US20190119774A1 - Flat steel product and method for the production thereof - Google Patents

Flat steel product and method for the production thereof Download PDF

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Publication number
US20190119774A1
US20190119774A1 US15/571,379 US201615571379A US2019119774A1 US 20190119774 A1 US20190119774 A1 US 20190119774A1 US 201615571379 A US201615571379 A US 201615571379A US 2019119774 A1 US2019119774 A1 US 2019119774A1
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Prior art keywords
flat steel
steel product
temperature
content
steel
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Richard G. Thiessen
Thomas Heller
Karsten Machalitza
Roland Sebald
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ThyssenKrupp Steel Europe AG
ThyssenKrupp AG
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ThyssenKrupp Steel Europe AG
ThyssenKrupp AG
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Assigned to THYSSENKRUPP AG, THYSSENKRUPP STEEL EUROPE AG reassignment THYSSENKRUPP AG ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: MACHALITZA, Karsten, HELLER, THOMAS, SEBALD, ROLAND, THIESSEN, RICHARD G.
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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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/22—Martempering
    • C—CHEMISTRY; METALLURGY
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • C23C2/04—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
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C23C2/29—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—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
    • 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
    • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
    • C23F17/00—Multi-step processes for surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by subclass C21D or C22F or class C25
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present disclosure generally relates to flat steel products with optimized strength and elongation characteristics, including methods for producing such flat steel products.
  • CA 2 734 976 A1 discloses a steel having good ductility and formability, which is to have a tensile strength of at least 980 MPa.
  • the steel comprises, as well as iron and unavoidable impurities (in % by weight), 0.17%-0.73% C, up to 3.0% Si, 0.5%-3.0% Mn, up to 0.1% P, up to 0.07% S, up to 3.0% Al and up to 0.010% N.
  • the sum total of the Al and Si contents is to be at least 0.7%.
  • the martensite content in the steel microstructure is to be 10%-90%, the proportion of residual austenite within the range of 5%-50%, and the proportion of ferritic bainite originating from “upper bainite” at least 5%.
  • “Upper bainite” refers here to a bainite in which fine carbide grains are distributed homogeneously, whereas these are not to be found in “lower bainite”. Higher contents of upper bainite of 17% or more are regarded as advantageous in order to generate the desired high residual austenite contents in the microstructure.
  • EP 2 524 970 A1 additionally discloses a flat steel product having a tensile strength R m of at least 1200 MPa and consisting of a steel which, as well as Fe and unavoidable impurities, contains (in % by weight) C: 0.10%-0.50%, Si: 0.1%-2.5%, Mn: 1.0%-3.5%, Al: up to 2.5%, P: up to 0.020%, S: up to 0.003%, N: up to 0.02%, and optionally one or more of the elements “Cr, Mo, V, Ti, Nb, B and Ca” in the following contents: Cr: 0.1%-0.5%, Mo: 0.1%-0.3%, V: 0.01%-0.1%, Ti: 0.001%-0.15%, Nb: 0.02%-0.05%.
  • the flat steel product has a microstructure having (in area %) less than 5% ferrite, less than 10% bainite, 5%-70% unannealed martensite, 5%-30% residual austenite and 25%-80% annealed martensite, with at least 99% of the iron carbides present in the annealed martensite having a size of less than 500 nm. Owing to its minimized proportion of overannealed martensite, a flat steel product having such characteristics has optimized formability.
  • EP 2 524 970 A1 likewise discloses a process for producing a flat steel product of the type elucidated above.
  • a flat steel product having the aforementioned composition is heated at a heating rate ⁇ H1 , ⁇ H2 of at least 3° C./s to an austenitization temperature T HZ above the A 3 temperature of the steel of the flat steel product and of not more than 960° C.
  • the flat steel product is kept at that temperature for an austenitization period t HZ of 20-180 s, in order then to be cooled to a cooling finish temperature.
  • the latter is greater than the martensite finish temperature and less than the martensite start temperature, the cooling being effected at a cooling rate at least equal to a minimum cooling rate determined as a function of the alloy contents of the steel.
  • the flat steel product is kept at the cooling finish temperature for 10-60 s, in order then to be heated at a heating rate of 2-80° C./s to a partitioning temperature of 400-500° C. This may be followed by an isothermal hold of the flat steel product at the partitioning temperature over up to 500 s. Subsequently, the flat steel product is cooled down at a cooling rate of 3-25° C./s.
  • the heating and the optional additional holding at the partitioning temperature result in enrichment of the residual austenite in the microstructure of the flat steel product with carbon from the oversaturated martensite.
  • This operation also is referred to in the art as “partitioning of the carbon” or “partitioning”.
  • the partitioning can be conducted as early as during the heating, as what is called “ramped partitioning”, by means of holding at the partitioning temperature after the heating (called “isothermal partitioning”), or by means of a combination of isothermal and ramped partitioning.
  • the slower heating rate which is the aim in ramped partitioning as compared with isothermal partitioning permits particularly exact actuation of the partitioning temperature specified in each case with a reduced energy input.
  • the steels having the characteristics and having been processed as elucidated above are among what are called the “AHSS steels” (advanced high strength steels).
  • One example object of the present disclosure is to provide a flat steel product that has not just an optimized combination of high strength and elongation, but also, coupled with improved use properties such as good suitability for welding, surface characteristics and suitability for coating with a metallic protective coating, has a microstructure that assures optimized formability irrespective of the direction of forming.
  • a flat steel product of the invention accordingly features a tensile strength R m of at least 950 MPa, a yield point of at least 800 MPa and an elongation at break A 50 determined according to DIN EN ISO 6892, sample shape 1, of at least 8%.
  • a flat steel product of the invention consists here of a steel consisting of, as well as iron and unavoidable impurities, (in % by weight)
  • the invention is based on the finding that, through the choice of a suitable alloy, it is possible to obtain a flat steel product in which a microstructure comprising minimum residual austenite contents at most and characterized by a high content of annealed martensite and by ultrafinely distributed unannealed martensite results in a high strength coupled with very good deformability.
  • Typical tensile strengths R m of flat steel products of the invention are 950-1300 MPa, coupled with a yield point which is at least 800 MPa and can reach as far as the respective tensile strength.
  • the elongation A 50 of flat steel products of the invention is typically 8%-20%.
  • a flat steel product of the invention in the hole expanding test according to ISO 16630, regularly achieves hole expansion ratios of at least 30%.
  • Carbon has several important functions in the steel of the invention. Firstly, the C content plays a major role in the formation of austenite and adjustment of the A 3 temperature. An adequate C content enables full austenitization even at temperatures of less than 930° C. In the subsequent quenching, the residual austenite is stabilized by carbon. This stabilization can be assisted by an additional heat treatment step as envisaged by the invention in the process of the invention. The strength of the martensite is also greatly affected by the C content of the steel. On the other hand, the martensite start temperature is shifted to ever lower temperatures with rising C content, which leads to challenges in the production.
  • the invention envisages, in the steel of a flat steel product of the invention, a C content of 0.05%-0.2% by weight, especially at least 0.065% by weight of C, and in practice the positive effect of C in the steel of the invention can be exploited in a particularly reliable manner when the C content is 0.07%-0.19% by weight.
  • the carbon equivalent CE should be not more than 1.1% by weight, in order to assure good weldability. Particularly good suitability for welding can be assured in that the CE value is limited to not more than 1.0% by weight. However, the CE value should not be less than 0.254% by weight and especially not less than 0.29% by weight, in order to obtain the effect of the alloy elements that affect the calculation of the carbon equivalent CE and are envisaged in accordance with the invention.
  • the presence of silicon in the steel of a flat steel product of the invention suppresses the formation of cementite, which would bind carbon that would then no longer be available for the stabilization of the residual austenite, and which would worsen the elongation.
  • the same effect can also be achieved by including Al in the alloy.
  • a minimum of 0.2% by weight of Si should be present in the steel envisaged in accordance with the invention.
  • Si contents of more than 1.5% by weight would have an adverse effect on the surface quality of a flat steel product of the invention.
  • the Si content is 0.2%-1.5% by weight, and in practice Si contents of at least 0.25% by weight or at most 0.95% by weight have been found to be particularly favorable and those of at most 0.63% by weight to be very particularly favorable.
  • Aluminum is added to the steel of a flat steel product of the invention in steel production for deoxidation and for binding of any nitrogen present.
  • Al can additionally also be used for the suppression of cementite.
  • the Al content of a steel envisaged for a flat steel product of the invention is limited to 0.01%-1.5% by weight. If low austenitization temperatures are to be assured, it may be appropriate to limit the Al content to a maximum of 0.44% by weight, especially to 0.1% by weight.
  • higher Al contents have an adverse effect on castability in steel production.
  • aluminum can be bound by nitrogen to give aluminum nitride.
  • the sum total of the contents of Al and Si in the steel of a flat steel product of the invention can be limited to not more than 1.7% by weight, and particularly favorable upper limits here have been found to be not more than 1.5% by weight, especially not more than 1.0% by weight, particularly with regard to optimization of suitability for welding.
  • advantageous upper limits for the sum total of the contents of Al and Si have likewise been found to be not more than 1.0% by weight, especially not more than 0.4% by weight.
  • Manganese is important for the hardenability of the steel of a flat steel product of the invention and additionally prevents the formation of unwanted pearlite during the cooling.
  • the presence of Mn thus enables the formation of a starting microstructure (martensite and residual austenite) suitable for the formation of the microstructure stipulated in accordance with the invention.
  • too high a Mn concentration would have an adverse effect on the elongation and weldability of the steel. Therefore, the range envisaged for the Mn content in accordance with the invention is 1.0%-3.0% by weight, especially at least 1.5% by weight or at most 2.4% by weight.
  • Phosphorus has an adverse effect on the weldability of a flat steel product of the invention.
  • the P content should be as low as possible, but at least should not exceed 0.02% by weight, and should especially be less than 0.02% by weight or less than 0.018% by weight.
  • the presence of effective contents of sulfur in the steel of a flat steel product of the invention would lead to formation of sulfides, especially MnS or (Mn,Fe)S, which would have an adverse effect on the elongation.
  • the S content of the steel should be kept as low as possible, but at least should not be higher than 0.005% by weight, especially less than 0.005% by weight or less than 0.003% by weight.
  • the N content of the steel of a flat steel product of the invention is limited to not more than 0.008% by weight.
  • the N content, for avoidance of any adverse effect should be below 0.008% by weight, especially less than 0.006% by weight.
  • Chromium in contents of up to 1.0% by weight can optionally be utilized in the steel envisaged in accordance with the invention as an effective inhibitor of pearlite, and additionally contributes to strength.
  • contents of more than 1.0% by weight of Cr there is the risk of marked grain boundary oxidation.
  • at least 0.05% by weight is required.
  • the presence of Cr has a particularly favorable effect in the steel of a flat steel product of the invention when at least 0.15% by weight of Cr is present, and an optimal effect is achieved at contents of up to 0.8% by weight.
  • the steel of a flat steel product of the invention may additionally also contain molybdenum in contents of 0.05%-0.2% by weight. Mo in these contents likewise particularly effectively suppresses the formation of unwanted pearlite.
  • the steel of a flat steel product of the invention may additionally optionally contain contents of one or more micro alloy elements, in order to promote strength through the formation of very finely divided carbides. It has been found that contents of Ti and Nb are particularly suitable for this purpose.
  • Ti contents of at least 0.005% by weight and Nb contents of at least 0.001% by weight each lead, alone or in combination with one another, to freezing of the particle and phase boundaries during the heat treatment that a flat steel product of the invention undergoes in the course of production thereof in accordance with the invention.
  • Ti can additionally be utilized for binding of the nitrogen present in the steel, in order to enable an effect of other alloy elements, especially boron. It has been found that particularly advantageous Ti contents are those of at least 0.02% by weight. However, too high a concentration of micro alloy elements would lead to carbides of excessive dimensions, which could initiate cracks at high degrees of deformation.
  • the Ti content of the steel of a flat steel product of the invention is limited to not more than 0.2% by weight and the Nb content thereof to not more than 0.05% by weight, and it is found to be advantageous for avoidance of adverse effects of the presence of micro alloy elements when the sum of the contents of Nb and Ti does not exceed 0.2% by weight.
  • the boron likewise optionally present in the steel of a flat steel product of the invention segregates to the phase boundaries and attenuates their movement. This leads to a fine-grain microstructure, which has an advantageous effect on the mechanical properties.
  • Ti can be included in the steel alloy, as mentioned above.
  • the steel envisaged in accordance with the invention must contain at least 0.0001% by weight of B. In the case of contents of more than 0.005% by weight, no further increase in the positive effect of B can be identified.
  • the flat steel product of the invention may have been provided with a metallic protective coating. This may especially have been applied by melt dip coating. Suitable coatings here for a flat steel product of the invention are especially Zn-based coatings.
  • the process of the invention for producing a high-strength flat steel product comprises the following operating steps:
  • FIG. 1 The principle of the procedure of the invention is illustrated in the diagram appended as FIG. 1 .
  • a flat steel product consisting of a steel having the above-elucidated composition.
  • the flat steel product provided may especially be a cold-rolled flat steel product.
  • the flat steel product in the first step is heated at a heating rate ⁇ H1 of 5-25 K/s up to an inflection temperature T W of 200-400° C.
  • ⁇ H1 for the productivity of the process have been found to be at least 5 K/s, while a heating rate ⁇ H1 of more than 25 K/s has been found to be very energy-intensive and costly.
  • the heating in the second step is continued at a heating rate ⁇ H2 of 2-10 K/s until the austenitization temperature T HZ has been attained.
  • the alloy elements present in the flat steel product can diffuse within the flat steel product during heating operation.
  • the heating rate increases, there is a decrease in the time available for the diffusion process and hence for the homogenization of the alloy element distribution of the flat steel product.
  • Inhomogeneously distributed alloy elements can lead to locally different microstructure transformations.
  • values for the heating rate ⁇ H2 of less than 2 K/s have been found to be unfavorable for the economic viability of the process.
  • the heating to the austenitization temperature can also be effected in one run with a constant heating rate of 5-10 K/s. In that case, the heating rates ⁇ H1 and ⁇ H2 in operating step b) are the same.
  • the austenitization temperature T HZ must be above the A 3 temperature.
  • the A 3 temperature is dependent on the analysis and can be estimated by the following empirical equation (alloy contents used in % by weight):
  • the alloying of the steel selected in accordance with the invention permits restriction of the austenitization temperature T HZ to a maximum of 950° C. and hence allows the operating costs incurred for the performance of the process of the invention to be limited.
  • the austenitization period t HZ over which the flat steel product is kept at the austenitization temperature T HZ in operating step c) is limited to 5-15 seconds, where the austenitization period t HZ may be less than 15 s in order to avoid any unwanted grain growth.
  • step d there follows controlled and gradual cooling of the flat steel product proceeding from the austenitization period t HZ .
  • This cooling can extend over 50-300 seconds and has to end at an intermediate temperature T K no lower than 680° C., in order to avoid the unwanted formation of ferrite.
  • the upper limit in the intermediate temperature T K is preferably at temperatures of not more than A 3 , and is typically restricted to 775° C., since, in the case of higher intermediate temperatures T K , the cooling output required for the subsequent cooling is disproportionately high and thus puts the economic viability of the process into question.
  • the flat steel product in operating step e), is quenched to an analysis-dependent cooling finish temperature T Q at a high cooling rate ⁇ Q .
  • the high cooling rate ⁇ Q can be achieved, for example, with modern gas jet cooling.
  • the minimum cooling rate ⁇ Q necessary to avoid ferritic and bainitic transformation is more than 30 K/s.
  • the range within which the cooling finish temperature T Q lies is limited at the upper end by the martensite start temperature T MS , and at the lower end by a temperature which is 175° C. below the martensite start temperature T MS ((T MS ⁇ 175° C.) ⁇ T Q ⁇ T MS ).
  • the martensite start temperature can be estimated by means of the following equation (alloy contents used in % by weight):
  • the flat steel product is kept at the cooling finish temperature T Q for a holding period t Q of 10-60 seconds, in order to establish the microstructure.
  • a martensitic microstructure is obtained with up to 30% residual austenite.
  • the amount of martensite produced in this step depends essentially on the degree to which the cooling finish temperature is below the martensite start temperature T MS .
  • the holding period t Q is at least 10 seconds, in order to assure homogenization of the temperature in the flat steel product and hence a homogeneous microstructure. In the case of longer holding periods of more than 60 seconds, the homogenization of the temperature is complete.
  • the holding period t Q is not more than 60 seconds, in order to increase the productivity of the process.
  • the heat treatment of the flat steel product conducted in operating step g) has the aim of controlled redistribution of the carbon such that the microstructure of the flat steel product obtained on conclusion of the process consists essentially of two different kinds of martensite, namely an annealed martensite and an unannealed martensite.
  • operating step g) comprises two process variants g.1) and g.2), of which the first variant g.1) leads to an uncoated flat steel product of the invention and the second variant g.2) to a flat steel product of the invention provided with a Zn coating.
  • the temperature regime in each of the variants g.1), g.2) of the operating step g) is chosen such that the existing residual austenite present in the microstructure is enriched with carbon from the oversaturated martensite.
  • the formation of carbides and the breakdown of residual austenite is deliberately suppressed via the inventive limitation of the total treatment period t BT . This period is 10-1000 seconds in order to enable sufficient redistribution of the carbon.
  • the treatment of the flat steel product in operating step g) comprises keeping the flat steel product over the entire treatment period t BT at a treatment temperature T B at least equal to the cooling finish temperature T Q and not higher than 550° C., and a cooling finish temperature T Q of not more than 500° C. has been found to be particularly favorable.
  • the treatment temperature T B may also be higher than the cooling finish temperature T Q .
  • the flat steel product, proceeding from the cooling finish temperature T Q is heated to the respective treatment temperature T B , where the heating should be effected at a heating rate ⁇ B1 of less than 80 K/s.
  • the flat steel product is brought to a treatment temperature T B of 400-500° C. at a heating rate ⁇ B1 of less than 80 K/s, in order to enrich the residual austenite with carbon from the oversaturated martensite.
  • the formation of carbides and the breakdown of residual austenite are deliberately suppressed by the inventive limitation of the total treatment period t BT , which in this variant g.2) of operating step g) is composed of the heating time t BR required for the heating and the holding period t BI over which the flat steel product is kept under isothermal conditions at the temperature T B . Given a sufficiently gradual heating rate ⁇ B1 , the isothermal hold can also be dispensed with, and so the holding period t BI can be “0”.
  • the flat steel product after the heating and the optional hold at the treatment temperature T B , undergoes a melt dip-coating operation in which it is coated with a Zn coating.
  • the treatment temperature T B can be chosen such that it corresponds to the inlet temperature at which the flat steel product is to enter the respective melt bath.
  • the treatment temperatures T B are in the range of 450-500° C.
  • This melt bath typically comprises, as well as zinc and unavoidable impurities, a total of up to 3.0% by weight of one or more elements from the group consisting of Al, Mg, Si, Pb, Ti, Ni, Cu, B and Mn.
  • the flat steel product, on conclusion of operating step g), for new production of martensite is cooled in a controlled manner at a cooling rate ⁇ B2 of more than 5 K/s, the cooling rates typically being not more than 50 K/s.
  • ⁇ B2 is more than 5 K/s, in order to avoid the formation of pearlite and ferrite.
  • the process of the invention can be conducted in a continuous run in conventional calcining systems or belt coating systems that are typically provided for the purpose.
  • the flat steel product of the invention has a microstructure consisting of
  • microstructure of a flat steel product of the invention with a mean grain size of less than 2 ⁇ m, is very fine and can barely be assessed by means of standard light-optical microscopy. Therefore, an assessment by means of scanning electron microscopy (SEM) with a minimum of 5000-fold magnification is recommended.
  • SEM scanning electron microscopy
  • the maximum permissible residual austenite content can be determined only with difficulty by light microscopy or scanning electron microscopy. Therefore, a quantitative determination of the residual austenite by means of x-ray diffraction (XRD) is recommended (according to ASTM E975), by which the residual austenite content is reported in % by volume.
  • XRD x-ray diffraction
  • Another measure that can be employed for the quality of the mechanical properties of a flat steel product of the invention is the distortion of the crystal lattice.
  • This lattice distortion is very important for the initial resistance to plastic deformation.
  • a suitable method for the measurement and quantification of lattice distortion is electron backscatter diffraction (EBSD).
  • EBSD electron backscatter diffraction
  • a useful EBSD evaluation method is what is called the kernel average misorientation (KAM—further description in the handbook “OIM Analysis v5.31” from EDAX Inc., 91 McKee Drive, Mahwah, N.J. 07430, USA), wherein the orientation of a measurement point is compared with the neighboring points.
  • KAM kernel average misorientation
  • the KAM of the third adjacent points is evaluated.
  • a flat steel product of the invention must have a mean KAM value from a measurement region of at least 75 ⁇ m ⁇ 75 ⁇ m of more than 1.200, preferably more than 1.250
  • % C is the respective C content, % Si the respective Si content, % Mn the respective Mn content, % Cr the respective Cr content, % Mo the respective Mo content and % Al the respective Al content of the steels A-I.
  • Samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60 manufactured from steels A-I have undergone the process sequence shown in FIG. 1 . These have firstly been heated at a heating rate ⁇ H1 to an inflection temperature T W and then at a heating rate ⁇ H2 to an austenitization temperature T HZ , each of which was above the A 3 temperature of the respective steel but lower than 950° C. The samples thus heated have subsequently been kept at the austenitization temperature T HZ over an austenitization period t HZ and then cooled to an intermediate temperature T K over a cooling period t K .
  • accelerated cooling at a cooling rate ⁇ Q has set in, in which the samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60 have been cooled to a cooling finish temperature T Q which, for each of samples 1-7, 11, 12, 16, 17, 19-23, 28-31, 33-35, 39, 40 and 43-60, was up to 175° C. lower and, for sample 18, higher than the martensite start temperature T MS of the respective steel A-I of samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60.
  • Samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60 have been kept at the cooling finish temperature T Q for a holding period t Q of 10-60 s.
  • Samples 1-7, 11, 12, 16, 17, 19-23, 28-31, 33-35, 39, 40 and 43-48 were subsequently heated at a heating rate ⁇ B1 over a heating time t BR to a treatment temperature T B at which they have been kept over an additional holding period t BI in some experiments.
  • sample 18 was cooled to the treatment temperature T B . This was followed by cooling to room temperature at a cooling rate ⁇ B2 .
  • Samples 49-60 after being cooled down to the cooling finish temperature T Q and held at T Q for the holding period t Q in an isothermal manner without heating, were kept at the treatment temperature T B over a holding period t BI . For samples 49-60 too, this was followed by cooling to room temperature at a cooling rate ⁇ B2 .
  • samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60 in the cases where the treatment temperature T B was at a level of about 450° C. sufficient for entry into a Zn melt bath, could have passed through a melt bath. In the context of the experiments, however, this has been dispensed with, and so it did not affect the results of the study.
  • Comparative examples B11 and D28 by contrast, illustrate the effect of an insufficient austenitization temperature T HZ .
  • the microstructure has not been fully austenitized, and so too much ferrite forms in the microstructure. This leads to extremely localized damage and early failure during forming.
  • Comparative example D29 shows how austenitization for too long a period at high temperatures can adversely affect formability.
  • Comparative examples A3 and C19 show that, in the case of excessively low cooling rates ⁇ Q , the desired yield point is not attained, which is attributable to the fact that ferrite formation could not be adequately prevented.
  • Comparative example C18 which was produced with too high a cooling finish temperature T Q , shows a yield point below that desired and low hole expansion ratios. These are attributable to an elevated level of ferrite and bainite in the microstructure.
  • Comparative examples E33-E35 and E56-E58 show a yield point and strength below those desired, which is attributable to the composition not in accordance with the invention and too high a ferrite content in the microstructure obtained.
  • the high ferrite content is caused by inadequate prevention of carbide formation as a result of too low a silicon content and too low a content of aluminum and silicon in relation to carbon, manganese and chromium, and hence too high a ⁇ factor.
  • comparative examples F39, F40, F59 and F60 show the effects of too low a ⁇ factor, which also leads to departures from the microstructure desired.
  • the minimum strength was attained in some cases, but the yield point and the hole expansion here are not within the target range.
  • Comparative example G43 makes it clear that too high a factor leads to excessively high residual austenite contents and reduced formability, which is manifested in poor hole expansion values ⁇ 1, ⁇ 2.
  • Comparative example 148 illustrates that too low a cooling rate ⁇ B2 leads to increased ferrite formation and hence to low yield points.

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