EP2020451A1 - Method of manufacturing sheets of steel with high levels of strength and ductility, and sheets produced using same - Google Patents

Method of manufacturing sheets of steel with high levels of strength and ductility, and sheets produced using same Download PDF

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Publication number
EP2020451A1
EP2020451A1 EP07290908A EP07290908A EP2020451A1 EP 2020451 A1 EP2020451 A1 EP 2020451A1 EP 07290908 A EP07290908 A EP 07290908A EP 07290908 A EP07290908 A EP 07290908A EP 2020451 A1 EP2020451 A1 EP 2020451A1
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EP
European Patent Office
Prior art keywords
steel
sheet
composition
hot
content
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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EP07290908A
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German (de)
French (fr)
Inventor
Pascal Drillet
Damien Ormston
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ArcelorMittal France SA
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ArcelorMittal France SA
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Priority to EP07290908A priority Critical patent/EP2020451A1/en
Priority to UAA201001690A priority patent/UA98798C2/en
Priority to KR1020127034336A priority patent/KR20130010030A/en
Priority to KR1020147007669A priority patent/KR20140044407A/en
Priority to CN2008801040865A priority patent/CN101784688B/en
Priority to KR1020107003457A priority patent/KR20100037147A/en
Priority to RU2010105699/02A priority patent/RU2451764C2/en
Priority to PL08830766T priority patent/PL2171112T3/en
Priority to US12/669,188 priority patent/US20100221573A1/en
Priority to ES08830766T priority patent/ES2375429T3/en
Priority to CA2694069A priority patent/CA2694069C/en
Priority to EP08830766A priority patent/EP2171112B1/en
Priority to JP2010516534A priority patent/JP5298127B2/en
Priority to PCT/FR2008/000993 priority patent/WO2009034250A1/en
Priority to AT08830766T priority patent/ATE534756T1/en
Priority to KR1020157029946A priority patent/KR20150123957A/en
Priority to BRPI0814514A priority patent/BRPI0814514B1/en
Priority to KR1020187002754A priority patent/KR101892423B1/en
Priority to ARP080103095A priority patent/AR067594A1/en
Publication of EP2020451A1 publication Critical patent/EP2020451A1/en
Priority to ZA201000290A priority patent/ZA201000290B/en
Priority to MA32523A priority patent/MA31525B1/en
Priority to US14/575,475 priority patent/US10214792B2/en
Priority to US15/879,944 priority patent/US10428400B2/en
Withdrawn legal-status Critical Current

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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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • 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
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—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
    • C22C38/06—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/12—Aluminium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/12—All metal or with adjacent metals
    • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
    • Y10T428/12736—Al-base component
    • Y10T428/1275—Next to Group VIII or IB metal-base component
    • Y10T428/12757—Fe
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/12—All metal or with adjacent metals
    • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
    • Y10T428/12771—Transition metal-base component
    • Y10T428/12785—Group IIB metal-base component
    • Y10T428/12792—Zn-base component
    • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

  • the invention relates to the manufacture of hot rolled sheets of so-called "multiphase" steels, simultaneously having a very high strength and a deformation capacity for carrying out cold forming operations.
  • the invention more specifically relates to predominantly bainitic microstructure steels having a strength greater than 800 MPa and an elongation rate greater than 10% rupture.
  • the automotive industry is in particular a preferred field of application for these hot-rolled steel sheets.
  • TRIP Transform Induced Plasticity
  • the present invention aims to solve the problems mentioned above. It aims to provide a hot-rolled steel having a mechanical strength greater than 800 MPa together with an elongation rate greater than 10% rupture, both in long direction and in cross-direction relative to rolling.
  • the invention also aims at providing a steel that is not very sensitive to damage during cutting by a mechanical method.
  • the invention also aims to provide a method of manufacturing a steel in the uncoated, electrogalvanized or galvanized state. This therefore requires that the mechanical characteristics of this steel are insensitive to the thermal cycles associated with dipping zinc coating processes.
  • composition of the steel preferably comprises the content being expressed by weight: 0.050% ⁇ C ⁇ 0.070%
  • the composition comprises, the content being expressed by weight: 0.070% ⁇ C ⁇ 0.090%
  • the composition comprises: 1.4% ⁇ Mn ⁇ 1.8%.
  • the composition comprises: 0.020% ⁇ Al ⁇ 0.040%.
  • the composition of the steel preferably comprises: 0.12% ⁇ V ⁇ 0.16%. In a preferred embodiment, the composition of the steel comprises 0.18% ⁇ Mo ⁇ 0.30%.
  • the composition comprises: Nb ⁇ 0.005%
  • the composition comprises: 0.20% ⁇ Cr ⁇ 0.45%
  • the microstructure of the steel preferably comprises, as a surface fraction, at least 80% of higher bainite, the optional complement being constituted by lower bainite, martensite and residual austenite, the sum of the martensite and residual austenite contents being lower. at 5%.
  • the invention also relates to a beam welded assembly with high energy density made from a steel sheet according to one of the above modes.
  • the subject of the invention is also a process for manufacturing a hot-rolled steel sheet with a resistance greater than 800 MPa and an elongation at break of greater than 10%, according to which a steel of the above composition is supplied, a semi-finished product is poured at a temperature above 1150 ° C.
  • the semi-finished product is hot rolled to a temperature T FL in a temperature range where the microstructure of the steel is entirely austenitic so as to obtain a sheet.
  • This is then cooled to a cooling rate V R of 75 and 200 ° C./s, then the sheet is reeled at a temperature T bob of between 500 and 600 ° C.
  • the end of rolling temperature T FL is between 870 and 930 ° C.
  • the cooling rate V R is between 80 and 150 ° C / s.
  • the sheet is pickled, then optionally skin-passed, and then coated with zinc or zinc alloy.
  • the coating is made by dipping.
  • the invention also relates to the use of a hot-rolled steel sheet according to one of the above modes, or manufactured by a method according to one of the above modes for the manufacture of parts of structure or reinforcement elements, in the automotive field.
  • carbon plays an important role in the formation of the microstructure and in the mechanical properties.
  • the carbon content is between 0.050 and 0.090% by weight: Below 0.050%, sufficient strength can not be obtained. Beyond 0.090%, the microstructure formed consists mainly of lower bainite, this structure being characterized by the presence of carbides precipitated within the slats of bainitic ferrite: the resistance thus obtained is high, but the elongation is then significantly reduced. it.
  • the carbon content is between 0.050 and 0.070%.
  • figure 1 illustrates the influence of carbon content on the long-term elongation of LASER beam splicing welds: A particularly high elongation at break of 17-23% is associated with a carbon content of 0.050 at 0.070%.
  • the carbon content is greater than 0.070% and less than or equal to 0.090%: even if this range does not lead to such a high ductility, the elongation at break of the LASER welds is greater than 15%, and remains comparable to that of the base steel sheet.
  • manganese increases the quenchability and avoids the formation of ferrite cooling after rolling.
  • Manganese also helps to deoxidize steel during liquid phase processing.
  • the addition of manganese also contributes to effective solid solution hardening and increased strength.
  • the manganese is between 1.4 and 1.8%: thus forming a completely bainitic structure without risk of appearance of harmful band structure.
  • aluminum is an effective element for the deoxidation of steel. This efficiency is obtained in a particularly economical and stable manner when the aluminum content is between 0.020 and 0.040%.
  • silicon contributes to liquid phase deoxidation and hardening in solid solution.
  • An addition of silicon above 0.3% causes the formation of strongly adherent oxides and the possible appearance of surface defects, due in particular to a lack of wettability in dip galvanizing operations.
  • molybdenum retards bainitic transformation during cooling after rolling, contributes to hardening by solid solution and refines the size of bainitic slats.
  • the molybdenum content is less than or equal to 0.40% to prevent excessive formation of quenching structures. This limited molybdenum content also makes it possible to lower the manufacturing cost.
  • the molybdenum content is greater than or equal to 0.18% and less than or equal to 0.30%. In this way, the level is ideally adjusted to avoid the formation of ferrite or perlite in the steel sheet on the cooling table after hot rolling.
  • Phosphorus is a known element to segregate at grain boundaries. Its content must be limited to 0.025% in order to maintain sufficient hot ductility.
  • the composition may comprise chromium in an amount of less than or equal to 0.45%. Thanks to the other elements of the composition and to the process according to the invention, its presence is however not absolutely necessary, which has the advantage of avoiding expensive additions.
  • the steel contains less than 0.005% Ti and less than 0.020% Nb.
  • these elements fix too much nitrogen in the form of nitrides or carbonitrides. There is not enough nitrogen available to precipitate with vanadium. In addition, excessive precipitation of niobium would increase the hot hardness and would not easily allow the realization of thin-rolled hot-rolled sheets.
  • the niobium content is less than 0.005%
  • Vanadium is an important element according to the invention: the steel contains a vanadium content of between 0.12 and 0.22%. Compared to a vanadium-free steel, the increase in strength due to a hardening precipitation of carbonitrides can be up to 300 MPa. Below 0.12%, there is no significant effect on the mechanical tensile characteristics. Beyond 0.22% vanadium, under the conditions of manufacture according to the invention, a saturation of the effect on the mechanical characteristics is noted. A content of less than 0.22% thus makes it possible to obtain high mechanical characteristics in a very economical manner with respect to steels which contain higher levels of vanadium.
  • microstructure refinement and structural hardening are particularly effective.
  • the nitrogen content is greater than or equal to 0.003% in order to obtain a precipitation of vanadium carbonitrides in a sufficient quantity.
  • the nitrogen content is less than or equal to 0.009% to avoid the presence of solid solution nitrogen or the formation of larger carbonitrides, which would reduce ductility.
  • the rest of the composition consists of unavoidable impurities resulting from the preparation, such as for example Sb, Sn, As.
  • microstructural percentages above are surface fractions that can be measured on polished and etched sections.
  • the microstructure therefore has no primary or proeutectoid ferrite: it then has a great homogeneity since the property gap Mechanics between the matrix (upper bainite) and the other possible constituents (lower bainite and martensite) is weak. During a mechanical stress, the deformations are distributed homogeneously. Dislocation accumulation does not occur at the interfaces between the constituents and premature damage is avoided, contrary to what can be noted in structures with a significant amount of primary ferrite, a phase whose flow limit is very low. , or martensite with a very high level of resistance. In this way, the steel according to the invention has a particular aptitude for certain demanding deformation modes such as the expansion of holes, the mechanical stress of cut edges, the folding.
  • the cast semifinished products are first brought to a temperature higher than 1150 ° C. to reach at any point a temperature favorable to the high deformations which the steel will undergo during rolling.
  • the hot rolling step of these semi-finished products starting at more than 1150 ° C. can be done directly after casting so well. that an intermediate heating step is not necessary in this case.
  • the semi-finished product is hot-rolled in a temperature range where the structure of the steel is totally austenitic up to an end-of-rolling temperature T FL .
  • the temperature T FL is preferably between 870 and 930 ° C to obtain a grain size adapted to the bainitic transformation that follows.
  • Cooling is then carried out at a speed V R of between 75 and 200 ° C / s: a minimum speed of 75 ° C / s prevents the formation of proeutectoid ferrite and perlite, while a speed V R of 200 ° C / s or less prevents formation excessive martensite.
  • the speed V R is between 80 and 150 ° C / s:
  • a minimum speed of 80 ° C / s leads to the formation of upper bainite with a very small slat size, associated with excellent mechanical properties.
  • a speed of less than 150 ° C / s makes it possible to avoid, for the most part, the formation of martensite.
  • the sheet can be used in the bare state or coated. Depending on the use envisaged, the sheet is scoured after rolling according to a method known per se, so as to obtain a surface state suitable for promoting the implementation of the subsequent coating.
  • the sheet may be subjected to a slight cold deformation, usually less than 1% ("skin-pass").
  • the sheet is then coated with zinc or aluminum. an alloy based on zinc, for example by electrogalvanizing or dip galvanizing.
  • electrogalvanizing or dip galvanizing it has been demonstrated that the particular microstructure of the steel, composed mainly of higher bainite, is not very sensitive to the thermal conditions of the subsequent galvanizing treatment, so that the mechanical characteristics of the sheets coated with dipping have a high great stability even in case of untimely fluctuation of these conditions.
  • the sheet in the galvanized state therefore has mechanical characteristics very similar to those in the naked state.
  • the microstructure of steel I1 illustrated in figure 2 comprises more than 80% higher bainite, the remainder being lower bainite and MA compounds.
  • the total content of martensite and residual austenite is less than 5%.
  • the size of the old austenitic grains and bainitic batten bundles is about 10 microns.
  • the limitation of the size of the batten packets and the strong disorientation between the adjacent packets results in a high resistance to the propagation of any microcracks. Due to the small difference in hardness between the various constituents of the microstructure, the steel is not very sensitive to damage during cutting by a mechanical process.
  • Steel R1 with too high a carbon content and a too low vanadium content, has insufficient breaking elongation.
  • Steel R2 has a carbon content and phosphorus too high, its winding temperature is also too low. As a result, its elongation at break is also significantly less than 10%.
  • LASER autogenous welded joints were made under the following conditions: power: 4.5kW, welding speed: 2.5m / min.
  • the lengthwise elongation of the LASER welds of I-1 steel is 17%, whereas it is 10 and 13% respectively for the R-1 and R-2 steels. These values lead, particularly for steel R1, to difficulties in stamping welded joints.
  • Steel sheets I1 according to the invention were also galvanized under the following conditions: after heating at 680 ° C., the sheets were cooled to 455 ° C. and then quenched in a Zn bath at this temperature and finally cooled. at room temperature.
  • the invention allows the manufacture of steels bainitic matrix steels without excessive addition of expensive elements. These combine high strength and high ductility.
  • the steel sheets according to the invention are used profitably for the manufacture of structural parts or reinforcement elements in the automotive field and general industry.

Abstract

Hot-rolled steel sheet having a strength of greater than 800 MPa, and an elongation at break of greater than 10%, comprises (wt.%): carbon (0.05-0.09); manganese (1-2); aluminum (0.015-0.05); silicon (0.1-0.3); molybdenum (0.1-0.4); sulfur (= 0.01); phosphorus (= 0.025); nitrogen (0.003-0.009); vanadium (0.12-0.22); titanium (= 0.009); niobium (= 0.02); and optionally chromium (= 0.45), where the rest of the composition is constituted of iron and unavoidable impurities resulting from elaboration. Independent claims are included for: (1) a weld assembly done by the steel sheet, where the sheet is welded by beam at high energy density; and (2) a preparation of the hot-rolled steel sheet comprising supplying the steel composition, casting of semi finished product from the steel, subjecting the semi finished product to a temperature of greater than 1150[deg] C, hot-rolling the semi finished product at end temperature of rolling (TFL), where the microstructure of the steel is entirely austenitic to obtain the sheet, then cooling the sheet at a cooling speed (VR) of 75-200[deg] C/second, and coiling the sheet at a winding temperature (Tbob) of 500-600[deg] C.

Description

L'invention concerne la fabrication de tôles laminées à chaud d'aciers dits « multiphasés », présentant simultanément une très haute résistance et une capacité de déformation permettant de réaliser des opérations de mise en forme à froid. L'invention concerne plus précisément des aciers à microstructure majoritairement bainitique présentant une résistance supérieure à 800 MPa et un taux d'allongement à rupture supérieur à 10%. L'industrie automobile constitue en particulier un domaine privilégié d'application de ces tôles d'aciers laminées à chaud.The invention relates to the manufacture of hot rolled sheets of so-called "multiphase" steels, simultaneously having a very high strength and a deformation capacity for carrying out cold forming operations. The invention more specifically relates to predominantly bainitic microstructure steels having a strength greater than 800 MPa and an elongation rate greater than 10% rupture. The automotive industry is in particular a preferred field of application for these hot-rolled steel sheets.

Il existe en particulier dans cette industrie un besoin continu d'allègement des véhicules et d'accroissement de la sécurité. C'est ainsi que l'on a proposé différentes familles d'aciers pour répondre aux besoins croissants :

  • On a tout d'abord proposé des aciers comportant des éléments de micro-alliage dont le durcissement est obtenu simultanément par précipitation et par affinement de la taille de grains. Le développement de ces aciers a été suivi par celui d'aciers « Dual-Phase » où la présence de martensite au sein d'une matrice ferritique permet d'obtenir une résistance supérieure à 450MPa associée à une bonne aptitude au formage à froid.
In particular, there is a continuing need in this industry for lighter vehicles and increased safety. Thus, different families of steels have been proposed to meet the growing needs:
  • Steels were first proposed comprising micro-alloy elements whose hardening is obtained simultaneously by precipitation and by grain size refinement. The development of these steels was followed by that of "Dual-Phase" steels where the presence of martensite within a ferritic matrix makes it possible to obtain a strength greater than 450 MPa combined with a good cold forming ability.

Pour obtenir des niveaux de résistance supérieurs, on a développé des aciers présentant un comportement «TRIP» (Transformation Induced Plasticity ») avec des combinaisons de propriétés (résistance-aptitude à la déformation) avantageuses : ces propriétés sont liées à la structure de ces aciers constituée d'une matrice ferritique comportant de la bainite et de l'austénite résiduelle. Sous l'effet d'une déformation, l'austénite résiduelle d'une pièce en acier TRIP se transforme progressivement en martensite, ce qui se traduit par une consolidation importante et retarde l'apparition d'une striction.To obtain higher resistance levels, steels with a "Transform Induced Plasticity" (TRIP) behavior have been developed with advantageous combinations of properties (resistance-ability to deformation): these properties are related to the structure of these steels. consisting of a ferritic matrix comprising bainite and residual austenite. Under the effect of a deformation, the residual austenite of a TRIP steel part gradually changes to martensite, which results in a significant consolidation and delays the appearance of a necking.

Pour atteindre simultanément un rapport limite d'élasticité/résistance élevé, une résistance encore plus importante, c'est à dire un niveau supérieur à 800 MPa, on a développé des aciers multiphasés à structure majoritairement bainitiques ; dans l'industrie automobile ou dans l'industrie générale, ces aciers sont utilisés avec profit pour la fabrication de pièces structurales. L'aptitude à la mise en forme de ces pièces requiert cependant simultanément un allongement suffisant. Cette exigence peut également être requise lorsque les pièces sont soudées puis mises en forme : dans ce cas, les joints soudés doivent présenter une aptitude suffisante à la mise en forme et ne pas conduire à des ruptures prématurées au niveau des assemblages.To simultaneously achieve a high yield strength / resistance ratio, an even greater strength, that is to say a level greater than 800 MPa, we developed multiphase steels predominantly structure bainitic; in the automotive industry or in the general industry, these steels are used profitably for the manufacture of structural parts. The ability to shape these parts, however, simultaneously requires sufficient elongation. This requirement may also be required when the parts are welded and then shaped: in this case, the welded joints must have a sufficient fitness for shaping and not lead to premature fractures at the joints.

La présente invention a pour but de résoudre les problèmes évoqués ci-dessus. Elle vise à mettre à disposition un acier laminé à chaud présentant une résistance mécanique supérieure à 800 MPa conjointement avec un taux d'allongement à rupture supérieur à 10%, aussi bien en sens long qu'en sens travers par rapport au laminage.The present invention aims to solve the problems mentioned above. It aims to provide a hot-rolled steel having a mechanical strength greater than 800 MPa together with an elongation rate greater than 10% rupture, both in long direction and in cross-direction relative to rolling.

L'invention vise également à mettre à disposition un acier peu sensible à l'endommagement lors de la découpe par un procédé mécanique.The invention also aims at providing a steel that is not very sensitive to damage during cutting by a mechanical method.

Elle vise également à disposer d'un acier présentant une bonne aptitude à la mise en forme d'assemblages soudés fabriqués à partir de cet acier, en particulier d'assemblages obtenus par soudage LASER.It also aims to have a steel having a good aptitude for shaping welded joints made from this steel, in particular assemblies obtained by welding LASER.

L'invention vise également à disposer d'un procédé de fabrication d'un acier à l'état non revêtu, électrozingué ou galvanisé. Ceci nécessite donc que les caractéristiques mécaniques de cet acier soient peu sensibles aux cycles thermiques associés aux procédés de revêtement de zinc au trempé.The invention also aims to provide a method of manufacturing a steel in the uncoated, electrogalvanized or galvanized state. This therefore requires that the mechanical characteristics of this steel are insensitive to the thermal cycles associated with dipping zinc coating processes.

L'invention vise également à disposer d'un acier laminé à chaud disponible même en faible épaisseur, c'est à dire par exemple entre 1 et 5mm. La dureté à chaud de l'acier ne doit donc pas être trop élevée pour faciliter le laminage. Dans ce but, l'invention a pour objet une tôle d'acier laminée à chaud de résistance supérieure à 800 MPa, d'allongement à rupture supérieur à 10%, dont la composition comprend, les teneurs étant exprimées en poids :

  • 0,050% ≤ C ≤ 0,090%, 1%≤ Mn ≤ 2%, 0,015% ≤ Al ≤ 0,050 %, 0,1%≤Si≤ 0,3%, 0,10% ≤ Mo ≤ 0,40%, S ≤ 0,010%, P≤ 0,025%, 0,003%≤N≤0,009%, 0, 12% ≤V≤ 0,22%, Ti≤ 0,005%, Nb≤ 0,020% et à titre optionnel, Cr≤ 0,45%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration.
The invention also aims to have a hot rolled steel available even in small thickness, that is to say for example between 1 and 5mm. The hot hardness of the steel must not be too high to facilitate rolling. For this purpose, the subject of the invention is a hot-rolled steel sheet with a strength of greater than 800 MPa and an elongation at break greater than 10%, the composition of which comprises the contents being expressed by weight:
  • 0.050% ≤ C ≤ 0.090%, 1% ≤ Mn ≤ 2%, 0.015% ≤ Al ≤ 0.050%, 0.1% ≤Si ≤ 0.3%, 0.10% ≤ Mo ≤ 0.40%, S ≤ 0.010%, P≤0.025%, 0.003% ≤N≤0.009%, 0, 12% ≤V≤0.22%, Ti≤0.005%, Nb≤0.020% and optionally, Cr≤0.45%, the the remainder of the composition being made of iron and unavoidable impurities resulting from the elaboration.

La composition de l'acier comprend préférentiellement, la teneur étant exprimée en poids : 0,050% ≤ C ≤ 0,070%The composition of the steel preferably comprises the content being expressed by weight: 0.050% ≤ C ≤ 0.070%

A titre préféré, la composition comprend, la teneur étant exprimée en poids : 0,070% <C ≤ 0,090%Preferably, the composition comprises, the content being expressed by weight: 0.070% <C ≤ 0.090%

Selon un mode préféré, la composition comprend : 1,4% ≤ Mn ≤ 1,8%.In a preferred embodiment, the composition comprises: 1.4% ≤ Mn ≤ 1.8%.

A titre préféré, la composition comprend : 0,020% ≤ Al ≤ 0,040 %.Preferably, the composition comprises: 0.020% ≤ Al ≤ 0.040%.

La composition de l'acier comprend préférentiellement : 0,12% ≤ V ≤ 0,16 %. Selon un mode préféré, la composition de l'acier comprend 0,18% ≤ Mo ≤ 0,30 %.The composition of the steel preferably comprises: 0.12% ≤ V ≤ 0.16%. In a preferred embodiment, the composition of the steel comprises 0.18% ≤ Mo ≤ 0.30%.

A titre préféré, la composition comprend : Nb ≤ 0,005 %By way of preference, the composition comprises: Nb ≤ 0.005%

Préférentiellement, la composition comprend : 0,20% ≤ Cr ≤ 0,45%Preferably, the composition comprises: 0.20% ≤ Cr ≤ 0.45%

La microstructure de l'acier comprend préférentiellement, en fraction surfacique, au moins 80% de bainite supérieure, le complément éventuel étant constitué de bainite inférieure, de martensite et d'austénite résiduelle, la somme des teneurs en martensite et en austénite résiduelle étant inférieure à 5%.The microstructure of the steel preferably comprises, as a surface fraction, at least 80% of higher bainite, the optional complement being constituted by lower bainite, martensite and residual austenite, the sum of the martensite and residual austenite contents being lower. at 5%.

L'invention a également pour objet un assemblage soudé par faisceau à haute densité d'énergie réalisé à partir d'une tôle d'acier selon l'un des modes ci-dessus.The invention also relates to a beam welded assembly with high energy density made from a steel sheet according to one of the above modes.

L'invention a également pour objet un procédé de fabrication d'une tôle d'acier laminée à chaud de résistance supérieure à 800 MPa, d'allongement à rupture supérieur à 10%, selon lequel on approvisionne un acier de composition ci-dessus, on coule un demi-produit qu'on porte à une température supérieure à 1150°C. On lamine à chaud le demi-produit jusqu'à une température TFL dans un domaine de température où la microstructure de l'acier est entièrement austénitique de façon à obtenir une tôle. On refroidit ensuite celle-ci à une vitesse de refroidissement VR comprise 75 et 200°C/s, puis on bobine la tôle à une température Tbob comprise entre 500 et 600°C.The subject of the invention is also a process for manufacturing a hot-rolled steel sheet with a resistance greater than 800 MPa and an elongation at break of greater than 10%, according to which a steel of the above composition is supplied, a semi-finished product is poured at a temperature above 1150 ° C. The semi-finished product is hot rolled to a temperature T FL in a temperature range where the microstructure of the steel is entirely austenitic so as to obtain a sheet. This is then cooled to a cooling rate V R of 75 and 200 ° C./s, then the sheet is reeled at a temperature T bob of between 500 and 600 ° C.

Selon un mode préféré, la température de fin de laminage TFL est comprise entre 870 et 930°C.According to a preferred embodiment, the end of rolling temperature T FL is between 870 and 930 ° C.

A titre préférentiel, la vitesse de refroidissement VR est comprise entre 80 et 150°C/s.Preferably, the cooling rate V R is between 80 and 150 ° C / s.

Préférentiellement, la tôle est décapée, puis optionnellement skin-passée, puis revêtue de zinc ou d'alliage de zinc.Preferably, the sheet is pickled, then optionally skin-passed, and then coated with zinc or zinc alloy.

Selon un mode préféré, le revêtement est réalisé au trempé.In a preferred embodiment, the coating is made by dipping.

L'invention a également pour objet l'utilisation d'une tôle d'acier laminée à chaud selon l'un des modes ci-dessus, ou fabriquée par un procédé selon l'un des modes ci-dessus pour la fabrication de pièces de structure ou d'éléments de renfort, dans le domaine automobile.The invention also relates to the use of a hot-rolled steel sheet according to one of the above modes, or manufactured by a method according to one of the above modes for the manufacture of parts of structure or reinforcement elements, in the automotive field.

D'autres caractéristiques et avantages de l'invention apparaîtront au cours de la description ci-dessous, donnée à titre d'exemple et faite en référence aux figures annexées ci-jointes selon lesquelles :

  • La figure 1 illustre l'influence de la teneur en carbone sur l'allongement en sens long de soudures de raboutage réalisées par faisceau LASER
  • La figure 2 illustre la microstructure d'un acier selon l'invention
Other features and advantages of the invention will become apparent from the description below, given by way of example and with reference to the appended accompanying figures in which:
  • The figure 1 illustrates the influence of carbon content on long-term elongation of LASER beam splicing welds
  • The figure 2 illustrates the microstructure of a steel according to the invention

En ce qui concerne la composition chimique de l'acier, le carbone joue un rôle important sur la formation de la microstructure et sur les propriétés mécaniques.With regard to the chemical composition of steel, carbon plays an important role in the formation of the microstructure and in the mechanical properties.

Selon l'invention, la teneur en carbone est comprise entre 0,050 et 0,090% en poids : Au dessous de 0,050%, une résistance suffisante ne peut pas être obtenue. Au delà de 0,090%, la microstructure formée est constituée majoritairement de bainite inférieure, cette structure étant caractérisée par la présence de carbures précipités au sein des lattes de ferrite bainitique : la résistance ainsi obtenue est élevée, mais l'allongement est alors notablement réd u it.According to the invention, the carbon content is between 0.050 and 0.090% by weight: Below 0.050%, sufficient strength can not be obtained. Beyond 0.090%, the microstructure formed consists mainly of lower bainite, this structure being characterized by the presence of carbides precipitated within the slats of bainitic ferrite: the resistance thus obtained is high, but the elongation is then significantly reduced. it.

Selon un mode particulier de l'invention, la teneur en carbone est comprise entre 0,050 et 0,070% La figure 1 illustre l'influence de la teneur en carbone sur l'allongement en sens long de soudures de raboutage par faisceau LASER : Un allongement à rupture particulièrement élevé, de l'ordre de 17 à 23% est associé à une teneur en carbone allant de 0,050 à 0,070%. Ces valeurs d'allongement élevées permettent d'assurer que des tôles soudées par LASER pourront être embouties de façon satisfaisante, même en tenant compte d'éventuelles imperfections locales, telles que des singularités géométriques de cordons de soudure entraînant des concentrations de contraintes, ou des microporosités au sein du métal fondu. Par rapport à des aciers à 0,12%C de l'art antérieur, il était attendu que la réduction du carbone améliore la soudabilité. Cependant, on a mis en évidence qu'un abaissement important de la teneur en carbone permet non seulement d'obtenir un allongement à rupture élevé, mais encore de maintenir simultanément la résistance mécanique à un niveau supérieur à 800MPa, ce qui n'était pas attendu pour des teneurs aussi basses que 0,050%C.According to a particular embodiment of the invention, the carbon content is between 0.050 and 0.070%. figure 1 illustrates the influence of carbon content on the long-term elongation of LASER beam splicing welds: A particularly high elongation at break of 17-23% is associated with a carbon content of 0.050 at 0.070%. These high elongation values make it possible to ensure that LASER-welded sheets can be stamped satisfactorily, even taking into account any local imperfections, such as geometrical singularities of weld seams resulting in stress concentrations, or microporosities within the molten metal. Compared to 0.12% C steels of the prior art, carbon reduction was expected to improve weldability. However, it has been demonstrated that a significant lowering of the carbon content not only makes it possible to obtain a high breaking elongation, but also simultaneously maintains the mechanical strength at a level greater than 800 MPa, which was not expected for contents as low as 0.050% C.

Selon un autre mode préféré, la teneur en carbone est supérieure à 0,070% et inférieure ou égale à 0,090% : même si cette gamme ne conduit pas à une ductilité aussi élevée, l'allongement à rupture des soudures LASER est supérieure à 15%, et reste comparable à celui de la tôle d'acier de base.According to another preferred embodiment, the carbon content is greater than 0.070% and less than or equal to 0.090%: even if this range does not lead to such a high ductility, the elongation at break of the LASER welds is greater than 15%, and remains comparable to that of the base steel sheet.

En quantité comprise entre 1 et 2% en poids, le manganèse augmente la trempabilité et permet d'éviter la formation de ferrite au refroidissement après laminage. Le manganèse contribue également à désoxyder l'acier lors de l'élaboration en phase liquide. L'addition de manganèse participe également à un durcissement efficace en solution solide et à l'obtention d'une résistance accrue. Préférentiellement, le manganèse est compris entre 1,4 et 1,8% : on forme de la sorte une structure totalement bainitique sans risque d'apparition de structure en bandes néfaste.In an amount of between 1 and 2% by weight, manganese increases the quenchability and avoids the formation of ferrite cooling after rolling. Manganese also helps to deoxidize steel during liquid phase processing. The addition of manganese also contributes to effective solid solution hardening and increased strength. Preferentially, the manganese is between 1.4 and 1.8%: thus forming a completely bainitic structure without risk of appearance of harmful band structure.

Dans une gamme de teneurs comprises entre 0,015% et 0,050%, l'aluminium est un élément efficace pour la désoxydation de l'acier. Cette efficacité est obtenue de façon particulièrement économique et stable lorsque la teneur en aluminium est comprise entre 0,020 et 0,040%.In a range of contents between 0.015% and 0.050%, aluminum is an effective element for the deoxidation of steel. This efficiency is obtained in a particularly economical and stable manner when the aluminum content is between 0.020 and 0.040%.

En quantité supérieure ou égale à 0,1%, le silicium contribue à la désoxydation en phase liquide et au durcissement en solution solide. Une addition de silicium au delà de 0,3% provoque cependant la formation d'oxydes fortement adhérents et l'apparition éventuelle de défauts de surface, dus notamment à un manque de mouillabilité dans les opérations de galvanisation au trempé.In an amount greater than or equal to 0.1%, silicon contributes to liquid phase deoxidation and hardening in solid solution. An addition of silicon above 0.3%, however, causes the formation of strongly adherent oxides and the possible appearance of surface defects, due in particular to a lack of wettability in dip galvanizing operations.

En quantité supérieure ou égale à 0,10%, le molybdène retarde la transformation bainitique lors du refroidissement après laminage, contribue au durcissement par solution solide et affine la taille des lattes bainitiques. Selon l'invention, la teneur en molybdène est inférieure ou égale à 0,40% pour éviter la formation excessive de structures de trempe. Cette teneur limitée en molybdène permet également d'abaisser le coût de fabrication.In an amount greater than or equal to 0.10%, molybdenum retards bainitic transformation during cooling after rolling, contributes to hardening by solid solution and refines the size of bainitic slats. According to the invention, the molybdenum content is less than or equal to 0.40% to prevent excessive formation of quenching structures. This limited molybdenum content also makes it possible to lower the manufacturing cost.

Selon un mode préféré, la teneur en molybdène est supérieure ou égale à 0,18% et inférieure ou égale à 0,30%. De la sorte, le niveau est idéalement ajusté pour éviter la formation de ferrite ou de perlite dans la tôle d'acier sur la table de refroidissement après laminage à chaud.In a preferred embodiment, the molybdenum content is greater than or equal to 0.18% and less than or equal to 0.30%. In this way, the level is ideally adjusted to avoid the formation of ferrite or perlite in the steel sheet on the cooling table after hot rolling.

En quantité supérieure à 0,010%, le soufre tend à précipiter en quantité excessive sous forme de sulfures de manganèse qui réduisent fortement l'aptitude à la mise en forme.In excess of 0.010%, sulfur tends to precipitate excessively in the form of manganese sulphides which greatly reduce the shaping ability.

Le phosphore est un élément connu pour ségréger aux joints de grains. Sa teneur doit être limitée à 0,025% de façon à maintenir une ductilité à chaud suffisante.Phosphorus is a known element to segregate at grain boundaries. Its content must be limited to 0.025% in order to maintain sufficient hot ductility.

A titre optionnel, la composition peut comporter du chrome en quantité inférieure ou égale à 0,45%. Grâce aux autres éléments de la composition et au procédé selon l'invention, sa présence n'est cependant pas absolument nécessaire, ce qui présente l'avantage d'éviter des additions coûteuses.As an option, the composition may comprise chromium in an amount of less than or equal to 0.45%. Thanks to the other elements of the composition and to the process according to the invention, its presence is however not absolutely necessary, which has the advantage of avoiding expensive additions.

Une addition de chrome entre 0,20 et 0,45% peut être effectuée en complément des autres éléments augmentant la trempabilité.Addition of chromium between 0.20 and 0.45% can be carried out in addition to the other elements increasing the quenchability.

Selon l'invention, l'acier contient moins de 0,005%Ti et moins de 0,020%Nb Dans le cas contraire, ces éléments fixent une quantité trop importante d'azote sous forme de nitrures ou de carbonitrures. Il ne reste pas alors suffisamment d'azote disponible pour précipiter avec le vanadium. De plus, une précipitation excessive de niobium augmenterait la dureté à chaud et ne permettrait pas aisément la réalisation de tôles laminées à chaud de faible épaisseur.According to the invention, the steel contains less than 0.005% Ti and less than 0.020% Nb. In the opposite case, these elements fix too much nitrogen in the form of nitrides or carbonitrides. There is not enough nitrogen available to precipitate with vanadium. In addition, excessive precipitation of niobium would increase the hot hardness and would not easily allow the realization of thin-rolled hot-rolled sheets.

Selon un mode particulièrement économique, la teneur en niobium est inférieure à 0,005%In a particularly economical mode, the niobium content is less than 0.005%

Le vanadium est un élément important selon l'invention : l'acier contient une teneur en vanadium comprise entre 0,12 et 0,22%. Par rapport à un acier sans vanadium, l'augmentation de la résistance grâce à une précipitation durcissante de carbonitrures peut aller jusqu'à 300MPa. Au dessous de 0,12%, on ne note pas d'effet significatif sur les caractéristiques mécaniques de traction. Au delà de 0,22% de vanadium, dans les conditions de fabrication selon l'invention, on note une saturation de l'effet sur les caractéristiques mécaniques. Une teneur inférieure à 0,22% permet donc d'obtenir des caractéristiques mécaniques élevées de façon très économique par rapport à des aciers qui comporteraient des teneurs plus élevées en vanadium.Vanadium is an important element according to the invention: the steel contains a vanadium content of between 0.12 and 0.22%. Compared to a vanadium-free steel, the increase in strength due to a hardening precipitation of carbonitrides can be up to 300 MPa. Below 0.12%, there is no significant effect on the mechanical tensile characteristics. Beyond 0.22% vanadium, under the conditions of manufacture according to the invention, a saturation of the effect on the mechanical characteristics is noted. A content of less than 0.22% thus makes it possible to obtain high mechanical characteristics in a very economical manner with respect to steels which contain higher levels of vanadium.

Pour une teneur en vanadium comprise entre 0,13 et 0,15%, on obtient un affinement de la microstructure et un durcissement structural tout particulièrement efficaces.For a vanadium content between 0.13 and 0.15%, microstructure refinement and structural hardening are particularly effective.

Selon l'invention, la teneur en azote est supérieure ou égale à 0,003% pour obtenir une précipitation de carbonitrures de vanadium en quantité suffisante. Cependant, la teneur en azote est inférieure ou égale à 0,009% pour éviter la présence d'azote en solution solide ou la formation de carbonitrures de taille plus importante, qui réduiraient la ductilité.According to the invention, the nitrogen content is greater than or equal to 0.003% in order to obtain a precipitation of vanadium carbonitrides in a sufficient quantity. However, the nitrogen content is less than or equal to 0.009% to avoid the presence of solid solution nitrogen or the formation of larger carbonitrides, which would reduce ductility.

Le reste de la composition est constitué d'impuretés inévitables résultant de l'élaboration, telles que par exemple Sb, Sn, As.The rest of the composition consists of unavoidable impurities resulting from the preparation, such as for example Sb, Sn, As.

La microstructure de l'acier selon l'invention est constituée :

  • d'au moins 80% de bainite supérieure, cette structure étant constituée de lattes de ferrite bainitique et de carbures situés entre ces lattes, la précipitation intervenant lors de la transformation bainitique. Cette matrice présente des propriétés de résistance élevées, combinées à une ductilité importante
  • en complément éventuel, la structure contient :
  • De la bainite inférieure, dont la précipitation de carbures intervient au sein des lattes ferritiques ; par rapport à la bainite supérieure, la bainite inférieure présente une résistance un peu plus importante, mais une ductilité moins grande.
  • Eventuellement de la martensite. Celle-ci est fréquemment associée à de l'austénite résiduelle sous forme de composés « M-A » (martensite-austénite résiduelle) La teneur totale en martensite et en austénite résiduelle doit être limitée à 5% pour ne pas diminuer la ductilité.
The microstructure of the steel according to the invention consists of:
  • at least 80% higher bainite, this structure consisting of bainitic ferrite slats and carbides located between these laths, the precipitation occurring during the bainitic transformation. This matrix has high strength properties, combined with high ductility
  • as a possible complement, the structure contains:
  • Lower bainite, the precipitation of carbides intervenes within the ferritic slats; compared with the upper bainite, the lower bainite has a somewhat greater resistance, but a smaller ductility.
  • Possibly martensite. This is frequently associated with residual austenite in the form of "MA" compounds (residual martensite-austenite). The total content of martensite and residual austenite must be limited to 5% in order not to reduce the ductility.

Les pourcentages microstructuraux ci-dessus correspondent aux fractions surfaciques que l'on peut mesurer sur des coupes polies et attaquées.The microstructural percentages above are surface fractions that can be measured on polished and etched sections.

La microstructure ne comporte donc pas de ferrite primaire ou proeutectoïde : elle présente alors une grande homogénéité puisque l'écart de propriétés mécaniques entre la matrice (bainite supérieure) et les autres constituants éventuels (bainite inférieure et martensite) est faible. Lors d'une sollicitation mécanique, les déformations se répartissent de façon homogène. Une accumulation de dislocations n'intervient pas au niveau des interfaces entre les constituants et un endommagement prématuré est évité, contrairement à ce qui peut être noté dans des structures comportant une quantité significative de ferrite primaire, phase dont la limite d'écoulement est très faible, ou de martensite à très haut niveau de résistance. De la sorte, l'acier selon l'invention présente une aptitude particulière à certains modes de déformation exigeants tels que l'expansion de trous, la sollicitation mécanique de bords découpés, le pliage.The microstructure therefore has no primary or proeutectoid ferrite: it then has a great homogeneity since the property gap Mechanics between the matrix (upper bainite) and the other possible constituents (lower bainite and martensite) is weak. During a mechanical stress, the deformations are distributed homogeneously. Dislocation accumulation does not occur at the interfaces between the constituents and premature damage is avoided, contrary to what can be noted in structures with a significant amount of primary ferrite, a phase whose flow limit is very low. , or martensite with a very high level of resistance. In this way, the steel according to the invention has a particular aptitude for certain demanding deformation modes such as the expansion of holes, the mechanical stress of cut edges, the folding.

La mise en oeuvre du procédé de fabrication d'une tôle laminée à chaud selon l'invention est la suivante :

  • On approvisionne un acier de composition selon l'invention, puis on procède à la coulée d'un demi-produit à partir de cet acier. Cette coulée peut être réalisée en lingots, ou en continu sous forme de brames d'épaisseur de l'ordre de 200mm. On peut également effectuer la coulée sous forme de brames minces de quelques dizaines de millimètres d'épaisseur, ou de bandes minces, entre cylindres d'acier contra-rotatifs.
The method of manufacturing a hot-rolled sheet according to the invention is implemented as follows:
  • A steel composition is supplied according to the invention, and then a semi-finished product is cast from this steel. This casting may be carried out in ingots, or continuously in the form of slabs of thickness of the order of 200 mm. The casting can also be carried out in the form of thin slabs of a few tens of millimeters thick, or thin strips, between contra-rotating steel rolls.

Les demi-produits coulés sont tout d'abord portés à une température supérieure à 1150°C pour atteindre en tout point une température favorable aux déformations élevées que va subir l'acier lors du laminage. Naturellement, dans le cas d'une coulée directe de brames minces ou de bandes minces entre cylindres contra-rotatifs, l'étape de laminage à chaud de ces demi-produits débutant à plus de 1150°C peut se faire directement après coulée si bien qu'une étape de réchauffage intermédiaire n'est pas nécessaire dans ce cas.The cast semifinished products are first brought to a temperature higher than 1150 ° C. to reach at any point a temperature favorable to the high deformations which the steel will undergo during rolling. Naturally, in the case of a direct casting of thin slabs or thin strips between contra-rotating rolls, the hot rolling step of these semi-finished products starting at more than 1150 ° C. can be done directly after casting so well. that an intermediate heating step is not necessary in this case.

On lamine à chaud le demi-produit dans un domaine de température où la structure de l'acier est totalement austénitique jusqu'à une température de fin de laminage TFL. La température TFL est comprise préférentiellement entre 870 et 930°C pour obtenir une taille de grain adaptée à la transformation bainitique qui va suivre.The semi-finished product is hot-rolled in a temperature range where the structure of the steel is totally austenitic up to an end-of-rolling temperature T FL . The temperature T FL is preferably between 870 and 930 ° C to obtain a grain size adapted to the bainitic transformation that follows.

On effectue ensuite un refroidissement à une vitesse VR comprise entre 75 et 200°C/s : une vitesse minimale de 75°C/s permet d'éviter la formation de ferrite proeutectoïde et de perlite, alors qu'une vitesse VR inférieure ou égale à 200°C/s permet d'éviter la formation excessive de martensite.Cooling is then carried out at a speed V R of between 75 and 200 ° C / s: a minimum speed of 75 ° C / s prevents the formation of proeutectoid ferrite and perlite, while a speed V R of 200 ° C / s or less prevents formation excessive martensite.

D'une façon optimale, la vitesse VR est comprise entre 80 et 150°C/s : Une vitesse minimale de 80°C/s conduit à la formation de bainite supérieure avec une taille de lattes très réduite, associée à d'excellentes propriétés mécaniques. Une vitesse inférieure à 150°C/s permet d'éviter très majoritairement la formation de martensite.Optimally, the speed V R is between 80 and 150 ° C / s: A minimum speed of 80 ° C / s leads to the formation of upper bainite with a very small slat size, associated with excellent mechanical properties. A speed of less than 150 ° C / s makes it possible to avoid, for the most part, the formation of martensite.

La gamme de vitesse de refroidissement selon l'invention peut être obtenue au moyen d'une pulvérisation d'eau ou d'un mélange air-eau, en fonction de l'épaisseur de la tôle, à la sortie du laminoir finisseur.

  • Après cette phase de refroidissement rapide, la tôle laminée à chaud est bobinée à une température Tbob comprise entre 500 et 600°C. La transformation bainitique se produit pendant cette phase de bobinage ; de la sorte, on évite la formation de ferrite proeutectoïde ou de perlite causée par une température de bobinage trop élevée, et on évite également la formation de constituants de trempe qui serait causée par une température de bobinage trop basse. De plus, la précipitation de carbonitrures intervenant dans cette gamme de température de bobinage permet également d'obtenir un durcissement supplémentaire.
The cooling rate range according to the invention can be obtained by means of a water spray or an air-water mixture, depending on the thickness of the sheet, at the output of the finishing mill.
  • After this rapid cooling phase, the hot-rolled sheet is wound at a T bob temperature of between 500 and 600 ° C. The bainitic transformation occurs during this winding phase; in this way, the formation of proeutectoid ferrite or perlite caused by a too high winding temperature is avoided, and the formation of quenching constituents which is caused by a too low winding temperature is also avoided. In addition, the precipitation of carbonitrides occurring in this winding temperature range also makes it possible to obtain additional hardening.

La tôle peut être utilisée à l'état nu ou revêtu. Selon l'utilisation envisagée, on décape la tôle après laminage selon un procédé connu en soi, de façon à obtenir un état de surface propre à favoriser la mise oeuvre du revêtement ultérieur.The sheet can be used in the bare state or coated. Depending on the use envisaged, the sheet is scoured after rolling according to a method known per se, so as to obtain a surface state suitable for promoting the implementation of the subsequent coating.

Afin d'effacer le palier observé lors d'un essai mécanique de traction, la tôle peut être éventuellement soumise à une légère déformation à froid, usuellement inférieure à 1% (« skin-pass ») La tôle est ensuite revêtue de zinc ou d'un alliage à base de zinc, par exemple par électrozingage ou par galvanisation au trempé. Dans ce dernier cas, on a mis en évidence que la microstructure particulière de l'acier, composée majoritairement de bainite supérieure, est peu sensible aux conditions thermiques du traitement ultérieur de galvanisation, si bien que les caractéristiques mécaniques des tôles revêtues au trempé présentent une grande stabilité même en cas de fluctuation intempestive de ces conditions. La tôle à l'état galvanisé présente donc des caractéristiques mécaniques très similaires de celles à l'état nu.In order to erase the bearing observed during a mechanical tensile test, the sheet may be subjected to a slight cold deformation, usually less than 1% ("skin-pass"). The sheet is then coated with zinc or aluminum. an alloy based on zinc, for example by electrogalvanizing or dip galvanizing. In the latter case, it has been demonstrated that the particular microstructure of the steel, composed mainly of higher bainite, is not very sensitive to the thermal conditions of the subsequent galvanizing treatment, so that the mechanical characteristics of the sheets coated with dipping have a high great stability even in case of untimely fluctuation of these conditions. The sheet in the galvanized state therefore has mechanical characteristics very similar to those in the naked state.

Exemple :Example:

On a élaboré des aciers dont la composition figure au tableau ci-dessous, exprimée en pourcentage pondéral. Outre l'acier I-1 ayant servi à la fabrication de tôles selon l'invention, on a indiqué à titre de comparaison la composition d'aciers R-1 et R-2 ayant servi à la fabrication de tôles de référence. Tableau 1 Compositions d'aciers (% poids). I= Selon l'invention. R= référence Valeurs soulignées : Non conforme à l'invention. Acier C (%) Mn (%) Si (%) Al(%) S(%) P(%) Mo (%) Cr (%) N(%) V (%) Nb (%) I-1 0,070 1,604 0,218 0,028 0,002 0,014 0,313 0,400 0,006 0,150 - I2 0,072 1,592 0,204 0,031 0,003 0,024 0,200 0,414 0,006 0,211 0,017 R1 0.125 1,670 0,205 0,030 0,002 0,025 0,307 0,414 0,004 0.105 - R2 0.102 1,680 0,204 0,023 0,002 0.028 0,315 0,408 0,007 0,205 - Steels have been developed, the composition of which is given in the table below, expressed in percentage by weight. In addition to the steel I-1 used for the production of sheets according to the invention, the composition of R-1 and R-2 steels used for the manufacture of reference sheets has been indicated for comparison. Table 1 Compositions of steel (% by weight). I = according to the invention. R = reference Underlined values: Not in accordance with the invention. Steel VS (%) Mn (%) Yes (%) Al (%) S (%) P (%) Mo (%) Cr (%) NOT(%) V (%) Nb (%) I-1 0,070 1,604 0.218 0,028 0,002 0.014 0.313 0,400 0.006 0,150 - I2 0,072 1,592 0,204 0.031 0,003 0,024 0,200 0.414 0.006 0.211 0,017 R1 0125 1,670 0.205 0,030 0,002 0,025 0.307 0.414 0,004 0105 - R2 0102 1,680 0,204 0,023 0,002 0028 0,315 0,408 0,007 0.205 -

Des demi-produits correspondant aux compositions ci-dessus ont été réchauffés à 1220°C et laminés à chaud jusqu'à une épaisseur de 2,3 mm dans un domaine où la structure est entièrement austénitique. Les conditions de fabrication de ces aciers (température de fin de laminage TFL, vitesse de refroidissement VR, température de bobinage Tbob) sont indiquées au tableau 2 Tableau 2 Conditions de fabrication. Valeurs soulignées : non conforme à l'invention Acier TFL(°C) VR(°C/s) Tbob(°C) I1 910 80 520 I2 875 80 600 R1 880 80 520 R2 885 100 450 Semi-finished products corresponding to the above compositions were reheated to 1220 ° C and hot rolled to a thickness of 2.3 mm in a field where the structure is fully austenitic. The manufacturing conditions of these steels (end of rolling temperature T FL , cooling rate V R , winding temperature T bob ) are indicated in table 2 Table 2 Manufacturing conditions. Underlined values: not in accordance with the invention Steel T FL (° C) V R (° C / s) T bob (° C) I1 910 80 520 I2 875 80 600 R1 880 80 520 R2 885 100 450

Les propriétés mécaniques de traction obtenues (limite d'élasticité Re, résistance Rm, allongement à rupture A) ont été portées au tableau 3 ci-dessous. Tableau 3 : Caractéristiques mécaniques (sens long par rapport au laminage) : Valeurs soulignées : non conforme à l'invention Acier Re(MPa) Rm (MPa) Allongement à rupture A (%) I1 820 880 11 I2 767 831 16 R1 740 835 8 R2 870 927 7,5 The tensile mechanical properties obtained (elastic limit Re, resistance Rm, elongation at break A) were given in Table 3 below. Table 3: Mechanical characteristics (long direction with respect to rolling): Underlined values: not according to the invention Steel Re (MPa) Rm (MPa) Elongation at break A (%) I1 820 880 11 I2 767 831 16 R1 740 835 8 R2 870 927 7.5

Les valeurs élevées des caractéristiques mécaniques sont obtenues aussi bien en sens long qu'en sens travers par rapport au laminage pour les aciers selon l'invention.The high values of the mechanical characteristics are obtained both in the long direction and in the transverse direction with respect to rolling for the steels according to the invention.

La microstructure de l'acier I1 illustrée à la figure 2 comprend plus de 80% de bainite supérieure, le reste étant constitué de bainite inférieure et de composés M-A. La teneur totale en martensite et en austénite résiduelle est inférieure à 5%. La taille des anciens grains austénitiques et des paquets de lattes bainitiques est d'environ 10 micromètres. La limitation de la taille des paquets de lattes et la forte désorientation entre les paquets adjacents a pour conséquence une grande résistance à la propagation d'éventuelles microfissures. Grâce à la faible différence de dureté entre les différents constituants de la microstructure, l'acier est peu sensible à l'endommagement lors de la découpe par un procédé mécanique.The microstructure of steel I1 illustrated in figure 2 comprises more than 80% higher bainite, the remainder being lower bainite and MA compounds. The total content of martensite and residual austenite is less than 5%. The size of the old austenitic grains and bainitic batten bundles is about 10 microns. The limitation of the size of the batten packets and the strong disorientation between the adjacent packets results in a high resistance to the propagation of any microcracks. Due to the small difference in hardness between the various constituents of the microstructure, the steel is not very sensitive to damage during cutting by a mechanical process.

L'acier R1, présentant une teneur en carbone trop élevée et une teneur en vanadium trop faible, a un allongement à rupture insuffisant. L'acier R2, présente une teneur en carbone et en phosphore trop élevée, sa température de bobinage est également trop faible. En conséquence, son allongement à rupture est également nettement inférieur à 10%.Steel R1, with too high a carbon content and a too low vanadium content, has insufficient breaking elongation. Steel R2, has a carbon content and phosphorus too high, its winding temperature is also too low. As a result, its elongation at break is also significantly less than 10%.

Des joints soudés autogènes LASER ont été réalisés dans les conditions suivantes : puissance : 4,5kW, vitesse de soudage : 2,5m/mn. L'allongement en sens long des soudures LASER de l'acier I-1 est de 17%, alors qu'il est de 10 et 13% respectivement pour les aciers R-1 et R-2. Ces valeurs conduisent, particulièrement pour l'acier R1, à des difficultés lors d'emboutissage de joints soudés.LASER autogenous welded joints were made under the following conditions: power: 4.5kW, welding speed: 2.5m / min. The lengthwise elongation of the LASER welds of I-1 steel is 17%, whereas it is 10 and 13% respectively for the R-1 and R-2 steels. These values lead, particularly for steel R1, to difficulties in stamping welded joints.

Des tôles d'acier I1 selon l'invention ont été également galvanisées dans les conditions suivantes : après chauffage à 680°C, les tôles ont été refroidies à 455°C puis revêtues au trempé dans un bain de Zn à cette température et enfin refroidies à température ambiante. Les caractéristiques mécaniques des tôles galvanisées sont les suivantes : Re=824MPa, Rm=879MPa, A=12%. Ces propriétés sont pratiquement identiques à celles de la tôle non revêtue, ce qui indique que la microstructure des aciers selon l'invention est très stable vis-à-vis des cycles thermiques de galvanisation.Steel sheets I1 according to the invention were also galvanized under the following conditions: after heating at 680 ° C., the sheets were cooled to 455 ° C. and then quenched in a Zn bath at this temperature and finally cooled. at room temperature. The mechanical characteristics of the galvanized sheets are as follows: Re = 824 MPa, Rm = 879 MPa, A = 12%. These properties are virtually identical to those of the uncoated sheet, which indicates that the microstructure of the steels according to the invention is very stable vis-à-vis the thermal cycles of galvanization.

Ainsi, l'invention permet la fabrication de tôles d'aciers à matrice bainitique sans addition excessive d'éléments coûteux. Celles-ci allient une haute résistance et une ductilité élevée. Les tôles d'aciers selon l'invention sont utilisées avec profit pour la fabrication de pièces de structure ou d'éléments de renfort dans le domaine automobile et de l'industrie générale.Thus, the invention allows the manufacture of steels bainitic matrix steels without excessive addition of expensive elements. These combine high strength and high ductility. The steel sheets according to the invention are used profitably for the manufacture of structural parts or reinforcement elements in the automotive field and general industry.

Claims (17)

Tôle d'acier laminée à chaud de résistance supérieure à 800 MPa, d'allongement à rupture supérieur à 10%, dont la composition comprend, les teneurs étant exprimées en poids : 0,050% ≤C≤ 0,090% 1%≤Mn≤2% 0,015% ≤ AI ≤ 0,050 % 0,1 %≤Si≤ 0,3% 0,10%≤ Mo ≤0,40% S≤ 0,010% P≤ 0,025% 0,003%≤N≤0,009% 0, 12% ≤ V ≤ 0,22% Ti≤ 0,005% Nb≤ 0,020% et à titre optionnel, Cr≤ 0,45% le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaborationHot-rolled steel sheet with a strength greater than 800 MPa, elongation at break greater than 10%, the composition of which comprises the contents being expressed by weight: 0.050% ≤C≤ 0.090% 1% ≤Mn≤2% 0.015% ≤ AI ≤ 0.050% 0.1% ≤Si≤ 0.3% 0.10% ≤ Mo ≤ 0.40% S≤ 0.010% P≤ 0.025% 0.003% ≤N≤0,009% 0, 12% ≤ V ≤ 0.22% Ti≤ 0.005% Nb≤ 0.020% and as an option, Cr≤ 0.45% the remainder of the composition consisting of iron and unavoidable impurities resulting from the elaboration Tôle d'acier selon la revendication 1, caractérisée en ce que la composition dudit acier comprend, la teneur étant exprimée en poids : 0,050% ≤C ≤ 0,070% Sheet steel according to claim 1, characterized in that the composition of said steel comprises, the content being expressed by weight: 0.050% ≤C ≤ 0.070% Tôle d'acier selon la revendication 1, caractérisée en ce que la composition dudit acier comprend, la teneur étant exprimée en poids : 0,070% <C ≤ 0,090% Sheet steel according to claim 1, characterized in that the composition of said steel comprises, the content being expressed by weight: 0.070% <C ≤ 0.090% Tôle d'acier selon l'une quelconque des revendications 1 à 3, caractérisée en ce que la composition dudit acier comprend, la teneur étant exprimée en poids : 1,4%≤Mn ≤1,8% Sheet steel according to any one of claims 1 to 3, characterized in that the composition of said steel comprises, the content being expressed by weight: 1.4% ≤Mn ≤1.8% Tôle d'acier selon l'une quelconque des revendications 1 à 4, caractérisée en ce que la composition dudit acier comprend, la teneur étant exprimée en poids : 0,020% ≤ Al ≤ 0,040 % Sheet steel according to any one of claims 1 to 4, characterized in that the composition of said steel comprises, the content being expressed by weight: 0.020% ≤ Al ≤ 0.040% Tôle d'acier selon l'une quelconque des revendications 1 à 5, caractérisée en ce que la composition dudit acier comprend, la teneur étant exprimée en poids : 0,12%≤V≤0,16% Steel sheet according to any one of claims 1 to 5, characterized in that the composition of said steel comprises, the content being expressed by weight: 0.12% ≤V≤0,16% Tôle d'acier selon l'une quelconque des revendications 1 à 6, caractérisée en ce que la composition dudit acier comprend, la teneur étant exprimée en poids : 0,18% ≤ Mo ≤ 0,30 % Sheet steel according to any one of claims 1 to 6, characterized in that the composition of said steel comprises, the content being expressed by weight: 0.18% ≤ Mo ≤ 0.30% Tôle d'acier selon l'une quelconque des revendications 1 à 7, caractérisée en ce que la composition dudit acier comprend, la teneur étant exprimée en poids : Nb ≤ 0,005 % Sheet steel according to any one of claims 1 to 7, characterized in that the composition of said steel comprises, the content being expressed by weight: Nb ≤ 0.005% Tôle d'acier selon l'une quelconque des revendications 1 à 8, caractérisée en ce que la composition dudit acier comprend, la teneur étant exprimée en poids : 0,20% ≤ Cr ≤ 0,45% Steel sheet according to any one of claims 1 to 8, characterized in that the composition of said steel comprises, the content being expressed by weight: 0.20% ≤ Cr ≤ 0.45% Tôle d'acier selon l'une quelconque des revendications 1 à 9, caractérisée en ce que la microstructure dudit acier comprend, en fraction surfacique, au moins 80% de bainite supérieure, le complément éventuel étant constitué de bainite inférieure, de martensite et d'austénite résiduelle, la somme des teneurs en martensite et en austénite résiduelle étant inférieure à 5%Steel sheet according to any one of claims 1 to 9, characterized in that the microstructure of said steel comprises, as a surface fraction, at least 80% of higher bainite, the optional supplement consisting of lower bainite, martensite and residual austenite, the sum of the martensite and residual austenite being less than 5% Assemblage soudé réalisé à partir d'au moins une tôle d'acier selon l'une quelconque des revendications 1 à 10, caractérisé en ce que l'au moins une tôle est soudée par faisceau à haute densité d'énergieWelded joint made from at least one steel sheet according to any one of claims 1 to 10, characterized in that the at least one sheet is welded by a high energy density beam Procédé de fabrication d'une tôle d'acier laminée à chaud de résistance supérieure à 800 MPa, d'allongement à rupture supérieur à 10%, selon lequel : - on approvisionne un acier de composition selon l'une quelconque des revendications 1 à 9, - on procède à la coulée d'un demi-produit à partir de cet acier, - on porte ledit demi-produit à une température supérieure à 1150°C - on lamine à chaud ledit demi-produit jusqu'à une température TFL , dans un domaine de température où la microstructure de l'acier est entièrement austénitique de façon à obtenir une tôle, puis - on refroidit ladite tôle de telle sorte que la vitesse de refroidissement VR soit comprise 75 et 200°C/s, puis - on bobine ladite tôle à une température Tbob comprise entre 500 et 600°C A process for producing a hot-rolled steel sheet with a resistance greater than 800 MPa and a tensile elongation greater than 10%, wherein: a steel of composition according to any one of Claims 1 to 9 is supplied, - the casting of a half-product from this steel, said half-product is carried at a temperature above 1150 ° C. said half-product is hot-rolled to a temperature T FL , in a temperature range where the microstructure of the steel is entirely austenitic so as to obtain a sheet, and then said sheet is cooled so that the cooling rate V R is between 75 and 200 ° C./s, and then said sheet is reeled at a temperature T bob of between 500 and 600 ° C. Procédé de fabrication d'une tôle d'acier laminée à chaud selon la revendication 12 caractérisé en ce que la température de fin de laminage TFL est comprise entre 870 et 930°CA method of manufacturing a hot-rolled steel sheet according to claim 12 characterized in that the end-of-rolling temperature T FL is between 870 and 930 ° C Procédé de fabrication d'une tôle d'acier laminée à chaud selon la revendication 12 ou 13 caractérisé en ce que la vitesse de refroidissement VR est comprise entre 80 et 150°C/sA method of manufacturing a hot-rolled steel sheet according to claim 12 or 13, characterized in that the cooling rate V R is between 80 and 150 ° C / sec. Procédé de fabrication selon lequel une tôle fabriquée selon l'une quelconque des revendications 12 à 14 est décapée, puis optionnellement skin-passée, puis revêtue de zinc ou d'alliage le zincA manufacturing method according to which a sheet manufactured according to any one of claims 12 to 14 is etched, then optionally skin-passed, and then coated with zinc or alloy zinc Procédé de fabrication d'une tôle d'acier selon la revendication 15, caractérisé en ce que ledit revêtement est réalisé au trempéA method of manufacturing a steel sheet according to claim 15, characterized in that said coating is made by dipping Utilisation d'une tôle d'acier laminée à chaud selon l'une quelconque des revendications 1 à 10, ou fabriquée par un procédé selon l'une quelconque des revendications 12 à 16, pour la fabrication de pièces de structure ou d'éléments de renfort, dans le domaine automobile.Use of a hot-rolled steel sheet according to any one of claims 1 to 10, or manufactured by a process according to any one of claims 12 to 16, for the manufacture of structural parts or structural elements. reinforcement, in the automotive field.
EP07290908A 2007-07-19 2007-07-19 Method of manufacturing sheets of steel with high levels of strength and ductility, and sheets produced using same Withdrawn EP2020451A1 (en)

Priority Applications (23)

Application Number Priority Date Filing Date Title
EP07290908A EP2020451A1 (en) 2007-07-19 2007-07-19 Method of manufacturing sheets of steel with high levels of strength and ductility, and sheets produced using same
EP08830766A EP2171112B1 (en) 2007-07-19 2008-07-09 Method for producing steel sheets having high resistance and ductility characteristics, and sheets thus obtained
KR1020187002754A KR101892423B1 (en) 2007-07-19 2008-07-09 Process for manufacturing steel sheet having high tensile strength and ductility characteristics, and sheet thus produced
KR1020147007669A KR20140044407A (en) 2007-07-19 2008-07-09 Process for manufacturing steel sheet having high tensile strength and ductility characteristics, and sheet thus produced
CN2008801040865A CN101784688B (en) 2007-07-19 2008-07-09 Method for producing steel sheets having high resistance and ductility characteristics, and sheets thus obtained
KR1020107003457A KR20100037147A (en) 2007-07-19 2008-07-09 Method for producing steel sheets having high resistance and ductility characteristics, and sheets thus obtained
RU2010105699/02A RU2451764C2 (en) 2007-07-19 2008-07-09 High strength and plasticity steel sheets and method of manufacturing the same
PL08830766T PL2171112T3 (en) 2007-07-19 2008-07-09 Method for producing steel sheets having high resistance and ductility characteristics, and sheets thus obtained
US12/669,188 US20100221573A1 (en) 2007-07-19 2008-07-09 Process for manufacturing steel sheet having high tensile strength and ductility characteristics, and sheet thus produced
ES08830766T ES2375429T3 (en) 2007-07-19 2008-07-09 PROCEDURE FOR MANUFACTURING STEEL SHEETS WITH HIGH CHARACTERISTICS? RESISTANCE AND DUCTILITY STICES, AND SHEETS SO? PRODUCED
CA2694069A CA2694069C (en) 2007-07-19 2008-07-09 Method for producing steel sheets having high resistance and ductility characteristics, and sheets thus obtained
UAA201001690A UA98798C2 (en) 2007-07-19 2008-07-09 hot-rolled steel sheet or part, method FOR THEIR PRODUCTION AND USE, welded joints from steel sheets or parts
JP2010516534A JP5298127B2 (en) 2007-07-19 2008-07-09 Method for producing a steel sheet having high resistance characteristics and ductility characteristics and the steel sheet thus obtained
KR1020157029946A KR20150123957A (en) 2007-07-19 2008-07-09 Process for manufacturing steel sheet having high tensile strength and ductility characteristics, and sheet thus produced
AT08830766T ATE534756T1 (en) 2007-07-19 2008-07-09 METHOD FOR PRODUCING A STEEL SHEET HAVING VERY HIGH STRENGTH AND FLEXIBILITY PROPERTIES AND SHEETS PRODUCED BY THIS PROCESS
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