least 5 K/s, in particular at least 6 K/s, preferably at least 8 K/s, in particular at least 10 K/s, preferably at least 15 K/s.
  • the average heating rate r furnace is to be understood as the average heating rate from 30°C to 700°C.
  • the standardized average heating ⁇ norm is at least 5 Kmm/s, in particular at least 8 Kmm/s, preferably at least 10 Kmm/s.
  • the maximum standardized average heating is 15 Kmm/s, in particular a maximum of 14 Kmm/s, preferably a maximum of 15 Kmm/s.
  • the average heating ⁇ is the product of the average heating rate in Kelvin per second from 30 °C to 700 °C and the sheet thickness in millimeters.
  • the heating takes place in a furnace with a furnace temperature T furnace of at least Ac3 + 10 K, preferably at least 850 °C, preferably at least 880 °C, particularly preferably at least 900 °C, in particular at least 920 °C, and at most 1000 °C, preferably at most 950 °C, particularly preferably at most 950 °C.
  • the dew point of the furnace atmosphere in the furnace is preferably at least -20 °C, preferably at least -15 °C, in particular at least -5 °C, particularly preferably at least 0 °C and at most +25 °C, preferably at most +20 °C, in particular at most +15 °C.
  • the heating in step b) is carried out stepwise in areas with different temperatures.
  • the heating takes place in a roller hearth furnace with different heating zones.
  • the heating takes place in a first heating zone with a temperature (so-called furnace inlet temperature) of at least 650 °C, preferably at least 680 °C, in particular at least 720 °C.
  • the maximum temperature in the first heating zone is preferably 900 °C, in particular a maximum of 850 °C.
  • the maximum temperature of all heating zones in the furnace is preferably a maximum of 1200 °C, in particular a maximum of 1000 °C, preferably a maximum of 950 °C, particularly preferably a maximum of 950 °C.
  • the total time in the furnace t furnace which is made up of a heating time and a holding time, is preferably at least 2 minutes, in particular at least 5 minutes, preferably at least 4 minutes for both variants (constant furnace temperature, step-by-step heating). Furthermore, the total time in the furnace for both variants is preferably a maximum of 20 minutes, in particular a maximum of 15 minutes, preferably a maximum of 12 minutes, in particular a maximum of 8 minutes. Longer total times in the furnace have the advantage of ensuring uniform austenitization of the sheet metal blank. On the other hand, holding for too long above Ac3 leads to grain coarsening, which has a negative effect on the mechanical properties.
  • the blank heated in this way is removed from the respective heating device, which can be, for example, a conventional heating furnace, an equally known induction heating device or a conventional device for keeping steel components hot, and transported into the forming tool so quickly that its temperature upon arrival in the tool is at least partially above Ms+100°C, in particular above Ms+300°C, preferably above 600°C, in particular above 650°C, particularly preferably above 700°C.
  • Ms here denotes the martensite start temperature.
  • the temperature is at least partially above the AC1 temperature.
  • the temperature is in particular a maximum of 900°C.
  • step c) the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably a maximum of 20 seconds, in particular within a maximum of 15 seconds. Such rapid transport is necessary to avoid excessive cooling prior to forming.
  • the tool When inserting the blank, the tool typically has a temperature between room temperature (RT) and 200 °C, preferably between 20 °C and 180 °C, in particular between 50 °C and 150 °C.
  • the tool can also have a temperature slightly below room temperature when inserting the blank, for example if the cooling water used is slightly colder (e.g. 15°C).
  • the tool therefore has a temperature of between 10°C and 200°C when the blank is inserted.
  • the tool can be tempered at least in some areas to a temperature TWZ of at least 200°C, in particular at least 300°C, in order to only partially harden the component.
  • the tool temperature t WZ is preferably a maximum of 600°C, in particular a maximum of 550°C. It only needs to be ensured that the tool temperature twz is below the desired target temperature T Ziel .
  • the residence time in the tool twz is preferably at least 2 s, in particular at least 5 s, particularly preferably at least 5 s.
  • the maximum residence time in the tool is preferably 25 s, in particular a maximum of 20 s, preferably a maximum of 10 s.
  • the target temperature T target of the sheet metal part is at least partially below 400°C, preferably below 300°C, in particular below 250°C, preferably below 200°C, particularly preferably below 180°C, in particular below 150°C.
  • the target temperature T target of the sheet metal part is particularly preferably below Ms-50°C, where Ms denotes the martensite start temperature.
  • the target temperature of the sheet metal part is preferably at least 20°C, particularly preferably at least 50°C.
  • AC1[°C] (739 wt% - 22*%C - 7*%Mn + 2*%Si + 14*%Cr + 13*%Mo - 13*%Ni + 20*%V )[°C/wt%]
  • AC3[°C] (902 wt% - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55*%V)[°C/wt%] to be calculated, where %C denotes the C content, %Si the Si content, %Mn the Mn content, %Cr the Cr content, %Mo the Mo content, %Ni the Ni content and +%V the vanadium content of the respective steel ( Brandis H 1975 TEW-Techn. Ber. 1 8-10 ).
  • the blank is not only formed into the sheet metal part, but simultaneously quenched to the target temperature.
  • the cooling rate in the tool to the target temperature is in particular at least 20 K/s, preferably at least 30 K/s, in particular at least 50 K/s, and in special designs at least 100 K/s.
  • the sheet metal part is cooled to a cooling temperature T AB of less than 100 °C within a cooling time t AB of 0.5 to 600 s. This is usually done by air cooling.
  • Figure 1 shows a grain representation of the reconstructed austenite.
  • the slabs were first rough-rolled to an intermediate product with a thickness of 40 mm, whereby the intermediate products, which in hot strip rolling can also be referred to as pre-strips, each had an intermediate product temperature T2 at the end of the rough-rolling phase.
  • the pre-strips were fed to the finish rolling immediately after rough-rolling so that the intermediate product temperature T2 corresponds to the initial rolling temperature for the finish rolling phase.
  • the pre-strips were rolled out to hot strips with a final thickness of 5 - 7 mm and the respective final rolling temperatures T3 given in Table 2 , cooled to the respective coiler temperature and wound into coils at the respective coiler temperature T4 and then cooled in still air.
  • the hot-rolled strips were descaled conventionally by pickling before being subjected to cold rolling with the cold rolling grades specified in Table 2.
  • the cold-rolled flat steel products were heated in a continuous annealing furnace to a respective annealing temperature T5 and held at annealing temperature for 100 s each before being cooled to their respective immersion temperature T6 at a cooling rate of 1 K/s.
  • the cold-rolled strips were passed through a molten coating bath at temperature T7 at their respective immersion temperature T6.
  • the composition of the coating bath is given in Table 3.
  • the coated strips were blown off conventionally, creating overlays with varying layer thicknesses (see Table 3).
  • the strips were first cooled to 600 °C at an average cooling rate of 10-15 K/s.
  • the strips were cooled for the cooling times T mT and T nT specified in Table 2. Between 450 °C and 400 °C and below 220 °C, the strips were cooled at a cooling rate of 5 - 15 K/s.
  • Table 4 shows which steel variant (see Table 1) was combined with which process variant (see Table 2) and which coating (see Table 3) .
  • the thickness of the steel strips produced was between 1.4 mm and 1.7 mm in all tests.
  • the following material properties were determined during the tensile test: the type of yield point, which is designated Re for a pronounced yield point and Rp for a continuous yield point, and for a continuous yield point the value for the proof stress Rp0.2, for a pronounced yield point the values for the lower yield point ReL, the upper yield point ReH and the difference between the upper and lower yield points ⁇ Re, the tensile strength Rm, the uniform elongation Ag and the elongation at break A80. All specimens exhibit a continuous yield strength Rp and a uniform elongation Ag of at least 11.5%. Therefore, the yield strength Rp0.2 is specified for all specimens.
  • Table 4 also lists the properties of the fine precipitates in the microstructure of the flat steel product.
  • These precipitates are niobium carbonitride and titanium carbonitride, both of which contribute to grain refinement.
  • the precipitates are determined using electron-optical and X-ray images (TEM and EDX) based on carbon extraction replicas (known in the technical literature as "carbon extraction replicas").
  • the carbon extraction replicas were created from longitudinal sections (20 x 30 mm). The magnification during the measurement ranges between 10,000 and 200,000 times. Based on these images, the precipitates can be divided into coarse and fine precipitates. All precipitates with a diameter of less than 30 nm are referred to as fine precipitates. The remaining precipitates are referred to as coarse precipitates.
  • the proportion of fine precipitates to the total number of precipitates in the measurement field is determined by simple counting.
  • the average diameter of the fine precipitates is also calculated using computer-assisted image analysis. In the samples according to the invention, the proportion of fine precipitates is more than 90%.
  • the average diameter of the fine precipitates is also less than 12 nm.
  • Blanks were cut from the steel strips produced in this way and used for further tests.
  • sheet metal part samples 1 - 8 in the form of 200 x 300 mm 2 plates were hot-pressed from the respective blanks.
  • the blanks were heated in a heating device, for example, in a conventional heating furnace, from room temperature at an average heating rate r furnace (between 30 °C and 700 °C) in a furnace with a furnace temperature T furnace .
  • the total duration in the furnace, which includes heating and holding, is designated t furnace .
  • the dew point of the furnace atmosphere was -5 °C in all cases.
  • the blanks were then removed from the heating device and placed in a forming tool having a temperature T wz .
  • the transfer time t trans comprising removal from the heating device, transport to the tool, and insertion into the tool, was between 5 and 14 s.
  • the temperature T einlg of the blanks upon insertion into the forming tool was above the respective martensite start temperature +100 °C in all cases.
  • the blanks were formed into the respective sheet metal parts in the forming tool, with the sheet metal parts being cooled in the tool at a cooling rate rwz.
  • the residence time in the tool is designated twz.
  • Table 5 shows the parameters mentioned for different variants, where "RT" abbreviates room temperature.
  • Table 5 shows very different variants for the forming process. While variant II, for example, results in almost complete formation of a martensitic microstructure, the comparatively slow cooling of variant X with the high tool temperature T WZ leads to a modified microstructure formation with high ferrite contents, which results in a higher elongation at fracture A80.
  • Table 6 summarizes the overall results for the obtained sheet metal parts.
  • the first columns indicate the sample number, the steel grade according to Table 1, the process variant according to Table 2, the coating according to Table 2 , and the hot forming variant according to Table 5.
  • the following columns show the yield strength Rp02, the tensile strength Rm, the ratio of yield strength to tensile strength (yield strength ratio), and the elongation at break A80. These values were determined according to DIN EN ISO 6892-1 specimen shape 2 (Appendix B Table B1) on specimens transverse to the rolling direction. The determined bending angle was determined according to VDA standard 238-100 with a bending axis transverse to the rolling direction.
  • the determined bending angle is calculated from the punch travel using the formula specified in the standard (the determined bending angle (also referred to as the maximum bending angle) is the bending angle at which the force in the bending test is at its maximum).
  • the determined bending angle is given in Table 7. To determine the corrected bending angle, these numerical values must be multiplied by the square root of the sheet thickness, which is given in Table 4. Table 7 also shows the Vickers hardness HV1. This was determined in accordance with DIN EN ISO 6507 (2018.07).
  • the mechanical properties in Table 6 were determined after a cathodic dip coating was applied to the formed sheet metal part. During this coating process, the sheet metal parts were heated to 170 °C and held at this temperature for 20 minutes. The components were then cooled to room temperature in ambient air.
  • microstructural properties of the sheet metal part are listed in Table 7.
  • the microstructural fractions are given in area %. All examples according to the invention have a martensite content of more than 90%.
  • Table 7 also lists the properties of the fine precipitates in the microstructure. These precipitates are niobium carbonitride and titanium carbonitride, both of which contribute to grain refinement.
  • the precipitates are determined using electron-optical and X-ray images (TEM and EDX) based on carbon extraction replicas (known in the literature as "carbon extraction replicas"). The carbon extraction replicas were created from longitudinal sections (20 x 30 mm). The magnification during the measurement ranges between 10,000 and 200,000 times. Based on these images, the precipitates can be divided into coarse and fine precipitates. All precipitates with a diameter of less than 30 nm are referred to as fine precipitates. The remaining precipitates are referred to as coarse precipitates.
  • the proportion of fine precipitates to the total number of precipitates in the measurement field is determined by simple counting.
  • the average diameter of the fine precipitates is also calculated using computer-assisted image analysis.
  • the proportion of fine precipitates is more than 90%.
  • the average diameter of the fine precipitates is also less than 11 nm.
  • Figure 1 a corresponding reconstruction of the austenite.
  • the average diameter of the former austenite grains is 7.5 ⁇ m.
  • the average grain diameter of the former austenite grains is below 14 ⁇ m.

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WO2019223854A1 (fr) 2018-05-22 2019-11-28 Thyssenkrupp Steel Europe Ag Pièce façonnée en tôle composée d'acier et présentant une résistance élevée à la traction, et procédé de fabrication de ladite pièce

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WO2019223854A1 (fr) 2018-05-22 2019-11-28 Thyssenkrupp Steel Europe Ag Pièce façonnée en tôle composée d'acier et présentant une résistance élevée à la traction, et procédé de fabrication de ladite pièce

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EP4388141A1 (fr) 2024-06-26
WO2023020932A1 (fr) 2023-02-23
EP4388141C0 (fr) 2024-12-18
EP4520848A3 (fr) 2025-05-21
US20240352550A1 (en) 2024-10-24

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