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US20100221573A1 - Process for manufacturing steel sheet having high tensile strength and ductility characteristics, and sheet thus produced - Google Patents

Process for manufacturing steel sheet having high tensile strength and ductility characteristics, and sheet thus produced Download PDF

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Publication number
US20100221573A1
US20100221573A1 US12/669,188 US66918808A US2010221573A1 US 20100221573 A1 US20100221573 A1 US 20100221573A1 US 66918808 A US66918808 A US 66918808A US 2010221573 A1 US2010221573 A1 US 2010221573A1
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Prior art keywords
sheet
steel
steel sheet
composition
temperature
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Inventor
Pascal Drillet
Damien Ormston
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ArcelorMittal France SA
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ArcelorMittal France SA
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Assigned to ARCELORMITTAL FRANCE reassignment ARCELORMITTAL FRANCE ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: DRILLET, PASCAL, ORMSTON, DAMIEN
Publication of US20100221573A1 publication Critical patent/US20100221573A1/en
Priority to US14/575,475 priority Critical patent/US10214792B2/en
Priority to US15/879,944 priority patent/US10428400B2/en
Abandoned legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—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
    • 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 sheet or parts made of what are called “multiphase” steels having simultaneously a very high tensile strength and a deformability enabling cold or warm forming operations to be carried out.
  • the invention relates more specifically to steels having a predominantly bainitic microstructure having a tensile strength greater than 800 MPa and an elongation at break greater than 10%.
  • the automotive industry constitutes in particular a preferential field of application of such hot-rolled steel sheet.
  • multiphase steels having a predominantly bainitic structure have been developed.
  • these steels have been profitably used to manufacture structural parts.
  • the formability of these parts requires at the same time a sufficient elongation. This requirement may also apply when the parts are welded and then formed. In this case, welded joints must have a sufficient formability and not result in premature fractures at the joints.
  • the object of the present invention is to solve the abovementioned problems by providing a hot-rolled steel sheet having a tensile strength greater than 800 MPa together with an elongation at break greater than 10%, both in the rolling direction and in the transverse direction.
  • the invention also provides a steel sheet that is largely insensitive to damage when being cut by a mechanical process.
  • the aim of the invention is also to provide a steel sheet having a good capability for forming welded assemblies manufactured from this steel, in particular assemblies obtained by laser welding.
  • the aim of the invention is also to provide a process for manufacturing a steel sheet in the uncoated, electrogalvanized or galvanized, or aluminium-coated state. This therefore requires the mechanical properties of this steel to be largely insensitive to the thermal cycles associated with continuous zinc hot-dip coating processes.
  • the aim of the invention is also to provide a hot-rolled steel sheet or part available even with a small thickness, i.e. for example between 1 and 5 mm.
  • the hot hardness of the steel must therefore not be too high in order to facilitate the rolling.
  • one subject of the invention is a hot-rolled steel sheet or part having a tensile strength 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 balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of said sheet or said part comprising, as a surface fraction, at least 80% upper bainite, the possible complement consisting of lower bainite, martensite and residual austenite, the sum of the martensite and residual austenite contents being less than 5%.
  • 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%.
  • the composition of the steel comprises: 0.18% ⁇ Mo ⁇ 0.30%.
  • the composition comprises: Nb ⁇ 0.005%.
  • the composition comprises: 0.20% ⁇ C ⁇ 0.45%.
  • the sheet or part is coated with a zinc-based or aluminium-based coating.
  • Another subject of the invention is a steel part with a composition and a microstructure defined above, characterized in that it is obtained by heating at a temperature T of between 400 and 690° C., then warm-drawing in a temperature range of between 350° C. and (T-20° C.) and then finally cooling down to ambient temperature.
  • Another subject of the invention is an assembly welded by a high-energy-density beam, produced from a steel sheet or part according to one of the above embodiments.
  • Another subject of the invention is a process for manufacturing a hot-rolled steel sheet or part having a tensile strength greater than 800 MPa and an elongation at break greater than 10%, in which a steel of the above composition is provided, a semi-finished product is cast, which is heated to a temperature above 1150° C.
  • the semi-finished product is hot-rolled to a temperature T ER in a temperature range in which the microstructure of the steel is entirely austenitic so as to obtain a sheet.
  • the latter is then cooled at a cooling rate V c of between 75 and 200° C./s, and then the sheet is coiled at a temperature T coil of between 500 and 600° C.
  • the end-of-rolling temperature T ER is between 870 and 930° C.
  • the cooling rate V c is between 80 and 150° C./s.
  • the sheet is pickled, then optionally skin-passed and then coated with zinc or a zinc alloy.
  • the coating is carried out continuously by hot-dip coating.
  • Another subject of the invention is a process for manufacturing a warm-drawn part, in which a steel sheet according to one of the above features is provided, or manufactured by a process according to one of the above features, then said sheet is cut so as to obtain a blank.
  • the blank is partly or completely heated to a temperature T of between 400 and 690° C., where it is maintained for a time of less than 15 minutes so as to obtain a heated blank, then the heated blank is drawn at a temperature of between 350 and T-20° C. in order to obtain a part that is cooled down to ambient temperature at a rate V′ c .
  • the rate V′ c is between 25 and 100° C./s.
  • Another subject of the invention is the use of a hot-rolled steel sheet according to one of the above embodiments, or manufactured by a process according to one of the above embodiments, for the manufacture of structural parts or reinforcing elements in the automotive field.
  • FIG. 1 illustrates the influence of the carbon content on the elongation in the longitudinal direction of butt-welded joints produced using a laser beam
  • FIG. 2 illustrates the microstructure of a steel sheet or part according to the invention
  • FIG. 3 illustrates the microstructure of a warm-drawn steel part according to the invention.
  • the carbon content 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%, insufficient strength cannot be achieved. Above 0.090%, the microstructure formed consists predominantly of lower bainite, this structure being characterized by the presence of carbides precipitated within the ferrite-bainite laths: the mechanical strength thus obtained is high, but the elongation is then considerably reduced.
  • the carbon content is between 0.050 and 0.070%.
  • FIG. 1 illustrates the influence of the carbon content on the elongation in the longitudinal direction of butt-welded joints produced by a laser beam. A particularly high elongation at break of around 17 to 23% is associated with a carbon content ranging from 0.050 to 0.070%. These high elongation values ensure that laser-welded sheets can be satisfactorily drawn, even when taking into account possible local imperfections such as geometrical singularities of weld beads causing stress concentrations, or microporosities within the melted metal. Compared with 0.12% C steels of the prior art, it was expected that the reduction in carbon content would improve the weldability.
  • the carbon content is greater than 0.070% but does not exceed 0.090%. Even though this range does not result in as high a ductility, the elongation at break of laser welds is greater than 15% and remains comparable with that of the base steel sheet.
  • Manganese in an amount of between 1 and 2% by weight, increases the hardenability and prevents the formation of ferrite upon cooling after rolling. Manganese also contributes to deoxidizing the steel in the liquid phase during smelting. The addition of manganese also contributes to effective solid-solution hardening and to obtaining a higher strength. Preferably, the manganese content is between 1.4 and 1.8%: in this way, a completely bainitic structure is formed without the risk of a deleterious banded structure appearing.
  • Aluminium within a content range between 0.015% and 0.050%, is an effective element for deoxidizing the steel. This effectiveness is obtained in a particularly inexpensive and stable manner when the aluminium content is between 0.020 and 0.040%.
  • Silicon in an amount not exceeding 0.1%, contributes to deoxidation in the liquid phase and to hardening in solid solution.
  • an addition of silicon in excess of 0.3% causes the formation of highly adherent oxides and to the possible appearance of surface defects due in particular to the lack of wettability in the hot-galvanizing operations.
  • Molybdenum in an amount not exceeding 0.10%, retards the bainite transformation during cooling after rolling, contributes to solid-solution hardening and refines the size of the bainite laths.
  • the molybdenum content does not exceed 0.40% so as to prevent the excessive formation of hardening structures. This limited molybdenum content also makes it possible to lower the manufacturing cost.
  • the molybdenum content is equal to or greater than 0.18% but does not exceed 0.30%.
  • the level is ideally adjusted so as to prevent the formation of ferrite or pearlite in the steel sheet on the cooling table after hot rolling.
  • Sulphur in an amount greater than 0.010%, tends to precipitate excessively in the form of manganese sulphides which greatly reduce the formability.
  • Phosphorus is an element known to segregate at grain boundaries. Its content must be limited to 0.025% so as to maintain a sufficient hot ductility.
  • the composition may contain chromium in an amount not exceeding 0.45%. Thanks to the other elements of the composition and to the process according to the invention, its presence is not however absolutely necessary, this being an advantage as it avoids costly additions.
  • chromium between 0.20 and 0.45% may be made as a complement to the other elements that increase the hardenability: below 0.20%, the effect on hardenability is not as pronounced, while above 0.45% the coatability may be reduced.
  • the steel contains less than 0.005% Ti and less than 0.020% Nb. If this is not the case, these elements fix too large an amount of nitrogen in the form of nitrides or carbonitrides. There then remains insufficient nitrogen available for precipitating with vanadium. In addition, an excessive precipitation of niobium would increase the hot hardness and would not enable thin hot-rolled sheet products to be easily produced.
  • the niobium content is less than 0.005%.
  • Vanadium is an important element according to the invention—the steel has a vanadium content of between 0.12 and 0.22%. Compared with a steel containing no vanadium, the increase in strength thanks to a hardening precipitation of carbonitrides may be up to 300 MPa. Below 0.12%, a significant effect on the tensile mechanical properties is noted. Above 0.22% vanadium, under the manufacturing conditions according to the invention, a saturation of the effect on the mechanical properties is noted. A content of less than 0.22% therefore makes it possible to obtain high mechanical properties very economically compared with steels having higher vanadium contents. For a vanadium content of between 0.13 and 0.15%, the refinement of the microstructure and the structure hardening obtained are most particularly effective.
  • the nitrogen content is greater than or equal to 0.003% in order to precipitate vanadium carbonitrides in sufficient quantity.
  • the nitrogen content is less than or equal to 0.009% in order to prevent nitrogen from going into solid solution or to prevent the formation of larger carbonitrides, which would reduce the ductility.
  • the remainder of the composition consists of inevitable impurities resulting from the smelting, such as for example Sb, Sn and As.
  • microstructure of the steel sheet or part according to the invention consists of:
  • microstructural percentages correspond to surface fractions that can be measured on polished and etched sections.
  • the microstructure therefore contains no primary or proeutectoid ferrite—it is therefore very homogeneous since the variation in mechanical properties between the matrix (upper bainite) and the other possible constituents (lower bainite and martensite) is small.
  • the deformations are distributed uniformly. Dislocation accumulation does not occur at the interfaces between the constituents and premature damage is avoided, unlike what may be observed in structures having a significant quantity of primary ferrite, in which phase the yield point is very low, or martensite having a very high strength level.
  • the steel sheet according to the invention is particularly capable of undergoing certain demanding modes of deformation, such as the expansion of holes, the mechanical stressing of cut edges and folding.
  • the cast semi-finished products are firstly heated to a temperature above 1150° C., so as to reach at any point a temperature favourable to the high deformations that the steel will undergo during rolling.
  • the step of hot-rolling these semi-finished products may be carried out directly after casting so that an intermediate reheating step is in this case unnecessary.
  • the semi-finished product is hot-rolled in a temperature range in which the structure of the steel is fully austenitic down to an end-of-rolling temperature T ER .
  • the temperature T ER is preferably between 870 and 930° C. so as to obtain a grain size suitable for the bainitic transformation that follows.
  • the product is cooled at a rate V c of between 75 and 200° C./s.
  • a minimum rate of 75° C./s prevents the formation of pearlite and proeutectoid ferrite, while a rate V c not exceeding 200° C./s prevents excessive formation of martensite.
  • the rate V c is between 80 and 150° C./s.
  • a minimum rate of 80° C./s leads to the formation of upper bainite with a very small lath size, combined with excellent mechanical properties.
  • a rate below 150° C./s prevents the formation of martensite fairly considerably.
  • the cooling rate range according to the invention may be obtained by means of a water or air/water mixture spray, depending on the thickness of the sheet, at the exit of the finishing mill.
  • the hot-rolled sheet is coiled at a temperature T coil of between 500 and 600° C.
  • T coil of between 500 and 600° C.
  • the bainitic transformation takes place during this coiling phase.
  • the formation of proeutectoid ferrite or pearlite, caused by too high a cooling temperature is prevented, as is also the formation of hardening constituents that would be caused by too low a coiling temperature.
  • the precipitation of carbonitrides occurring within this coiling temperature range enables additional hardening to be obtained.
  • the sheet may be used in the bare state or coated state.
  • the coating may for example be a coating based on zinc or aluminium.
  • the sheet is pickled after rolling using a process known per se, so as to obtain a surface finish conducive to implementing the subsequent coating operation.
  • the sheet may optionally be subjected to a slight cold deformation, usually of less than 1% (skin pass).
  • the sheet is then coated with zinc or with a zinc-based alloy, for example by electrogalvanizing or by continuous hot-dipped galvanizing.
  • electrogalvanizing or by continuous hot-dipped galvanizing.
  • the particular microstructure of the steel composed predominantly of lower bainite, is insensitive to the thermal conditions of the subsequent galvanizing treatment, so that the mechanical properties of the continuously hot-dipped coated sheet are very stable even in the event of inopportune fluctuations in these conditions.
  • the sheet in the galvanized state therefore has mechanical properties very similar to those in the uncoated state.
  • the sheet is cut by processes known per se so as to obtain blanks suitable for the forming operation.
  • the particular microstructure of the steels according to the invention leads to very stable mechanical properties (strength, elongation) upon warm-drawing—this is because a variation in the drawing temperature or in the cooling rate after drawing does not result in a significant modification in the microstructure or in the precipitates, such as carbonitrides.
  • an inopportune modification or a fluctuation in the heating parameters (soak temperature or soak time) or in the cooling parameters (better or worse contact between the part and the tool) therefore does not result in the parts thus produced being scrapped.
  • microstructure after warm-drawing is very similar to the microstructure before drawing. This way, if not the entire blank is heated and warm-drawn, but only a portion (the portion to be drawn having been locally heated by an appropriate means, for example by induction heating), the microstructure and the properties of the final part will be very homogeneous in its various portions.
  • the microstructure of steel I1 illustrated in FIG. 2 comprises more than 80% upper bainite, the remainder consisting of lower bainite and M-A compounds.
  • the total content of martensite and residual austenite is less than 5%.
  • the size of the prior austenitic grains and of the packets of bainite laths is about 10 microns.
  • the limitation in size of the packets of laths and the pronounced misorientation between adjacent packets has the result that there is a great resistance to the propagation of any microcracks. Thanks to the small difference in hardness between the various constituents of the microstructure, the steel is largely insensitive to damage when being cut by a mechanical process.
  • the sheet of steel R1 having too high a carbon content and too low a vanadium content, has an insufficient elongation at break.
  • the steel R2 has too high a carbon content and too high a phosphorus content, and its coiling temperature is also too low. Consequently, its elongation at break is substantially below 10%.
  • Welding joints produced by autogenous laser welding were produced under the following conditions: power: 4.5 kW; welding speed: 2.5 m/min.
  • the elongation in the longitudinal direction of the laser-welded joints of steel I-1 was 17%, whereas it was 10% and 13% for steels R-1 and R-2 respectively. These values result, in particular in the case of steel R1, in difficulties when drawing welded joints.
  • Sheets of steel I1 according to the invention are also galvanized under the following conditions: after heating to 680° C., the sheets were cooled down to 455° C. and then continuously hot-dip coated in a Zn bath at this temperature, and finally cooled down to ambient temperature.
  • the rate V′ c denotes the average cooling rate between the temperature T and ambient temperature.
  • the tensile strength R m of the parts thus obtained is indicated in Table 4.
  • the parts drawn according to the conditions of the invention will have a low sensitivity to a variation in the manufacturing conditions: after heating to 400° C., the final strength may vary little (by 10 MPa) when the heating time and/or the cooling rate are modified.
  • the strength of the part obtained is greater than 800 MPa.
  • the invention makes it possible to manufacture sheets or parts made of steels having a bainitic matrix without excessive addition of expensive elements. These sheets or parts combine high strength with high ductility.
  • the steel sheets according to the invention are advantageously used to manufacture structural parts or reinforcing elements in the automotive field and general industry.

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  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
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  • Crystallography & Structural Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
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  • Heat Treatment Of Steel (AREA)
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EP07290908A EP2020451A1 (fr) 2007-07-19 2007-07-19 Procédé de fabrication de tôles d'acier à hautes caractéristiques de résistance et de ductilité, et tôles ainsi produites
EP07290908.8 2007-07-19
PCT/FR2008/000993 WO2009034250A1 (fr) 2007-07-19 2008-07-09 Procede de fabrication de tôles d'acier a hautes caracteristiques de resistance et de ductilite, et tôles ainsi produites

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US20150361532A1 (en) * 2013-05-09 2015-12-17 Hyundai Hysco Co., Ltd. Hot stamping product with enhanced toughness and method for manufacturing the same
WO2016005780A1 (fr) * 2014-07-11 2016-01-14 Arcelormittal Investigación Y Desarrollo Sl Tôle d'acier laminée à chaud et procédé de fabrication associé
US20180209011A1 (en) * 2015-07-17 2018-07-26 Salzgitter Flachstahl Gmbh Method of producing a hot strip of a bainitic multi-phase steel having a zn-mg-al coating, and a corresponding hot strip
US10995383B2 (en) 2014-07-03 2021-05-04 Arcelormittal Method for producing a high strength coated steel sheet having improved strength and ductility and obtained sheet
EP3816316A4 (fr) * 2018-06-27 2022-06-15 Baoshan Iron & Steel Co., Ltd. Tôle d'acier laminée à chaud à ultra-haute résistance et bande d'acier ayant de bonnes propriétés de fatigue et d'alésage et procédé de fabrication associé
US11377803B2 (en) * 2016-06-28 2022-07-05 Vigor Industrial Llc Method for manufacturing an orthotropic deck panel
US11492676B2 (en) 2014-07-03 2022-11-08 Arcelormittal Method for producing a high strength coated steel sheet having improved strength, ductility and formability
US11555226B2 (en) 2014-07-03 2023-01-17 Arcelormittal Method for producing a high strength steel sheet having improved strength and formability and obtained sheet
US11618931B2 (en) 2014-07-03 2023-04-04 Arcelormittal Method for producing a high strength steel sheet having improved strength, ductility and formability
US20230295787A1 (en) * 2020-07-31 2023-09-21 Baoshan Iron & Steel Co., Ltd. Steel plate for torsion beam and manufacturing method therefor, and torsion beam and manufacturing method therefor
US12509740B2 (en) 2019-12-17 2025-12-30 Arcelormittal Hot rolled and steel sheet and a method of manufacturing thereof

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WO2012127125A1 (fr) 2011-03-24 2012-09-27 Arcelormittal Investigatión Y Desarrollo Sl Tôle d'acier laminée à chaud et procédé de fabrication associé
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BR112016014435A2 (pt) 2014-01-06 2017-08-08 Nippon Steel & Sumitomo Metal Corp Material de aço e processo para fabricar o mesmo
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Cited By (19)

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JP2012092358A (ja) * 2010-10-22 2012-05-17 Jfe Steel Corp 成形性および強度上昇能に優れた温間成形用薄鋼板およびそれを用いた温間成形方法
US9920408B2 (en) * 2013-05-09 2018-03-20 Hyundai Steel Company Hot stamping product with enhanced toughness and method for manufacturing the same
US20150361532A1 (en) * 2013-05-09 2015-12-17 Hyundai Hysco Co., Ltd. Hot stamping product with enhanced toughness and method for manufacturing the same
JP2015163730A (ja) * 2014-01-28 2015-09-10 株式会社神戸製鋼所 加工硬化能が大きく一様伸びと溶接性に優れた低降伏比高強度鋼板およびその製造方法
US11555226B2 (en) 2014-07-03 2023-01-17 Arcelormittal Method for producing a high strength steel sheet having improved strength and formability and obtained sheet
US11492676B2 (en) 2014-07-03 2022-11-08 Arcelormittal Method for producing a high strength coated steel sheet having improved strength, ductility and formability
US10995383B2 (en) 2014-07-03 2021-05-04 Arcelormittal Method for producing a high strength coated steel sheet having improved strength and ductility and obtained sheet
US11618931B2 (en) 2014-07-03 2023-04-04 Arcelormittal Method for producing a high strength steel sheet having improved strength, ductility and formability
WO2016005780A1 (fr) * 2014-07-11 2016-01-14 Arcelormittal Investigación Y Desarrollo Sl Tôle d'acier laminée à chaud et procédé de fabrication associé
US11447844B2 (en) * 2014-07-11 2022-09-20 Arcelormittal Manufacturing method for hot rolled steel sheet
US11512364B2 (en) 2015-07-17 2022-11-29 Salzgitter Flachstahl Gmbh Method for producing a hot strip of a bainitic multi-phase steel having a Zn—Mg—Al coating, and a corresponding hot strip
US20180209011A1 (en) * 2015-07-17 2018-07-26 Salzgitter Flachstahl Gmbh Method of producing a hot strip of a bainitic multi-phase steel having a zn-mg-al coating, and a corresponding hot strip
US11377803B2 (en) * 2016-06-28 2022-07-05 Vigor Industrial Llc Method for manufacturing an orthotropic deck panel
US11578380B2 (en) 2018-06-27 2023-02-14 Baoshan Iron & Steel Co., Ltd. Ultrahigh-strength hot-rolled steel sheet and steel strip having good fatigue and reaming properties and manufacturing method therefor
EP3816316A4 (fr) * 2018-06-27 2022-06-15 Baoshan Iron & Steel Co., Ltd. Tôle d'acier laminée à chaud à ultra-haute résistance et bande d'acier ayant de bonnes propriétés de fatigue et d'alésage et procédé de fabrication associé
AU2019296099B2 (en) * 2018-06-27 2025-02-27 Baoshan Iron & Steel Co., Ltd. Ultrahigh-strength hot-rolled steel sheet and steel strip having good fatigue and reaming properties and manufacturing method therefor
US12509740B2 (en) 2019-12-17 2025-12-30 Arcelormittal Hot rolled and steel sheet and a method of manufacturing thereof
US20230295787A1 (en) * 2020-07-31 2023-09-21 Baoshan Iron & Steel Co., Ltd. Steel plate for torsion beam and manufacturing method therefor, and torsion beam and manufacturing method therefor
EP4180548A4 (fr) * 2020-07-31 2024-05-15 Baoshan Iron & Steel Co., Ltd. Plaque d'acier pour barre de torsion et son procédé de fabrication, ainsi que barre de torsion et son procédé de fabrication

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EP2171112A1 (fr) 2010-04-07
MA31525B1 (fr) 2010-07-01
PL2171112T3 (pl) 2012-04-30
US20180148806A1 (en) 2018-05-31
US10214792B2 (en) 2019-02-26
EP2020451A1 (fr) 2009-02-04
US20150203932A1 (en) 2015-07-23
KR20140044407A (ko) 2014-04-14
US10428400B2 (en) 2019-10-01
JP5298127B2 (ja) 2013-09-25
CA2694069C (fr) 2013-05-21
KR20180014843A (ko) 2018-02-09
RU2451764C2 (ru) 2012-05-27
US20180163282A9 (en) 2018-06-14
BRPI0814514B1 (pt) 2019-09-03
WO2009034250A1 (fr) 2009-03-19
EP2171112B1 (fr) 2011-11-23
KR20100037147A (ko) 2010-04-08
UA98798C2 (ru) 2012-06-25
CA2694069A1 (fr) 2009-03-19
KR20150123957A (ko) 2015-11-04
AR067594A1 (es) 2009-10-14
ZA201000290B (en) 2010-10-27
BRPI0814514A2 (pt) 2015-02-03
CN101784688B (zh) 2011-11-23
CN101784688A (zh) 2010-07-21
ES2375429T3 (es) 2012-02-29
JP2010533791A (ja) 2010-10-28
RU2010105699A (ru) 2011-08-27
ATE534756T1 (de) 2011-12-15
KR101892423B1 (ko) 2018-08-27

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Effective date: 20100223

STCB Information on status: application discontinuation

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