US20230151468A1 - Hot-Rolled Flat Steel Product and Method for the Production Thereof - Google Patents

Hot-Rolled Flat Steel Product and Method for the Production Thereof Download PDF

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Publication number
US20230151468A1
US20230151468A1 US17/920,533 US202017920533A US2023151468A1 US 20230151468 A1 US20230151468 A1 US 20230151468A1 US 202017920533 A US202017920533 A US 202017920533A US 2023151468 A1 US2023151468 A1 US 2023151468A1
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Prior art keywords
hot
flat steel
steel
mass
steel product
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US17/920,533
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English (en)
Inventor
Nicholas Winzer
Ekaterina Bocharova
Roland Sebald
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ThyssenKrupp Steel Europe AG
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ThyssenKrupp Steel Europe AG
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Assigned to THYSSENKRUPP STEEL EUROPE AG reassignment THYSSENKRUPP STEEL EUROPE AG ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: SEBALD, ROLAND, BOCHAROVA, EKATERINA, WINZER, Nicholas
Publication of US20230151468A1 publication Critical patent/US20230151468A1/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/46—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting
    • B21B1/463—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting in a continuous process, i.e. the cast not being cut before rolling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B21B3/02—Rolling special iron alloys, e.g. stainless steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
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    • 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
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • 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
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    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • 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
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    • 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
    • C—CHEMISTRY; METALLURGY
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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
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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
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    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
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    • 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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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium

Definitions

  • the invention relates to a hot-rolled flat steel product comprising a steel substrate and a corrosion protection layer applied thereto by hot-dip coating on zinc.
  • the invention relates to a method for producing such a flat steel product.
  • Flat steel products are understood in the present text as rolled products of which the length and width are each significantly greater than their thickness. These include in particular steel strips and steel sheets.
  • impurities of a steel, zinc or other alloy refers to technically unavoidable materials accompanying the steel, which can enter the steel during production or cannot be removed completely therefrom, but the contents of which are in any case so small that they have no influence on the properties of the steel.
  • the image analysis for the quantitative determination of structure takes place optically by means of light optical microscopy (“LOM”) with 200 to 2000 times and with a scanning electron microscope (“SEM”) with 2000 to 20,000 times magnification.
  • LOM light optical microscopy
  • SEM scanning electron microscope
  • the distribution of manganese (Mn) in the structure of the steel substrate of a flat steel product according to the invention has been determined by wavelength-dispersive X-ray microrange analysis (WDX) of the structure, which has been described, for example, by Reimer L. (1998) in “Elemental Analysis and Imaging with X-Rays” appearing in Scanning Electron Microscopy, Springer Series in Optical Sciences, vol. 45, Springer, Berlin, Heidelberg.
  • WDX wavelength-dispersive X-ray microrange analysis
  • High-load passenger car and truck components such as crash structures and chassis of automobile bodies, require a hot-dip galvanized steel sheet with a thickness of more than 1.5 mm and a tensile strength of more than 590 MPa.
  • CP-W complex phase steels
  • Dual-phase steels which consist of a combination of hard (e.g., martensite or bainite) and soft (e.g., ferrite) phases, are suitable for complex components due to their combination of high strength and good deformability.
  • hard e.g., martensite or bainite
  • soft e.g., ferrite
  • DP-K cold-rolled dual-phase steels
  • the maximum sheet thickness of hot-dip galvanized DP-K steels is generally limited to 2 mm.
  • DP-W hot-rolled dual-phase steels
  • One possibility would be to perform annealing and then galvanizing of a hot-rolled strip in a hot-dip galvanizing line with an annealing cycle typical for DP-K (i.e., partial austenitization in the intercritical temperature range, i.e., in the temperature range lying between the Ac1 and Ac3 temperatures of the respective steel, where ⁇ - and ⁇ -Fe are produced in equilibrium).
  • This procedure is similar to the manufacturing process of a DP-K steel except for the cold-rolling step.
  • High-strength multi-phase steel with minimum tensile strengths of 580 MPa is known from DE 10 2012 013 113 A1.
  • the steel should preferably have a dual-phase structure and make it possible to produce cold-rolled or hot-rolled steel strips with improved forming properties, from which, in particular, parts for lightweight vehicle construction can be produced.
  • the known multi-phase steel consists of, in % by mass, 0.075% ⁇ C ⁇ 0.105%, 0.600% ⁇ Si ⁇ 0.800%, 1.000% ⁇ Mn ⁇ 2.250%, 0.280% ⁇ Cr ⁇ 0.480%, 0.010% ⁇ Al ⁇ 0.060%, ⁇ 0.020% P, ⁇ 0.0100% N, ⁇ 0.0150% S and the remainder consisting of iron and impurities.
  • a further high-strength multi-phase steel with a minimum tensile strength of 580 MPa is the steel known from DE 10 2012 006 017 A1.
  • parts for lightweight vehicle construction are to be formed from such steel strips in particular.
  • the known steel consists of, in % by mass, 0.075% ⁇ C ⁇ 0.105%, 0.200% ⁇ Si ⁇ 0.300%, 1.000% ⁇ Mn ⁇ 2.000%, 0.280% ⁇ Cr ⁇ 0.480%, 0.010% ⁇ Al ⁇ 0.060%, up to 0.020% P, 0.005% ⁇ Nb ⁇ 0.025%, up to 0.0100% N, up to 0.0050% S, and the remainder consisting of iron and technically unavoidable impurities.
  • the steel known from DE 10 2013 013 067 A1 is also among the type of known multi-phase steels explained above, which preferably have a dual-phase structure and are intended to be suitable for cold-rolled or hot-rolled steel strip with improved forming properties.
  • This known steel should have a yield-to-tensile ratio of not more than 73% and, in % by mass, 0.075% ⁇ C ⁇ 0.105%, 0.600% ⁇ Si ⁇ 0.800%, 1.000% ⁇ Mn ⁇ 1.900%, 0.100% ⁇ Cr ⁇ 0.700%, 0.010% ⁇ Al ⁇ 0.060%, 0.0020% ⁇ N ⁇ 0.0120%, ⁇ 0.0030% S, 0.005% ⁇ Nb ⁇ 0.050%, 0.005% ⁇ Ti ⁇ 0.050%, 0.0005% ⁇ B ⁇ 0.0040%, ⁇ 0.200% Mo, ⁇ 0.040% Cu, ⁇ 0.040% Ni and the remainder consisting of iron and unavoidable impurities.
  • the object arises to develop a flat steel product that not only has optimized mechanical properties, but is also particularly suitable for application of a Zn-based corrosion protection layer by hot-dip coating.
  • the invention has achieved this object by a flat steel product that comprises a steel substrate at least 1.5 mm thick, which consists of, in % by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V: 0.005% ⁇ %Ti+%V ⁇ 0.15%, and optionally one element or a plurality of the elements from the group “Al, Cr, Mo, B” with the specification that their contents, if present, are dimensioned as follows: Al: 0.01-1.5%, Cr and Mo, wherein the following applies for the sum of the contents %Cr+%Mo of Cr and Mo: 0.02 ⁇ %Mo+%Cr ⁇ 1.4%, B: 0.0005-0.005%, and the remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less
  • a corrosion protection layer based on zinc is applied to at least one of the surfaces by hot-dip coating
  • the invention should specify a method with which the production of flat steel products obtained according to the invention is reliably achieved.
  • a hot-rolled steel substrate in the form of a steel strip in at least the following sub-steps: A.1) melting a steel melt, which consists of, in % by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V: 0.005% ⁇ %Ti+%V ⁇ 0.15%, and optionally one element or a plurality of the elements from the group “Al, Cr, Mo, B” with the specification that their contents, if present, are dimensioned as follows: Al: 0.01-1.5%, Cr and Mo, wherein the following applies for the sum of the contents %Cr+%Mo of Cr and Mo: 0.02 ⁇ %Mo+%Cr ⁇ 1.4%, B: 0.0005-0.005%, and the remainder consisting of iron and unavoidable impurities, wherein the
  • A.4 hot-rolling the preliminary product to form a hot-rolled steel strip, wherein the final temperature of the hot rolling is at least 840-980° C. and the thickness of the hot-rolled steel strip is 1.5-10 mm; A.5) cooling the hot-rolled steel strip to a coiling temperature that is 510-640° C.; and A.6) coiling the hot-rolled steel strip cooled to the coiling temperature.
  • the steel substrate which is present in the form of a hot-rolled steel strip, is coated with a corrosion protection coating based on zinc in at least the following sub-steps, which are passed through continuously: B.1) optional pickling of the hot-rolled steel strip; B.2) heating the hot-rolled steel strip with a heating rate of 0.5-100° C./s to an annealing temperature of 750-950° C. and holding the hot-rolled steel strip at the annealing temperature over an annealing period of 10-1000 s;
  • B.3 cooling the hot-rolled steel strip at a cooling rate of 0.5-100° C./s to a bath entry temperature BET, for which BT ⁇ BET ⁇ (BT+20° C.) applies, wherein the temperature of the zinc melt bath is referred to as BT, which is 450-480° C.;
  • B.4 passing the hot-rolled steel strip cooled down to the bath entry temperature BET through the zinc melt bath, which consists of up to 5% by mass Mg, up to 10% by mass Al, the remainder of Zn and unavoidable impurities; B.5) cooling the obtained flat steel product with a cooling rate of 0.5-100° C./s; and B.6) optional skin-pass rolling of the flat steel product with a degree of skin passing of 0.3-2.0%.
  • the invention thus provides a hot-rolled flat steel product that comprises a steel substrate and a corrosion protection layer based on zinc (Zn) applied thereto by hot-dip coating.
  • the steel of the steel substrate of a flat steel product according to the invention consists of, in % by mass,
  • the steel substrate of a flat steel product according to the invention is at least 1.5 mm thick and has a structure that consists of, in % by area, in total 50-90% ferrite and bainitic ferrite, 5-50% martensite, 2-15% residual austenite and up to 10% other structural constituents that are unavoidable due to production.
  • a flat steel product according to the invention has a yield point Rp0.2 of at least 290 MPa, a tensile strength Rm of at least 490 MPa and an elongation at break A80 that is determined by the following formula (1):
  • a 80[%] B ⁇ Rm/ 37 where 31 ⁇ B ⁇ 51.
  • a flat steel product according to the invention can be produced by passing through at least the following work steps:
  • a preheating temperature of at least 1150° C. is necessary in work step A.1 in order to completely homogenize the structure of the preliminary product.
  • the microstructure of the preliminary product would be inherited by the hot strip subsequently produced, so that the Mn segregations desired according to the invention could not be formed.
  • the alloying elements would be bound in deposits, so that their effects on the mechanical properties of a flat steel product according to the invention could not develop.
  • a hot-rolling end temperature of at least 840° C. is required to be able to roll the preliminary product alloyed according to the invention in a reliable manner to form a hot-rolled steel strip.
  • the rolling forces would be too high and, as a result, the risk of damage to the rolls of the roll stands used for hot rolling would increase disproportionately.
  • a hot-rolling end temperature of at least 880° C. can be provided.
  • the hot-rolling end temperature should not exceed 980° C., since hot-rolling end temperatures lying above this upper limit cannot be realized in practice.
  • the hot-rolled steel strip according to the invention must be at least 1.5 mm thick, so that the Mn segregations desired according to the invention can form in the structure after hot rolling. With smaller strip thicknesses, the hot-rolled steel strip would experience excessively strong deformations during hot rolling, which in turn would result in an undesired homogenization of the Mn distribution in the structure of the hot-rolled steel strip.
  • a steel strip with a thickness of more than 10 mm cannot be used for the intended use. Therefore, the maximum strip thickness is limited to 10 mm.
  • the coiling temperature at which the hot-rolled steel strip, which forms the steel substrate of the flat steel product according to the invention, is coiled is at least 510° C. in order to secure the formation of Mn segregations during the cooling of the hot-rolled steel strip in the coil.
  • Higher coiling temperatures can promote this process, such that coiling temperatures of at least 530° C., in particular at least 550° C., are particularly advantageous.
  • An excessively high coiling temperature would trigger the risk of pronounced grain boundary oxidation.
  • the coiling temperature is limited to 640° C., preferably 620° C.
  • the hot-rolled steel strip can, if necessary, be pickled in a conventional manner, in order to remove scale present on the steel strip or to prepare the surface of the steel strip for the work steps carried out below.
  • the hot-rolled steel strip is first heated to an annealing temperature at a heating rate of 0.5-100° C. per second in a pre-heating stage.
  • the heating rate must lie within this window in order to ensure sufficient conversion of the structure, in particular its complete recrystallization.
  • an annealing temperature 750-950° C. and a holding time of 10-1000 seconds are required.
  • the structure would not crystallize completely with the result that, during the subsequent cooling, insufficient austenite would be available to form the desired martensite proportion of the structure.
  • An unrecrystallized steel substrate would also result in a pronounced anisotropy of the mechanical properties of a flat steel product according to the invention.
  • Cooling from the annealing temperature to the zinc bath entry temperature BET likewise takes place at a cooling rate of 0.5 to 100° C. per second.
  • the bath entry temperature BET is at least equal to and at most 20° C. higher than the melt bath temperature, in order to prevent the melt bath temperature from changing substantially by the entry of the hot-rolled steel strip.
  • a further heat treatment can follow the hot-dip coating, with which the hot-dip coated flat steel product is heated up to 550° C., in order to burn in the previously applied corrosion protection layer.
  • the flat steel product obtained is cooled to room temperature at a cooling rate of 0.5-100° C./s.
  • the flat steel product thus produced can optionally be subjected to a conventional skin-pass rolling, in order to optimize its dimensional accuracy and surface properties.
  • the degree of skin passing set here is typically at least 0.3% and at most 2.0%, wherein degrees of skin passing of at least 0.5% have proven to be particularly practical.
  • a degree of skin passing of less than 0.3% leads to a lower surface roughness of the corrosion protection layer, which would have a negative influence on the formability of the flat steel product.
  • the yield point Rp0.2 is increased and the elongation at break A80 is reduced, so that an elongation at break according to formula 1 could not be achieved.
  • a flat steel product that comprises a steel substrate that is alloyed according to the invention and has a structure according to the invention achieves high elongation at break values in the hot-rolled state, which are comparable with the elongations at break A80, which conventionally cold-rolled flat steel products of the type explained at the outset have (“DP-K steels”), which have similar strengths.
  • elongation at break values A80 can regularly be achieved, for which the parameter B in formula (1) is at least in the range 31-51, preferably 36-46.
  • the combination of high strength and high elongation at break values results from the proportion of 2-15% by area residual austenite present in the steel substrate of a flat steel product according to the invention, wherein residual austenite proportions of at least 5% by area are regularly present in the structure of the steel substrate of a flat steel product according to the invention and have a positive effect on the mechanical properties of the flat steel product.
  • the residual austenite contents that can be established in the flat steel product according to the invention are thus significantly higher than with a cold-rolled flat steel product with a comparable alloy.
  • the presence of larger residual austenite proportions in the structure is a result of the inheritance of Mn segregations that are present in the steel substrate, hot-rolled according to the invention, of a flat steel product according to the invention and that are maintained via the annealing treatment, which the flat steel product passes through for its hot-dip coating. It could thus be shown that, in the manner according to the invention of producing a flat steel product according to the invention after coiling (sub-step A.6 of the method according to the invention) and before hot-dip coating (work step B of the method according to the invention), the hot-rolled steel substrate has a highly anisotropic and inhomogeneous structure with a high pearlite content, which is present in line form.
  • Wavelength-dispersive X-ray microrange analyses of the structure result in the fact that Mn in the pearlite lines is segregated and the Mn segregations are present in a highly anisotropic and inhomogeneous distribution after coiling and before hot-dip coating.
  • the steel substrate of a flat steel product according to the invention passes through an annealing (sub-step B.2 of the method according to the invention) before entry into the melt bath, during which it is kept at the annealing temperature over a period of time.
  • the annealing temperature and the annealing duration are coordinated with one another in such a manner that there is no redistribution of the Mn segregations.
  • both the conversion temperature and the residual austenite content after cooling are distributed in a more inhomogeneous manner in comparison to hot-rolled flat steel products that were coiled at lower temperatures in deviation from the specification of the invention.
  • the structural regions of the steel substrate in which there is a higher Mn concentration transform more easily and thus retain more austenite after cooling than the structural regions in which a lower Mn concentration is present. They convert at higher temperatures or not at all, whereby a higher proportion of the original ferrite is maintained there.
  • the inhomogeneity of the Mn distribution in the steel substrate of a fully processed flat steel product according to the invention can be quantified by the total surface proportion of the structure of the steel substrate in which an Mn concentration (in % by mass) is present which is more than 15% higher than the average value of the Mn concentrations in the entire structure of the flat steel product.
  • the sum of the surface proportions of the structure of the steel substrate of a flat steel product according to the invention which have an Mn concentration that is more than 15% higher than the average value of the Mn concentration in the entire structure is referred to as “X.”
  • X is at least 10%, in particular at least 12%, advantageously at least 15% of the total structure.
  • the surface proportions forming the sum X can be evaluated using a WDX measurement, wherein typically the Mn concentration is determined over a measurement surface of at least 200 ⁇ 200 ⁇ m with a step size of 0.5 ⁇ m.
  • the steel of the steel substrate of a flat steel product according to the invention present in the course of the production according to the invention as a hot-rolled steel strip is composed as follows:
  • Carbon (C) is present in the steel substrate of a flat steel product according to the invention in contents of 0.04-0.23% by mass.
  • C is an essential element for the formation of martensite and austenite, which are required in order to achieve the strength properties required by a flat steel product according to the invention.
  • the steel according to the invention contains at least 0.04% by mass, wherein the desired effect is achieved particularly reliably at C contents of at least 0.07% by mass.
  • An excessively high C content would have a negative effect on the welding behavior of the flat steel product.
  • the weldability of a steel decreases with the level of its C content.
  • the C content of the steel according to the invention is therefore limited to a maximum of 0.23% by mass, in particular to a maximum of 0.20% by mass, wherein the negative effects of the presence of C can be particularly reliably avoided at contents of at most 0.17% by mass.
  • Si Silicon
  • Si is present in the steel substrate of a flat steel product according to the invention in contents of 0.04-0.54% by mass. Si is required to suppress the formation of pearlite in the structure during annealing, which would have a negative effect on the mechanical properties of the end product. A minimum content of 0.04% by mass Si is required for this purpose. An excessively high Si content also prevents the formation of pearlite during coiling and thus the segregation of Mn in the structure of the steel substrate. A significant segregation of Mn during coiling is necessary to achieve a high sum X and the desired mechanical properties. An excessively high Si content would likewise impair the surface quality of a flat steel product according to the invention. For these reasons, the upper limit of the Si content is limited to 0.54% by mass.
  • Aluminum (Al) can optionally be added to the steel substrate of a flat steel product according to the invention in contents of 0.01-1.5% by mass, in order to contribute to the suppression of the formation of pearlite. Even if Al is used in the usual manner for deoxidation of the melt, a minimum Al content of 0.01% by mass results. However, an excessively high Al content can have a negative effect on the castability of the steel and worsen the coating behavior during the hot-dip coating. Such negative influences of the presence of Al in the steel of the substrate of a flat steel product according to the invention can thereby be avoided particularly reliably in that the Al content is limited to at most 1.0% by mass, in particular at most 0.5% by mass.
  • Manganese (Mn) is present in the steel substrate of a flat steel product according to the invention in contents of 1.4-2.9% by mass. Mn is a mixed crystal element that contributes to the strength of the material. The presence of Mn in the steel of the substrate of a flat steel product according to the invention additionally stabilizes the austenite in the structure of the substrate.
  • the special feature of the alloy concept according to the invention in combination with the production according to the invention of a flat steel product according to the invention consists in that a flat steel product according to the invention is an optimal combination of high tensile strength and high elongation at break as a result of the segregation of Mn in the pearlite lines of the steel substrate after coiling, which is also maintained if the flat steel product has been annealed for the hot-dip coating and has passed through the hot-dip bath.
  • Mn contents of at least 1.4% by mass are required, wherein it is favorable with regard to the reliability with which the positive influence of Mn on the properties of a flat steel product according to the invention is established when the Mn content is at least 1.5% by mass.
  • an excessively high Mn concentration would also have a negative effect on weldability. Therefore, the upper limit of the Mn content of the steel substrate of a flat steel product according to the invention is limited to 2.9% by mass, preferably 2.5% by mass, wherein the amount of Mn for the properties of a flat steel product according to the invention can be utilized particularly effectively at Mn contents of up to 2.2% by mass.
  • Chromium (Cr) and molybdenum (Mo) can be added to the steel of the steel substrate of a flat steel product according to the invention as optional elements for increasing strength.
  • Cr and/or Mo increases the formation of martensite with respect to pearlite during the cooling of the flat steel product from the intercritical region in a continuous coating line. If these effects are to be utilized, contents of Cr and Mo that in total amount to at least 0.02% by mass, in particular at least 0.05% by mass, are required. In the case of excessively high Cr contents, however, the risk of pronounced grain boundary oxidation would be increased. An excessively high Mo content is also to be avoided for reasons of cost.
  • the upper limit of the total content of Cr and Mo is therefore set to 1.4% by mass, preferably 1.0% by mass.
  • Cr and Mo do not necessarily have to occur in combination, but can also each be added alone to the steel in contents of 0.02 to 1.4% by mass, in particular 0.05-1.0% by mass, as specified according to the invention, in order to achieve the effects explained.
  • particularly favorable effects result when Cr and Mo are present together, each in effective contents, as long as the sum of such contents is within the limits according to the invention.
  • At least one of the elements of titanium (Ti) and vanadium (V) is present as a required constituent in the steel of the steel substrate of a flat steel product according to the invention in contents of 0.005-0.15% by mass, wherein, here as well, it applies that an optimal effect of such elements occurs when Ti and V are each present together in effective contents.
  • Ti and V are micro-alloying elements that cause the formation of fine precipitates in the steel. Such precipitates prevent the coarsening of the austenite grains at temperatures that are higher than the Ar1 temperature of the steel, and in this manner lead to the refinement of the structure.
  • a finer structure favors the segregation of Mn that is desired according to the invention during the coiling carried out in the course of the production of a flat steel product according to the invention, because the distance over which Mn diffuses is reduced by the presence of Ti and/or V.
  • Ti-containing and V-containing precipitates also contribute to the strength of a flat steel product according to the invention by dispersion hardening.
  • Ti and/or V contents of at least 0.005% by mass in total are required. At contents above 0.15% by mass, the presence of Ti and/or V no longer results in any particular increase with regard to the properties desired according to the invention. Rather, Ti and V can be utilized particularly effectively if the sum of their contents is at most 0.1% by mass.
  • the content of niobium (Nb) is limited to less than 0.005% by mass, so that, if niobium is present at all, it is among the impurities that are technically ineffective. Higher Nb contents would lead to the formation of fine Nb precipitates, which would bring about susceptibility to crack formation during continuous casting or in the case of the slab cooling or reheating. Therefore, the Nb content is preferably limited to less than 0.003% by mass, in particular less than 0.002% by mass.
  • Boron (B) can likewise optionally be added to the steel of the steel substrate of a flat steel product according to the invention in contents of 0.0005-0.005% by mass, in order to prevent the formation of ferrite from the intercritical region in the course of the cooling carried out during the production of the flat steel product.
  • B promotes the formation of bainite, which leads to an increase in strength.
  • a minimum content of 0.0005% by mass B is required, but excessively high B content can lead to undesired embrittlement. Therefore, according to the invention, the upper limit of the B content, if B is added, is set to not more than 0.005% by mass, in particular 0.002% by mass.
  • Phosphorus (P) is among the undesired, but technically generally unavoidable impurities of the steel of the steel substrate of a flat steel product according to the invention and should therefore be as low as possible. P proves to be disadvantageous in particular with regard to weldability.
  • the P content according to the invention is limited to less than 0.02% by mass, preferably less than 0.01% by mass, in particular less than 0.005% by mass.
  • S Sulfur
  • MnS or (Mn, Fe)S MnS or (Mn, Fe)S
  • the S content according to the invention is limited to less than 0.005% by mass, preferably less than 0.002% by mass.
  • N also includes the undesired, but technically generally unavoidable impurities of the steel of the steel substrate of a flat steel product according to the invention and should therefore be as low as possible.
  • N forms, for example, nitrides with aluminum or titanium. In the case of higher N contents, this would lead to coarse precipitates that could be harmful to the formability of the flat steel product. Therefore, the N content is limited according to the invention to less than 0.01% by mass, preferably less than 0.005% by mass.
  • the upper limit of the Ca content is limited to at most 0.005% by mass, preferably at most 0.002% by mass,
  • Copper (Cu), nickel (Ni), tin (Sn), arsenic (As), cobalt (Co), zirconium (Zr), lanthanum (La) and/or cerium (Ce) are alloying elements that are also among the impurities of the steel of the steel substrate of a flat steel product according to the invention, the presence of which is undesirable per se.
  • the Cu content is limited to not more than 0.2% by mass
  • the Ni content is limited to not more than 0.1% by mass
  • the Sn content is limited to not more than 0.05% by mass
  • the As content is limited to not more than 0.02% by mass
  • the Co content is limited to not more than 0.02% by mass
  • the Zr content is limited to not more than 0.0002% by mass
  • the La content is limited to not more than 0.0002% by mass
  • the Ce content is limited to not more than 0.0002% by mass.
  • Oxygen (O) is also an undesirable impurity, since in the presence of larger amounts of O, oxide deposits are formed, which have a negative effect both on the mechanical properties of the flat steel product and on the castability and rollability of the steel of its steel substrate. Therefore, the content of oxygen is limited to at most 0.005% by mass, preferably 0.002% by mass.
  • Hydrogen (H) is also among the undesirable impurities of the steel of the steel substrate of a flat steel product according to the invention.
  • H is highly mobile on interstitial sites in the steel and can lead to cracking in the core during cooling from hot rolling, in particular in ultrahigh-strength steels. Therefore, the content of H in the steel of the steel substrate of a flat steel product according to the invention is reduced to a maximum of 0.001% by mass, preferably a maximum of 0.0006% by mass, more preferably a maximum of 0.0004% by mass, most preferably a maximum of 0.0002% by mass.
  • the corrosion protection coating of a flat steel product according to the invention consists of zinc (Zn) in its main proportion and can otherwise be composed in a conventional manner.
  • the corrosion protection layer can contain up to 20% by mass Fe, up to 5% by mass Mg and up to 10% by mass Al. Typically, if they are each present, at least 5% by mass Fe, at least 1% by mass Mg and/or at least 1% by mass Al is provided, in order to achieve optimal usage properties of corrosion protection.
  • steels A-1 were melted and cast into slabs, the composition of which is specified in Table 1. Contents of an alloying element that are so small that they are “0” in the technical sense, i.e., are so small that they have no influence on the properties of the steel, are referred to in Table 1 by the entry “-”.
  • the slabs were heated through in a preheating furnace in which a preheating temperature VT prevailed.
  • the preheated slabs were hot rolled in a conventional manner to form hot-rolled steel strips W1-W35, wherein the hot rolling was ended at an end rolling temperature ET.
  • the hot-rolled steel strips W1-W35 obtained in this manner were coiled in a likewise conventional manner starting from a coiling temperature HT in a likewise conventional manner to each form a coil. If necessary, they were cooled to the coiling temperature HT in a conventional manner for this purpose before coiling.
  • the hot-rolled steel strips W1-W35 were coated with a Zn-based corrosion protection layer by hot-dip coating.
  • they were subjected in each case to one of six variants a-f of an annealing treatment and a melt application, in which they were heated in a pre-heating stage with a heating rate HR to an annealing temperature GT, at which they were subsequently held over an annealing period of 40 s to 100 s in each case.
  • the hot-rolled steel strips W1-W35 were cooled with a cooling rate KR1 to a bath entry temperature BET, which was in each case equal to the bath temperature of the melt bath, through which the hot strips were passed after the respective annealing treatment a-f.
  • the melt bath consisted of at least 99% by mass Zn.
  • the now complete flat steel products emerging from the melt bath and produced on the basis of the hot-rolled steel strips W1-W35 were subsequently cooled to room temperature at a cooling rate KR2.
  • the parameters of heating rate HR, annealing temperature GT, cooling rate KR1, bath entry temperature BET and cooling rate KR2 belonging to the variants a-f of the annealing treatment and the melt application are recorded in Table 3.
  • the hot-rolled steel strip W3 contains too little Mn, so that Mn in the pearlite lines of the hot strip structure did not segregate to sufficient degrees. This resulted in a lower residual austenite content and therefore to a relatively low elongation at break A80 of the flat steel product produced from the hot-rolled steel strip W3. As a result, parameter B was below 31.

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