EP2840159B1 - Method for producing a steel component - Google Patents

Method for producing a steel component Download PDF

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Publication number
EP2840159B1
EP2840159B1 EP13181374.3A EP13181374A EP2840159B1 EP 2840159 B1 EP2840159 B1 EP 2840159B1 EP 13181374 A EP13181374 A EP 13181374A EP 2840159 B1 EP2840159 B1 EP 2840159B1
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European Patent Office
Prior art keywords
flat steel
steel product
vol
content
rolled
Prior art date
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EP13181374.3A
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German (de)
French (fr)
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EP2840159A1 (en
EP2840159B8 (en
Inventor
Brigitte Hammer
Thomas Heller
Frank Hisker
Rudolf Kawalla
Grzegorz Korpala
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ThyssenKrupp Steel Europe AG
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ThyssenKrupp Steel Europe AG
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Priority to ES13181374.3T priority Critical patent/ES2636780T3/en
Application filed by ThyssenKrupp Steel Europe AG filed Critical ThyssenKrupp Steel Europe AG
Priority to EP13181374.3A priority patent/EP2840159B8/en
Priority to PCT/EP2014/067571 priority patent/WO2015024903A1/en
Priority to JP2016535447A priority patent/JP6606075B2/en
Priority to US14/913,592 priority patent/US10301700B2/en
Priority to CN201480046408.0A priority patent/CN105518175B/en
Priority to KR1020167006903A priority patent/KR20160047495A/en
Publication of EP2840159A1 publication Critical patent/EP2840159A1/en
Publication of EP2840159B1 publication Critical patent/EP2840159B1/en
Application granted granted Critical
Publication of EP2840159B8 publication Critical patent/EP2840159B8/en
Priority to JP2019072123A priority patent/JP2019151932A/en
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • 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
    • 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
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • 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
    • C21D8/0205—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
    • 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
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • 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
    • 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
    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
    • C23C30/005—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process on hard metal substrates
    • 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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/10—Modifying the physical properties of iron or steel by deformation by cold working of the whole cross-section, e.g. of concrete reinforcing bars
    • 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
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below

Definitions

  • the invention relates to a method for producing a steel component, which has a tensile strength Rm of more than 1200 MPa and an elongation at break A50 of at least 6%.
  • Steel components produced according to the invention are distinguished by a very high strength in combination with good elongation properties and, as such, are particularly suitable as components for motor vehicle bodies.
  • flat steel product here by a rolling process produced steel sheets or steel strips and divided therefrom boards and the like understood.
  • Steel components of the type according to the invention are produced by a forming process from such flat steel products.
  • alloy contents are stated here only in “%”, this always means “% by weight”, unless expressly stated otherwise.
  • breaking elongation A50 breaking elongation A80
  • tensile strength Rm tensile strength
  • the hot strip obtained is cooled at a cooling rate of 20 - 150 ° C / sec to a reel temperature of 300 - 550 ° C, in which it is wound into a coil.
  • the cooling starts within 2 seconds after the end of the hot rolling.
  • the hot rolled strip thus obtained shall have a fine bainitic structure with a bainite content of at least 90%, the average grain size of which shall not exceed 3,0 ⁇ m, the ratio of the longest axis length to the shortest axis length of the grains not exceeding 1, 5 and the length of the longest axis of the grains should not exceed 10 microns.
  • the remainder of the structure not occupied by bainite should consist of tempered martensite, which is very similar in its appearance and properties to bainite.
  • Hot rolled strips produced and produced in this manner have tensile strengths of 850 - 1103 MPa at an elongation of 15 - 23%.
  • a method for producing a steel sheet having a tensile strength of at least 1470 MPa in which the product of Elongation and tensile strength is at least 29000 MPa%.
  • the steel constituting the steel sheet contains, in addition to iron and unavoidable impurities (in% by weight) 0.30-0.73% C, up to 3.0% Si, up to 3.0% Al, wherein the sum of the Si and Al contents is at least 0.7%, 0.2-8.0% Cr, up to 10.0% Mn, the sum of the Cr and Mn contents being at least 1.0% up to 0.1% P, up to 0.07% S and up to 0.010% N.
  • the steel sheet thus composed is processed such that the martensite area fraction in relation to the total microstructure of the steel is in the range of 15-90%. is and the content of retained austenite of the structure is 10 - 50%. At least 50% of the martensite should be present as tempered martensite and the area fraction of tempered martensite should be at least 10%. If present in the structure, at the same time the area ratio of polygonal ferrites present in the structure should be at most 10%.
  • a hot rolled steel strip as set forth by heating a steel precursor such as a slab to 1000-1300 ° C and thereafter rolling it to a hot strip at a hot rolling end temperature of 870-950 ° C.
  • the resulting hot strip is then wound at a reel temperature of 350 - 720 ° C to form a coil. After coiling, pickling followed by cold rolling takes place at degrees of deformation of 40-90%.
  • the cold-rolled strip thus obtained is annealed for 15-1000 seconds at a temperature where it has a purely austenitic structure, and then at a cooling rate of at least 3 ° C / s cooled to a temperature ranging from below the martensite start temperature and reaching a temperature lower than 150 ° C lower temperature range to produce tempered martensite in the structure of the steel sheet. Thereafter, the cold rolled steel strip is heated to 340-500 ° C over a period of 15-1000 seconds to stabilize the retained austenite.
  • the cold-rolled steel sheets thus produced reached tensile strengths of more than 1600 MPa at an elongation of up to 27%.
  • the object of the invention was to specify a method which makes it possible in a simple way to produce complex shaped components from flat steel products of the type described above.
  • this object has been achieved by carrying out the operations specified in claim 1 for the production of high-strength steel components having good elongation properties.
  • the method according to the invention is suitable for producing a steel component which has a tensile strength Rm of more than 1200 MPa and an elongation at break A50 of at least 6%.
  • the method according to the invention comprises the following steps: Providing a flat steel product containing, in addition to iron and unavoidable impurities (in% by weight): C: 0.10 - 0.60%, Si: 0,4 - 2,5%, al: up to 3.0% Mn: 0.4 - 3.0%, Ni: up to 1%, Cu: up to 2.0%, Not a word: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%, wherein the microstructure of the flat steel product consists of at least 10% by volume of retained austenite comprising globular retained austenite islands with a particle size of at least 1 ⁇ m, Heating the flat steel product to a forming temperature which is 150-400 ° C, -
  • the invention is based on the recognition that a component which is produced by forming a 150-400 ° C warm flat steel product of the type according to the invention, after a subsequent cooling to room temperature over the strength of the original Stahlflachöns significantly increased strength with almost unchanged elongation properties has.
  • the elongation properties of a component produced according to the invention thus do not decrease, in spite of the increased strength, compared to a component formed at room temperature.
  • the invention thus results from a pre-deformation at 150 - 400 ° C a significant increase in strength with unchanged extensibility of each component obtained.
  • the increase in strength achieved by the forming according to the invention is considerable.
  • the expansion properties of the component obtained according to the invention correspond to the elongation properties of the component formed at room temperature, so that the component produced according to the invention is particularly suitable for use in automobile bodies due to its deformation behavior.
  • the reason for the increase in strength achieved by the procedure according to the invention, according to the findings of the invention, is that in the structure of the present invention processed flat steel product present globular retained austenite, which is characterized by a grain size of at least 1 micron, under the load of the forming in accordance with the invention Temperature range of 150 - 400 ° C in film-like retained austenite and bainitic ferrite or below the martensite starting temperature converted into martensite. During the forming in the temperature range concerned, the globular retained austenite present in the steel flat product thus contributes to increasing the elongation at. After the forming and cooling of the component of the steel according to the invention then shows higher tensile strengths due to the additionally formed ferritic bainite or martensite.
  • the parts of film-like retained austenite which remain unchanged over cooling ensure the good residual elongation achieved after the transformation. This effect can be used particularly reliably if the flat steel product for the inventive transformation to the component is heated to 200-400 ° C., in particular 200-300 ° C.
  • the method according to the invention is particularly suitable for converting flat steel products which are provided with a metallic protective coating into components.
  • the metallic protective layer is at most slightly influenced by the invention taking place heating.
  • the protective coating may be a conventional zinc, zinc alloy, aluminum or aluminum alloy, magnesium or magnesium alloy coating.
  • Si Due to the presence of Si in contents of 0.4-2.5% by weight and Al in contents of up to 3% by weight in the flat steel product processed according to the invention, carbide formation in the bainite can be suppressed and consequently the residual austenite stabilized by dissolved carbon become.
  • Si contributes to solid solution hardening. In order to avoid potentially harmful influences of Si, the Si content may be limited to 2.0 wt%. In order to use Si as a mixed-crystal former for increasing the strength, it may be expedient for the flat steel product processed according to the invention to contain at least 1% by weight of Si.
  • Al can partially replace the Si content in the steel processed according to the invention.
  • a minimum content of 0.4 wt .-% Al may be provided. This is especially true when the hardness or tensile strength of the steel is to be set to a lower value in favor of improved ductility by the addition of Al.
  • the optionally additionally present contents of Cu, Cr and Ni also contribute to the formation of bainite.
  • the martensite start temperature can be lowered and the tendency of the bainite to convert to pearlite or cementite can be suppressed.
  • Cr at levels up to the upper limit of not more than 2% by weight given in accordance with the invention promotes the ferritic transformation, whereby optimum effects of the presence of Cr in a flat steel product according to the invention result if the Cr content is reduced to 1.5% by weight. is limited.
  • Ti, V or Nb can support the formation of fine-grained microstructures and promote ferritic transformation.
  • these micro-alloying elements contribute to increasing the hardness by forming precipitates.
  • the positive effects of Ti, V and Nb in the flat steel product processed according to the invention can then be achieved particularly effectively use, if their content is in each case in the range of 0.002 to 0.15 wt .-%, in particular 0.14 wt .-% does not exceed.
  • the formation of the structure provided according to the invention can be ensured, in particular, by the contents of the steel flat product of Mn, Cr, Ni, Cu and C processed according to the invention having the following condition 1 ⁇ 0 . 5 % Mn + 0 . 167 % Cr + 0 . 125 % Ni + 0 . 125 % Cu + 1 . 334 % C ⁇ 2 meet, where with% Mn the respective Mn content in wt .-%, with% Cr of the respective Cr content in wt .-%, with% Ni of the respective Ni content in wt .-%, with% Cu of the respective Cu content in wt .-% and with% C of the respective C content in wt .-% are designated.
  • hot-rolled or cold-rolled flat steel products having a composition corresponding to the specifications according to the invention are suitable as the starting material for the process according to the invention.
  • eligible hot rolled flat steel products and a process for their preparation are the subject of the European patent application EP 12 17 83 30.2 , the content of which is hereby expressly incorporated into the disclosure of the present patent application.
  • the hot-rolled flat steel products produced according to this patent application are characterized by an optimum combination of elongation properties and strength.
  • This combination of properties can be achieved in a particularly reliable way that the structure of flat steel products processed according to the invention, in addition to optionally present fractions of up to 5% by volume of ferrite and up to 10% by volume of martensite, to at least 60% by volume of bainite and the balance of retained austenite, the retained austenite content being at least 10% by volume, at least a portion of the retained austenite is in block form and the blocks of the retained austenite present in block form at least 98% have a mean diameter of less than 5 microns.
  • obtained hot-rolled flat steel product has a two-phase dominated microstructure of which one dominant component is bainite and its second dominant component is retained austenite.
  • small amounts of martensite and ferrite may be present, but their contents are too low to have an influence on the properties of the hot-rolled steel flat product.
  • blocky retained austenite is referred to as the ratio of length / width, ie longest extent / thickness, of 1 to 5 in the structural components of retained austenite present in the structure.
  • retained austenite is referred to as "film-like” if the ratio of length / width is greater than 5 for the retained austenite accumulations present in the microstructure and the width of the respective microstructure constituents of retained austenite is less than 1 ⁇ m. Accordingly, film-like retained austenite is typically present as a finely distributed lamella.
  • a cold-rolled flat steel product suitable for carrying out the process according to the invention as starting material and a process for producing such a cold-rolled steel flat product are the subject of the European patent application 12 17 83 32.8 , the contents of which are hereby expressly included in the disclosure of the present patent application.
  • the microstructure of the cold-rolled steel flat product preferably consists of at least 20% by volume of bainite, 10 to 35% by volume of retained austenite and the remainder of martensite. It goes without saying that in the structure of the flat steel product technically unavoidable traces of other structural constituents can be present. Accordingly, such a cold-rolled flat steel product suitable for the processing according to the invention has a three-phase structure whose dominant constituent is bainite and which furthermore consists of retained austenite and the remainder of martensite.
  • the bainite content is at least 50% by volume, in particular at least 60% by volume, and the residual austenite content is in the range from 10 to 25% by volume, the remainder of the microstructure in each case also being filled with martensite.
  • the optimum martensite content is at least 10% by volume.
  • retained austenite In addition to the main components “Bainite”, “retained austenite” and “martensite” may be present in the cold-rolled steel flat product processed according to the invention contents of other microstructure constituents, but their proportions are too low to have an influence on the properties of the cold-rolled steel flat product.
  • the retained austenite is predominantly film-like with small globular islands of blocky retained austenite with a grain size ⁇ 5 ⁇ m in the case of such a flat steel product suitable for processing according to the invention, so that the retained austenite has a high stability and, consequently, a low tendency to undesirable transformation into martensite ,
  • the C content of the retained austenite is typically more than 1.0% by weight.
  • the above-mentioned martensite starting temperature ie the temperature at which martensite forms in steel processed according to the invention, can be determined in each case according to the article " Thermodynamic Exatrapolation and Martensite Start-Temperature of Substitutionally Alloyed Steels "by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180 explained procedure can be calculated.
  • the molten steel has been conventionally cast into slabs which have subsequently been heated to a reheating temperature TDC in a manner also conventional.
  • the heated slabs were hot rolled in a likewise conventional hot rolling mill to hot strips W1 - W4 with a thickness of 2.0 mm each.
  • the hot strips W1-W4 emerging from the hot rolling scale each had a hot rolling end temperature ET, from which they have been accelerated at a cooling rate KR to a coiling temperature HT. At this reel temperature HT, the hot strips W1 - W4 have been wound into coils.
  • the coils were then each cooled in a temperature range whose upper limit was determined by the respective reel temperature HT and the lower limit by the martensite starting temperature MS calculated for the steel S1.
  • the calculation of the martensite start temperature MS was carried out according to the article " Thermodynamic Exatrapolation and Martensite Start-Temperature of Substituted Alloyed Steels "by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180 explained procedure.
  • the duration over which the coil was cooled in the temperature range defined in the manner described above was such that the hot strips thus obtained each had a structure consisting of bainite and retained austenite, in which the proportions of others Structural constituents were present at most in ineffective, against "0" going amounts.
  • Table 3 also shows the mechanical properties tensile strength Rm, yield strength Rp, elongation at break A80, quality Rm * A80 and the respective retained austenite content RA determined for the individual hot strips W1-W4.
  • Samples of the steel flat products obtained in the form of the hot strips W1-W4 are then heated to a forming temperature UT lying in the range of 200-250 ° C. and formed into a single component with a degree of deformation of up to 15%.
  • the elongation at break A50 of the samples was> 30%, so that it was also possible to image complex shaped elements without the risk of crack formation in the temperature range of the forming process according to the invention.
  • the components converted from the samples of the hot strips W1-W4 were air-cooled to room temperature and their breaking elongation A50 and their tensile strength Rm were determined.
  • the tensile strength Rm of the samples formed according to the invention was 80-120 MPa higher than the samples converted at room temperature.
  • Fig. 2 is a section of a structural sample shown, which has been removed at room temperature from the component, which has been formed from the consisting of the steel S1 hot strip W2 in accordance with the invention at temperatures of 200 - 250 ° C.
  • Fig. 3a, 3b are in each case 20,000-fold magnification cut-outs of a structural sample of steel S1 existing steel component before ( Fig. 3a ) and after ( Fig. 3b ) reproduced the deformation of the invention.
  • FIG. 4a, 4b corresponding photographs of the structural samples of the existing steel S1 steel component ( Fig. 4a ) and after ( Fig. 4b ) of the inventive transformation in 50000-fold magnification.
  • FIG. 3a Comparison of FIG. 3a with the FIG. 3b and the FIG. 4a with the FIG. 4b clearly show the changes that are caused by a deformation according to the invention.
  • a flat steel product which, in addition to iron and unavoidable impurities (in% by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0% Mn: 0.4 - 3.0%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5 %, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%, the structure of the flat steel product consisting of at least 10% by volume of retained austenite, the globular retained austenite islands having a grain size of at least 1 micron.
  • the flat steel product is heated to 150-400 ° C forming temperature and formed at the forming temperature with a degree of deformation which is at most equal to the uniform strain Ag, to the component. Finally, the steel flat product thus obtained is cooled.
  • Such molded at elevated temperatures component has over the same flat steel product, but molded at room temperature components significantly increased strength.
  • Table 1 stolen C Si al Mn Ni Cu Cr other S1 0.48 1.5 0.02 1.48 0.034 1.51 0.9 In% by weight, Remaining iron and unavoidable impurities hot strip OT [° C] E T [° C] KR [° C / s] HT [° C] MS [° C] W1 1150 970 20 350 245 W2 1200 1000 10 400 315 W3 1200 1000 20 450 270 W4 1150 1000 20 500 230 hot strip Rm [MPa] Rp [MPa] A80 [%] RM * A80 [MPa * %] RA [vol.%] W1 1357 807 22.2 27387 36 W2 1318 751 17.8 21328 17 W3 1217 821 25.8 28544 32 W4 1345 889 21.0 25677 30

Description

Die Erfindung betrifft ein Verfahren zum Herstellen eines Stahlbauteils, das eine Zugfestigkeit Rm von mehr als 1200 MPa und eine Bruchdehnung A50 von mindestens 6 % aufweist.The invention relates to a method for producing a steel component, which has a tensile strength Rm of more than 1200 MPa and an elongation at break A50 of at least 6%.

Erfindungsgemäß hergestellte Stahlbauteile zeichnen sich durch eine sehr hohe Festigkeit in Kombination mit guten Dehnungseigenschaften aus und sind als solche insbesondere als Bauteile für Kraftfahrzeugkarosserien geeignet.Steel components produced according to the invention are distinguished by a very high strength in combination with good elongation properties and, as such, are particularly suitable as components for motor vehicle bodies.

Unter dem Begriff "Stahlflachprodukt" werden hier durch einen Walzprozess erzeugte Stahlbleche oder Stahlbänder sowie davon abgeteilte Platinen und desgleichen verstanden. Stahlbauteile der erfindungsgemäßen Art werden durch einen Umformprozess aus solchen Stahlflachprodukten hergestellt.The term "flat steel product" here by a rolling process produced steel sheets or steel strips and divided therefrom boards and the like understood. Steel components of the type according to the invention are produced by a forming process from such flat steel products.

Sofern hier Legierungsgehalte lediglich in "%" angegeben sind, ist damit immer "Gew.-%" gemeint, sofern nicht ausdrücklich etwas anderes angegeben ist.If alloy contents are stated here only in "%", this always means "% by weight", unless expressly stated otherwise.

Wenn hier von "Bruchdehnung A50", "Bruchdehnung A80" oder "Zugfestigkeit Rm" die Rede ist, so sind damit die gemäß DIN EN 6892-1 ermittelten mechanischen Kennwerte gemeint.If the term "breaking elongation A50", "breaking elongation A80" or "tensile strength Rm" is used, this refers to the mechanical characteristic values determined in accordance with DIN EN 6892-1.

Aus der US 6,364,968 B1 ist ein Verfahren zum Herstellen eines warmgewalzten Stahlblechs bekannt, das bei einer Dicke von nicht mehr als 3,5 mm eine gleichmäßige Verteilung seiner mechanischen Eigenschaften und ein besonders gutes Lochaufweitungsverhalten aufweisen soll. Das Verfahren sieht dabei vor, dass eine Bramme, die (in Gew.-%) 0,05 - 0,30 % C, 0,03 - 1,0 % Si, 1,5 - 3,5 % Mn, bis zu 0,02 % P, bis zu 0,005 % S, bis zu 0,150 % al, bis zu 0,0200 % N sowie alternativ oder in Kombination 0,003 - 0,20 % Nb oder 0,005 - 0,20 % Ti, aufweist, auf bis zu 1200 °C erwärmt wird und anschließend mit einer Warmwalzendtemperatur von mindestens 800 °C, insbesondere 950 - 1050 °C, zu einem Warmband warmgewalzt wird. Anschließend wird das erhaltene Warmband mit einer Abkühlrate von 20 - 150 °C/sec auf eine Haspeltemperatur von 300 - 550 °C abgekühlt, bei der es zu einem Coil gewickelt wird. Die Abkühlung setzt dabei innerhalb von 2 Sekunden nach Ende des Warmwalzens ein. Das so erhaltene Warmband soll ein feines bainitisches Gefüge mit einem Bainit-Anteil von mindestens 90 % besitzen, dessen mittlere Korngröße 3,0 µm nicht überschreitet, wobei das Verhältnis der Länge der längsten Achse zur Länge der kürzesten Achse der Körner nicht mehr als 1,5 und die Länge der längsten Achse der Körner nicht mehr als 10 µm betragen soll. Der nicht vom Bainit eingenommene Rest des Gefüges soll aus angelassenem Martensit bestehen, der hinsichtlich seiner Erscheinung und seiner Eigenschaften dem Bainit sehr ähnlich ist. In dieser Weise erzeugte und beschaffene Warmbänder weisen Zugfestigkeiten von 850 - 1103 MPa bei einer Dehnung von 15 - 23 % auf.From the US 6,364,968 B1 is a method for producing a hot-rolled steel sheet known in a Thickness of not more than 3.5 mm should have a uniform distribution of its mechanical properties and a particularly good Lochaufweitungsverhalten. The process envisages that a slab containing (in% by weight) 0.05-0.30% C, 0.03-1.0% Si, 1.5-3.5% Mn, up to 0.02% P, up to 0.005% S, up to 0.150% al, up to 0.0200% N, and alternatively or in combination 0.003-0.20% Nb or 0.005-0.20% Ti, up to is heated to 1200 ° C and then hot rolled at a hot rolling end temperature of at least 800 ° C, in particular 950 - 1050 ° C, to a hot strip. Subsequently, the hot strip obtained is cooled at a cooling rate of 20 - 150 ° C / sec to a reel temperature of 300 - 550 ° C, in which it is wound into a coil. The cooling starts within 2 seconds after the end of the hot rolling. The hot rolled strip thus obtained shall have a fine bainitic structure with a bainite content of at least 90%, the average grain size of which shall not exceed 3,0 μm, the ratio of the longest axis length to the shortest axis length of the grains not exceeding 1, 5 and the length of the longest axis of the grains should not exceed 10 microns. The remainder of the structure not occupied by bainite should consist of tempered martensite, which is very similar in its appearance and properties to bainite. Hot rolled strips produced and produced in this manner have tensile strengths of 850 - 1103 MPa at an elongation of 15 - 23%.

Aus der EP 2 546 382 A1 ist zudem ein Verfahren zur Herstellung eines Stahlblechs mit einer Zugfestigkeit von mindestens 1470 MPa bekannt, bei dem das Produkt aus Dehnung und Zugfestigkeit mindestens 29000 MPa% beträgt. Der Stahl, aus dem das Stahlblech besteht, enthält dabei neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,30 - 0,73 % C, bis zu 3,0 % Si, bis zu 3,0 % Al, wobei die Summe der Si- und Al-Gehalte mindestens 0,7 % beträgt, 0,2 - 8,0 % Cr, bis zu 10,0 % Mn, wobei die Summe der Cr- und Mn-Gehalte mindestens 1,0 % beträgt, bis zu 0,1 % P, bis zu 0,07 % S sowie bis zu 0,010 % N. Das derart zusammengesetzte Stahlblech wird derart verarbeitet, dass der Flächenanteil an Martensit bezogen auf das gesamte Mikrogefüge des Stahls im Bereich von 15 - 90 % liegt und der Gehalt an Restaustenit des Gefüges 10 - 50 % beträgt. Dabei sollen mindestens 50 % des Martensits als angelassener Martensit vorliegen und der Flächenanteil des angelassenen Martensits mindestens 10 % sein. Sofern im Gefüge vorhanden, soll gleichzeitig das Flächenverhältnis von im Gefüge anwesenden polygonalen Ferriten höchstens 10 % betragen.From the EP 2 546 382 A1 In addition, a method for producing a steel sheet having a tensile strength of at least 1470 MPa is known, in which the product of Elongation and tensile strength is at least 29000 MPa%. The steel constituting the steel sheet contains, in addition to iron and unavoidable impurities (in% by weight) 0.30-0.73% C, up to 3.0% Si, up to 3.0% Al, wherein the sum of the Si and Al contents is at least 0.7%, 0.2-8.0% Cr, up to 10.0% Mn, the sum of the Cr and Mn contents being at least 1.0% up to 0.1% P, up to 0.07% S and up to 0.010% N. The steel sheet thus composed is processed such that the martensite area fraction in relation to the total microstructure of the steel is in the range of 15-90%. is and the content of retained austenite of the structure is 10 - 50%. At least 50% of the martensite should be present as tempered martensite and the area fraction of tempered martensite should be at least 10%. If present in the structure, at the same time the area ratio of polygonal ferrites present in the structure should be at most 10%.

Um dies zu erreichen, wird gemäß der EP 2 546 382 A1 zunächst ein in der angegebenen Weise zusammengesetztes warmgewalztes Stahlband erzeugt, indem ein Stahlvormaterial, wie eine Bramme, auf 1000 - 1300 °C erwärmt wird und darauf folgend bei einer 870 - 950 °C betragenden Warmwalzendtemperatur zu einem Warmband gewalzt wird. Das erhaltene Warmband wird anschließend bei einer Haspeltemperatur von 350 - 720 °C zu einem Coil gewickelt. Nach dem Haspeln erfolgt ein Beizen mit anschließendem Kaltwalzen bei Verformungsgraden von 40 - 90 %. Das so erhaltene kaltgewalzte Band wird für 15 - 1000 Sekunden bei einer Temperatur geglüht, in dem es ein rein austenitisches Gefüge besitzt, und dann mit einer Abkühlgeschwindigkeit von mindestens 3 °C/s auf eine Temperatur abgekühlt, die in einem unterhalb der Martensitstarttemperatur beginnenden und bis zu einer 150 °C niedrigeren Temperatur reichenden Temperaturbereich liegt, um angelassenen Martensit im Gefüge des Stahlblechs zu erzeugen. Daraufhin wird das kaltgewalzte Stahlband über eine Dauer von 15 - 1000 Sekunden auf 340 - 500 °C erwärmt, um den vorhandenen Restaustenit zu stabilisieren. Die so erzeugten kaltgewalzten Stahlbleche erreichten Zugfestigkeiten von mehr als 1600 MPa bei einer Dehnung von bis zu 27 %.To achieve this, according to the EP 2 546 382 A1 first producing a hot rolled steel strip as set forth by heating a steel precursor such as a slab to 1000-1300 ° C and thereafter rolling it to a hot strip at a hot rolling end temperature of 870-950 ° C. The resulting hot strip is then wound at a reel temperature of 350 - 720 ° C to form a coil. After coiling, pickling followed by cold rolling takes place at degrees of deformation of 40-90%. The cold-rolled strip thus obtained is annealed for 15-1000 seconds at a temperature where it has a purely austenitic structure, and then at a cooling rate of at least 3 ° C / s cooled to a temperature ranging from below the martensite start temperature and reaching a temperature lower than 150 ° C lower temperature range to produce tempered martensite in the structure of the steel sheet. Thereafter, the cold rolled steel strip is heated to 340-500 ° C over a period of 15-1000 seconds to stabilize the retained austenite. The cold-rolled steel sheets thus produced reached tensile strengths of more than 1600 MPa at an elongation of up to 27%.

Vor dem Hintergrund des voranstehend erläuterten Standes der Technik bestand die Aufgabe der Erfindung darin, ein Verfahren anzugeben, das auf einfache Weise die Herstellung komplex geformter Bauteile aus Stahlflachprodukten der voranstehend erläuterten Art ermöglicht.Against the background of the prior art explained above, the object of the invention was to specify a method which makes it possible in a simple way to produce complex shaped components from flat steel products of the type described above.

Erfindungsgemäß ist diese Aufgabe dadurch gelöst worden, dass zur Herstellung von hochfesten und gute Dehnungseigenschaften aufweisenden Stahlbauteilen die in Anspruch 1 angegebenen Arbeitsschritte durchlaufen werden.According to the invention, this object has been achieved by carrying out the operations specified in claim 1 for the production of high-strength steel components having good elongation properties.

Vorteilhafte Ausgestaltungen der Erfindung sind in den abhängigen Ansprüchen angegeben und werden nachfolgend wie der allgemeine Erfindungsgedanke im Einzelnen erläutert.Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

Das erfindungsgemäße Verfahren ist zum Herstellen eines Stahlbauteils geeignet, das eine Zugfestigkeit Rm von mehr als 1200 MPa und eine Bruchdehnung A50 von mindestens 6 % besitzt. Zu diesem Zweck umfasst das erfindungsgemäße Verfahren folgende Arbeitsschritte:
- Bereitstellen eines Stahlflachprodukts, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%): C: 0,10 - 0,60 %, Si: 0,4 - 2,5 %, Al: bis zu 3,0 % Mn: 0,4 - 3,0 %, Ni: bis zu 1 %, Cu: bis zu 2,0 %, Mo: bis zu 0,4 %, Cr: bis zu 2 %, Co: bis zu 1,5 %, Ti: bis zu 0,2 %, Nb: bis zu 0,2 %, V: bis zu 0,5 %, enthält, wobei das Gefüge des Stahlflachprodukts zu mindestens 10 Vol.-% aus Restaustenit besteht, der globulare Restaustenitinseln mit einer Korngröße von mindestens 1 µm umfasst,
- Erwärmen des Stahlflachprodukts auf eine Umformtemperatur, die 150 - 400 °C beträgt,
- Umformen des auf die Umformtemperatur erwärmten Stahlflachprodukts zu dem Bauteil mit einem höchstens bis zur Gleichmaßdehnung Ag reichenden Umformgrad, in der Praxis auch Umformdehnung oder Verformungsgrad genannt,
- Abkühlen des erhaltenen Bauteils.The method according to the invention is suitable for producing a steel component which has a tensile strength Rm of more than 1200 MPa and an elongation at break A50 of at least 6%. For this purpose, the method according to the invention comprises the following steps:
Providing a flat steel product containing, in addition to iron and unavoidable impurities (in% by weight): C: 0.10 - 0.60%, Si: 0,4 - 2,5%, al: up to 3.0% Mn: 0.4 - 3.0%, Ni: up to 1%, Cu: up to 2.0%, Not a word: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%, wherein the microstructure of the flat steel product consists of at least 10% by volume of retained austenite comprising globular retained austenite islands with a particle size of at least 1 μm,
Heating the flat steel product to a forming temperature which is 150-400 ° C,
- Forming the heated to the forming temperature flat steel product to the component with a maximum extent to the uniform expansion Ag-reaching degree of deformation, in practice also called forming strain or deformation,
- Cooling of the resulting component.

Die Erfindung geht von der Erkenntnis aus, dass ein Bauteil, das durch Umformen eines 150 - 400 °C warmen Stahlflachprodukts der erfindungsgemäß beschaffenen Art hergestellt wird, nach einer anschließenden Abkühlung auf Raumtemperatur eine gegenüber der Festigkeit des ursprünglichen Stahlflachprodukts deutlich erhöhte Festigkeit bei nahezu unveränderten Dehnungseigenschaften besitzt.The invention is based on the recognition that a component which is produced by forming a 150-400 ° C warm flat steel product of the type according to the invention, after a subsequent cooling to room temperature over the strength of the original Stahlflachprodukts significantly increased strength with almost unchanged elongation properties has.

In Folge der Erwärmung in dem erfindungsgemäß vorgegebenen Temperaturbereich steigt die Dehnbarkeit des erfindungsgemäß verarbeiteten Stahlflachprodukts deutlich an, so dass ohne besonderen Aufwand und bei minimierter Gefahr der Entstehung von Rissen vorgebeugt und Bauteilformen erzeugt werden können, die eine besonders komplexe Gestalt besitzen. Praktische Versuche haben hier ergeben, dass Stahlflachprodukte der erfindungsgemäß bereitgestellten Art im Temperaturbereich, in dem erfindungsgemäß die Umformung erfolgen soll, regelmäßig eine Bruchdehnung A50 von mindestens 30 % erreichen, wogegen die Bruchdehnung A50 des Bauteils bei Raumtemperatur gegenüber dem als Ausgangsprodukt dienenden Stahlflachprodukt unverändert im Bereich von typischerweise 22 % liegt.As a result of the heating in the temperature range specified according to the invention, the extensibility of the inventively processed steel flat product increases significantly, so that without special effort and minimized risk of the formation of cracks can be prevented and component shapes can be generated, which have a particularly complex shape. Practical experiments have shown here that flat steel products of the present invention provided in the temperature range in which the deformation is to take place, regularly reach an elongation at break A50 of at least 30%, whereas the elongation at break A50 of the component at room temperature compared to the starting as flat steel product unchanged in the range typically of 22%.

Überraschender Weise nehmen somit die Dehnungseigenschaften eines erfindungsgemäß hergestellten Bauteils trotz der gestiegenen Festigkeit im Vergleich zu einem bei Raumtemperatur geformten Bauteil nicht ab. Die Erfindung ergibt somit durch eine Vorverformung bei 150 - 400 °C eine deutliche Festigkeitssteigerung bei unveränderter Dehnbarkeit des jeweils erhaltenen Bauteils.Surprisingly, the elongation properties of a component produced according to the invention thus do not decrease, in spite of the increased strength, compared to a component formed at room temperature. The invention thus results from a pre-deformation at 150 - 400 ° C a significant increase in strength with unchanged extensibility of each component obtained.

Für die nach der Umformung erfolgende Abkühlung muss kein besonderer Aufwand getrieben werden. So kann die Abkühlung des Stahlflachprodukts nach dem Umformen an ruhender Luft erfolgen.No cooling is required to cool down after forming. Thus, the cooling of the flat steel product can take place after forming in still air.

Die durch die erfindungsgemäß vorgenommene Umformung erzielte Steigerung der Festigkeit ist beträchtlich. So konnte nachgewiesen werden, dass durch eine Bauteilumformung von 15 %, die bei erfindungsgemäß erhöhten Temperaturen durchgeführt worden ist, regelmäßig die Zugfestigkeit um ca. 80 - 120 MPa gegenüber der Zugfestigkeit von Proben gesteigert werden konnte, die ebenfalls mit einem Umformgrad von 15 %, jedoch bei Raumtemperatur umgeformt worden sind. Gleichzeitig entsprechen die Dehnungseigenschaften des erfindungsgemäß erhaltenen Bauteils den Dehnungseigenschaften des bei Raumtemperatur umgeformten Bauteils, so dass das erfindungsgemäß erzeugte Bauteil aufgrund seines Verformungsverhaltens insbesondere für den Einsatz in Automobilkarosserien geeignet ist.The increase in strength achieved by the forming according to the invention is considerable. Thus, it could be demonstrated that by a component transformation of 15%, which was carried out at elevated temperatures according to the invention, it was possible to regularly increase the tensile strength by about 80-120 MPa compared with the tensile strength of samples which likewise had a degree of deformation of 15%, but have been reformed at room temperature. At the same time, the expansion properties of the component obtained according to the invention correspond to the elongation properties of the component formed at room temperature, so that the component produced according to the invention is particularly suitable for use in automobile bodies due to its deformation behavior.

Der Grund für die durch die erfindungsgemäße Vorgehensweise erzielte Festigkeitssteigerung besteht nach den Erkenntnissen der Erfindung darin, dass sich im Gefüge des erfindungsgemäß verarbeiteten Stahlflachprodukts vorhandener globularer Restaustenit, der durch eine Korngröße von mindestens 1 µm gekennzeichnet ist, unter der Last der Umformung in dem erfindungsgemäß vorgegebenen Temperaturbereich von 150 - 400 °C in filmartigen Restaustenit und bainitischen Ferrit bzw. unterhalb von der Martensitstarttemperatur in Martensit umwandelt. Während der Umformung im betreffenden Temperaturbereich trägt somit der im Stahlflachprodukt vorhandene globulare Restaustenit zur Steigerung der Dehnung bei. Nach der Umformung und Abkühlung des Bauteils zeigt der erfindungsgemäß verarbeitete Stahl dann höhere Zugfestigkeiten in Folge des zusätzlich gebildeten ferritischen Bainits bzw. Martensits. Die über die Abkühlung unverändert erhalten bleibenden Anteile an filmartigem Restaustenit gewährleisten die nach der Umformung erreichte gute Restdehnung. Besonders sicher lässt sich dieser Effekt nutzen, wenn das Stahlflachprodukt für die erfindungsgemäße Umformung zu dem Bauteil auf 200 - 400 °C, insbesondere 200 - 300 °C, erwärmt wird.The reason for the increase in strength achieved by the procedure according to the invention, according to the findings of the invention, is that in the structure of the present invention processed flat steel product present globular retained austenite, which is characterized by a grain size of at least 1 micron, under the load of the forming in accordance with the invention Temperature range of 150 - 400 ° C in film-like retained austenite and bainitic ferrite or below the martensite starting temperature converted into martensite. During the forming in the temperature range concerned, the globular retained austenite present in the steel flat product thus contributes to increasing the elongation at. After the forming and cooling of the component of the steel according to the invention then shows higher tensile strengths due to the additionally formed ferritic bainite or martensite. The parts of film-like retained austenite which remain unchanged over cooling ensure the good residual elongation achieved after the transformation. This effect can be used particularly reliably if the flat steel product for the inventive transformation to the component is heated to 200-400 ° C., in particular 200-300 ° C.

Aufgrund der vergleichbar niedrigen Temperaturen, bei denen erfindungsgemäß die Umformung durchgeführt wird, eignet sich das erfindungsgemäße Verfahren insbesondere dazu, Stahlflachprodukte, die mit einer metallischen Schutzbeschichtung versehen sind, zu Bauteilen umzuformen. Die metallische Schutzschicht wird durch die erfindungsgemäß erfolgende Erwärmung allenfalls geringfügig beeinflusst. Dabei kann es sich bei der Schutzbeschichtung beispielsweise um eine konventionelle Zink-, Zinklegierungs-, Aluminium- oder Aluminiumlegierungs-, Magnesium- oder Magnesiumlegierungsbeschichtung handeln.Due to the comparatively low temperatures at which the deformation is carried out according to the invention, the method according to the invention is particularly suitable for converting flat steel products which are provided with a metallic protective coating into components. The metallic protective layer is at most slightly influenced by the invention taking place heating. For example, the protective coating may be a conventional zinc, zinc alloy, aluminum or aluminum alloy, magnesium or magnesium alloy coating.

Die Zusammensetzung eines erfindungsgemäß verarbeiteten Stahlflachprodukts ist unter Berücksichtigung folgender Gesichtspunkte gewählt worden:

  • Kohlenstoff in Gehalten von 0,1 - 0,6 Gew.-% verzögert im Stahl des erfindungsgemäß verarbeiteten Stahlflachprodukts die Umwandlung zu Ferrit/Perlit, senkt die Martensitstarttemperatur MS und trägt zur Erhöhung der Härte bei. Um diese positiven Effekte zu nutzen, kann der C-Gehalt des erfindungsgemäßen Stahlflachprodukts auf mindestens 0,25 Gew.-%, insbesondere mindestens 0,27 Gew.-%, mindestens 0,28 Gew.-% oder mindestens 0,3 Gew.-%, gesetzt werden, wobei sich die durch den vergleichbar hohen Kohlenstoffgehalt erzielten Effekte dann besonders sicher nutzen lassen, wenn der C-Gehalt im Bereich von > 0,25 - 0,5 Gew.-%, insbesondere 0,27 - 0,4 Gew.-% oder 0,28 - 0,4 Gew.-%, liegt.
The composition of a flat steel product processed according to the invention has been chosen taking into account the following aspects:
  • Carbon in contents of 0.1-0.6% by weight retards conversion to ferrite / pearlite in the steel of the steel flat product processed according to the invention, lowers the martensite start temperature MS and contributes to the increase in hardness. To use these positive effects, the C content of the flat steel product according to the invention to at least 0.25 wt .-%, in particular at least 0.27 wt .-%, at least 0.28 wt .-% or at least 0.3 wt .-%, are set, wherein the be used by the comparably high carbon content effects particularly safe when the C content in the range of> 0.25 to 0.5 wt .-%, in particular 0.27 to 0.4 wt .-% or 0.28 - 0.4 wt .-%, is.

Durch die Anwesenheit von Si in Gehalten von 0,4 - 2,5 Gew.-% und Al in Gehalten von bis zu 3 Gew.-% im erfindungsgemäß verarbeiteten Stahlflachprodukt kann die Karbidbildung im Bainit unterdrückt und damit einhergehend der Restaustenit durch gelösten Kohlenstoff stabilisiert werden. Zudem trägt Si zur Mischkristallverfestigung bei. Um möglicherweise schädliche Einflüsse von Si zu vermeiden, kann der Si-Gehalt auf 2,0 Gew.-% beschränkt werden. Um Si als Mischkristallbildner zur Steigerung der Festigkeit zu nutzen, kann es zweckmäßig sein, wenn das erfindungsgemäß verarbeitete Stahlflachprodukt mindestens 1 Gew.-% Si enthält.Due to the presence of Si in contents of 0.4-2.5% by weight and Al in contents of up to 3% by weight in the flat steel product processed according to the invention, carbide formation in the bainite can be suppressed and consequently the residual austenite stabilized by dissolved carbon become. In addition, Si contributes to solid solution hardening. In order to avoid potentially harmful influences of Si, the Si content may be limited to 2.0 wt%. In order to use Si as a mixed-crystal former for increasing the strength, it may be expedient for the flat steel product processed according to the invention to contain at least 1% by weight of Si.

Al kann im erfindungsgemäß verarbeiteten Stahl den Si-Gehalt zum Teil ersetzen. Hierzu kann ein Mindestgehalt von 0,4 Gew.-% Al vorgesehen sein. Dies gilt insbesondere dann, wenn durch die Zugabe von Al die Härte oder Zugfestigkeit des Stahls zu Gunsten einer verbesserten Verformbarkeit auf einen niedrigeren Wert eingestellt werden soll.Al can partially replace the Si content in the steel processed according to the invention. For this purpose, a minimum content of 0.4 wt .-% Al may be provided. This is especially true when the hardness or tensile strength of the steel is to be set to a lower value in favor of improved ductility by the addition of Al.

Die positiven Einflüsse der gleichzeitigen Anwesenheit von Al und Si können dann besonders effektiv genutzt werden, wenn die Gehalte an Si und Al innerhalb der erfindungsgemäß vorgegebenen Grenzen die Bedingung %Si + 0,8%Al > 1,2 Gew.-% oder sogar die Bedingung %Si + 0,8%Al > 1,5 Gew.-% (mit %Si: jeweiliger Si-Gehalt in Gew.-%, %Al: jeweiliger Al-Gehalt in Gew.-%) erfüllen.The positive effects of the simultaneous presence of Al and Si can be used particularly effectively if the contents of Si and Al within the invention predetermined limits, the condition% Si + 0.8% Al> 1.2 wt .-% or even the condition% Si + 0.8% Al> 1.5 wt .-% (with% Si: respective Si content in % By weight,% Al: respective Al content in% by weight).

Mn in Gehalten von mindestens 0,4 Gew.-% und bis zu 3,0 Gew.-%, insbesondere bis zu 2,5 Gew.-% oder 2,0 Gew.-%, fördert im erfindungsgemäß verarbeiteten Stahl die Bainitbildung, wobei die optional zusätzlich vorhandenen Gehalte an Cu, Cr und Ni ebenfalls zur Bildung von Bainit beitragen. Abhängig von den jeweils anderen Bestandteilen des erfindungsgemäß verarbeiteten Stahls kann es dabei zweckmäßig sein, den Mn-Gehalt auf maximal 1,6 Gew.-% oder 1,5 Gew.-% zu beschränken.Mn in contents of at least 0.4% by weight and up to 3.0% by weight, in particular up to 2.5% by weight or 2.0% by weight, promotes bainitization in the steel processed according to the invention, The optionally additionally present contents of Cu, Cr and Ni also contribute to the formation of bainite. Depending on the respective other constituents of the steel processed according to the invention, it may be expedient to limit the Mn content to a maximum of 1.6% by weight or 1.5% by weight.

Durch die optionale Zugabe von Cr kann die Martensitstarttemperatur abgesenkt und die Neigung des Bainits zur Umwandlung in Perlit oder Zementit unterdrückt werden. Des Weiteren fördert Cr in Gehalten bis zur erfindungsgemäß vorgegebenen Obergrenze von maximal 2 Gew.-% die ferritische Umwandlung, wobei sich optimale Wirkungen der Anwesenheit von Cr in einem erfindungsgemäßen Stahlflachprodukt dann ergeben, wenn der Cr-Gehalt auf 1,5 Gew.-% beschränkt ist.By the optional addition of Cr, the martensite start temperature can be lowered and the tendency of the bainite to convert to pearlite or cementite can be suppressed. Furthermore, Cr at levels up to the upper limit of not more than 2% by weight given in accordance with the invention promotes the ferritic transformation, whereby optimum effects of the presence of Cr in a flat steel product according to the invention result if the Cr content is reduced to 1.5% by weight. is limited.

Durch die optionale Zugabe von Ti, V oder Nb kann die Entstehung von feinkörnigem Gefüge unterstützt und die ferritische Umwandlung gefördert werden. Darüber hinaus tragen diese Mikrolegierungselemente durch die Bildung von Ausscheidungen zur Steigerung der Härte bei. Besonders effektiv lassen sich die positiven Wirkungen von Ti, V und Nb im erfindungsgemäß verarbeiteten Stahlflachprodukt dann nutzen, wenn ihr Gehalt jeweils im Bereich von 0,002 - 0,15 Gew.-% liegt, insbesondere 0,14 Gew.-% nicht überschreitet.The optional addition of Ti, V or Nb can support the formation of fine-grained microstructures and promote ferritic transformation. In addition, these micro-alloying elements contribute to increasing the hardness by forming precipitates. The positive effects of Ti, V and Nb in the flat steel product processed according to the invention can then be achieved particularly effectively use, if their content is in each case in the range of 0.002 to 0.15 wt .-%, in particular 0.14 wt .-% does not exceed.

Die Bildung des erfindungsgemäß vorgesehenen Gefüges lässt sich insbesondere dadurch gewährleisten, dass die Gehalte des erfindungsgemäß verarbeiteten Stahlflachprodukts an Mn, Cr, Ni, Cu und C die folgende Bedingung 1 < 0 , 5 % Mn + 0 , 167 % Cr + 0 , 125 % Ni + 0 , 125 % Cu + 1 , 334 % C < 2

Figure imgb0001
erfüllen, wobei mit %Mn der jeweilige Mn-Gehalt in Gew.-%, mit %Cr der jeweilige Cr-Gehalt in Gew.-%, mit %Ni der jeweilige Ni-Gehalt in Gew.-%, mit %Cu der jeweilige Cu-Gehalt in Gew.-% und mit %C der jeweilige C-Gehalt in Gew.-% bezeichnet sind.The formation of the structure provided according to the invention can be ensured, in particular, by the contents of the steel flat product of Mn, Cr, Ni, Cu and C processed according to the invention having the following condition 1 < 0 . 5 % Mn + 0 . 167 % Cr + 0 . 125 % Ni + 0 . 125 % Cu + 1 . 334 % C < 2
Figure imgb0001
 meet, where with% Mn the respective Mn content in wt .-%, with% Cr of the respective Cr content in wt .-%, with% Ni of the respective Ni content in wt .-%, with% Cu of the respective Cu content in wt .-% and with% C of the respective C content in wt .-% are designated.

Als Ausgangsprodukt für das erfindungsgemäße Verfahren eignen sich grundsätzlich warm- oder kaltgewalzte Stahlflachprodukte mit einer den erfindungsgemäßen Vorgaben entsprechenden Zusammensetzung. Hierzu in Frage kommende warmgewalzte Stahlflachprodukte und ein Verfahren zu ihrer Herstellung sind Gegenstand der Europäischen Patentanmeldung EP 12 17 83 30.2 , deren Inhalt hiermit ausdrücklich in die Offenbarung der vorliegenden Patentanmeldung einbezogen wird.In principle, hot-rolled or cold-rolled flat steel products having a composition corresponding to the specifications according to the invention are suitable as the starting material for the process according to the invention. For this purpose, eligible hot rolled flat steel products and a process for their preparation are the subject of the European patent application EP 12 17 83 30.2 , the content of which is hereby expressly incorporated into the disclosure of the present patent application.

Wie in der genannten Europäischen Patentanmeldung EP 12 17 83 30.2 erläutert, zeichnen sich die gemäß dieser Patentanmeldung erzeugten warmgewalzten Stahlflachprodukte durch eine optimale Kombination aus Dehnungseigenschaften und Festigkeit aus. Diese Eigenschaftskombination kann dadurch besonders sicher erreicht werden, dass das Gefüge von erfindungsgemäß verarbeiteten Stahlflachprodukten, neben optional vorhandenen Anteilen von bis zu 5 Vol.-% Ferrit und bis zu 10 Vol.-% Martensit, zu mindestens 60 Vol.-% aus Bainit und als Rest aus Restaustenit besteht, wobei der Restaustenitgehalt mindestens 10 Vol.-% beträgt, zumindest ein Teil des Restaustenits in blockiger Form vorliegt und die Blöcke des in blockiger Form vorliegenden Restaustenits zu mindestens 98 % einen mittleren Durchmesser von weniger als 5 µm aufweisen.As in the cited European patent application EP 12 17 83 30.2 As explained, the hot-rolled flat steel products produced according to this patent application are characterized by an optimum combination of elongation properties and strength. This combination of properties can be achieved in a particularly reliable way that the structure of flat steel products processed according to the invention, in addition to optionally present fractions of up to 5% by volume of ferrite and up to 10% by volume of martensite, to at least 60% by volume of bainite and the balance of retained austenite, the retained austenite content being at least 10% by volume, at least a portion of the retained austenite is in block form and the blocks of the retained austenite present in block form at least 98% have a mean diameter of less than 5 microns.

Ein gemäß der EP 12 17 83 30.2 beschaffenes warmgewalztes Stahlflachprodukt weist dementsprechend ein von zwei Phase dominiertes Gefüge auf, dessen einer dominierender Bestandteil Bainit und dessen zweiter dominierender Bestandteil Restaustenit ist. Neben diesen beiden Hauptkomponenten können geringe Anteile an Martensit und Ferrit vorhanden sein, deren Gehalte jedoch zu gering sind, um einen Einfluss auf die Eigenschaften des warmgewalzten Stahlflachprodukts zu haben.One according to the EP 12 17 83 30.2 Accordingly, obtained hot-rolled flat steel product has a two-phase dominated microstructure of which one dominant component is bainite and its second dominant component is retained austenite. In addition to these two main components, small amounts of martensite and ferrite may be present, but their contents are too low to have an influence on the properties of the hot-rolled steel flat product.

Von "blockartigem" Restaustenit spricht man in diesem Zusammenhang dann, wenn bei den im Gefüge vorhandenen Gefüge-Bestandteilen an Restaustenit das Verhältnis aus Länge/Breite, d. h. längste Ausdehnung/Dicke, 1 bis 5 beträgt. Dagegen wird Restaustenit als "filmartig" bezeichnet, wenn bei den im Gefüge vorhandenen Restaustenitansammlungen das Verhältnis Länge/Breite größer als 5 ist und die Breite der jeweiligen Gefüge-Bestandteile an Restaustenit kleiner als 1 µm ist. Filmartiger Restaustenit liegt dementsprechend typischerweise als fein verteilte Lamelle vor.In this context, "blocky" retained austenite is referred to as the ratio of length / width, ie longest extent / thickness, of 1 to 5 in the structural components of retained austenite present in the structure. On the other hand, retained austenite is referred to as "film-like" if the ratio of length / width is greater than 5 for the retained austenite accumulations present in the microstructure and the width of the respective microstructure constituents of retained austenite is less than 1 μm. Accordingly, film-like retained austenite is typically present as a finely distributed lamella.

Ein Verfahren zum Herstellen eines als Ausgangsprodukt für das erfindungsgemäße Verfahren geeigneten warmgewalzten Stahlflachprodukts umfasst folgende Arbeitsschritte:

  • Bereitstellen eines Vorprodukts in Form einer Bramme, Dünnbramme oder eines gegossenen Bands, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%): 0,10 - 0,60 % C, 0,4 - 2,0 % Si, bis zu 2,0 % Al, 0,4 - 2,5 % Mn, bis zu 1 % Ni, bis zu 2,0 % Cu, bis zu 0,4 % Mo, bis zu 2 % Cr, bis zu 0,2 % Ti, bis zu 0,2 % Nb und bis zu 0,5 % V enthält;
  • Warmwalzen des Vorprodukts zu einem Warmband in einem oder mehreren Walzstichen, wobei das erhaltene Warmband beim Verlassen des letzten Walzstichs eine Warmwalzendtemperatur von mindestens 880 °C aufweist;
  • beschleunigtes Abkühlen des erhaltenen Warmbands mit einer Abkühlrate von mindestens 5 °C/s auf eine Haspeltemperatur, die zwischen der Martensitstarttemperatur MS und 600 °C liegt;
  • Haspeln des Warmbands zu einem Coil;
  • Abkühlen des Coils, wobei die Temperatur des Coils während der Abkühlung zur Bildung von Bainit solange in einem Temperaturbereich gehalten wird, dessen Obergrenze gleich der Bainitstarttemperatur BS, ab der Bainit im Gefüge des Warmbands entsteht, und dessen Untergrenze gleich der Martensitstarttemperatur MS ist, ab der Martensit im Gefüge des Warmbands entsteht, bis mindestens 60 Vol.-% des Gefüges des Warmbands aus Bainit bestehen.
A method for producing a hot-rolled flat steel product suitable as a starting material for the method according to the invention comprises the following steps:
  • Providing a precursor in the form of a slab, thin slab or a cast strip, which in addition to iron and unavoidable impurities (in% by weight): 0.10-0.60% C, 0.4-2.0% Si, up to 2.0% Al, 0.4-2.5% Mn, up to 1% Ni, up to 2.0% Cu, up to 0.4% Mo, up to 2% Cr, up to 0.2% Ti, up to 0.2% Nb and up to 0.5% V;
  • Hot rolling the precursor into a hot strip in one or more rolling passes, the resulting hot strip having a hot rolling finish temperature of at least 880 ° C when leaving the last pass;
  • accelerated cooling of the obtained hot strip at a cooling rate of at least 5 ° C / s to a coiler temperature which is between the martensite start temperature MS and 600 ° C;
  • Coiling the hot strip into a coil;
  • Cooling of the coil, wherein the temperature of the coil is maintained during cooling to form bainite in a temperature range whose upper limit equal to the bainite start temperature BS, from the bainite in the structure of the hot strip, and whose lower limit is equal to the martensite start temperature MS, from the Martensite in the structure of the hot strip is produced until at least 60% by volume of the structure of the hot strip of bainite.

Ein für die Durchführung des erfindungsgemäßen Verfahrens als Ausgangsprodukt geeignetes kaltgewalztes Stahlflachprodukt und ein Verfahren zur Herstellung eines solchen kaltgewalzten Stahlflachprodukts sind Gegenstand der Europäischen Patentanmeldung 12 17 83 32.8 , deren Inhalt hiermit ebenfalls ausdrücklich in die Offenbarung der vorliegenden Patentanmeldung einbezogen wird.A cold-rolled flat steel product suitable for carrying out the process according to the invention as starting material and a process for producing such a cold-rolled steel flat product are the subject of the European patent application 12 17 83 32.8 , the contents of which are hereby expressly included in the disclosure of the present patent application.

Bei einer unter die erfindungsgemäß vorgegebene Stahlzusammensetzung fallenden Legierung besteht das Gefüge des kaltgewalzten Stahlflachprodukts vorzugweise zu mindestens 20 Vol.-% aus Bainit, zu 10 - 35 Vol.-% aus Restaustenit und als Rest aus Martensit. Dabei versteht es sich von selbst, dass im Gefüge des Stahlflachprodukts technisch unvermeidbare Spuren anderer Gefügebestandteile vorhanden sein können. Ein derartiges für die erfindungsgemäße Verarbeitung geeignetes kaltgewalztes Stahlflachprodukt weist dementsprechend ein dreiphasiges Gefüge auf, dessen dominierender Bestandteil Bainit ist und das darüber hinaus aus Restaustenit sowie als Rest aus Martensit besteht. Optimaler Weise liegt der Bainitanteil bei mindestens 50 Vol.-%, insbesondere mindestens 60 Vol.-%, und der Restaustenitanteil im Bereich von 10 - 25 Vol.-%, wobei auch hier der Rest des Gefüges jeweils durch Martensit aufgefüllt ist. Der optimale Martensitanteil beträgt mindestens 10 Vol.-%. Ein derart zusammengesetztes Gefüge bewirkt bei der für ein erfindungsgemäß verarbeitetes kaltgewalztes Stahlflachprodukt geforderten hohen Zugfestigkeit Rm von typischerweise mindestens 1400 MPa und einer Bruchdehnung A80 von mindestens 5 % ein optimales Produkt Rm x A80 von Dehnung und Zugfestigkeit. Neben den Hauptkomponenten "Bainit", "Restaustenit" und "Martensit" können im kaltgewalzten erfindungsgemäß verarbeiteten Stahlflachprodukt Gehalte an anderen Gefügebestandteilen vorhanden sein, deren Anteile jedoch zu gering sind, um einen Einfluss auf die Eigenschaften des kaltgewalzten Stahlflachprodukts zu haben. Der Restaustenit liegt bei einem derart beschaffenen, für die erfindungsgemäße Verarbeitung geeigneten Stahlflachprodukt überwiegend filmartig mit kleinen globularen Inseln von blockigem Restaustenit mit einer Korngröße < 5 µm vor, so dass der Restaustenit eine hohe Stabilität und damit einhergehend eine geringe Neigung zur unerwünschten Umwandlung in Martensit besitzt. Der C-Gehalt des Restaustenits beträgt dabei typischerweise mehr als 1,0 Gew.-%.In the case of an alloy falling below the steel composition specified in accordance with the invention, the microstructure of the cold-rolled steel flat product preferably consists of at least 20% by volume of bainite, 10 to 35% by volume of retained austenite and the remainder of martensite. It goes without saying that in the structure of the flat steel product technically unavoidable traces of other structural constituents can be present. Accordingly, such a cold-rolled flat steel product suitable for the processing according to the invention has a three-phase structure whose dominant constituent is bainite and which furthermore consists of retained austenite and the remainder of martensite. Optionally, the bainite content is at least 50% by volume, in particular at least 60% by volume, and the residual austenite content is in the range from 10 to 25% by volume, the remainder of the microstructure in each case also being filled with martensite. The optimum martensite content is at least 10% by volume. Such a composite structure in the case of the high tensile strength Rm of typically at least 1400 MPa and an elongation at break A80 of at least 5% required for a cold-rolled flat steel product processed according to the invention produces an optimum product Rm x A80 of elongation and tensile strength. In addition to the main components "Bainite", "retained austenite" and "martensite" may be present in the cold-rolled steel flat product processed according to the invention contents of other microstructure constituents, but their proportions are too low to have an influence on the properties of the cold-rolled steel flat product. The retained austenite is predominantly film-like with small globular islands of blocky retained austenite with a grain size <5 μm in the case of such a flat steel product suitable for processing according to the invention, so that the retained austenite has a high stability and, consequently, a low tendency to undesirable transformation into martensite , The C content of the retained austenite is typically more than 1.0% by weight.

Ein Verfahren zum Herstellen eines solcherart beschaffenen, erfindungsgemäß verarbeiteten kaltgewalzten Stahlflachprodukts umfasst folgende Arbeitsschritte:

  • Bereitstellen eines Vorprodukts in Form einer Bramme, Dünnbramme oder eines gegossenen Bands, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) C: 0,10 - 0,60 %, Si: 0,4 - 2,5 %, Al: bis zu 3,0 %, Mn: 0,4 - 3,0 %, Ni: bis zu 1,0 %, Cu: bis zu 2,0 %, Mo: bis zu 0,4 %, Cr: bis zu 2 %, Co: bis zu 1,5 %, Ti: bis zu 0,2 %, Nb: bis zu 0,2 %, V: bis zu 0,5 % enthält;
  • Warmwalzen des Vorprodukts zu einem Warmband in einem oder mehreren Walzstichen, wobei das erhaltene Warmband beim Verlassen des letzten Walzstichs eine Warmwalzendtemperatur von mindestens 830 °C aufweist;
  • Haspeln des erhaltenen Warmbands bei einer Haspeltemperatur, die zwischen der Warmwalzendtemperatur und 560 °C liegt;
  • Kaltwalzen des Warmbands zu einem Kaltband mit einem Kaltwalzgrad von mindestens 30 %;
  • Wärmebehandeln des erhaltenen Kaltbands, wobei das Kaltband im Zuge der Wärmebehandlung
    • auf eine mindestens 800 °C betragende Glühtemperatur erwärmt wird,
    • optional über eine Glühdauer von 50 - 150 s bei der Glühtemperatur gehalten wird,
    • ausgehend von der Glühtemperatur mit einer mindestens 8 °C/s betragenden Abkühlgeschwindigkeit auf eine Haltetemperatur abgekühlt wird, die in einem Haltetemperaturbereich liegt, dessen Obergrenze 470 °C beträgt und dessen Untergrenze höher ist als die Martensitstarttemperatur MS, ab der Martensit im Gefüge des Kaltbands entsteht, und
    • im Haltetemperaturbereich über einen Zeitraum gehalten wird, der ausreicht, um im Gefüge des Kaltbands mindestens 20 Vol.-% Bainit zu bilden.
A method for producing a cold-rolled flat steel product produced in accordance with the invention and comprises the following steps:
  • Providing a precursor in the form of a slab, thin slab or cast strip which, in addition to iron and unavoidable impurities (in% by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al : up to 3.0%, Mn: 0.4 - 3.0%, Ni: up to 1.0%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%;
  • Hot rolling the precursor into a hot strip in one or more rolling passes, the resulting hot strip having a hot rolling end temperature of at least 830 ° C when leaving the last pass;
  • Coiling the resulting hot strip at a reel temperature that is between the hot rolling end temperature and 560 ° C;
  • Cold rolling the hot strip to a cold strip having a cold rolling degree of at least 30%;
  • Heat treating the cold strip obtained, wherein the cold strip during the heat treatment
    • is heated to a minimum of 800 ° C annealing temperature,
    • is held at the annealing temperature over an annealing period of 50 - 150 s,
    • is cooled starting from the annealing temperature with a cooling rate of at least 8 ° C / s to a holding temperature, which is in a holding temperature range whose upper limit is 470 ° C and whose lower limit is higher than the martensite start temperature MS, starting from the martensite in the structure of the cold strip , and
    • held in the holding temperature range for a period sufficient to form at least 20 vol .-% bainite in the structure of the cold strip.

Die voranstehend erwähnte Martensitstarttemperatur, d. h. die Temperatur, ab der sich in erfindungsgemäß verarbeitetem Stahl Martensit bildet, kann jeweils gemäß der im Artikel " Thermodynamic Exatrapolation and Martensite-Start-Temperature of Substitutionally Alloyed Steels" von H. Bhadeshia, erschienen in Metal Science 15 (1981), Seiten 178 -180 erläuterten Vorgehensweise berechnet werden.The above-mentioned martensite starting temperature, ie the temperature at which martensite forms in steel processed according to the invention, can be determined in each case according to the article " Thermodynamic Exatrapolation and Martensite Start-Temperature of Substitutionally Alloyed Steels "by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180 explained procedure can be calculated.

Nachfolgend wird die Erfindung anhand von Ausführungsbeispielen erläutert. Es zeigen:

Fig. 1
ein Diagramm, in dem für vier warmgewalzte Stahlflachprodukte derselben Zusammensetzung S1 in erfindungsgemäßer Weise erzeugten Bauteile B1,B2,B3,B4 die Bruchdehnung A50 über die Zugfestigkeit Rm aufgetragen ist;
Fig. 2
eine Abbildung einer Gefügeprobe des Bauteils B4;
Fig. 3a,3b
Abbildungen einer Gefügeprobe des Stahlflachprodukts, aus dem das Bauteil B4 geformt ist, in 20000-facher Vergrößerung und zwar vor (Fig. 3a) und nach (Fig. 3b) der Umformung;
Fig. 4a,4b
Abbildungen einer Gefügeprobe des Stahlflachprodukts, aus dem das Bauteil B4 geformt ist, in 50000-facher Vergrößerung und zwar vor (Fig. 4a) und nach (Fig. 4b) der Umformung.
The invention will be explained below with reference to exemplary embodiments. Show it:
Fig. 1
a diagram in which is plotted for four hot rolled flat steel products of the same composition S1 in accordance with the invention produced components B1, B2, B3, B4, the elongation at break A50 on the tensile strength Rm;
Fig. 2
an illustration of a structural sample of the component B4;
Fig. 3a, 3b
Illustrations of a structural sample of the flat steel product from which the component B4 is formed, magnified 20,000 times before ( Fig. 3a ) and after ( Fig. 3b ) the transformation;
Fig. 4a, 4b
Illustrations of a structural sample of the flat steel product from which the component B4 is formed, magnified 50,000 times before ( Fig. 4a ) and after ( Fig. 4b ) of the transformation.

Es ist ein Stahl mit der in Tabelle 1 angegebenen Zusammensetzung erschmolzen worden.A steel having the composition shown in Table 1 has been melted.

Die Stahlschmelze ist auf konventionelle Weise zu Brammen vergossen worden, die anschließend auf ebenso konventionelle Weise auf eine Wiedererwärmungstemperatur OT erwärmt worden sind.The molten steel has been conventionally cast into slabs which have subsequently been heated to a reheating temperature TDC in a manner also conventional.

Die erwärmten Brammen sind in einer ebenfalls konventionellen Warmwalzstaffel zu Warmbändern W1 - W4 mit einer Dicke von jeweils 2,0 mm warmgewalzt worden.The heated slabs were hot rolled in a likewise conventional hot rolling mill to hot strips W1 - W4 with a thickness of 2.0 mm each.

Die aus der Warmwalzstaffel austretenden Warmbänder W1 - W4 wiesen jeweils eine Warmwalzendtemperatur ET auf, von der ausgehend sie mit einer Abkühlrate KR auf eine Haspeltemperatur HT beschleunigt abgekühlt worden sind. Bei dieser Haspeltemperatur HT sind die Warmbänder W1 - W4 zu Coils gewickelt worden.The hot strips W1-W4 emerging from the hot rolling scale each had a hot rolling end temperature ET, from which they have been accelerated at a cooling rate KR to a coiling temperature HT. At this reel temperature HT, the hot strips W1 - W4 have been wound into coils.

Die Coils sind anschließend jeweils in einem Temperaturbereich abgekühlt worden, dessen Obergrenze durch die jeweilige Haspeltemperatur HT und dessen Untergrenze durch die für den Stahl S1 berechnete Martensitstarttemperatur MS festgelegt war. Die Berechnung der Martensitstarttemperatur MS erfolgte dabei gemäß der im Artikel " Thermodynamic Exatrapolation and Martensite-Start-Temperature of Substitutionally Alloyed Steels" von H. Bhadeshia, erschienen in Metal Science 15 (1981), Seiten 178 -180 erläuterten Vorgehensweise.The coils were then each cooled in a temperature range whose upper limit was determined by the respective reel temperature HT and the lower limit by the martensite starting temperature MS calculated for the steel S1. The calculation of the martensite start temperature MS was carried out according to the article " Thermodynamic Exatrapolation and Martensite Start-Temperature of Substituted Alloyed Steels "by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180 explained procedure.

Die Dauer, über die das Coil in dem in der voranstehend beschriebenen Weise definierten Temperaturbereich abgekühlt worden ist, war so bemessen, dass die so erhaltenen Warmbänder jeweils ein aus Bainit und Restaustenit bestehendes Gefüge aufwiesen, in dem die Anteile anderer Gefügebestandteile allenfalls in unwirksamen, gegen "0" gehende Mengen vorhanden waren.The duration over which the coil was cooled in the temperature range defined in the manner described above was such that the hot strips thus obtained each had a structure consisting of bainite and retained austenite, in which the proportions of others Structural constituents were present at most in ineffective, against "0" going amounts.

Die jeweiligen Betriebsparameter Wiedererwärmungstemperatur OT, Warmwalzendtemperatur ET, Abkühlrate KR, Haspeltemperatur HT und Martensitstarttemperatur MS sind in Tabelle 2 angegeben.The respective operating parameters reheating temperature TDC, hot rolling end temperature ET, cooling rate KR, reeling temperature HT and martensite starting temperature MS are given in Table 2.

In Tabelle 3 sind darüber hinaus die für die einzelnen Warmbänder W1 - W4 ermittelten mechanischen Eigenschaften Zugfestigkeit Rm, Streckgrenze Rp, Bruchdehnung A80, Güte Rm*A80 sowie der jeweilige Restaustenitgehalt RA angegeben.Table 3 also shows the mechanical properties tensile strength Rm, yield strength Rp, elongation at break A80, quality Rm * A80 and the respective retained austenite content RA determined for the individual hot strips W1-W4.

Proben der so erhaltenen, in Form der Warmbänder W1 - W4 vorliegenden Stahlflachprodukte sind anschließend auf eine im Bereich von 200 - 250 °C liegende Umformtemperatur UT erwärmt und mit einem Umformgrad von bis zu 15 % zu jeweils einem Bauteil umgeformt worden. Bei der Temperatur UT war die Bruchdehnung A50 der Proben > 30 %, so dass in dem erfindungsgemäßen Temperaturbereich der Umformung auch die Abbildung von komplexen Formelementen ohne die Gefahr einer Rissbildung möglich war.Samples of the steel flat products obtained in the form of the hot strips W1-W4 are then heated to a forming temperature UT lying in the range of 200-250 ° C. and formed into a single component with a degree of deformation of up to 15%. At the temperature UT, the elongation at break A50 of the samples was> 30%, so that it was also possible to image complex shaped elements without the risk of crack formation in the temperature range of the forming process according to the invention.

Nach dem Umformen im Temperaturbereich von 200 - 250 °C sind die aus den Proben der Warmbänder W1 - W4 15 % umgeformten Bauteile an Luft auf Raumtemperatur abgekühlt und ihre Bruchdehnung A50 sowie ihre Zugfestigkeit Rm bestimmt worden.After forming in the temperature range of 200-250 ° C., the components converted from the samples of the hot strips W1-W4 were air-cooled to room temperature and their breaking elongation A50 and their tensile strength Rm were determined.

Zum Vergleich sind weitere Proben der Warmbänder W1 - W4 bei Raumtemperatur RT, d. h. kalt, zu den jeweiligen Bauteilen umgeformt worden. Auch an den so geformten Bauteilen ist die Bruchdehnung A50 und die Zugfestigkeit Rm bestimmt worden.For comparison, further samples of the hot strips W1 - W4 at room temperature RT, ie cold, have been converted to the respective components. Also on the so shaped Components, the elongation at break A50 and the tensile strength Rm has been determined.

Es zeigte sich, dass nach der Abkühlung auf Raumtemperatur bei im Wesentlichen konstanten Werten der Bruchdehnung A50 die Zugfestigkeit Rm der erfindungsgemäß umgeformten Proben um jeweils 80 - 120 MPa höher lag als bei den bei Raumtemperatur umgeformten Proben.It was found that, after cooling to room temperature at essentially constant values of the elongation at break A50, the tensile strength Rm of the samples formed according to the invention was 80-120 MPa higher than the samples converted at room temperature.

In Fig. 2 ist ein Ausschnitt einer Gefügeprobe dargestellt, die bei Raumtemperatur aus dem Bauteil entnommenen worden ist, das aus dem aus dem Stahl S1 bestehenden Warmband W2 in erfindungsgemäßer Weise bei Temperaturen von 200 - 250 °C geformt worden ist. Deutlich zu erkennen ist dort der durch die Umformung im genannten Temperaturbereich aus den zuvor globulitischen Restaustenitinseln entstandene, filmartig vorliegende Restaustenit RAf.In Fig. 2 is a section of a structural sample shown, which has been removed at room temperature from the component, which has been formed from the consisting of the steel S1 hot strip W2 in accordance with the invention at temperatures of 200 - 250 ° C. Clearly recognizable there is the residual austenite RAf formed by the transformation in the temperature range mentioned above from the previously globulitic retained austenite islands.

In Fig. 3a,3b sind in jeweils 20000-facher Vergrößerung Ausschnitte einer Gefügeprobe des aus dem Stahl S1 bestehenden Stahlbauteils vor (Fig. 3a) und nach (Fig. 3b) der erfindungsgemäßen Umformung wiedergegeben.In Fig. 3a, 3b are in each case 20,000-fold magnification cut-outs of a structural sample of steel S1 existing steel component before ( Fig. 3a ) and after ( Fig. 3b ) reproduced the deformation of the invention.

In Fig. 4a,4b finden sich entsprechende Aufnahmen der Gefügeproben des aus dem Stahl S1 bestehenden Stahlbauteils vor (Fig. 4a) und nach (Fig. 4b) der erfindungsgemäßen Umformung in 50000-facher Vergrößerung.In Fig. 4a, 4b corresponding photographs of the structural samples of the existing steel S1 steel component ( Fig. 4a ) and after ( Fig. 4b ) of the inventive transformation in 50000-fold magnification.

Auch der Vergleich der Figur 3a mit der Figur 3b und der Figur 4a mit der Figur 4b zeigen deutlich die Veränderungen, die mit einer erfindungsgemäßen Verformung bewirkt werden.Also the comparison of FIG. 3a with the FIG. 3b and the FIG. 4a with the FIG. 4b clearly show the changes that are caused by a deformation according to the invention.

Das erfindungsgemäße Verfahren erlaubt somit auf einfache Weise die Herstellung eines komplex geformten Stahlbauteils mit einer Zugfestigkeit Rm > 1200 MPa und einer Bruchdehnung A50 > 6 %. Hierzu wird erfindungsgemäß ein Stahlflachprodukt bereitgestellt, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) C: 0,10 - 0,60 %, Si: 0,4 - 2,5 %, Al: bis zu 3,0 % Mn: 0,4 - 3,0 %, Ni: bis zu 1 %, Cu: bis zu 2,0 %, Mo: bis zu 0,4 %, Cr: bis zu 2 %, Co: bis zu 1,5 %, Ti: bis zu 0,2 %, Nb: bis zu 0,2 %, V: bis zu 0,5 %, enthält, wobei das Gefüge des Stahlflachprodukts zu mindestens 10 Vol.-% aus Restaustenit besteht, der globulare Restaustenitinseln mit einer Korngröße von mindestens 1 µm umfasst. Das Stahlflachprodukt wird auf eine 150 - 400 °C betragende Umformtemperatur erwärmt und bei der Umformtemperatur mit einem Umformgrad, der höchstens gleich der Gleichmaßdehnung Ag ist, zu dem Bauteil umgeformt. Abschließend wird das so erhaltene Stahlflachprodukt abgekühlt. Ein derart bei erhöhten Temperaturen geformtes Bauteil besitzt gegenüber aus demselben Stahlflachprodukt, jedoch bei Raumtemperatur geformten Bauteilen eine deutlich gesteigerte Festigkeit. Tabelle 1 Stahl C Si Al Mn Ni Cu Cr Sonstige S1 0,48 1,5 0,02 1,48 0,034 1,51 0,9 Angaben in Gew.-%,
Rest Eisen und unvermeidbare Verunreinigungen Tabelle 2 Warmband OT [°C] ET [°C] KR [°C/s] HT [°C] MS [°C] W1 1150 970 20 350 245 W2 1200 1000 10 400 315 W3 1200 1000 20 450 270 W4 1150 1000 20 500 230 Tabelle 3 Warmband Rm [MPa] Rp [MPa] A80 [%] RM*A80 [MPa*%] RA [Vol.-%] W1 1357 807 22,2 27387 36 W2 1318 751 17,8 21328 17 W3 1217 821 25,8 28544 32 W4 1345 889 21,0 25677 30 The method according to the invention thus makes it possible in a simple way to produce a complex-shaped steel component with a tensile strength Rm> 1200 MPa and an elongation at break A50> 6%. For this purpose, according to the invention, a flat steel product is provided which, in addition to iron and unavoidable impurities (in% by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0% Mn: 0.4 - 3.0%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5 %, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%, the structure of the flat steel product consisting of at least 10% by volume of retained austenite, the globular retained austenite islands having a grain size of at least 1 micron. The flat steel product is heated to 150-400 ° C forming temperature and formed at the forming temperature with a degree of deformation which is at most equal to the uniform strain Ag, to the component. Finally, the steel flat product thus obtained is cooled. Such molded at elevated temperatures component has over the same flat steel product, but molded at room temperature components significantly increased strength. Table 1 stole C Si al Mn Ni Cu Cr other S1 0.48 1.5 0.02 1.48 0.034 1.51 0.9 In% by weight,
Remaining iron and unavoidable impurities hot strip OT [° C] E T [° C] KR [° C / s] HT [° C] MS [° C] W1 1150 970 20 350 245 W2 1200 1000 10 400 315 W3 1200 1000 20 450 270 W4 1150 1000 20 500 230 hot strip Rm [MPa] Rp [MPa] A80 [%] RM * A80 [MPa * %] RA [vol.%] W1 1357 807 22.2 27387 36 W2 1318 751 17.8 21328 17 W3 1217 821 25.8 28544 32 W4 1345 889 21.0 25677 30

Claims (10)

  1. A method for producing a steel component which has a tensile strength Rm of greater than 1200 MPa and an elongation at break A50 of greater than 6%, comprising the following production steps:
    - providing a flat steel product having the following composition (in wt%): C: 0.10 - 0.60%, Si: 0.4 - 2.5%, Al: up to 3.0%, Mn: 0.4 - 3.0%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%,
    the remainder iron and unavoidable impurities,
    wherein the structure of the flat steel product is composed of at least 10 vol% residual austenite which comprises globular residual austenite islands with a grain size of at least 1 µm,
    - heating the flat steel product to a forming temperature, which is 150 - 400 °C,
    - forming the flat steel product heated to the forming temperature into the component with a degree of forming which does not exceed the uniform elongation Ag,
    - cooling the formed flat steel product.
  2. A method according to claim 1, characterised in that the prepared flat steel product is provided with a metallic protective coating.
  3. A method according to one of claims 1 or 2, characterised in that the prepared flat steel product is a hot-rolled steel strip or sheet.
  4. A method according to claim 3, characterised in that the structure of the hot-rolled flat steel product contains at least 60 vol% bainite and at least 10 vol% residual austenite as well as optionally up to 5 vol% ferrite and up to 10 vol% martensite and in that at least part of the residual austenite is in block form and at least 98% of the blocks of the residual austenite present in block form have a mean diameter of less than 5 µm.
  5. A flat steel product according to claim 4, characterised in that its Mn, Cr, Ni, Cu and C contents fulfil the following condition: 1 < 0.5 % Mn + 0.167 % Cr + 0.125 % Ni + 0.125 % Cu + 1.334 % C < 2
    Figure imgb0003
    with %Mn: respective Mn content in wt%
    %Cr: respective Cr content in wt%
    %Ni: respective Ni content in wt%
    %Cu: respective Cu content in wt%
    %C: respective C content in wt%.
  6. A method according to one of the preceding claims, characterised in that the prepared flat steel product is a cold-rolled steel strip or sheet.
  7. A method according to claim 6, characterised in that the structure of the cold-rolled flat steel product contains at least 20 vol% bainite, 10 - 35 vol% residual austenite and at least 10 vol% martensite.
  8. A method according to claim 7, characterised in that the cold-rolled flat steel product contains at least 50 vol% bainite.
  9. A method according to one of the preceding claims, characterised in that the sum of the Al and Si contents of the prepared flat steel product is at least 1.5 wt%.
  10. A method according to one of the preceding claims, characterised in that the cooling of the flat steel product, which takes place after the forming, takes place in still air.
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EP2840159B8 (en) 2017-07-19
JP6606075B2 (en) 2019-11-13

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