EP3724359B1 - 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 Download PDF

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EP3724359B1
EP3724359B1 EP18825919.6A EP18825919A EP3724359B1 EP 3724359 B1 EP3724359 B1 EP 3724359B1 EP 18825919 A EP18825919 A EP 18825919A EP 3724359 B1 EP3724359 B1 EP 3724359B1
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hot
flat steel
steel product
max
slab
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German (de)
French (fr)
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EP3724359A1 (en
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Ingwer Denks
Joachim SCHÖTTLER
Christian PELZ
Patrick WITTELER
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Salzgitter Flachstahl GmbH
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Salzgitter Flachstahl GmbH
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • 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 flat steel product with high edge crack resistance and, at the same time, high bake hardening potential.
  • the invention also relates to a method for producing such a flat steel product.
  • the invention relates to flat steel products made of steels with a multiphase structure, which usually contains tempered bainite, and with a yield strength Rp0.2 in the range from 660 to 820 MPa, in particular for the production of components for automobile construction, which in addition to a high tensile strength of at least 760 MPa and an elongation at break A80 of at least 10% must have a high hole expansion capacity with a hole expansion ratio of more than 30% and a high bake hardening potential with a BH2 value of more than 30 MPa.
  • a bake hardening effect is understood to be a controlled aging process which is due to the carbon and / or nitrogen present in the steel in solution and which is associated with an increase in the yield point.
  • the bake hardening effect can be described with a BH2 value, which is defined as the increase in the yield point after a plastic pre-elongation of 2% and a subsequent heat treatment.
  • the dent resistance of a component can be increased by applying a suitable heat treatment after it has been formed into the component.
  • bainitic steels are steels that are characterized by a comparatively high yield point and tensile strength with a sufficiently high elongation for cold forming processes.
  • the structure typically consists of bainite with a proportion of ferrite.
  • the structure can occasionally contain small proportions of other phases, such as martensite and retained austenite.
  • Such a steel is used, for example, in the Offenlegungsschrift, among others DE 10 2012 002 079 A1 or WO2017012958 A1 disclosed.
  • the disadvantage here is that the hole expansion capacity is not yet sufficiently high.
  • the weight of the vehicles can be reduced by simultaneously improving the deformation behavior of the steels used and the component behavior during production and operation.
  • High-strength to ultra-high-strength steels must therefore meet comparatively high requirements with regard to their strength, ductility and energy absorption, especially when processing them, such as stamping, hot and cold forming, thermal quenching and tempering (e.g. air hardening, press hardening), welding and / or surface treatment , e.g. a metallic refinement, organic coating or painting.
  • stamping hot and cold forming
  • thermal quenching and tempering e.g. air hardening, press hardening
  • welding and / or surface treatment e.g. a metallic refinement, organic coating or painting.
  • a high-strength to ultra-high-strength steel with a single or multi-phase structure must therefore be used in order to ensure sufficient strength of the motor vehicle components and to meet the high forming and component requirements in terms of toughness, edge crack insensitivity, improved bending angle and bending radius, energy absorption and strengthening capacity and the like Bake hardening effect is sufficient.
  • the hole expansion capacity is a material property that describes the resistance of the material to crack initiation and crack propagation during forming operations in areas close to edges, such as when pulling collars.
  • the hole expansion test is regulated in ISO 16630, for example. Then holes punched in a sheet metal are widened by means of a mandrel.
  • the measured variable is the change in the hole diameter related to the initial diameter at which the first crack through the sheet occurs at the edge of the hole.
  • Improved edge crack insensitivity means an increased deformability of the sheet metal edges and can be described by an increased hole expansion capacity. This fact is known under the synonyms “Low Edge Crack” (LEC) or under “High Hole Expansion” (HHE) and xpand®.
  • the present invention is based on the object of creating a high-strength, hot-rolled flat steel product with good forming properties, in particular with high edge crack resistance and a high bake hardening potential, as well as a method for producing such a flat steel product which, based on the steel, is good Offer a combination of strength and forming properties.
  • the flat steel product according to the invention is also preferably characterized by a high hole expansion ratio of over 30% with a high tensile strength of 760 to 960 MPa and a high bake hardening potential BH2 of over 30 MPa.
  • the flat steel product contains the following alloy composition in% by weight in order to achieve particularly favorable combinations of properties: 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: at least 0.03 and particularly advantageous: C: 0.04 to 0.08, Si: 0.03 to 0.4, Mn: 1.4 up 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 to 0.05, Ti: up to 0.15 and Nb + Ti: min.0.03.
  • the second main constituent which is comparatively richer in carbon, is advantageously embedded in the form of an island in the comparatively lower carbon, first main constituent which forms the matrix.
  • the island size is about 1 ⁇ m in diameter, but in any case ⁇ 2 ⁇ m is 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 carbon-rich second main component embedded in the matrix in the form of an island Due to the proportion of the carbon-rich second main component embedded in the matrix in the form of an island, firstly the yield point in the area mentioned and secondly the bake hardening potential is set.
  • the metallurgical mechanism is that with the formation of the metastable structural components martensite, retained austenite and bainite, a large number of dislocations are generated, which cause a low yield strength.
  • dissolved carbon diffuses from the metastable structural components martensite, retained austenite and bainite into the previously created dislocations and causes the well-known increase in strength. Since no dissolved carbon is available in the pearlite, the carbon-rich component embedded in the matrix in the form of an island contains at least one of the metastable structural components martensite, 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 applications in shipbuilding, plant construction, infrastructure construction, in aerospace and household appliance technology are also conceivable.
  • the steel advantageously has a tensile strength Rm of 760 to 960 MPa, a yield strength Rp0.2 of 660 to 820 MPa, an elongation at break A80 of more than 10%, preferably more than 12%, a hole expansion ratio of more than 30% along the rolling direction. and a BH2 value of over 30 MPa.
  • 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 Mn increase the risk of center segregation, which significantly affects the ductility and thus the product quality to decrease. Lower contents ⁇ 1.0% by weight do not allow the required strength and toughness to be achieved with the aim of moderate analysis costs. An Mn content in the range between 1.4% by weight and 2.0% by weight is advantageous.
  • Aluminum AI Used for deoxidation in the steel mill process. The amount of AI used depends on the process. Therefore, no minimum AI content is given. An Al content of greater than 0.1% by weight significantly worsens the casting behavior in continuous casting. This results in a higher effort when potting.
  • Silicon Si One of the elements that enable the strength of steel to be increased in a cost-effective manner through solid solution strengthening.
  • Si reduces the surface quality of the hot strip by promoting firmly adhering scale on the reheated slab, which, with high Si contents, can only be removed with great effort or only inadequately. This is a particular disadvantage when it comes to subsequent galvanizing.
  • the Si content is therefore limited to a maximum of 0.8%, advantageously to 0.4%. If Si is largely dispensed with due to the surface issue, a lower limit of 0.03 is to be considered sensible, since comparatively high process costs arise in the steelworks if the Si content is further reduced.
  • Chromium Cr Improves strength and reduces the rate of corrosion, delays the formation of ferrite and pearlite and forms carbides.
  • the maximum content is set at less than 0.6% by weight, since higher contents result in a deterioration in ductility.
  • Molybdenum Mo Increases hardenability or reduces the critical cooling rate and thus promotes 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 in order to increase the strength of microalloyed structures.
  • Nickel Ni The use of even small amounts of Ni promotes ductility while maintaining the same strength. Because of the comparatively high costs, the content of Ni + Mo is limited to 0.5% by weight.
  • Phosphorus P is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness through solid solution strengthening and improves hardenability. As a rule, however, attempts are made to reduce the phosphorus content as much as possible, since it is, among other things, highly susceptible to segregation and to a great extent reduces the toughness. The accumulation of phosphorus at the grain boundaries can cause cracks to appear 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, through targeted measures that are precisely controlled on the process side, the use of small amounts of P can also be used to increase the strength in a cost-effective manner. For the reasons mentioned above, the phosphorus content is limited to less than 0.08% by weight.
  • Sulfur S Like phosphorus, it is bound as a trace element in iron ore. It is generally undesirable in steel, since it leads to undesirable inclusions of MnS, as a result of which the elongation and toughness properties are impaired. Attempts are therefore made to achieve the lowest possible amounts of sulfur in the melt and, if necessary, to convert the elongated inclusions into a more favorable geometric shape by means of a so-called Ca treatment. For the reasons mentioned above, the sulfur content is limited to less than 0.01% by weight.
  • Nitrogen N Is also an accompanying element from steel production. Steels with free nitrogen tend to have a strong aging effect. Even at low temperatures, nitrogen diffuses at dislocations and blocks them. It thus causes an increase in strength combined with a rapid loss of toughness. A setting of the nitrogen in the form of nitrides is possible, for example, by adding aluminum, niobium or titanium. As a result, however, the alloying elements mentioned are no longer available in the later process for the formation of new small precipitates that are very efficient in terms of strength. For the reasons mentioned above, the nitrogen content is limited to less than 0.01% by weight.
  • Micro-alloy elements are usually only added in very small amounts ( ⁇ 0.2% by weight per element). In contrast to the alloying elements, they work mainly through the formation of precipitates, but can also influence the properties in a dissolved state. Despite the small additions, micro-alloying elements influence the effective ones Manufacturing conditions as well as the processing and final properties of the product are strong.
  • Typical micro-alloy elements are, for example, niobium and titanium. These elements can be dissolved in the iron lattice and form carbides, nitrides and carbonitrides with carbon and nitrogen.
  • Nb and Ti depends in particular on the process control during hot rolling and the subsequent cooling process. With the addition of micro-alloy elements, the aim is to achieve grain refinement in the course of the process and to generate precipitates in the size range of nanometers. A minimum Nb + Ti content of 0.03% by weight is therefore a prerequisite for achieving the desired strength and elongation properties.
  • Niobium Nb The addition of niobium has a grain-refining effect due to the formation of carbides, which at the same time improves strength, toughness and elongation properties. At contents of more than 0.08% by weight, saturation behavior occurs, which is why a maximum content of less than or equal to 0.08% by weight is provided.
  • Titan Ti Has a grain-refining effect as a carbide former, which at the same time improves strength, toughness and elongation properties. Ti contents of more than 0.2% by weight deteriorate the ductility and the hole expansion capacity due to the formation of very coarse, primary TiN precipitates, which is why a maximum content of 0.2% by weight is specified.
  • ferritic-bainitic, microalloyed hot strip essentially retains its mechanical properties, although it is annealed - not as usual - at temperatures below Ac1 but at Ac1 ⁇ T ⁇ Ac1 + 100 ° C.
  • the temperature Ac1 describes the beginning of the transformation of the structure into austenite with slow heating according to the relevant standards. Ac1 is usually determined by dilatometric measurements.
  • both a high level of the hole expansion ratio of> 30% and a BH2 value of> 30 MPa are achieved in combination can.
  • a coiling temperature HT of less than 650 ° C, advantageously in the range of 450 ° C to 600 ° C, has an advantageous effect on the steel according to the invention, since the predominantly bainitic structure thus set has a high number of nucleation sites for the conversion to austenite at T> Ac1 provides and so the island diameter of the embedded second phase allows an average value of ⁇ 1 ⁇ m.
  • Below 450 ° C a comparatively high proportion of martensite is to be expected, which is disadvantageous after the heat treatment in terms of ductility and hole expansion capacity due to the internal structure.
  • the hot rolling end temperature for this steel is between 950 ° C. and Ar1 + 50 K, Ar1 describing the beginning of the conversion of austenite into ferrite during cooling.
  • Usual thickness ranges for slabs and thin slabs are between 35 mm and 450 mm. It is provided that the slab or thin slab is hot-rolled to form a hot strip with a thickness of 1.5 mm to 8 mm, preferably 1.8 mm to 4.5 mm.
  • the hot strip is coiled at a coiling temperature of preferably 450.degree. C. to 600.degree.
  • a coiling temperature preferably 450.degree. C. to 600.degree.
  • the hot-rolled Flat steel product is subjected to a heat treatment according to the invention in the temperature range Ac1 ⁇ T ⁇ Ac1 + 100 ° C. and is generally kept in this temperature range for 10 seconds to 10 minutes, possibly up to 48 hours, with higher temperatures being associated with shorter treatment times and vice versa.
  • the annealing is usually carried out in a continuous annealing (shorter annealing times), but can also take place, for example, in a hood annealing (longer annealing times).
  • the flat steel product is preferably hot-dip galvanized or electrolytically galvanized or coated with a metallic, inorganic or organic coating.
  • the annealing is preferably carried out in a continuous annealing plant upstream of the hot dip coating plant.
  • a hot-rolled flat steel product produced by the method according to the invention 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%. High strengths and thin sheet metal tend to be associated with lower elongation 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) compared to annealing below an Ac1 annealing temperature (comparison) in a radiant tube furnace (RTF).
  • RTF radiant tube furnace

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Description

Hochfestes, warmgewalztes Stahlflachprodukt mit hohem Kantenrisswiderstand und gleichzeitig hohem Bake-Hardening Potential, ein Verfahren zur Herstellung eines solchen StahlflachproduktsHigh-strength, hot-rolled flat steel product with high edge crack resistance and at the same time high bake hardening potential, a process for the production of such a flat steel product

Die Erfindung betrifft ein hochfestes, warmgewalztes Stahlflachprodukt mit hohem Kantenrisswiderstand und gleichzeitig hohem Bake-Hardening Potential. Des Weiteren betrifft die Erfindung ein Verfahren zur Herstellung eines solchen Stahlflachprodukts.The invention relates to a high-strength, hot-rolled flat steel product with high edge crack resistance and, at the same time, high bake hardening potential. The invention also relates to a method for producing such a 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 flat steel products made of steels with a multiphase structure, which usually contains tempered bainite, and with a yield strength Rp0.2 in the range from 660 to 820 MPa, in particular for the production of components for automobile construction, which in addition to a high tensile strength of at least 760 MPa and an elongation at break A80 of at least 10% must have a high hole expansion capacity with a hole expansion ratio of more than 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. 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. 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 oder WO2017012958 A1 offenbart.In general, a bake hardening effect (BH) is understood to be a controlled aging process which is due to the carbon and / or nitrogen present in the steel in solution and which is associated with an increase in the yield point. The bake hardening effect can be described with a BH2 value, which is defined as the increase in the yield point after a plastic pre-elongation of 2% and a subsequent heat treatment. With the bake hardening effect, for example, the dent resistance of a component can be increased by applying a suitable heat treatment after it has been formed into the component. According to EN 10346, bainitic steels are steels that are characterized by a comparatively high yield point and tensile strength with a sufficiently high elongation for cold forming processes. Due to the chemical composition, it is easy to weld. The structure typically consists of bainite with a proportion of ferrite. The structure can occasionally contain small proportions of other phases, such as martensite and retained austenite. Such a steel is used, for example, in the Offenlegungsschrift, among others DE 10 2012 002 079 A1 or WO2017012958 A1 disclosed.

Nachteilig ist hierbei allerdings ein noch nicht ausreichend hohes Lochaufweitevermögen.However, the disadvantage here is that the hole expansion capacity is not yet sufficiently high.

Der stark umkämpfte Automobilmarkt zwingt die Hersteller, stetig Lösungen zur Senkung des Flottenverbrauches und CO2-Abgasausstoßes unter Beibehaltung eines größtmöglichen Komforts und Insassenschutzes zu finden. Dabei spielt einerseits die Gewichtsreduktion aller Fahrzeugkomponenten eine entscheidende Rolle, 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.The highly competitive automotive market forces manufacturers to constantly find solutions to reduce fleet consumption and CO2 emissions while maintaining the greatest possible comfort and occupant protection. On the one hand, the weight reduction of all vehicle components plays a decisive role, on the other hand, the best possible behavior of the individual components under high static and dynamic stress, both during the 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 ultra-high-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 deformation 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 Beschichtung oder Lackierung.High-strength to ultra-high-strength steels must therefore meet comparatively high requirements with regard to their strength, ductility and energy absorption, especially when processing them, such as stamping, hot and cold forming, thermal quenching and tempering (e.g. air hardening, press hardening), welding and / or surface treatment , e.g. a metallic refinement, organic coating or painting.

Neu entwickelte Stähle müssen sich daher neben der verlangten Gewichtsreduzierung durch verringerte Blechdicken den zunehmenden Materialanforderungen an Dehngrenze, Zugfestigkeit, Verfestigungsverhalten und Bruchdehnung bei guten Verarbeitungseigenschaften, wie Umformbarkeit und Schweißbarkeit, stellen.In addition to the required weight reduction through reduced sheet thicknesses, newly developed steels must therefore 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 Bauteilanforderungen hinsichtlich Zähigkeit, Kantenrissunempfindlichkeit, verbessertem Biegewinkel und Biegeradius, Energieabsorption sowie Verfestigungsvermögen und dem Bake-Hardening-Effekt zu genügen.For such a reduction in sheet metal thickness, a high-strength to ultra-high-strength steel with a single or multi-phase structure must therefore be used in order to ensure sufficient strength of the motor vehicle components and to meet the high forming and component requirements in terms of toughness, edge crack insensitivity, improved bending angle and bending radius, energy absorption and strengthening capacity and the like Bake hardening effect is sufficient.

Auch wird zunehmend eine verbesserte Fügeeignung in Form von besserer allgemeiner Schweißbarkeit, wie einem größeren nutzbaren Schweißbereich beim Widerstandspunktschweißen und ein verbessertes Versagensverhalten der Schweißnaht (Bruchbild) unter mechanischer Beanspruchung sowie eine ausreichende Resistenz gegenüber verzögerter Rissbildung durch Wasserstoffversprödung gefordert.There is also increasingly an improved suitability for joining in the form of a better one general weldability, such as a larger usable welding area in resistance spot welding and an improved failure behavior of the weld seam (fracture pattern) under mechanical stress as well as sufficient resistance to delayed crack formation due to hydrogen embrittlement.

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 expansion capacity is a material property that describes the resistance of the material to crack initiation and crack propagation during forming operations in areas close to edges, such as when pulling collars.

Der Lochaufweiteversuch ist beispielsweise in der ISO 16630 normativ geregelt. Danach werden in ein Blech gestanzte Löcher mittels eines Dorns aufgeweitet. Die Messgröße ist die auf den Ausgangsdurchmesser bezogene Änderung des Lochdurchmessers bei der am Rand des Lochs der erste Riss durch das Blech auftritt.The hole expansion test is regulated in ISO 16630, for example. Then holes punched in a sheet metal are widened by means of a mandrel. The measured variable is the change in the hole diameter related to the initial diameter at which the first crack through the sheet occurs at the edge of the hole.

Eine verbesserte Kantenrissunempfindlichkeit bedeutet ein erhöhtes Umformvermögen 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.Improved edge crack insensitivity means an increased deformability of the sheet metal edges and can be described by an increased hole expansion capacity. This fact is known under the synonyms "Low Edge Crack" (LEC) or under "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.Proceeding from this, the present invention is based on the object of creating a high-strength, hot-rolled flat steel product with good forming properties, in particular with high edge crack resistance and a high bake hardening potential, as well as a method for producing such a flat steel product which, based on the steel, is good Offer a 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 Stahlflachprodukts 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 with the features of claim 1 and a method for producing a flat steel product with the features of claim 9. Advantageous refinements 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-%):

  • C: 0,04 bis 0,12
  • Si: 0,03 bis 0,8
  • Mn: 1 bis 2,5
  • 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
  • Cr: bis zu 0,6
According to the invention, a high-strength, hot-rolled flat steel 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% as well as a structure consisting of two main components, with a first main component of the structure having a proportion of at least 50%, consisting of one or more individual components of ferrite, tempered Bainite and tempered martensite each with less than 5% carbides, and a second main component of the structure having a proportion of 5% to a maximum of 50%, consisting of one or more individual components of martensite, retained austenite, bainite or pearlite, with the following chemical composition of Steel (in% by weight):
  • C: 0.04 to 0.12
  • Si: 0.03 to 0.8
  • Mn: 1 to 2.5
  • P: 0.08 or less
  • S: 0.01 or less
  • N: 0.01 or less
  • AI: up to 0.1
  • Ni + Mo: up to 0.5
  • Nb: up to 0.08
  • Ti: up to 0.2
  • Nb + Ti: 0.03 or more
  • 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.Remainder iron including unavoidable steel-accompanying elements, a good combination of strength, elongation and deformation properties. In addition, the production of this flat steel product according to the invention on the basis of 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 is also preferably characterized by a high hole expansion ratio of over 30% with a high tensile strength of 760 to 960 MPa and a high bake hardening potential BH2 of over 30 MPa.

In einer vorteilhaften Weiterbildung der Erfindung enthält das Stahlflachprodukt zur Erreichung besonders günstiger Eigenschaftskombinationen folgende Legierungszusammensetzung 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.In an advantageous development of the invention, the flat steel product contains the following alloy composition in% by weight in order to achieve particularly favorable combinations of properties: 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: at least 0.03 and particularly advantageous: C: 0.04 to 0.08, Si: 0.03 to 0.4, Mn: 1.4 up 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 to 0.05, Ti: up to 0.15 and 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 vergleichsweise höheren Kohlenstoffgehalt aufweist als der erste Hauptbestandteil.The use of the term “to” in the definition of the content ranges, such as 0.01 to 1% by weight, means that the benchmarks - in the example 0.01 and 1 - are included. The structure consists of two main components, with a first main component making up> = 50% with one or more structural components ferrite and tempered bainite and tempered martensite and with <5% carbides each and the second main component making up 5% -50% and consists of one or more structural components martensite, retained austenite, bainite or pearlite and preferably has a comparatively higher carbon content on average than the first main component.

Der vergleichsweise kohlenstoffreichere zweite Hauptbestandteil ist vorteilhaft inselförmig in den vergleichsweise kohlenstoffärmeren, die Matrix bildenden ersten Hauptbestandteil, eingebettet. Die Inselgröße ist mit ca. 1 µm Durchmesser, in jedem Falle aber < 2 µm vergleichsweise klein und die Inseln sind vorteilhaft gleichmäßig über die Banddicke verteilt. Die geringe Größe der Inseln und die homogene Verteilung des zweiten Hauptbestandteils tragen dabei zur Erreichung des hohen Lochaufweiteverhältnisses maßgeblich bei.The second main constituent, which is comparatively richer in carbon, is advantageously embedded in the form of an island in the comparatively lower carbon, first main constituent which forms the matrix. The island size is about 1 µm in diameter, but in any case <2 µm is 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.

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 Mechanismus besteht darin, dass mit der Bildung der metastabilen Gefügebestandteile 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, Restaustenit und Bainit.Due to the proportion of the carbon-rich second main component embedded in the matrix in the form of an island, firstly the yield point in the area mentioned and secondly the bake hardening potential is set. The metallurgical mechanism is that with the formation of the metastable structural components martensite, retained austenite and bainite, a large number of dislocations are generated, which cause a low yield strength. In the bake hardening process, dissolved carbon diffuses from the metastable structural components martensite, retained austenite and bainite into the previously created dislocations and causes the well-known increase in strength. Since no dissolved carbon is available in the pearlite, the carbon-rich component embedded in the matrix in the form of an island contains at least one of the metastable structural components martensite, retained austenite and bainite.

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.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 applications in shipbuilding, plant construction, infrastructure construction, in aerospace and household appliance technology are also 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 Bruchdehnung A80 von mehr als 10%, vorzugsweise mehr als 12%, ein Lochaufweitungsverhältnis von über 30% sowie einen BH2-Wert von über 30 MPa auf.The steel advantageously has a tensile strength Rm of 760 to 960 MPa, a yield strength Rp0.2 of 660 to 820 MPa, an elongation at break A80 of more than 10%, preferably more than 12%, a hole expansion ratio of more than 30% along the rolling direction. 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 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:

  • Kohlenstoff C: Wird benötigt zur Bildung von Karbiden, insbesondere im Zusammenhang 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.
Alloy elements are usually added to steel in order to specifically influence certain properties. An alloy element can influence different properties in different steels. The effect and interaction generally depends heavily on the amount, the presence of other alloying elements and the state of solution in the material. The relationships are varied 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 positive effects of the alloying elements used according to the invention are described below:
  • Carbon C: Is required for the formation of carbides, especially in connection with the so-called micro-alloy elements Nb, V and Ti, promotes the formation of martensite and bainite, stabilizes austenite and generally increases strength. Higher contents of C impair the welding properties and lead to a deterioration in the elongation and toughness properties, which is why a maximum content of less than 0.12% by weight, advantageously less than 0.08% by weight, is specified. In order to achieve sufficient strength of the material, a minimum addition of 0.04% by weight is required.

Mangan Mn: Stabilisiert den Austenit, erhöht die Festigkeit und die Zähigkeit und erhöht das Temperaturfenster für das Warmwalzen unterhalb der Rekristallisationsstopptemperatur. 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 Mn increase the risk of center segregation, which significantly affects the ductility and thus the product quality to decrease. Lower contents <1.0% by weight do not allow the required strength and toughness to be achieved with the aim of moderate analysis costs. An Mn content in the range between 1.4% by weight and 2.0% by weight is advantageous.

Aluminium AI: Wird für die Desoxidation im Stahlwerksprozess eingesetzt. Die Menge des eingesetzten AI ist prozessabhängig. Daher ist kein minimaler AI-Gehalt angegeben. Ein AI-Gehalt von größer 0,1 Gew.-% verschlechtert das Gießverhalten im Strangguss deutlich. Hierdurch entsteht ein höherer Aufwand beim Vergießen.Aluminum AI: Used for deoxidation in the steel mill process. The amount of AI used depends on the process. Therefore, no minimum AI content is given. An Al content of greater than 0.1% by weight significantly worsens the casting behavior in continuous casting. This results in a higher effort when potting.

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 enable the strength of steel to be increased in a cost-effective manner through solid solution strengthening. However, Si reduces the surface quality of the hot strip by promoting firmly adhering scale on the reheated slab, which, with high Si contents, can only be removed with great effort or only inadequately. This is a particular disadvantage when it comes to subsequent galvanizing. The Si content is therefore limited to a maximum of 0.8%, advantageously to 0.4%. If Si is largely dispensed with due to the surface issue, a lower limit of 0.03 is to be considered sensible, since comparatively high process costs arise in the steelworks if the Si content is further reduced.

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 the rate of corrosion, delays the formation of ferrite and pearlite and forms carbides. The maximum content is set at less than 0.6% by weight, since higher contents result in a deterioration in 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 hardenability or reduces the critical cooling rate and thus promotes 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 in order to increase the strength of microalloyed 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.Nickel Ni: The use of even small amounts of Ni promotes ductility while maintaining the same strength. Because of the comparatively high costs, the content of Ni + Mo is limited to 0.5% by weight.

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 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 Korngrenzen 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.Phosphorus P: is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness through solid solution strengthening and improves hardenability. As a rule, however, attempts are made to reduce the phosphorus content as much as possible, since it is, among other things, highly susceptible to segregation and to a great extent reduces the toughness. The accumulation of phosphorus at the grain boundaries can cause cracks to appear 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, through targeted measures that are precisely controlled on the process side, the use of small amounts of P can also be used to increase the strength in a cost-effective manner. For the reasons mentioned above, the phosphorus content is limited to less than 0.08% by weight.

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, since it leads to undesirable inclusions of MnS, as a result of which the elongation and toughness properties are impaired. Attempts are therefore made to achieve the lowest possible amounts of sulfur in the melt and, if necessary, to convert the elongated inclusions into a more favorable geometric shape by means of a so-called Ca treatment. For the reasons mentioned above, the sulfur content is limited to less than 0.01% by weight.

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 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.Nitrogen N: Is also an accompanying element from steel production. Steels with free nitrogen tend to have a strong aging effect. Even at low temperatures, nitrogen diffuses at dislocations and blocks them. It thus causes an increase in strength combined with a rapid loss of toughness. A setting of the nitrogen in the form of nitrides is possible, for example, by adding aluminum, niobium or titanium. As a result, however, the alloying elements mentioned are no longer available in the later process for the formation of new small precipitates that are very efficient in terms of strength. For the reasons mentioned above, the nitrogen content is limited to less than 0.01% by weight.

Mikrolegierungselemente werden in der Regel nur in sehr geringen Mengen zugegeben (< 0,2 Gew.-% pro Element). Sie wirken im Gegensatz zu den Legierungselementen hauptsächlich durch Ausscheidungsbildung können aber auch in gelöstem Zustand die Eigenschaften beeinflussen. Trotz der geringen Mengenzugaben beeinflussen Mikrolegierungselemente die zielführenden Herstellungsbedingungen sowie die Verarbeitungs- und Endeigenschaften des Produkts stark.Micro-alloy elements are usually only added in very small amounts (<0.2% by weight per element). In contrast to the alloying elements, they work mainly through the formation of precipitates, but can also influence the properties in a dissolved state. Despite the small additions, micro-alloying elements influence the effective ones Manufacturing conditions as well as the processing and final properties of the product are strong.

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-alloy elements are, for example, niobium and titanium. These elements can be dissolved in the iron lattice and form carbides, nitrides and carbonitrides with carbon and nitrogen.

Die Wirkung von Nb und Ti hängt insbesondere von der Prozessführung beim Warmwalzen und anschließenden Abkühlvorgang ab. Mit der Zugabe von Mikrolegierungselementen 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.The effect of Nb and Ti depends in particular on the process control during hot rolling and the subsequent cooling process. With the addition of micro-alloy elements, the aim is to achieve grain refinement in the course of the process and to generate precipitates in the size range of nanometers. A minimum Nb + Ti content of 0.03% by weight is therefore a prerequisite for achieving the desired strength and elongation properties.

Niob Nb: Die Zulegierung von Niob wirkt insbesondere durch die Bildung von Karbiden kornfeinend, wodurch gleichzeitig die Festigkeit, Zähigkeit und Dehnungseigenschaften 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.Niobium Nb: The addition of niobium has a grain-refining effect due to the formation of carbides, which at the same time improves strength, toughness and elongation properties. At contents of more than 0.08% by weight, saturation behavior occurs, 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.Titan Ti: Has a grain-refining effect as a carbide former, which at the same time improves strength, toughness and elongation properties. Ti contents of more than 0.2% by weight deteriorate the ductility and the hole expansion capacity due to the formation of very coarse, primary TiN precipitates, which is why a maximum content of 0.2% by weight is specified.

Ein erfindungsgemäßes Verfahren zur Herstellung des vorbeschriebenen, erfindungsgemäßen warmgewalzten Stahlflachprodukts, umfasst die Schritte:

  • Erschmelzen einer Stahlschmelze enthaltend (in Gewichts-%):
    • C: 0,04 bis 0,12
    • Si: 0,03 bis 0,8
    • Mn: 1 bis 2,5
    • P: max. 0,08
    • S: max. 0,01
    • N: max. 0,01
    • Al: 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
    • Cr: bis zu 0,6
    • Rest Eisen einschließlich unvermeidbarer stahlbegleitender Elemente,
  • Vergießen der Stahlschmelze zu einer Bramme oder Dünnbramme mittels eines horizontalen oder vertikalen Brammen- oder Dünnbrammengießverfahrens,
  • 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 Warmwalzens,
  • 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 Abkühlung den Beginn der Umwandlung und Ar1 den Abschluss der Umwandlung von Austenit in den Ferrit beschreibt,
  • Aufhaspeln des Warmbandes bei einer Haspeltemperatur in einem Temperaturbereich von 450°C bis 600°C,
  • Glühen des Warmbandes oberhalb Ac1 und unterhalb Ac1+100°C mit einer 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,
  • optionales Schmelztauchbeschichten des erwärmten Warmbandes nach dem Glühen und Abkühlen auf ≤ 500°C.
A method according to the invention for producing the above-described hot-rolled flat steel product according to the invention comprises the steps:
  • Melting a steel melt containing (in% by weight):
    • C: 0.04 to 0.12
    • Si: 0.03 to 0.8
    • Mn: 1 to 2.5
    • P: 0.08 or less
    • S: 0.01 or less
    • N: 0.01 or less
    • Al: up to 0.1
    • Ni + Mo: up to 0.5
    • Nb: up to 0.08
    • Ti: up to 0.2
    • Nb + Ti: 0.03 or more
    • Cr: up to 0.6
    • Remainder iron including unavoidable steel-accompanying elements,
  • Casting the molten steel into a slab or thin slab using a horizontal or vertical slab or thin slab casting process,
  • Reheating of the slab or thin slab to 1050 ° C to 1270 ° C and subsequent hot rolling of the slab or thin slab to form a hot strip with optional intermediate heating between individual rolling passes of hot rolling,
  • Rolling in the last pass at a final rolling temperature of less than 950 ° C and greater Ar1 + 50K, preferably less than 950 ° C and greater Ar3, where Ar3 describes the beginning of the transformation during cooling and Ar1 the end of the transformation of austenite into ferrite,
  • Coiling of the hot strip at a coiling temperature in a temperature range of 450 ° C to 600 ° C,
  • Annealing the hot strip above Ac1 and below Ac1 + 100 ° C. with an 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 5K / s to 20 K / s,
  • 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.In the context of the present investigations, it was found to be essential that the ferritic-bainitic, microalloyed hot strip essentially retains its mechanical properties, although it is annealed - not as usual - at temperatures below Ac1 but at Ac1 <T <Ac1 + 100 ° C.

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.The temperature Ac1 describes the beginning of the transformation of the structure into austenite with slow heating according to the relevant standards. Ac1 is usually determined by dilatometric measurements.

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.According to the invention, it was recognized that with an annealing of T <Ac1, the homogeneity of the ferritic-bainitic structure is largely retained and, in particular, the comparatively high level of the hole expansion ratio is maintained in the case of mainly bainitic structures. However, with annealing below Ac1, a BH2 value of> 30% cannot be achieved and a pronounced upper yield point of ReH> 820 MPa is formed, which is often viewed as problematic for the user. The cause is the blocking of dislocations through 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 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 µm erlaubt. Unterhalb von 450°C ist mit einem vergleichsweise hohen Anteil von Martensit zu rechnen, der nach der Wärmebehandlung hinsichtlich der Duktilität und des Lochaufweitevermögens aufgrund der inneren Struktur nachteilig ist.In the context of the invention, it was surprisingly found that when annealing in the temperature range Ac1 <T <Ac1 + 100 ° C., both a high level of the hole expansion ratio of> 30% and a BH2 value of> 30 MPa are achieved in combination can. A coiling temperature HT of less than 650 ° C, advantageously in the range of 450 ° C to 600 ° C, has an advantageous effect on the steel according to the invention, since the predominantly bainitic structure thus set has a high number of nucleation sites for the conversion to austenite at T> Ac1 provides and so the island diameter of the embedded second phase allows an average value of <1 µm. Below 450 ° C, a comparatively high proportion of martensite is to be expected, which is disadvantageous after the heat treatment in terms of ductility and hole expansion capacity 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.According to the invention, the hot rolling end temperature for this steel is between 950 ° C. and Ar1 + 50 K, Ar1 describing the beginning of the conversion of austenite into 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 Warmband mit einer Dicke von 1,5 mm bis 8 mm, vorzugsweise 1,8 mm bis 4,5 mm warmgewalzt wird.Usual thickness ranges for slabs and thin slabs are between 35 mm and 450 mm. It is provided that the slab or thin slab is hot-rolled to form a hot strip with a thickness of 1.5 mm to 8 mm, preferably 1.8 mm to 4.5 mm.

Das Warmband wird nach dem Warmwalzen erfindungsgemäß bei einer Haspeltemperatur von vorzugsweise 450°C bis 600°C aufgehaspelt. Zur Erreichung der geforderten Eigenschaftskombination für das Lochaufweitungsverhältnis, den BH2-Wert und der anderen mechanischen Eigenschaften, wird das warmgewalzte Stahlflachprodukt in einer erfindungsgemäßen Wärmebehandlung im Temperaturbereich 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.According to the invention, after hot rolling, the hot strip is coiled at a coiling temperature of preferably 450.degree. C. to 600.degree. To achieve the required combination of properties for the hole expansion ratio, the BH2 value and the other mechanical properties, the hot-rolled Flat steel product is subjected to a heat treatment according to the invention in the temperature range Ac1 <T <Ac1 + 100 ° C. and is generally kept in this temperature range for 10 seconds to 10 minutes, possibly up to 48 hours, with higher temperatures being associated with shorter treatment times and vice versa. The annealing is usually carried out in a continuous annealing (shorter annealing times), but can also take place, for example, in a hood annealing (longer annealing times).

Vorzugsweise wird das Stahlflachprodukt schmelztauch- oder elektrolytisch verzinkt oder metallisch, anorganisch oder organisch überzogen. Bei einer Schmelztauchbeschichtung erfolgt die Glühung vorzugsweise in einer der Schmelztauchbeschichtungsanlage vorgeschalteten Durchlaufglühanlage.The flat steel product is preferably hot-dip galvanized or electrolytically galvanized or coated with a metallic, inorganic or organic coating. In the case of hot dip coating, the annealing is preferably carried out in a continuous annealing plant upstream of the hot dip coating plant.

Ein nach dem erfindungsgemäßen Verfahren hergestelltes warmgewalztes 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.A hot-rolled flat steel product produced by the method according to the invention 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%. High strengths and thin sheet metal tend to be associated with lower elongation 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 statements made above regarding 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. Tabelle 1 Stahl C Si Mn P S N Al Mo Ti Nb A 0,08 0,5 1,9 0,01 0,001 0,006 0,08 0,15 0,13 0,05 B 0,06 0,6 1,9 0,01 0,004 0,004 0,06 0,19 0,11 0,04 The mechanical parameters as well as the values for the bake hardening (BH2) and the ratios for the hole expansion ratio (HER - hole expansion ratio) were determined on a hot strip produced according to the invention from two steels with different analyzes A and B according to Table 1. Table 1 stole C. Si Mn P. S. N Al Mon Ti Nb A. 0.08 0.5 1.9 0.01 0.001 0.006 0.08 0.15 0.13 0.05 B. 0.06 0.6 1.9 0.01 0.004 0.004 0.06 0.19 0.11 0.04

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 erfindungsgemäßen Glühung werden alle geforderten Kennwerte sicher erreicht. Tabelle 2 Haltezeit 5 s - 40 s, Abkühlrate 5 K/s - 20 K/s Stahl Dicke [mm] T(RTF) [°C] ΔT gegenüber AC1 [°C] ReL [MPa] ReH [MPa] Rp0,2 [MPa] Rm [MPa] A80 [%] BH2 [MPa] HER [%] A 2,2 680 40 842 905 855 874 14,3 16 49 Vergleich A 2,2 710 -10 842 898 858 880 14,2 9 45 Vergleich A 2,2 740 20 776 786 783 926 12,8 90 32 Erfindung A 2,2 770 50 keine keine 667 900 11,0 80 35 Erfindung B 2,2 680 44 829 859 839 883 14,0 13 72 Vergleich B 2,2 710 -14 837 861 839 886 13,6 17 51 Vergleich B 2,2 740 16 791 804 795 893 13,6 44 51 Erfindung B 2,2 770 46 keine keine 703 882 12,9 66 43 Erfindung B 2,2 800 76 656 666 668 828 12,4 82 54 Erfindung B 2,2 839 118 keine keine 565 771 15,3 65 55 Vergleich B 2,2 753 29 keine keine 727 873 12,7 76 65 Erfindung B 2,2 752 28 779 783 753 884 13 74 73 Erfindung B 3,4 680 44 894 929 892 926 14,9 25 53 Vergleich B 3,4 710 -14 883 909 890 924 14,6 34 59 Vergleich B 3,4 770 46 768 772 769 896 13,2 72 49 Erfindung B 3,4 800 76 718 742 719 834 14,1 87 50 Erfindung Table 2 shows the results for an annealing of the hot strip according to the invention at Ac1 <T <Ac1 + 100 ° C. (invention) compared to annealing below an Ac1 annealing temperature (comparison) in a radiant tube furnace (RTF). With the annealing according to the invention, all required characteristic values are reliably achieved. Table 2 Holding time 5 s - 40 s, cooling rate 5 K / s - 20 K / s stole Thickness [mm] T (RTF) [° C] ΔT compared to AC1 [° C] ReL [MPa] ReH [MPa] Rp0.2 [MPa] Rm [MPa] A80 [%] BH2 [MPa] HER [%] A. 2.2 680 40 842 905 855 874 14.3 16 49 comparison A. 2.2 710 -10 842 898 858 880 14.2 9 45 comparison A. 2.2 740 20th 776 786 783 926 12.8 90 32 invention A. 2.2 770 50 no no 667 900 11.0 80 35 invention B. 2.2 680 44 829 859 839 883 14.0 13th 72 comparison B. 2.2 710 -14 837 861 839 886 13.6 17th 51 comparison B. 2.2 740 16 791 804 795 893 13.6 44 51 invention B. 2.2 770 46 no no 703 882 12.9 66 43 invention B. 2.2 800 76 656 666 668 828 12.4 82 54 invention B. 2.2 839 118 no no 565 771 15.3 65 55 comparison B. 2.2 753 29 no no 727 873 12.7 76 65 invention B. 2.2 752 28 779 783 753 884 13th 74 73 invention B. 3.4 680 44 894 929 892 926 14.9 25th 53 comparison B. 3.4 710 -14 883 909 890 924 14.6 34 59 comparison B. 3.4 770 46 768 772 769 896 13.2 72 49 invention B. 3.4 800 76 718 742 719 834 14.1 87 50 invention

Claims (11)

  1. High-strength, hot-rolled flat steel product having a high resistance to edge cracks and made of a steel having an Rp0.2 proof stress of from 660 to 820 MPa, a BH2 value of over 30 MPa and a hole expansion ratio of over 30% measured according to ISO 16630, said steel having a microstructure consisting of two main components, wherein a first main component of the microstructure has a proportion of at least 50% and consists of one or more individual components of ferrite, tempered bainite and tempered martensite, each having less than 5% carbides, and wherein a second main component of the microstructure has a proportion of 5% to 50% and consists of one or more individual components of martensite, retained austenite, bainite or pearlite, the steel having the following chemical composition, in wt.%:
    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
    Al: 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 elements inevitably present in steel.
  2. Flat steel product according to claim 1, characterised in that the steel contains, in wt.%:
    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 0.08
    Ti: up to 0.2
    Nb+Ti: min. 0.03.
  3. Flat steel product according to either claim 1 or claim 2, characterised in that the steel contains, in wt.%:
    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
    Nb+Ti: min. 0.03.
  4. Flat steel product according to at least one of claims 1 to 3, characterised in that the second main component of the microstructure is embedded in the manner of an island in the first main component of the microstructure, which first main component is designed as a matrix.
  5. Flat steel product according to claim 4, characterised in that the island-like embedded elements have a size of less than 2 µm, preferably less than 1 µm.
  6. Flat steel product according to at least one of claims 1 to 5, characterised in that the tensile strength Rm of the flat steel product is from 760 to 960 MPa and the elongation at break A80 of the flat steel product is more than 10%, preferably more than 12%.
  7. Flat steel product according to at least one of claims 1 to 6, characterised in that it is hot-dip galvanised or electrogalvanised or is coated with a metallic, inorganic or organic coating.
  8. Flat steel product according to at least one of claims 1 to 7, characterised in that the second main component has, on average, a comparatively higher carbon content than the first main component.
  9. Method for producing a hot-rolled flat steel product according to at least one of claims 1 to 8, comprising the steps of:
    - smelting a steel melt containing, in wt.%:
    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
    Al: 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 elements inevitably present in steel,
    - casting the steel melt to form a slab or thin slab by means of a horizontal or vertical slab- or thin-slab casting method,
    - reheating the slab or thin slab to from 1050°C to 1250°C and subsequently hot-rolling the slab or thin slab to form a hot strip, with optional intermediate heating between individual hot-rolling passes,
    - in the last pass, rolling at a final rolling temperature of less than 950°C and greater than Ar3,
    - coiling the hot strip at a coiling temperature in the range of from 450°C to 600°C,
    - annealing the hot strip above Ac1 and below Ac1 +100°C with an annealing time of from 10 seconds to 10 minutes and an average cooling rate between the annealing temperature and 500°C of from 1 K/s to 150 K/s, preferably 5 K/s to 20 K/s,
    - optionally hot-dip coating the hot strip directly after the cooling process at the cooling stop temperature in a continuous hot-dip galvanizing line.
  10. Method according to claim 9, characterised in that the hot strip is rolled with a final rolling temperature of greater than Ar1+50°C.
  11. Method according to either claim 9 or claim 10, characterised in that the slab is hot-rolled to form a hot strip with a thickness of from 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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DE102021108448A1 (en) 2021-04-01 2022-10-06 Salzgitter Flachstahl Gmbh Steel strip made from a high-strength multi-phase steel and method for producing such a steel strip
CN113667894B (en) * 2021-08-13 2022-07-15 北京首钢冷轧薄板有限公司 800 MPa-grade dual-phase steel with excellent hole expansion performance and preparation method thereof
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