US20170152579A1 - Method for Producing a High Strength Coated Steel Sheet having Improved Strength, Ductility and Formability - Google Patents

Method for Producing a High Strength Coated Steel Sheet having Improved Strength, Ductility and Formability Download PDF

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US20170152579A1
US20170152579A1 US15/322,829 US201515322829A US2017152579A1 US 20170152579 A1 US20170152579 A1 US 20170152579A1 US 201515322829 A US201515322829 A US 201515322829A US 2017152579 A1 US2017152579 A1 US 2017152579A1
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steel sheet
temperature
coated steel
quenching
mpa
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Rashmi Ranjan Mohanty
Hyun Jo Jun
Dongwei Fan
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ArcelorMittal SA
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ArcelorMittal SA
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • 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
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a method for producing a high strength coated steel sheet having improved strength, ductility and formability and to the sheets obtained with the method.
  • galvanized or galvannealed sheets made of DP (dual phase) steels or TRIP (transformation induced plasticity) steels.
  • such steels which include a martensitic structure and/or some retained austenite and which contain about 0.2% of C, about 2% of Mn, about 1.7% of Si have a yield strength of about 750 MPa, a tensile strength of about 980 MPa, a total elongation of more than 8%.
  • These sheets are produced on continuous annealing line by quenching from an annealing temperature higher than Ac 3 transformation point, down to an overaging temperature above Ms transformation point and maintaining the sheet at the temperature for a given time. Then the sheet is galvanized or galvannealed.
  • the purpose of the present invention is to provide such sheet and a method to produce it.
  • the invention relates to a method for producing a high strength coated steel sheet having an improved ductility and an improved formability, the sheet having a yield strength YS of at least 800 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%, by heat treating and coating a steel sheet wherein the chemical composition of the steel contains in weight %:
  • the heat treatment and the coating comprise the following steps:
  • the chemical composition of the steel can, optionally, satisfy one or more of the following conditions: 0.17% ⁇ C ⁇ 0.21%, 1.3% ⁇ Si ⁇ 1.6% and 2.1% ⁇ Mn ⁇ 2.3%.
  • the hot dip coating step is a galvanizing step.
  • the hot dip coating step is a galvannealing step with an alloying temperature TGA between 480° C. and 510° C.
  • the cooling speed during the quenching is of at least 20° C./s, preferably at least 30° C./s.
  • the method further comprises, after the sheet is quenched to the quenching temperature and before heating the sheet up to the partitioning temperature PT, a step of holding the sheet at the quenching temperature for a holding time comprised between 2 s and 8 s, preferably between 3 s and 7 s.
  • the invention relates also to a coated steel sheet, the chemical composition of the steel containing in weight %:
  • the structure of the steel consists of 3% to 15% of residual austenite and 85% to 97% of martensite and bainite, without ferrite.
  • a least one face of the sheet comprises a metallic coating.
  • the sheet has a yield strength of at least 800 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%.
  • the chemical composition of the steel can, optionally, satisfy one or more of the following conditions: 0.17% ⁇ C ⁇ 0.21%, 1.3% ⁇ Si ⁇ 1.6 % and 2.1% ⁇ Mn ⁇ 2.3 %.
  • the at least one coated face is galvanized.
  • the at least one coated face is galvannealed.
  • the C content in the retained austenite is of at least 0.9%, still preferably of at least 1.0%, and up to 1.6%.
  • the average austenitic grain size i.e. the average grain size of the retained austenite, is preferably of 5 ⁇ m or less.
  • the average size of the grains or blocks of martensite and bainite is preferably of 10 ⁇ m or less.
  • the sheet is obtained by hot rolling and optionally cold rolling of a semi product which chemical composition contains, in weight %:
  • the Al content is limited to 0.05%. If the content of Al is above 0.5%, the austenitizing temperature will be too high to reach and the steel will become industrially difficult to process.
  • Ni, Mo, Cu, Nb, V, Ti, B, S, P and N at least are considered as residual elements which are unavoidable impurities. Therefore, their contents are less than 0.05% for Ni, 0.02% for Mo, 0.03% for Cu, 0.007% for V, 0.0010% for B, 0.005% for S, 0.02% for P and 0.010% for N.
  • Nb content is limited to 0.05% and Ti content is limited to 0.05% because above such values large precipitates will form and formability will decrease, making the 14% of total elongation more difficult to reach
  • the sheet is prepared by hot rolling and optionally cold rolling according to the methods known by those which are skilled in the art.
  • the heat treatment which is made preferably on a combined continuous annealing and hot dip coating line comprises the steps of:
  • annealing the sheet at an annealing temperature TA higher than the Ac 3 transformation point of the steel, and preferably higher than Ac 3 +15° C. i.e. higher than about 850° C. for the steel according to the invention, to ensure that the structure is completely austenitic, but less than 1000° C. in order not to coarsen too much the austenitic grains.
  • the sheet is maintained at the annealing temperature i.e. maintained between TA ⁇ 5° C. and TA +10° C., for a time sufficient to homogenize the chemical composition and the structure. This time is preferably of more than 30 s but does not need to be of more than 300 s.
  • the quenching temperature is between 250° C. and 350° C. in order to have just after quenching a structure consisting of martensite and austenite.
  • This structure contains at least 60% of martensite and contains a sufficient amount of austenite in order to be able to obtain a final structure i.e. after partitioning, coating and cooling to the room temperature, containing between 3 and 15% of residual austenite and between 85 and 97% of the sum of martensite and bainite without ferrite.
  • the cooling rate is higher than or equal to 20° C./s, still preferably higher than or equal to 30° C./s, for example of about 50° C./s. A cooling rate higher than 30° C./s is enough.
  • the partitioning temperature can be equal to the temperature at which the sheet must be heated in order to be hot dip coated, i.e. between 455° C. and 465° C.
  • the reheating rate can be high when the reheating is made by induction heater, but that reheating rate had no apparent effect on the final properties of the sheet.
  • the sheet is held at the quenching temperature for a holding time comprised between 2 s and 8 s, preferably between 3 s and 7 s.
  • Maintaining the sheet at the partitioning temperature means that during partitioning the temperature of the sheet remains between PT ⁇ 20° C. and PT +20° C.,
  • the hot dip coating may be, for example, galvanizing or galvannealing but all metallic hot dip coating is possible provided that the temperatures at which the sheet is brought to during coating remain less than 650° C.
  • the temperature of alloying TGA must not be too high to obtain good final mechanical properties. This temperature is preferably between 500° and 580° C.
  • the coated sheet is processed according to the known art.
  • the sheet is cool to the room temperature.
  • This treatment allows obtaining a final structure i.e. after partitioning, coating and cooling to the room temperature, containing between 3 and 15% of residual austenite and between 85 and 97% of the sum of martensite and bainite without ferrite.
  • this treatment allows obtaining an increased C content in the retained austenite, which is of at least 0.9%, preferably even of at least 1.0%, and up to 1.6%.
  • the average austenitic grain size is preferably of 5 ⁇ m or less, and the average size of the blocks of bainite or martensite is preferably of 10 ⁇ m or less.
  • the amount of retained austenite is for example of at least 7%.
  • coated sheets having a yield strength YS of at least 800 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER according to the ISO standard 16630:2009 of at least 30% can be obtained.
  • Samples of the sheet were heat treated by annealing, quenching and partitioning then galvanized or galvannealed, and the mechanical properties were measured.
  • TA is the annealing temperature
  • QT the quenching temperature
  • PT the partitioning temperature
  • Pt the time of maintaining at the partitioning temperature
  • TGA the temperature of alloying for the sheets that are galvannealed
  • YS the yield strength
  • Ts the tensile strength
  • UE is the uniform elongation
  • Te is the total elongation
  • HER is the hole elongation ratio measured according to the ISO standard 16630:2009.
  • RA % is the amount of retained austenite in the microstructure
  • RA grain size is the average austenite grain size
  • C % in RA is the C content in the retained austenite
  • BM grain size is the average size of the grains or blocks of martensite and bainite.
  • the partitioning temperature PT has to be near 460° C. i.e. the temperature for hot dip coating.
  • the partitioning temperature PT is 400° C. or below or 500° C. or above, in particular not within the range 430-480° C., the ductility is strongly reduced and is not sufficient.
  • the partitioning was made by heating up to 480° C. then linear cooling down to 460° C.
  • Examples 4 to 8 show that, with a partitioning temperature of 460° C. and a partitioning time between 10 s and 60 s, it is possible to obtain the desired properties on galvannealed sheets. These examples show also that it is preferable to have a partitioning time of less than 60 s, preferably about 30 s because with such partitioning time, the yield strength is higher than 1000 MPa while it is less than 1000 MPa when the partitioning time is 60 s.
  • the micrograph of the figure illustrates example 8 which contains 7.5% of retained austenite and 92.5% of martensite+bainite.
  • Examples 10 and 11 show that, when the partitioning temperature is above 460° C., the ductility is significantly reduced.
  • Example 9 shows that, on the contrary, when the partitioning temperature is 440° C., i.e. lower than 460° C., the properties and in particular ductility remain good.
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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
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