EP3724359A1 - High-strength, hot-rolled flat steel product with high edge crack resistance and simultaneously high bake-hardening potential, and method for producing a flat steel product of this kind - Google Patents

High-strength, hot-rolled flat steel product with high edge crack resistance and simultaneously high bake-hardening potential, and method for producing a flat steel product of this kind

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Publication number
EP3724359A1
EP3724359A1 EP18825919.6A EP18825919A EP3724359A1 EP 3724359 A1 EP3724359 A1 EP 3724359A1 EP 18825919 A EP18825919 A EP 18825919A EP 3724359 A1 EP3724359 A1 EP 3724359A1
Authority
EP
European Patent Office
Prior art keywords
hot
max
flat steel
steel product
strength
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.)
Granted
Application number
EP18825919.6A
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German (de)
French (fr)
Other versions
EP3724359B1 (en
Inventor
Ingwer Denks
Joachim SCHÖTTLER
Christian PELZ
Patrick WITTELER
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Salzgitter Flachstahl GmbH
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Salzgitter Flachstahl GmbH
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Publication of EP3724359A1 publication Critical patent/EP3724359A1/en
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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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/08—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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • 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
    • C23C2/024—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
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • 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
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • C21D2211/009—Pearlite
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/12—All metal or with adjacent metals
    • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
    • Y10T428/12771—Transition metal-base component
    • Y10T428/12785—Group IIB metal-base component
    • Y10T428/12792—Zn-base component
    • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

  • the invention relates to a high-strength, hot-rolled steel flat product with high edge crack resistance and at the same time high bake hardening potential.
  • the invention relates to a method for producing such
  • the invention relates to steel flat products of steels having a multiphase structure, which generally contains tempered bainite, and having a yield strength Rp0.2 in the range of 660 to 820 MPa, in particular for the production of components for the automotive industry, in addition to a high tensile strength of at least 760 MPa and an A80 elongation at break of at least 10% must have a high hole widening capacity with a hole widening ratio of over 30% and a high bake hardening potential with a BH2 value of more than 30 MPa.
  • BH bake-hardening effect
  • the bake hardening effect can be described as having a BH2 value defined as the increase in yield strength after a plastic pre-strain of 2% and a subsequent heat treatment.
  • the bake-hardening effect for example, the increase in the buckling strength of a component can be achieved by a suitable heat treatment takes place after shaping the component.
  • Bainitic steels are according to EN 10346 steels, which are characterized by a comparatively high yield strength and tensile strength at a sufficiently high elongation for cold forming processes. Due to the chemical composition a good weldability is given.
  • the microstructure typically consists of bainite with proportions of ferrite. Occasionally small proportions of other phases, such as martensite and retained austenite, may be present in the microstructure.
  • Such a steel is disclosed among others, for example, in the published patent application DE 10 2012 002 079 A1. The disadvantage here, however, is not yet sufficiently high Lochetzbergerweitever- to like.
  • the weight of the vehicles can be reduced by simultaneously improving the forming behavior of the steels used and the component behavior during production and operation.
  • High- to ultrahigh-strength steels must therefore meet comparatively high demands in terms of their strength, ductility and energy absorption, in particular during their processing, such as stamping, hot and cold forming, thermal quenching (eg air hardening, press hardening), welding and / or surface treatment , eg a metallic finish, organic coating or varnish.
  • Newly developed steels must therefore, in addition to the required weight reduction due to reduced sheet thicknesses, meet the increasing material requirements for yield strength, tensile strength, hardening behavior and elongation at break with good processing properties, such as formability and weldability.
  • the hole-expanding capacity is a material property that describes the resistance of the material to crack initiation and crack propagation during forming operations in near-edge areas, such as collaring.
  • the Lochetzweite brin is normatively regulated, for example, in ISO 16630. Thereafter, holes punched in a sheet metal are widened by means of a dome. The measured variable is the change in the output diameter of the
  • An improved edge crack resistance means increased forming capacity of the sheet edges and can be described by an increased Lochetzweiteabmögen. This situation is known under the synonyms “Low Edge Crack” (LEC) or “High Hole Expansion” (HHE) and xpand®.
  • the present invention based on the object to provide a high-strength, hot-rolled steel flat product with good forming properties, especially with high edge crack resistance and a high bake hardening potential and a method for producing such a flat steel product, based on the steel a good Combination of strength and forming properties.
  • a high strength hot rolled steel flat product with high edge crack resistance made from a steel with a yield strength Rp0.2 of 660 to 820 MPa, a BH2 value of over 30 MPa and a hole expansion ratio of over 30% and a structure consisting of two main components, wherein a first major component of the structure has a content of at least 50%, consisting of one or more individual components of ferrite, annealed Bainite and tempered martensite, each containing less than 5% of carbides, and wherein a second major constituent of the structure comprises from 5% to at most 50% of one or more constituents of martensite, retained austenite, bainite or perlite having the following chemical composition Steel (in% by weight):
  • Ni + Mo up to 0.5
  • the flat steel product according to the invention preferably also has a high hole expansion ratio of more than 30% with simultaneously high tensile strength of 760 to 960 MPa and high bake hardening potential BH2 of more than 30 MPa.
  • the flat steel product contains, in order to achieve particularly favorable combinations of properties, the following alloy composition in weight%: C: 0.04 to 0.08, Si: 0.03 to 0.4, Mn: 1.4 to 2 , 0, P: max. 0.08, S: max. 0.01, N: max. 0.01, AI: up to 0.1, Ni + Mo: up to 0.5, Nb: up to 0.08, Ti: up to 0.2, Nb + Ti: min. 0.03 and particularly advantageous: C: 0.04 to 0.08, Si: 0.03 to 0.4, Mn: 1, 4 to 2.0, P: max. 0.08, S: max. 0.01, N: max.
  • the comparatively carbon-rich second main constituent is advantageously embedded island-like in the comparatively low-carbon, first matrix constituent, the matrix.
  • the island size is about 1 pm in diameter, but in any case ⁇ 2 pm comparatively small and the islands are advantageously evenly distributed over the strip thickness. The small size of the islands and the homogeneous
  • Distribution of the second main component contribute significantly to achieving the high hole expansion ratio.
  • the proportion of the carbon-rich second main constituent embedded in the matrix in the matrix firstly sets the yield strength in said region and secondly the bake hardening potential.
  • the mechanism is that with the formation of the metastable structural constituents martensite, retained austenite and bainite, a large number of dislocations are produced which produce a low yield strength.
  • dissolved carbon diffuses from the metastable microstructure constituents martensite, retained austenite and bainite into the previously formed dislocations and causes the known increase in strength. Since no dissolved carbon is available in the pearlite, the carbon-rich constituent embedded in the matrix in the matrix contains at least one of the metastable microstructural constituents martensite,
  • the hot-rolled flat steel product according to the invention can be provided with a metallic or non-metallic coating and is particularly suitable for the production of components for vehicle construction in the automotive industry but also applications in the field of shipbuilding, plant construction, infrastructure construction, aerospace and domestic appliance technology are conceivable.
  • the steel has a tensile strength Rm of 760 to 960 MPa, a yield strength Rp0.2 of 660 to 820 MPa, a breaking elongation A80 of more than 10%, preferably more than 12%, along the rolling direction
  • Alloying elements are usually added to the steel in order to specifically influence certain properties.
  • An alloying element in different steels can influence different properties. The effect and interaction generally depends strongly on the amount, the presence of other alloying elements and the dissolution state in the material.
  • Carbon C is required to form carbides, especially in connection with the so-called micro-alloying elements Nb, V and Ti, promotes the formation of martensite and bainite, stabilizes the austenite and generally increases the strength. Higher contents of C deteriorate the welding properties and lead to the deterioration of the elongation and toughness properties, therefore a maximum content of less than 0.12 wt.%, Advantageously less than 0.08 wt.%, Is established. In order to achieve sufficient strength of the material, a minimum addition of 0.04 wt .-% is required.
  • Manganese Mn Stabilizes austenite, increases strength and toughness, and increases the temperature window for hot rolling below the recrystallization stop temperature. Higher contents of> 2.5% by weight of Mn increase the risk of medium segregations, which significantly increases the ductility and thus the product quality to decrease. Lower contents ⁇ 1, 0 wt .-% do not allow the achievement of the required strength and toughness at the desired moderate analysis costs.
  • Aluminum Al Used for deoxidation in the steelworks process. The amount of AI used depends on the process. Therefore, no minimum Al content is indicated. An Al content of greater than 0.1 wt .-% deteriorates the casting behavior in continuous casting significantly. This results in a higher cost when casting.
  • Silicon Si One of the elements that enables the increase in strength of steel by solid solution hardening in a cost-effective manner.
  • Si reduces the surface quality of the hot strip by the promotion of firmly adhering scale on the reheated slabs, which can be removed only at great expense or insufficiently at high Si levels. This is particularly disadvantageous in the subsequent galvanizing. Therefore, the Si content is limited to max. 0.8% limited, favorably to 0.4%. If Si is largely dispensed with on account of the surface issue, a lower limit of 0.03 is to be regarded as meaningful, since with further reduction of the Si content, comparatively high process costs occur on the steelwork side.
  • Chromium Cr Improves strength and reduces corrosion rate, retards ferrite and pearlite formation and forms carbides.
  • the maximum content is set at less than 0.6% by weight because higher contents result in deterioration of ductility.
  • Molybdenum Mo Increases the hardenability or reduces the critical cooling rate, thus promoting the formation of fine, bainitic structures. In addition, even the use of small amounts of Mo delays the coarsening of fine precipitates, which should be made as fine as possible to increase the strength of micro-alloyed structures.
  • Nickel Ni The use of even small amounts of Ni promotes ductility while maintaining strength. Due to the comparatively high cost of the content of Ni + Mo is limited to 0.5 wt .-%.
  • Phosphorus P is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness by solid solution strengthening and improves hardenability. However, it is usually tried to
  • Sulfur S Like phosphorus, it is bound as a trace element in iron ore. It is generally undesirable in steel because it leads to undesirable inclusions of MnS, thereby degrading the elongation and toughness properties. It is therefore an attempt to achieve the lowest possible amounts of sulfur in the melt and possibly to convert the elongated inclusions by a so-called Ca- treatment in a more favorable geometric shape. For the above reasons, the sulfur content is limited to less than 0.01 wt .-%.
  • Nitrogen N Is also an accompanying element of steelmaking. Steels with free nitrogen tend to have a strong aging effect. The nitrogen diffuses at low temperatures at dislocations and blocks them. It causes an increase in strength combined with a rapid loss of toughness. Curing of the nitrogen in the form of nitrides is possible, for example, by alloying aluminum, niobium or titanium. In the episode stand the mentioned
  • the nitrogen content is limited to less than 0.01 wt .-%.
  • Micro-alloying elements are usually added only in very small amounts ( ⁇ 0.2 wt .-% per element). They act in contrast to the alloying elements mainly by precipitation formation but can also affect the properties in a dissolved state. Despite the small quantity additions, micro-alloying elements influence the targeted ones Production conditions and the processing and final properties of the product.
  • Typical micro-alloying elements are, for example, niobium and titanium. These elements can be dissolved in the iron grid and form carbides, nitrides and carbonitrides with carbon and nitrogen.
  • Niobium Nb The alloying of niobium is particularly effective through the formation of
  • Carbides are grain-refining, which simultaneously improves the strength, toughness and elongation properties. At contents of more than 0.08% by weight, a saturation behavior sets in, which is why a maximum content of less than or equal to 0.08% by weight is provided.
  • Titanium Ti Grain-refining as a carbide former, which simultaneously improves strength, toughness and elongation properties. Contents of Ti exceeding 0.2 wt% deteriorate the ductility and the hole expanding ability by forming very coarse, primary TiN precipitates, therefore, a maximum content of 0.2 wt% is set.
  • a method according to the invention for the production of the above-described hot-rolled flat steel product according to the invention comprises the steps:
  • Ni + Mo up to 0.5
  • Annealing time of at least 1 s, preferably 5 s-40 s, and an average cooling rate between annealing temperature and 500 ° C. of 0.1 K / min to 150 K / s, preferably 5 K / s to 20 K / s,
  • ferritic-bainitic, microalloyed hot-rolled strip substantially retains the mechanical properties, although it is not annealed at temperatures below Ac1 but at Ac1 ⁇ T ⁇ Ac1 + 100 ° C., as usual.
  • the temperature Ac1 describes the beginning of the transformation of the microstructure into austenite with slow heating in accordance with relevant standards.
  • Ac1 is usually determined by dilatometric measurements. According to the invention, it has been recognized that the homogeneity of the ferritic-bainitic microstructure is largely retained with an annealing of T ⁇ Ac.sub.1, and thus, in particular, the comparatively high level of the hole widening ratio is maintained for mainly bainitic structures.
  • a BH2 value of> 30% can not be achieved and a pronounced upper yield strength of ReH> 820 MPa is formed, which is often regarded as problematic for the user.
  • the cause is the blocking of dislocations due to diffusion of atomically dissolved carbon during annealing at T ⁇ Ac1 or galvanizing at T> 400 ° C.
  • Hole expansion ratio of> 30%, as well as a BH2 value of> 30 MPa can be achieved in combination.
  • a reeling temperature HT of less than 650 ° C. advantageously in the range of 450 ° C. to 600 ° C., since the set predominantly bainitic structure has a high number of nucleation sites for the transformation into austenite at T> Ac1 and thus allows the island diameter of the stored second phase an average value of ⁇ 1 pm.
  • Below 450 ° C is with a comparatively high proportion of
  • Martensite which is disadvantageous after the heat treatment in terms of ductility and Lochetzweiteabmögens due to the internal structure.
  • the hot rolling end temperature in this steel according to the invention is between 950 ° C and Ar1 + 50 K, where Ar1 describes the beginning of the conversion of austenite into the ferrite during cooling.
  • Typical thickness ranges for slabs and thin slabs are between 35 mm to 450 mm. It is envisaged that the slab or thin slab to a
  • the hot strip is after the hot rolling according to the invention at a reel temperature of preferably 450 ° C to 650 ° C reeled.
  • a reel temperature of preferably 450 ° C to 650 ° C reeled.
  • this is hot rolled
  • a heat treatment according to the invention in the temperature range Ac1 ⁇ T ⁇ Ac1 subjected to a flat steel product in the temperature range + 100 ° C and held usually in this temperature range for 10 seconds to 10 minutes, possibly up to 48h, with higher temperatures being associated with shorter treatment times and vice versa.
  • the annealing is usually in a continuous annealing (shorter annealing times), but can also be done for example in a Haubenglühe (longer annealing times).
  • the flat steel product is hot-dip or electrolytically galvanized or metallic, inorganic or organic coated.
  • the annealing is preferably carried out in one of
  • Hot dip coating system upstream continuous annealing.
  • Flat steel product has a tensile strength Rm of the flat steel product of 760 to 960 MPa and an elongation at break A80 of more than 10%, preferably more than 12%. In this case, high strengths and small sheet thicknesses tend to be associated with lower elongations at break and vice versa.
  • Table 2 shows the results for an annealing of the hot strip according to the invention at Ac1 ⁇ T ⁇ Ac1 + 100 ° C. (invention) in comparison to annealing below an Ac1 annealing temperature (comparison) in a radiant tube furnace (RTF).
  • inventive annealing all required characteristics are achieved safely.

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Abstract

The invention relates to a high-strength, hot-rolled flat steel product with high edge crack resistance and simultaneously high bake-hardening potential made of a steel with a yield strength Rp 0.2 of 660 to 820 MPa, a BH2 value of more than 30 MPa, and a hole expansion ratio of more than 30%, and a structure consisting of two main constituents, wherein a first main constituent of the structure accounts for a proportion of at least 50%, consisting of one or more individual constituents of ferrite, tempered bainite, and tempered martensite, each with less than 5% carbides, and wherein a second main constituent of the structure accounts for a proportion of 5% to 50%, consisting of one or more individual constituents of martensite, residual austenite, bainite or perlite with the following chemical composition of the steel (in % by weight): C: 0.04 to 0.12; Si: 0.03 to 0.8; Mn: 1 to 2.5; P: max. 0.08; S: max. 0.01; N: max. 0.01; AI: up to 0.1; Ni+Mo: up to 0.5; Nb: up to 0.08; Ti: up to 0.2; Nb+Ti: min. 0.03; Cr: up to 0.6; the remainder being iron including unavoidable steel-associated elements. Furthermore, the invention relates to a method for producing a flat steel product of this type.

Description

Hochfestes, warmgewalztes Stahlflachprodukt mit hohem Kantenrisswiderstand und gleichzeitig hohem Bake-Hardening Potential, ein Verfahren zur Herstellung eines solchen Stahlflachprodukts  High strength hot rolled flat steel product with high edge crack resistance and high bake hardening potential, a process for producing such a flat steel product
Die Erfindung betrifft ein hochfestes, warmgewalztes Stahlflachprodukt mit hohem Kantenrisswiderstand und gleichzeitig hohem Bake-Hardening Potential. Des The invention relates to a high-strength, hot-rolled steel flat product with high edge crack resistance and at the same time high bake hardening potential. Of
Weiteren betrifft die Erfindung ein Verfahren zur Herstellung eines solchen Furthermore, the invention relates to a method for producing such
Stahlflachprodukts. Flat steel product.
Insbesondere betrifft die Erfindung Stahlflachprodukte aus Stählen mit einem mehrphasigen Gefüge, das in der Regel angelassenen Bainit enthält, und mit einer Dehngrenze Rp0,2 im Bereich von 660 bis 820 MPa, insbesondere zur Herstellung von Bauteilen für den Automobilbau, die neben einer hohen Zugfestigkeit von mindestens 760 MPa und einer Bruchdehnung A80 von mindestens 10% ein hohes Lochaufweitungsvermögen mit einem Lochaufweitungsverhältnis von über 30% sowie ein hohes Bake-Hardening-Potential mit einem BH2-Wert von über 30 MPa aufweisen müssen. In particular, the invention relates to steel flat products of steels having a multiphase structure, which generally contains tempered bainite, and having a yield strength Rp0.2 in the range of 660 to 820 MPa, in particular for the production of components for the automotive industry, in addition to a high tensile strength of at least 760 MPa and an A80 elongation at break of at least 10% must have a high hole widening capacity with a hole widening ratio of over 30% and a high bake hardening potential with a BH2 value of more than 30 MPa.
Allgemein versteht man unter einem Bake-Hardening-Effekt (BH) einen kontrollierten Alterungsprozess, der auf den im Stahl in Lösung vorhandenen Kohlen-und/oder Stickstoff zurückzuführen ist und mit einer Erhöhung der Streckgrenze einhergeht.In general, a bake-hardening effect (BH) is a controlled aging process, which is attributable to the carbon and / or nitrogen present in the steel in solution and is accompanied by an increase in the yield strength.
Der Bake-Hardening-Effekt kann mit einem BH2-Wert beschrieben werden, der als die Erhöhung der Streckgrenze nach einer plastischen Vordehnung von 2% und einer darauffolgenden Wärmebehandlung definiert ist. Mit dem Bake-Hardening-Effekt kann beispielsweise die Zunahme der Beulfestigkeit eines Bauteils erreicht werden, indem nach der Formung zum Bauteil eine geeignete Wärmebehandlung erfolgt. The bake hardening effect can be described as having a BH2 value defined as the increase in yield strength after a plastic pre-strain of 2% and a subsequent heat treatment. With the bake-hardening effect, for example, the increase in the buckling strength of a component can be achieved by a suitable heat treatment takes place after shaping the component.
Bainitische Stähle sind nach EN 10346 Stähle, die sich durch eine vergleichsweise hohe Streckgrenze und Zugfestigkeit bei einer ausreichend hohen Dehnung für Kaltumformprozesse auszeichnen. Aufgrund der chemischen Zusammensetzung ist eine gute Schweißbarkeit gegeben. Das Gefüge besteht typischerweise aus Bainit mit Anteilen von Ferrit. Es können im Gefüge vereinzelt geringe Anteile anderer Phasen, wie z.B. Martensit und Restaustenit, enthalten sein. Ein solcher Stahl wird neben anderen beispielsweise in der Offenlegungsschrift DE 10 2012 002 079 A1 offenbart. Nachteilig ist hierbei allerdings ein noch nicht ausreichend hohes Lochaufweitever- mögen. Bainitic steels are according to EN 10346 steels, which are characterized by a comparatively high yield strength and tensile strength at a sufficiently high elongation for cold forming processes. Due to the chemical composition a good weldability is given. The microstructure typically consists of bainite with proportions of ferrite. Occasionally small proportions of other phases, such as martensite and retained austenite, may be present in the microstructure. Such a steel is disclosed among others, for example, in the published patent application DE 10 2012 002 079 A1. The disadvantage here, however, is not yet sufficiently high Lochaufweitever- to like.
Der stark umkämpfte Automobilmarkt zwingt die Hersteller, stetig Lösungen zur Senkung des Flottenverbrauches und C02-Abgasausstoßes unter Beibehaltung eines größtmöglichen Komforts und Insassenschutzes zu finden. Dabei spielt einerseits die Gewichtsreduktion aller Fahrzeugkomponenten eine entscheidende Rolle, The highly competitive automotive market is forcing manufacturers to consistently find solutions to reduce fleet consumption and CO 2 emissions while maintaining maximum comfort and occupant safety. On the one hand, the weight reduction of all vehicle components plays a decisive role,
andererseits aber auch ein möglichst günstiges Verhalten der einzelnen Bauteile bei hoher statischer und dynamischer Beanspruchung sowohl während der Nutzung eines Automobils als auch im Crashfall. on the other hand, however, the most favorable behavior of the individual components with high static and dynamic stress both during use of an automobile and in the event of a crash.
Durch die Bereitstellung hochfester bis höchstfester Stähle mit Festigkeiten von bis zu 1200 MPa oder darüber und die Verringerung der Blechdicke, kann das Gewicht der Fahrzeuge durch gleichzeitig verbessertes Umformverhalten der eingesetzten Stähle sowie das Bauteilverhalten bei der Fertigung und im Betrieb reduziert werden. By providing high-strength to highest-strength steels with strengths of up to 1200 MPa or more and reducing the sheet thickness, the weight of the vehicles can be reduced by simultaneously improving the forming behavior of the steels used and the component behavior during production and operation.
Hoch- bis höchstfeste Stähle müssen daher vergleichsweise hohe Anforderungen hinsichtlich ihrer Festigkeit, Duktilität und Energieaufnahme erfüllen, insbesondere bei ihrer Verarbeitung, wie beispielsweise beim Stanzen, Warm- und Kaltumformen, beim thermischen Vergüten (z.B. Lufthärten, Presshärten), Schweißen und/oder einer Oberflächenbehandlung, z.B. einer metallischen Veredelung, organischen Beschich- tung oder Lackierung. High- to ultrahigh-strength steels must therefore meet comparatively high demands in terms of their strength, ductility and energy absorption, in particular during their processing, such as stamping, hot and cold forming, thermal quenching (eg air hardening, press hardening), welding and / or surface treatment , eg a metallic finish, organic coating or varnish.
Neu entwickelte Stähle müssen sich daher neben der verlangten Gewichtsreduzie- rung durch verringerte Blechdicken den zunehmenden Materialanforderungen an Dehngrenze, Zugfestigkeit, Verfestigungsverhalten und Bruchdehnung bei guten Verarbeitungseigenschaften, wie Umformbarkeit und Schweißbarkeit, stellen. Newly developed steels must therefore, in addition to the required weight reduction due to reduced sheet thicknesses, meet the increasing material requirements for yield strength, tensile strength, hardening behavior and elongation at break with good processing properties, such as formability and weldability.
Für eine solche Blechdickenverringerung muss daher ein hoch- bis höchstfester Stahl mit ein- oder mehrphasigem Gefüge verwendet werden, um ausreichende Festigkeit der Kraftfahrzeugbauteile sicherzustellen und um den hohen Umform- und For such a reduction in sheet thickness, therefore, a high to ultra high strength steel with single or multi-phase structure must be used to ensure sufficient strength of the motor vehicle components and the high forming and
Bauteilanforderungen hinsichtlich Zähigkeit, Kantenrissunempfindlichkeit, verbesser- tem Biegewinkel und Biegeradius, Energieabsorption sowie Verfestigungsvermögen und dem Bake-Hardening-Effekt zu genügen. Auch wird zunehmend eine verbesserte Fügeeignung in Form von besserer allgemeiner Schweißbarkeit, wie einem größeren nutzbaren Schweißbereich beim Widerstandspunktschweißen und ein verbessertes Versagensverhalten der Component requirements in terms of toughness, edge crack resistance, improved bending angle and bending radius, energy absorption and hardenability and the bake hardening effect to meet. There is also an increasing ability to join in the form of better general weldability such as a larger usable weld area in resistance spot welding and improved failure behavior of the
Schweißnaht (Bruchbild) unter mechanischer Beanspruchung sowie eine ausreichen- de Resistenz gegenüber verzögerter Rissbildung durch Wasserstoffversprödung gefordert. Weld seam (fracture pattern) under mechanical stress and sufficient resistance to delayed crack formation due to hydrogen embrittlement are required.
Das Lochaufweitevermögen ist eine Materialeigenschaft, welche die Beständigkeit des Materials gegen Risseinleitung und Rissausbreitung bei Umformoperationen in kantennahen Bereichen, wie zum Beispiel beim Kragenziehen, beschreibt. The hole-expanding capacity is a material property that describes the resistance of the material to crack initiation and crack propagation during forming operations in near-edge areas, such as collaring.
Der Lochaufweiteversuch ist beispielsweise in der ISO 16630 normativ geregelt. Danach werden in ein Blech gestanzte Löcher mittels eines Doms aufgeweitet. Die Messgröße ist die auf den Ausgangsdurchmesser bezogene Änderung des The Lochaufweiteversuch is normatively regulated, for example, in ISO 16630. Thereafter, holes punched in a sheet metal are widened by means of a dome. The measured variable is the change in the output diameter of the
Lochdurchmessers bei der am Rand des Lochs der erste Riss durch das Blech auftritt. Hole diameter at the edge of the hole, the first crack occurs through the sheet.
Eine verbesserte Kantenrissunempfindlichkeit bedeutet ein erhöhtes Umformvermö- gen der Blechkanten und kann durch ein erhöhtes Lochaufweitevermögen beschrieben werden. Dieser Sachverhalt ist unter den Synonymen„Low Edge Crack“ (LEC) bzw. unter„High Hole Expansion“ (HHE) sowie xpand® bekannt. An improved edge crack resistance means increased forming capacity of the sheet edges and can be described by an increased Lochaufweiteabmögen. This situation is known under the synonyms "Low Edge Crack" (LEC) or "High Hole Expansion" (HHE) and xpand®.
Hiervon ausgehend liegt der vorliegenden Erfindung die Aufgabe zu Grunde, ein hochfestes, warmgewalztes Stahlflachprodukt mit guten Umformeigenschaften, insbesondere mit hohem Kantenrisswiderstand und einem hohen Bake-Hardening- Potential sowie ein Verfahren zur Herstellung eines solchen Stahlflachproduktes zu schaffen, die bezogen auf den Stahl eine gute Kombination von Festigkeits- und Umformeigenschaften bieten. On this basis, the present invention based on the object to provide a high-strength, hot-rolled steel flat product with good forming properties, especially with high edge crack resistance and a high bake hardening potential and a method for producing such a flat steel product, based on the steel a good Combination of strength and forming properties.
Diese Aufgabe wird durch ein hochfestes, warmgewalztes Stahlflachprodukt mit den Merkmalen des Anspruchs 1 und ein Verfahren zur Herstellung eines Stahlflachpro- dukts mit den Merkmalen des Anspruchs 9 gelöst. Vorteilhafte Ausgestaltungen der Erfindung sind in den Unteransprüchen angegeben. This object is achieved by a high-strength, hot-rolled flat steel product having the features of claim 1 and a method for producing a flat steel product having the features of claim 9. Advantageous embodiments of the invention are specified in the subclaims.
Erfindungsgemäß bietet ein hochfestes, warmgewalztes Stahlflachprodukt mit hohem Kantenrisswiderstand, aus einem Stahl mit einer Dehngrenze Rp0,2 von 660 bis 820 MPa, einem BH2-Wert von über 30 MPa und einem Lochaufweitungsverhältnis von über 30% sowie einem Gefüge, bestehend aus zwei Hauptbestandteilen, wobei ein erster Hauptbestandteil des Gefüges einen Anteil von mindestens 50% aufweist, bestehend aus einem oder mehreren Einzelbestandteilen von Ferrit, angelassenem Bainit und angelassenem Martensit mit jeweils weniger als 5% Karbiden, und wobei ein zweiter Hauptbestandteil des Gefüges einen Anteil aus 5% bis höchstens 50% aufweist, bestehend aus einem oder mehreren Einzelbestandteilen von Martensit, Restaustenit, Bainit oder Perlit, mit folgender chemischer Zusammensetzung des Stahls (in Gewichts-%): In accordance with the present invention, a high strength hot rolled steel flat product with high edge crack resistance, made from a steel with a yield strength Rp0.2 of 660 to 820 MPa, a BH2 value of over 30 MPa and a hole expansion ratio of over 30% and a structure consisting of two main components, wherein a first major component of the structure has a content of at least 50%, consisting of one or more individual components of ferrite, annealed Bainite and tempered martensite, each containing less than 5% of carbides, and wherein a second major constituent of the structure comprises from 5% to at most 50% of one or more constituents of martensite, retained austenite, bainite or perlite having the following chemical composition Steel (in% by weight):
C: 0,04 bis 0,12  C: 0.04 to 0.12
Si: 0,03 bis 0,8 Si: 0.03 to 0.8
Mn: 1 bis 2,5 Mn: 1 to 2.5
P: max. 0,08 P: max. 0.08
S: max. 0,01 S: max. 0.01
N: max. 0,01 N: max. 0.01
AI: bis zu 0,1 AI: up to 0.1
Ni+Mo: bis zu 0,5 Ni + Mo: up to 0.5
Nb: bis zu 0,08 Nb: up to 0.08
Ti: bis zu 0,2 Ti: up to 0.2
Nb+Ti: min. 0,03 Nb + Ti: min. 0.03
Cr: bis zu 0,6 Cr: up to 0.6
Rest Eisen einschließlich unvermeidbarer stahlbegleitender Elemente, eine gute Kombination von Festigkeits-, Dehnungs- und Umformeigenschaften. Außerdem ist die Herstellung dieses erfindungsgemäßen Stahlflachproduktes auf der Basis der Legierungselemente C, Si, Mn, Nb und/oder Ti vergleichsweise kostengünstig.  Remaining iron including unavoidable steel-supporting elements, a good combination of strength, elongation and forming properties. In addition, the production of this flat steel product according to the invention based on the alloying elements C, Si, Mn, Nb and / or Ti is comparatively inexpensive.
Das erfindungsgemäße Stahlflachprodukt zeichnet sich vorzugsweise außerdem durch ein hohes Lochaufweitungsverhältnis von über 30% bei gleichzeitig hoher Zugfestigkeit von 760 bis 960 MPa und hohem Bake-Hardening-Potential BH2 von über 30 MPa aus. The flat steel product according to the invention preferably also has a high hole expansion ratio of more than 30% with simultaneously high tensile strength of 760 to 960 MPa and high bake hardening potential BH2 of more than 30 MPa.
In einer vorteilhaften Weiterbildung der Erfindung enthält das Stahlflachprodukt zur Erreichung besonders günstiger Eigenschaftskombinationen folgende Legierungszu- sammensetzung in Gewichts-%: C: 0,04 bis 0,08, Si: 0,03 bis 0,4, Mn: 1 ,4 bis 2,0, P: max. 0,08, S: max. 0,01 , N: max. 0,01 , AI: bis zu 0,1 , Ni+Mo: bis zu 0,5, Nb: bis zu 0,08, Ti: bis zu 0,2, Nb+Ti: min. 0,03 und besonders vorteilhaft: C: 0,04 bis 0,08, Si: 0,03 bis 0,4, Mn: 1 ,4 bis 2,0, P: max. 0,08, S: max. 0,01 , N: max. 0,01 , AI: bis zu 0,1 , Ni+Mo: bis zu 0,5, Nb: bis zu 0,05, Ti: bis zu 0,15 und Nb+Ti: min. 0,03. Die Verwendung des Begriffs„bis“ in der Definition der Gehaltsbereiche, wie beispielsweise 0,01 bis 1 Gew.-%, bedeutet, dass die Eckwerte - im Beispiel 0,01 und 1 - miteingeschlossen sind. Das Gefüge besteht aus zwei Hauptbestandteilen, wobei ein erster Hauptbestandteil einen Anteil von >= 50% ausmacht mit einem oder mehreren Gefügebestandteilen Ferrit und angelassenem Bainit und angelassenem Martensit und mit jeweils < 5% Karbiden und der zweite Hauptbestandteil einen Anteil aus 5%-50% ausmacht und aus einem oder mehreren Gefügebestandteilen Martensit, Restaustenit, Bainit oder Perlit besteht und vorzugsweise im Mittel einen vergleichs- weise höheren Kohlenstoffgehalt aufweist als der erste Hauptbestandteil. Der vergleichsweise kohlenstoffreichere zweite Hauptbestandteil ist vorteilhaft inselförmig in den vergleichsweise kohlenstoffärmeren, die Matrix bildenden ersten Hauptbestandteil, eingebettet. Die Inselgröße ist mit ca. 1 pm Durchmesser, in jedem Falle aber < 2 pm vergleichsweise klein und die Inseln sind vorteilhaft gleichmäßig über die Banddicke verteilt. Die geringe Größe der Inseln und die homogene In an advantageous development of the invention, the flat steel product contains, in order to achieve particularly favorable combinations of properties, the following alloy composition in weight%: C: 0.04 to 0.08, Si: 0.03 to 0.4, Mn: 1.4 to 2 , 0, P: max. 0.08, S: max. 0.01, N: max. 0.01, AI: up to 0.1, Ni + Mo: up to 0.5, Nb: up to 0.08, Ti: up to 0.2, Nb + Ti: min. 0.03 and particularly advantageous: C: 0.04 to 0.08, Si: 0.03 to 0.4, Mn: 1, 4 to 2.0, P: max. 0.08, S: max. 0.01, N: max. 0.01, Al: up to 0.1, Ni + Mo: up to 0.5, Nb: up to 0.05, Ti: up to 0.15 and Nb + Ti: min. 0.03. The use of the term "bis" in the definition of the content ranges, such as 0.01 to 1 wt .-% means that the benchmarks - in the example 0.01 and 1 - are included. The structure consists of two main constituents, with a first main constituent accounting for> = 50% with one or more structural constituents ferrite and tempered bainite and tempered martensite and <5% carbides each and the second major constituent accounting for 5% -50% and martensite, retained austenite, bainite or perlite consists of one or more microstructural constituents and preferably has on average a comparatively higher carbon content than the first main constituent. The comparatively carbon-rich second main constituent is advantageously embedded island-like in the comparatively low-carbon, first matrix constituent, the matrix. The island size is about 1 pm in diameter, but in any case <2 pm comparatively small and the islands are advantageously evenly distributed over the strip thickness. The small size of the islands and the homogeneous
Verteilung des zweiten Hauptbestandteils tragen dabei zur Erreichung des hohen Lochaufweiteverhältnisses maßgeblich bei. Distribution of the second main component contribute significantly to achieving the high hole expansion ratio.
Durch den Anteil des inselförmig in der Matrix eingebetteten kohlenstoffreicheren zweiten Hauptbestandteils wird erstens die Streckgrenze in dem genannten Bereich und zweitens das Bake-Hardening-Potential eingestellt. Der metallkundliche The proportion of the carbon-rich second main constituent embedded in the matrix in the matrix firstly sets the yield strength in said region and secondly the bake hardening potential. The metallurgical
Mechanismus besteht darin, dass mit der Bildung der metastabilen Gefügebestandtei- le Martensit, Restaustenit und Bainit eine Vielzahl von Versetzungen erzeugt werden, die eine niedrige Dehngrenze hervorrufen. Bei dem Bake-Hardening-Prozess diffundiert gelöster Kohlenstoff aus den metastabilen Gefügebestandteilen Martensit, Restaustenit und Bainit in die zuvor entstandenen Versetzungen und ruft die bekannte Festigkeitssteigerung hervor. Da im Perlit kein gelöster Kohlenstoff zur Verfügung steht, enthält der inselförmig in der Matrix eingebettete kohlenstoffreiche Bestandteil zumindest einen der metastabilen Gefügebestandteile Martensit,  The mechanism is that with the formation of the metastable structural constituents martensite, retained austenite and bainite, a large number of dislocations are produced which produce a low yield strength. In the bake-hardening process, dissolved carbon diffuses from the metastable microstructure constituents martensite, retained austenite and bainite into the previously formed dislocations and causes the known increase in strength. Since no dissolved carbon is available in the pearlite, the carbon-rich constituent embedded in the matrix in the matrix contains at least one of the metastable microstructural constituents martensite,
Restaustenit und Bainit. Das erfindungsgemäße warmgewalzte Stahlflachprodukt kann mit einem metallischen oder nichtmetallischen Überzug versehen werden und eignet sich insbesondere zur Erzeugung von Bauteilen für den Fahrzeugbau in der Automobilindustrie aber es sind auch Anwendungen im Bereich Schiffsbau, Anlagenbau, Infrastrukturbau, in der Luft- und Raumfahrt und Hausgerätetechnik denkbar. Retained austenite and bainite. The hot-rolled flat steel product according to the invention can be provided with a metallic or non-metallic coating and is particularly suitable for the production of components for vehicle construction in the automotive industry but also applications in the field of shipbuilding, plant construction, infrastructure construction, aerospace and domestic appliance technology are conceivable.
In vorteilhafter Weise weist der Stahl längs zur Walzrichtung eine Zugfestigkeit Rm von 760 bis 960 MPa, eine Dehngrenze Rp0,2 von 660 bis 820 MPa, eine Bruchdeh- nung A80 von mehr als 10%, vorzugsweise mehr als 12%, ein Advantageously, the steel has a tensile strength Rm of 760 to 960 MPa, a yield strength Rp0.2 of 660 to 820 MPa, a breaking elongation A80 of more than 10%, preferably more than 12%, along the rolling direction
Lochaufweitungsverhältnis von über 30% sowie einen BH2-Wert von über 30 MPa auf. Hole expansion ratio of over 30% and a BH2 value of over 30 MPa.
Legierungselemente werden dem Stahl in der Regel zugegeben, um gezielt bestimmte Eigenschaften zu beeinflussen. Dabei kann ein Legierungselement in verschiedenen Stählen unterschiedliche Eigenschaften beeinflussen. Die Wirkung und Wechselwirkung hängt im Allgemeinen stark von der Menge, der Anwesenheit weiterer Legierungselemente und dem Lösungszustand im Werkstoff ab. Die Alloying elements are usually added to the steel in order to specifically influence certain properties. An alloying element in different steels can influence different properties. The effect and interaction generally depends strongly on the amount, the presence of other alloying elements and the dissolution state in the material. The
Zusammenhänge sind vielseitig und komplex. Im Folgenden soll auf die Wirkung der Legierungselemente in der erfindungsgemäßen Legierung näher eingegangen werden. Nachfolgend werden die positiven Effekte der erfindungsgemäß verwendeten Legierungselemente beschrieben: Connections are versatile and complex. In the following, the effect of the alloying elements in the alloy according to the invention will be discussed in more detail. The following describes the positive effects of the alloying elements used according to the invention:
Kohlenstoff C: Wird benötigt zur Bildung von Karbiden, insbesondere im Zusammen- hang mit den sogenannten Mikrolegierungselementen Nb, V und Ti, fördert die Bildung von Martensit und Bainit, stabilisiert den Austenit und erhöht im Allgemeinen die Festigkeit. Höhere Gehalte an C verschlechtern die Schweißeigenschaften und führen zur Verschlechterung der Dehnungs- und Zähigkeitseigenschaften, weshalb ein maximaler Gehalt von weniger als 0,12 Gew.-%, vorteilhafter Weise von weniger als 0,08 Gew.-% festgelegt wird. Um eine ausreichende Festigkeit des Werkstoffs zu erreichen, ist eine Mindestzugabe von 0,04 Gew.-% erforderlich. Carbon C: is required to form carbides, especially in connection with the so-called micro-alloying elements Nb, V and Ti, promotes the formation of martensite and bainite, stabilizes the austenite and generally increases the strength. Higher contents of C deteriorate the welding properties and lead to the deterioration of the elongation and toughness properties, therefore a maximum content of less than 0.12 wt.%, Advantageously less than 0.08 wt.%, Is established. In order to achieve sufficient strength of the material, a minimum addition of 0.04 wt .-% is required.
Mangan Mn: Stabilisiert den Austenit, erhöht die Festigkeit und die Zähigkeit und erhöht das Temperaturfenster für das Warmwalzen unterhalb der Rekristallisations- stopptemperatur. Höhere Gehalte von > 2,5 Gew.-% Mn erhöhen das Risiko von Mittenseigerungen, die die Duktilität und somit die Produktqualität signifikant verringern. Geringere Gehalte < 1 ,0 Gew.-% erlauben nicht die Erreichung der erforderlichen Festigkeit und Zähigkeit bei angestrebten moderaten Analysekosten. Vorteilhaft ist ein Gehalt an Mn im Bereich zwischen 1 ,4 Gew.-% und 2,0 Gew.-%. Manganese Mn: Stabilizes austenite, increases strength and toughness, and increases the temperature window for hot rolling below the recrystallization stop temperature. Higher contents of> 2.5% by weight of Mn increase the risk of medium segregations, which significantly increases the ductility and thus the product quality to decrease. Lower contents <1, 0 wt .-% do not allow the achievement of the required strength and toughness at the desired moderate analysis costs. Advantageously, a content of Mn in the range between 1, 4 wt .-% and 2.0 wt .-%.
Aluminium AI: Wird für die Desoxidation im Stahlwerksprozess eingesetzt. Die Menge des eingesetzten AI ist prozessabhängig. Daher ist kein minimaler Al-Gehalt angegeben. Ein Al-Gehalt von größer 0,1 Gew.-% verschlechtert das Gießverhalten im Strangguss deutlich. Hierdurch entsteht ein höherer Aufwand beim Vergießen. Aluminum Al: Used for deoxidation in the steelworks process. The amount of AI used depends on the process. Therefore, no minimum Al content is indicated. An Al content of greater than 0.1 wt .-% deteriorates the casting behavior in continuous casting significantly. This results in a higher cost when casting.
Silizium Si: Gehört zu den Elementen, die die Festigkeitssteigerung von Stahl durch Mischkristallverfestigung auf kostengünstige Art und Weise ermöglichen. Allerdings verringert Si die Oberflächenqualität des Warmbandes durch die Förderung von festanhaftendem Zunder auf den wiedererwärmten Brammen, der bei hohen Si- Gehalten nur mit hohem Aufwand oder nur unzureichend entfernt werden kann. Das ist insbesondere beim anschließenden Verzinken von Nachteil. Daher ist der Si- Gehalt auf max. 0,8% begrenzt, vorteilhaft auf 0,4%. Wird auf Si aufgrund der Oberflächenthematik weitgehend verzichtet, ist eine Untergrenze von 0,03 als sinnvoll anzusehen, da bei weitergehender Reduzierung des Si-Gehaltes stahlwerksseitig vergleichsweise hohe Prozesskosten eintreten. Silicon Si: One of the elements that enables the increase in strength of steel by solid solution hardening in a cost-effective manner. However, Si reduces the surface quality of the hot strip by the promotion of firmly adhering scale on the reheated slabs, which can be removed only at great expense or insufficiently at high Si levels. This is particularly disadvantageous in the subsequent galvanizing. Therefore, the Si content is limited to max. 0.8% limited, favorably to 0.4%. If Si is largely dispensed with on account of the surface issue, a lower limit of 0.03 is to be regarded as meaningful, since with further reduction of the Si content, comparatively high process costs occur on the steelwork side.
Chrom Cr: Verbessert die Festigkeit und verringert die Korrosionsrate, verzögert die Ferrit- und Perlitbildung und bildet Karbide. Der maximale Gehalt wird mit kleiner 0,6 Gew.-% festgelegt, da höhere Gehalte eine Verschlechterung der Duktilität zur Folge haben. Chromium Cr: Improves strength and reduces corrosion rate, retards ferrite and pearlite formation and forms carbides. The maximum content is set at less than 0.6% by weight because higher contents result in deterioration of ductility.
Molybdän Mo: Erhöht die Härtbarkeit bzw. verringert die kritische Abkühlrate und fördert so die Bildung von feinen, bainitischen Gefügen. Darüber hinaus verzögert bereits der Einsatz von geringen Mengen von Mo die Vergröberung von feinen Ausscheidungen, die zur Festigkeitssteigerung von mikrolegierten Gefügen möglichst fein ausgebildet sein sollen. Molybdenum Mo: Increases the hardenability or reduces the critical cooling rate, thus promoting the formation of fine, bainitic structures. In addition, even the use of small amounts of Mo delays the coarsening of fine precipitates, which should be made as fine as possible to increase the strength of micro-alloyed structures.
Nickel Ni: Der Einsatz von bereits geringen Mengen von Ni fördert die Duktilität bei gleichbleibender Festigkeit. Aufgrund der vergleichsweise hohen Kosten wird der Gehalt von Ni + Mo auf 0,5 Gew.-% begrenzt. Phosphor P: Ist ein Spurenelement aus dem Eisenerz und wird im Eisengitter als Substitutionsatom gelöst. Phosphor steigert durch Mischkristallverfestigung die Härte und verbessert die Härtbarkeit. Es wird allerdings in der Regel versucht, den Nickel Ni: The use of even small amounts of Ni promotes ductility while maintaining strength. Due to the comparatively high cost of the content of Ni + Mo is limited to 0.5 wt .-%. Phosphorus P: is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness by solid solution strengthening and improves hardenability. However, it is usually tried to
Phosphorgehalt soweit wie möglich abzusenken, da er unter anderem stark seigerungsanfällig ist und im hohen Maße die Zähigkeit vermindert. Durch die Anlagerung von Phosphor an den Korngrenzen können Risse entlang der Korngren- zen beim Warmwalzen auftreten. Zudem setzt Phosphor die Übergangstemperatur von zähem zu sprödem Verhalten um bis zu 300 °C herauf. Allerdings kann durch gezielte, prozessseitig präzise gesteuerte Maßnahmen der Einsatz von geringen Mengen an P auch die kostengünstige Erhöhung der Festigkeit realisiert werden. Aus vorgenannten Gründen ist der Phosphorgehalt auf kleiner 0,08 Gew.-% begrenzt. Reduce phosphorus content as much as possible, as it is highly susceptible to segregation and greatly reduces its toughness. The addition of phosphorus to the grain boundaries can cause cracks along the grain boundaries during hot rolling. In addition, phosphorus increases the transition temperature from tough to brittle behavior by up to 300 ° C. However, the use of small amounts of P and the cost-effective increase of the strength can be realized by targeted, precise process-controlled measures. For the aforementioned reasons, the phosphorus content is limited to less than 0.08 wt .-%.
Schwefel S: Ist wie Phosphor als Spurenelement im Eisenerz gebunden. Er ist im Stahl im Allgemeinen unerwünscht, da er zu unerwünschten Einschlüssen von MnS führt, wodurch die Dehnungs- und Zähigkeitseigenschaften verschlechtert werden. Es wird daher versucht, möglichst geringe Mengen an Schwefel in der Schmelze zu erreichen und ggf. die langgestreckten Einschlüsse durch eine sogenannte Ca- Behandlung in eine günstigere geometrische Form zu überführen. Aus vorgenannten Gründen ist der Schwefelgehalt auf kleiner 0,01 Gew.-% begrenzt. Sulfur S: Like phosphorus, it is bound as a trace element in iron ore. It is generally undesirable in steel because it leads to undesirable inclusions of MnS, thereby degrading the elongation and toughness properties. It is therefore an attempt to achieve the lowest possible amounts of sulfur in the melt and possibly to convert the elongated inclusions by a so-called Ca- treatment in a more favorable geometric shape. For the above reasons, the sulfur content is limited to less than 0.01 wt .-%.
Stickstoff N: Ist ebenfalls ein Begleitelement aus der Stahlherstellung. Stähle mit freiem Stickstoff neigen zu einem starken Alterungseffekt. Der Stickstoff diffundiert schon bei geringen Temperaturen an Versetzungen und blockiert diese. Er bewirkt damit einen Festigkeitsanstieg verbunden mit einem rapiden Zähigkeitsverlust. Ein Abbinden des Stickstoffes in Form von Nitriden ist beispielsweise durch Zulegieren von Aluminium, Niob oder Titan möglich. In der Folge stehen die genannten Nitrogen N: Is also an accompanying element of steelmaking. Steels with free nitrogen tend to have a strong aging effect. The nitrogen diffuses at low temperatures at dislocations and blocks them. It causes an increase in strength combined with a rapid loss of toughness. Curing of the nitrogen in the form of nitrides is possible, for example, by alloying aluminum, niobium or titanium. In the episode stand the mentioned
Legierungselemente aber nicht mehr zur Neubildung von kleinen, hinsichtlich der Festigkeit sehr effizienten Ausscheidungen, im späteren Prozess zur Verfügung. Aus vorgenannten Gründen ist der Stickstoffgehalt auf kleiner 0,01 Gew.-% begrenzt. Alloy elements but no longer for the formation of small, in terms of strength very efficient precipitates, in the later process available. For the above reasons, the nitrogen content is limited to less than 0.01 wt .-%.
Mikrolegierungselemente werden in der Regel nur in sehr geringen Mengen zugegeben (< 0,2 Gew.-% pro Element). Sie wirken im Gegensatz zu den Legie- rungselementen hauptsächlich durch Ausscheidungsbildung können aber auch in gelöstem Zustand die Eigenschaften beeinflussen. Trotz der geringen Mengenzuga- ben beeinflussen Mikrolegierungselemente die zielführenden Herstellungsbedingungen sowie die Verarbeitungs- und Endeigenschaften des Produkts stark. Micro-alloying elements are usually added only in very small amounts (<0.2 wt .-% per element). They act in contrast to the alloying elements mainly by precipitation formation but can also affect the properties in a dissolved state. Despite the small quantity additions, micro-alloying elements influence the targeted ones Production conditions and the processing and final properties of the product.
Typische Mikrolegierungselemente sind zum Beispiel Niob und Titan. Diese Elemente können im Eisengitter gelöst werden und bilden mit Kohlenstoff und Stickstoff Carbide, Nitride und Carbonitride. Typical micro-alloying elements are, for example, niobium and titanium. These elements can be dissolved in the iron grid and form carbides, nitrides and carbonitrides with carbon and nitrogen.
Die Wirkung von Nb und Ti hängt insbesondere von der Prozessführung beim The effect of Nb and Ti depends in particular on the process control
Warmwalzen und anschließenden Abkühlvorgang ab. Mit der Zugabe von Mikrolegie- rungselementen wird angestrebt, im Laufe des Prozesses eine Kornfeinung zu erreichen und Ausscheidungen im Größenbereich von Nanometern zu erzeugen. Daher ist ein Mindestgehalt Nb+Ti von 0,03 Gew.-% Voraussetzung zum Erreichen der angestrebten Festigkeit und Dehnungseigenschaften. Hot rolling and subsequent cooling off. With the addition of micro-alloying elements, the aim is to achieve grain refining in the course of the process and to produce precipitates in the size range of nanometers. Therefore, a minimum content of Nb + Ti of 0.03 wt% is a prerequisite for achieving the desired strength and elongation properties.
Niob Nb: Die Zulegierung von Niob wirkt insbesondere durch die Bildung von Niobium Nb: The alloying of niobium is particularly effective through the formation of
Karbiden kornfeinend, wodurch gleichzeitig die Festigkeit, Zähigkeit und Dehnungsei- genschaften verbessert werden. Bei Gehalten von über 0,08 Gew.-% stellt sich ein Sättigungsverhalten ein, weshalb ein Maximalgehalt von kleiner gleich 0,08 Gew.-% vorgesehen ist. Carbides are grain-refining, which simultaneously improves the strength, toughness and elongation properties. At contents of more than 0.08% by weight, a saturation behavior sets in, which is why a maximum content of less than or equal to 0.08% by weight is provided.
Titan Ti: Wirkt als Karbidbildner kornfeinend, wodurch gleichzeitig die Festigkeit, Zähigkeit und Dehnungseigenschaften verbessert werden. Gehalte an Ti von über 0,2 Gew.-% verschlechtern die Duktilität und das Lochaufweitevermögen durch die Bildung sehr grober, primärer TiN Ausscheidungen, weshalb ein Maximalgehalt von 0,2 Gew.-% festgelegt wird. Titanium Ti: Grain-refining as a carbide former, which simultaneously improves strength, toughness and elongation properties. Contents of Ti exceeding 0.2 wt% deteriorate the ductility and the hole expanding ability by forming very coarse, primary TiN precipitates, therefore, a maximum content of 0.2 wt% is set.
Ein erfindungsgemäßes Verfahren zur Herstellung des vorbeschriebenen, erfindungs- gemäßen warmgewalzten Stahlflachprodukts, umfasst die Schritte: A method according to the invention for the production of the above-described hot-rolled flat steel product according to the invention comprises the steps:
- Erschmelzen einer Stahlschmelze enthaltend (in Gewichts-%):  Melting of a steel melt containing (in% by weight):
C: 0,04 bis 0,12  C: 0.04 to 0.12
Si: 0,03 bis 0,8 Si: 0.03 to 0.8
Mn: 1 bis 2,5 Mn: 1 to 2.5
P: max. 0,08 P: max. 0.08
S: max. 0,01 S: max. 0.01
N: max. 0,01 AI: bis zu 0,1 N: max. 0.01 AI: up to 0.1
Ni+Mo: bis zu 0,5 Ni + Mo: up to 0.5
Nb: bis zu 0,08 Nb: up to 0.08
Ti: bis zu 0,2 Ti: up to 0.2
Nb+Ti: min. 0,03 Nb + Ti: min. 0.03
Cr: bis zu 0,6 Cr: up to 0.6
Rest Eisen einschließlich unvermeidbarer stahlbegleitender Elemente,  Remainder of iron including unavoidable steel-accompanying elements,
- Vergießen der Stahlschmelze zu einer Bramme oder Dünnbramme mittels eines horizontalen oder vertikalen Brammen- oder Dünnbrammengießverfahrens,  Casting the molten steel into a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,
- Wiedererwärmen der Bramme oder Dünnbramme auf 1050 °C bis 1270 °C und anschließendes Warmwalzen der Bramme oder Dünnbramme zu einem Warmband mit optionalem Zwischenerwärmen zwischen einzelnen Walzstichen des Warmwal- zens, Reheating the slab or thin slab to 1050 ° C. to 1270 ° C. and then hot-rolling the slab or thin slab into a hot strip with optional intermediate heating between individual rolling passes of the hot rolling,
- Walzen im letzten Walzstich bei einer Endwalztemperatur von kleiner 950 °C und größer Ar1 +50K, bevorzugt bei kleiner 950 °C und größer Ar3, wobei Ar3 bei der - Rolling in the last pass at a final rolling temperature of less than 950 ° C and greater Ar1 + 50K, preferably at less than 950 ° C and greater Ar3, wherein Ar3 in the
Abkühlung den Beginn der Umwandlung und Ar1 den Abschluss der Umwandlung von Austenit in den Ferrit beschreibt, Cooling the beginning of the transformation and Ar1 describes the completion of the transformation of austenite into the ferrite,
- Aufhaspeln des Warmbandes bei einer Haspeltemperatur von unterhalb 650°C, bevorzugt in einem Temperaturbereich von 450°C bis 600°C,  Coiling of the hot strip at a coiler temperature of below 650 ° C., preferably in a temperature range of 450 ° C. to 600 ° C.,
- Glühen des Warmbandes oberhalb Ac1 und unterhalb Ac1 +100°C mit einer Annealing of the hot strip above Ac1 and below Ac1 + 100 ° C with a
Glühdauer von mindestens 1 s, bevorzugt 5 s - 40 s und einer mittleren Abkühlrate zwischen Glühtemperatur und 500°C von 0,1 K/min bis 150 K/s, bevorzugt 5K/s bis 20 K/s,  Annealing time of at least 1 s, preferably 5 s-40 s, and an average cooling rate between annealing temperature and 500 ° C. of 0.1 K / min to 150 K / s, preferably 5 K / s to 20 K / s,
- optionales Schmelztauchbeschichten des erwärmten Warmbandes nach dem Glühen und Abkühlen auf < 500°C.  - Optional hot-dip coating of the heated hot strip after annealing and cooling to <500 ° C.
Als wesentlich wurde im Rahmen der vorliegenden Untersuchungen gefunden, dass das ferritisch-bainitische, mikrolegierte Warmband im Wesentlichen die mechanischen Eigenschaften behält, obwohl es - nicht wie üblich - bei Temperaturen unterhalb Ac1 sondern bei Ac1 < T< Ac1 +100°C geglüht wird. It has been found to be essential in the present investigations that the ferritic-bainitic, microalloyed hot-rolled strip substantially retains the mechanical properties, although it is not annealed at temperatures below Ac1 but at Ac1 <T <Ac1 + 100 ° C., as usual.
Dabei beschreibt die Temperatur Ac1 den Beginn der Umwandlung des Gefüges in den Austenit bei langsamer Erwärmung gemäß einschlägiger Normen. Ac1 wird in der Regel durch dilatometrische Messungen bestimmt. Erfindungsgemäß wurde erkannt, dass bei einer Glühung von T < Ac1 zwar die Homogenität des ferritisch-bainitischen Gefüges weitgehend erhalten bleibt und so insbesondere das bei hauptsächlich bainitischen Gefügen vergleichsweise hohe Niveau des Lochaufweitungsverhältnisses gehalten wird. Allerdings ist bei einer Glühung unterhalb Ac1 ein BH2-Wert von > 30% nicht zu erreichen und es bildet sich eine ausgeprägte obere Streckgrenze von ReH > 820 MPa aus, die für den Anwender oft als problematisch angesehen wird. Ursache ist die Blockierung von Versetzungen durch Diffusion von atomar gelöstem Kohlenstoff bei der Glühung bei T < Ac1 bzw. Verzinkung bei T > 400°C. The temperature Ac1 describes the beginning of the transformation of the microstructure into austenite with slow heating in accordance with relevant standards. Ac1 is usually determined by dilatometric measurements. According to the invention, it has been recognized that the homogeneity of the ferritic-bainitic microstructure is largely retained with an annealing of T <Ac.sub.1, and thus, in particular, the comparatively high level of the hole widening ratio is maintained for mainly bainitic structures. However, with a calcination below Ac1, a BH2 value of> 30% can not be achieved and a pronounced upper yield strength of ReH> 820 MPa is formed, which is often regarded as problematic for the user. The cause is the blocking of dislocations due to diffusion of atomically dissolved carbon during annealing at T <Ac1 or galvanizing at T> 400 ° C.
Im Rahmen der Erfindung wurde überraschend gefunden, dass bei einer Glühung im Temperaturbereich von Ac1 < T< Ac1 +100°C, sowohl ein hohes Niveau des In the context of the invention, it has surprisingly been found that in the case of annealing in the temperature range of Ac1 <T <Ac1 + 100 ° C., both a high level of
Lochaufweitungsverhältnisses von > 30%, als auch ein BH2-Wert von > 30 MPa in Kombination erreicht werden kann. Vorteilhaft wirkt sich bei dem erfindungsgemäßen Stahl eine Haspeltemperatur HT von kleiner als 650°C, vorteilhaft im Bereich von 450°C bis 600°C aus, da das so eingestellte überwiegend bainitische Gefüge eine hohe Zahl an Keimstellen für die Umwandlung in Austenit bei T > Ac1 bereitstellt und so die Inseldurchmesser der eingelagerten Zweitphase einen Mittelwert von < 1 pm erlaubt. Unterhalb von 450°C ist mit einem vergleichsweise hohen Anteil von Hole expansion ratio of> 30%, as well as a BH2 value of> 30 MPa can be achieved in combination. Advantageously, in the case of the steel according to the invention, a reeling temperature HT of less than 650 ° C., advantageously in the range of 450 ° C. to 600 ° C., since the set predominantly bainitic structure has a high number of nucleation sites for the transformation into austenite at T> Ac1 and thus allows the island diameter of the stored second phase an average value of <1 pm. Below 450 ° C is with a comparatively high proportion of
Martensit zu rechnen, der nach der Wärmebehandlung hinsichtlich der Duktilität und des Lochaufweitevermögens aufgrund der inneren Struktur nachteilig ist. Martensite, which is disadvantageous after the heat treatment in terms of ductility and Lochaufweiteabmögens due to the internal structure.
Die Warmwalzendtemperatur liegt bei diesem Stahl erfindungsgemäß zwischen 950 °C und Ar1 + 50 K, wobei Ar1 den Beginn der Umwandlung von Austenit in den Ferrit bei der Abkühlung beschreibt. The hot rolling end temperature in this steel according to the invention is between 950 ° C and Ar1 + 50 K, where Ar1 describes the beginning of the conversion of austenite into the ferrite during cooling.
Übliche Dickenbereiche für Brammen und Dünnbrammen liegen zwischen 35 mm bis 450 mm. Es ist vorgesehen, dass die Bramme oder Dünnbramme zu einem Typical thickness ranges for slabs and thin slabs are between 35 mm to 450 mm. It is envisaged that the slab or thin slab to a
Warmband mit einer Dicke von 1 ,5 mm bis 8 mm, vorzugsweise 1 ,8 mm bis 4,5 mm warmgewalzt wird. Hot strip with a thickness of 1, 5 mm to 8 mm, preferably 1, 8 mm to 4.5 mm hot rolled.
Das Warmband wird nach dem Warmwalzen erfindungsgemäß bei einer Haspeltem- peratur von vorzugsweise 450°C bis 650°C aufgehaspelt. Zur Erreichung der geforderten Eigenschaftskombination für das Lochaufweitungsverhältnis, den BH2- Wert und der anderen mechanischen Eigenschaften, wird das warmgewalzte Stahlflach produkt in einer erfindungsgemäßen Wärmebehandlung im Temperaturbe- reich Ac1 < T< Ac1 +100°C unterzogen und in der Regel für 10 Sekunden bis 10 Minuten, möglicherweise bis 48h, in diesem Temperaturbereich gehalten, wobei höhere Temperaturen kürzeren Behandlungszeiten und umgekehrt zugeordnet werden. Die Glühung wird in der Regel in einer Durchlaufglühe (kürzere Glühzeiten), kann aber auch Beispielsweise in einer Haubenglühe (längere Glühzeiten) erfolgen. The hot strip is after the hot rolling according to the invention at a reel temperature of preferably 450 ° C to 650 ° C reeled. To achieve the required property combination for the hole expansion ratio, the BH2 value and the other mechanical properties, this is hot rolled In a heat treatment according to the invention, in the temperature range Ac1 <T <Ac1 subjected to a flat steel product in the temperature range + 100 ° C and held usually in this temperature range for 10 seconds to 10 minutes, possibly up to 48h, with higher temperatures being associated with shorter treatment times and vice versa. The annealing is usually in a continuous annealing (shorter annealing times), but can also be done for example in a Haubenglühe (longer annealing times).
Vorzugsweise wird das Stahlflachprodukt schmelztauch- oder elektrolytisch verzinkt oder metallisch, anorganisch oder organisch überzogen. Bei einer Schmelztauchbe- Schichtung erfolgt die Glühung vorzugsweise in einer der Preferably, the flat steel product is hot-dip or electrolytically galvanized or metallic, inorganic or organic coated. In a hot dip coating, the annealing is preferably carried out in one of
Schmelztauchbeschichtungsanlage vorgeschalteten Durchlaufglühanlage.  Hot dip coating system upstream continuous annealing.
Ein nach dem erfindungsgemäßen Verfahren hergestelltes warmgewalztes A hot rolled by the process according to the invention
Stahlflachprodukt weist eine Zugfestigkeit Rm des Stahlflachprodukts von 760 bis 960 MPa und eine Bruchdehnung A80 von mehr als 10%, vorzugsweise mehr als 12% auf. Hierbei sind hohen Festigkeiten und geringen Blechdicken tendenziell niedrigeren Bruchdehnungen zuzuordnen und umgekehrt. Flat steel product has a tensile strength Rm of the flat steel product of 760 to 960 MPa and an elongation at break A80 of more than 10%, preferably more than 12%. In this case, high strengths and small sheet thicknesses tend to be associated with lower elongations at break and vice versa.
In Bezug auf weitere Vorteile wird auf die vorstehenden Ausführungen zu dem erfindungsgemäßen Stahl verwiesen. With regard to further advantages, reference is made to the above statements on the steel according to the invention.
An einem erfindungsgemäß hergestellten Warmband aus zwei Stählen mit unterschiedlichen Analysen A und B gemäß Tabelle 1 , wurden die mechanischen Kennwerte, sowie die Werte für das Bake-Hardening (BH2) und die Verhältnisse für die Lochaufweitung (HER - hole expansion ratio) ermittelt. On a hot rolled strip produced according to the invention from two steels with different analyzes A and B according to Table 1, the mechanical characteristics, as well as the values for the bake hardening (BH2) and the ratios for hole expansion (HER hole expansion ratio) were determined.
Tabelle 1 Tabelle 2 zeigt die Ergebnisse für eine erfindungsgemäße Glühung des Warmbandes bei Ac1 < T< Ac1 +100°C (Erfindung) im Vergleich zu einer Glühung unterhalb einer Ac1 -Glühtemperatur (Vergleich) in einem Strahlrohrofen (RTF). Bei der erfindungs- gemäßen Glühung werden alle geforderten Kennwerte sicher erreicht. Table 2 shows the results for an annealing of the hot strip according to the invention at Ac1 <T <Ac1 + 100 ° C. (invention) in comparison to annealing below an Ac1 annealing temperature (comparison) in a radiant tube furnace (RTF). In the inventive annealing all required characteristics are achieved safely.
Tabelle 2 Table 2

Claims

Patentansprüche claims
1. Hochfestes, warmgewalztes Stahlflachprodukt mit hohem Kantenrisswiderstand, aus einem Stahl mit einer Dehngrenze Rp0,2 von 660 bis 820 MPa, einem BH2-Wert von über 30 MPa und einem Lochaufweitungsverhältnis von über 30%, sowie einem Gefüge bestehend aus zwei Hauptbestandteilen, wobei ein erster Hauptbestandteil des Gefüges einen Anteil von mindestens 50% aufweist, bestehend aus einem oder mehreren Einzelbestandteilen von Ferrit, angelassenem Bainit und angelassenem Martensit mit jeweils weniger als 5% Karbiden, und wobei ein zweiter Hauptbestand- teil des Gefüges einen Anteil aus 5% bis 50% aufweist, bestehend aus einem oder mehreren Einzelbestandteilen von Martensit, Restaustenit, Bainit oder Perlit mit folgender chemischer Zusammensetzung des Stahls (in Gewichts-%): A high strength hot rolled flat steel product having high edge crack resistance, made of a steel having a yield strength Rp0.2 of 660 to 820 MPa, a BH2 value of over 30 MPa and a hole expansion ratio of over 30%, and a structure consisting of two main components, wherein a first major constituent of the microstructure has a proportion of at least 50%, consisting of one or more individual constituents of ferrite, tempered bainite and tempered martensite, each with less than 5% carbides, and wherein a second main constituent of the microstructure comprises a proportion of 5% to 50%, consisting of one or more individual constituents of martensite, retained austenite, bainite or pearlite with the following chemical composition of the steel (in% by weight):
C: 0,04 bis 0,12  C: 0.04 to 0.12
Si: 0,03 bis 0,8 Si: 0.03 to 0.8
Mn: 1 bis 2,5 Mn: 1 to 2.5
P: max. 0,08 P: max. 0.08
S: max. 0,01 S: max. 0.01
N: max. 0,01 N: max. 0.01
AI: bis zu 0,1 AI: up to 0.1
Ni+Mo: bis zu 0,5 Ni + Mo: up to 0.5
Nb: bis zu 0,08 Nb: up to 0.08
Ti: bis zu 0,2 Ti: up to 0.2
Nb+Ti: min. 0,03 Nb + Ti: min. 0.03
Cr: bis zu 0,6 Cr: up to 0.6
Rest Eisen einschließlich unvermeidbarer stahlbegleitender Elemente. Remainder of iron including unavoidable steel-accompanying elements.
2. Stahlflachprodukt nach Anspruch 1 , dadurch gekennzeichnet, dass der Stahl (in Gewichts-%) enthält: 2. Flat steel product according to claim 1, characterized in that the steel contains (in% by weight):
C: 0,04 bis 0,08  C: 0.04 to 0.08
Si: 0,03 bis 0,4 Si: 0.03 to 0.4
Mn: 1 ,4 bis 2,0 Mn: 1, 4 to 2.0
P: max. 0,08 P: max. 0.08
S: max. 0,01 S: max. 0.01
N: max. 0,01 N: max. 0.01
AI: bis zu 0,1 Ni+Mo: bis zu 0,5 AI: up to 0.1 Ni + Mo: up to 0.5
Nb: bis zu 0,08 Nb: up to 0.08
Ti: bis zu 0,2 Ti: up to 0.2
Nb+Ti: min. 0,03. Nb + Ti: min. 0.03.
3. Stahlflachprodukt nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass der Stahl (in Gewichts-%) enthält: 3. Flat steel product according to claim 1 or 2, characterized in that the steel contains (in% by weight):
C: 0,04 bis 0,08  C: 0.04 to 0.08
Si: 0,03 bis 0,4 Si: 0.03 to 0.4
Mn: 1 ,4 bis 2,0 Mn: 1, 4 to 2.0
P: max. 0,08 P: max. 0.08
S: max. 0,01 S: max. 0.01
N: max. 0,01 N: max. 0.01
AI: bis zu 0,1 AI: up to 0.1
Ni+Mo: bis zu 0,5 Ni + Mo: up to 0.5
Nb: bis zu 0,05 Nb: up to 0.05
Ti: bis zu 0,15 Ti: up to 0.15
Nb+Ti: min. 0,03. Nb + Ti: min. 0.03.
4. Stahlflachprodukt nach mindestens einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, dass der zweite Hauptbestandteil des Gefüges inselförmig in den als Matrix ausgebildeten ersten Hauptbestandteil des Gefüges eingelagert ist. 4. Flat steel product according to at least one of claims 1 to 3, characterized in that the second main component of the structure is insular embedded in the matrix formed as the first main component of the structure.
5. Stahlflachprodukt nach Anspruch 4, dadurch gekennzeichnet, dass die inselförmi- gen Einlagerungen eine Größe von weniger als 2 pm, bevorzugt von weniger als 1 pm, aufweisen. 5. Flat steel product according to claim 4, characterized in that the insular deposits have a size of less than 2 pm, preferably less than 1 pm.
6. Stahlflachprodukt nach mindestens einem der Ansprüche 1 bis 5, dadurch gekennzeichnet, dass die Zugfestigkeit Rm des Stahlflachprodukts 760 bis 960 MPa beträgt und die Bruchdehnung A80 des Stahlflachprodukts mehr als 10%, vorzugs- weise mehr als 12% beträgt. 6. Flat steel product according to at least one of claims 1 to 5, characterized in that the tensile strength Rm of the flat steel product is 760 to 960 MPa and the elongation at break A80 of the flat steel product is more than 10%, preferably more than 12%.
7. Stahlflachprodukt nach mindestens einem der Ansprüche 1 bis 6, dadurch gekennzeichnet, dass dieses schmelztauch- oder elektrolytisch verzinkt ist oder metallisch, anorganisch oder organisch überzogen ist. 7. Flat steel product according to at least one of claims 1 to 6, characterized in that this is hot-dip or electrolytically galvanized or metallic, inorganic or organic coated.
8. Stahlflachprodukt nach mindestens einem der Ansprüche 1 bis 7, dadurch gekennzeichnet, dass der zweite Hauptbestandteil im Mittel einen vergleichsweise höheren Kohlenstoffgehalt aufweist als der erste Hauptbestandteil. 8. Flat steel product according to at least one of claims 1 to 7, characterized in that the second main component has on average a comparatively higher carbon content than the first main component.
9. Verfahren zur Herstellung eines warmgewalzten Stahlflachprodukts, nach mindestens einem der Ansprüche 1 bis 8 umfassend die Schritte: 9. A process for producing a hot rolled flat steel product according to any one of claims 1 to 8, comprising the steps of:
- Erschmelzen einer Stahlschmelze enthaltend (in Gewichts-%):  Melting of a steel melt containing (in% by weight):
C: 0,04 bis 0,12  C: 0.04 to 0.12
Si: 0,03 bis 0,8 Si: 0.03 to 0.8
Mn: 1 bis 2,5 Mn: 1 to 2.5
P: max. 0,08 P: max. 0.08
S: max. 0,01 S: max. 0.01
N: max. 0,01 N: max. 0.01
AI: bis zu 0,1 AI: up to 0.1
Ni+Mo: bis zu 0,5  Ni + Mo: up to 0.5
Nb: bis zu 0,08 Nb: up to 0.08
Ti: bis zu 0,2 Ti: up to 0.2
Nb+Ti: min. 0,03 Nb + Ti: min. 0.03
Cr: bis zu 0,6 Cr: up to 0.6
Rest Eisen einschließlich unvermeidbarer stahlbegleitender Elemente,  Remainder of iron including unavoidable steel-accompanying elements,
- Vergießen der Stahlschmelze zu einer Bramme oder Dünnbramme mittels eines horizontalen oder vertikalen Brammen- oder Dünnbrammengießverfahrens, Casting the molten steel into a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,
- Wiedererwärmen der Bramme oder Dünnbramme auf 1050 °C bis 1250 °C und anschließendes Warmwalzen der Bramme oder Dünnbramme zu einem Warmband mit optionalem Zwischenerwärmen zwischen einzelnen Walzstichen des Warmwal- zens, Reheating the slab or thin slab to 1050 ° C. to 1250 ° C. and then hot-rolling the slab or thin slab into a hot strip with optional intermediate heating between individual rolling passes of the hot rolling,
- Walzen im letzten Walzstich bei einer Endwalztemperatur von kleiner 950 °C und größer Ar3,  Rolling in the last pass at a final rolling temperature of less than 950 ° C and greater than Ar3,
- Aufhaspeln des Warmbandes bei einer Haspeltemperatur von unterhalb 650°C, bevorzugt im Bereich von 450°C bis 600°C, Coiling the hot-rolled strip at a reeling temperature of below 650 ° C., preferably in the range from 450 ° C. to 600 ° C.,
- Glühen des Warmbandes oberhalb Ac1 und unterhalb Ac1 +100°C mit einer Glühdauer von 10 Sekunden bis 10 Minuten und einer mittleren Abkühlrate zwischen Glühtemperatur und 500°C von 1 K/s bis 150 K/s, bevorzugt 5 K/s bis 20 K/s, - optionales Schmelztauchbeschichten des Warmbandes direkt im Anschluss des Abkühlvorgangs auf Kühlstopptemperatur in einer kontinuierlichen Feuerverzinkungs- anlage. Annealing of the hot strip above Ac1 and below Ac1 + 100 ° C with an annealing time of 10 seconds to 10 minutes and an average cooling rate between annealing temperature and 500 ° C from 1 K / s to 150 K / s, preferably 5 K / s to 20 K / s, - optional hot - dip dip coating of the hot strip directly after the Cooling process to cooling stop temperature in a continuous hot dip galvanizing plant.
10. Verfahren nach Anspruch 9, dadurch gekennzeichnet, dass das Warmband mit einer Endwalztemperatur von größer Ar1 +50°C gewalzt wird. 10. The method according to claim 9, characterized in that the hot strip with a final rolling temperature of greater than Ar1 + 50 ° C is rolled.
1 1. Verfahren nach Anspruch 9 oder 10, dadurch gekennzeichnet, dass die Bramme zu einem Warmband mit einer Dicke von 1 ,5 mm bis 8 mm, bevorzugt 1 ,8 mm bis 4,5 mm warmgewalzt wird. 1 1. The method of claim 9 or 10, characterized in that the slab is hot rolled into a hot strip having a thickness of 1, 5 mm to 8 mm, preferably 1, 8 mm to 4.5 mm.
EP18825919.6A 2017-12-15 2018-12-11 High-strength, hot-rolled flat steel product with high edge crack resistance and simultaneously high bake-hardening potential and method for producing a flat steel product of this kind Active EP3724359B1 (en)

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