EP4308736A1 - Bande, feuille ou ébauche d'acier et procédé de production d'une pièce formée à chaud ou d'une pièce préformée traitée à chaud - Google Patents

Bande, feuille ou ébauche d'acier et procédé de production d'une pièce formée à chaud ou d'une pièce préformée traitée à chaud

Info

Publication number
EP4308736A1
EP4308736A1 EP22718071.8A EP22718071A EP4308736A1 EP 4308736 A1 EP4308736 A1 EP 4308736A1 EP 22718071 A EP22718071 A EP 22718071A EP 4308736 A1 EP4308736 A1 EP 4308736A1
Authority
EP
European Patent Office
Prior art keywords
steel
hot
formed part
blank
temperature
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.)
Pending
Application number
EP22718071.8A
Other languages
German (de)
English (en)
Inventor
Radhakanta RANA
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Tata Steel Ijmuiden BV
Original Assignee
Tata Steel Ijmuiden BV
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Tata Steel Ijmuiden BV filed Critical Tata Steel Ijmuiden BV
Publication of EP4308736A1 publication Critical patent/EP4308736A1/fr
Pending legal-status Critical Current

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Classifications

    • 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
    • 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
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die 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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C22C38/001—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
    • C22C38/06—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—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
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
    • 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
    • C21D2211/005—Ferrite
    • 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

  • This invention relates to a steel strip, sheet or blank for producing a hot-formed part or a heat-treated pre-formed part, and to a method for hot-forming a steel blank or heat-treating a pre-formed part into an article and the use thereof.
  • the steel In cold-stamping or cold-forming processes, the steel is shaped into a product at near-room temperature.
  • Steel products produced in this way are for instance dual phase (DP) steels which have a ferritic-martensitic microstructure.
  • DP steels display a high ultimate tensile strength, their bendability and yield strength are low which is undesirable since these reduce crash performance in service.
  • a steel typically used for hot-stamping is 22MnB5 steel.
  • This boron steel can be reheated in a furnace to austenitize usually between 870 and 940 °C, transferred from furnace to the hot-stamping press, and stamped into the desired part geometry, while the part is cooled at the same time.
  • the advantage of such boron steel parts produced this way is that they display high yield strength and ultimate tensile strength for anti- intrusive crashworthiness due to their fully martensitic microstructure achieved by pressquenching, but at the same time they display a low bendability and ductility which in turn result in a limited toughness and bending fracture resistance and thus a poor impact- energy absorptive crashworthiness.
  • Fracture toughness measurement is a useful tool to indicate the crash energy absorption of steels. When the fracture toughness parameters are high, generally a good crash behaviour is obtained. In view of the above, it will be clear that there is a need for steel parts that display an excellent ultimate tensile strength, and at the same time an excellent yield strength, bendability and fracture toughness, and in turn excellent crash energy absorption.
  • Indirect Process a blank is formed, trimmed, and formed in cold condition.
  • the preformed part is later heated and quenched to obtain the required properties.
  • the preformed part can be given a hot calibration to achieve the final shape in the hot press after heating and quenched.
  • Alloying elements are elements that are deliberately added or allowed in the steel to provide a desirable effect. Boron and niobium are deemed not to provide a desirable effect in the steel according to the invention. Any boron or niobium that is present in the steel is to be qualified as inevitable impurities resulting from the Basic Oxygen Steelmaking (BOS) or the Electric Arc Furnace (EAF) process.
  • BOS Basic Oxygen Steelmaking
  • EAF Electric Arc Furnace
  • inevitable or unavoidable impurities mean that the low alloy steel does not contain any deliberately added elements other than those specified, and that the inevitable or unavoidable impurities are present only as a result of a technical or economical inability to remove them completely from the steel melt during the BOS- or EAF-process.
  • the present invention relates to a steel strip, sheet or blank for producing hot-formed parts or a heat-treated pre-formed part is provided, which comprises the following composition in weight % (wt.%):
  • the hot-formed part produced from the steel strip, sheet or blank in accordance with the present invention displays an excellent combination of yield strength, tensile strength, ductility, bendability and fracture toughness, and thereby impact-energy absorptive crashworthiness when compared to conventional hot-formed boron steels.
  • Examples of automotive components that can be made from these steels are the front and back longitudinal bars and the B-pillar.
  • a cold-formed dual phase steel e.g. DP800
  • a hot- formed 22MnB5 steel is used.
  • the DP800 steel exhibits a lower energy absorption, and using a higher strength steel (Ultimate Tensile Strength > 800 MPa) will enable more weight saving through downgauging and enhanced passenger safety by higher crash energy absorption.
  • Ultimate Tensile Strength > 800 MPa Ultra-high strength
  • 22MnB5 a higher strength steel
  • ⁇ 500 MPa ⁇ 500 MPa
  • the two steel blanks are joined by laser welding before hot-forming and then the hybrid blank (known as laser welded blank or LWB in abbreviation) is formed into the B-pillar.
  • the hybrid blank referred as laser welded blank or LWB in abbreviation
  • the steel chemistry and the hot-forming thermal cycle are the most critical steps to achieve the desired microstructures, properties and performance.
  • Most of the alloying elements (C, Mn, Mo) deliberately added in the steel of invention are to increase the hardenability so that ferritic and/or pearlitic transformation during press cooling of the formed part are avoided.
  • the presence of ferrite and pearlite are detrimental to achieve high strength, high bendability and high toughness and therefore undesired.
  • Bainite which does not have high strength difference with martensite can also be optionally present which has been utilised in this invention in the martensitic matrix to improve the toughness and bendability, and thus energy absorption capacity, while not adversely affecting the desired strength level.
  • Carbon is added as the main strengthening element and also to promote hardenability.
  • C increases strength by distorting the cubic BCC lattice and converting it to a BCT lattice.
  • In bainite which is a mixture of ferrite and carbides, C strengthens the ferrite by solid solution hardening. According to the invention the carbon content is 0.07 to 0.20 wt.%.
  • Manganese and silicon are added also to increase the hardenability.
  • Mn causes solid solution hardening as well.
  • Mn below the limit will give insufficient hardenability and above the limit will make the steel difficult to process - as for example in terms of rolling, hot dip galvanising.
  • Si also causes solid solution strengthening.
  • the hardenability of the alloy is further increased by adding small amount of Mo to the steel.
  • Mo in small amount in steel is very effective in delaying ferritic, pearlitic and bainitic transformations. Mo also refines the grain size.
  • the Mo content in the steel is at most 1.0 wt.%.
  • the Mo content in the steel is at most 0.80 wt.%, and preferably is at most 0.50 wt.%.
  • the Mo content in the steel is at most 0.20 wt.%.
  • the Mo content in the steel is preferably kept higher than 0.1 wt.%. If Mo content is less than 0.1 wt.%, its hardenability effect may not be sufficient to give the desired microstructure for this invention.
  • Austenitizing reheating is done above the Ac3 temperature and intercritical reheating is done at a temperature chosen in between Acl and Ac3, where Acl is the temperature where austenite starts to form during heating of the steel and Ac3 is the temperature where the ferrite to austenite transformation completes during heating.
  • the chromium content in the steel is at most 0.050 wt.%, and preferably it is present only as an inevitable impurity, i.e. it is not added as an alloying element.
  • the chromium content in the steel is at most 0.040 wt.%, and preferably is at most 0.020 wt.%. in some embodiment the chromium content in the steel is at most 0.010 wt.%.
  • the chromium content in the steel is lower than 0.0001 wt %, preferably there is no chromium present in the steel according to the invention.
  • the copper content in the steel is at most 0.050 wt.%, and preferably it is present only as an inevitable impurity, i.e. it is not added as an alloying element. Preferably there is no copper present in the steel according to the invention.
  • titanium and/or niobium and/or boron and/or vanadium are present only as an inevitable impurity, which means that Ti and/or Nb is at most 0.005 wt.%, V is at most 0.010 wt.% and B is at most 0.0005 wt.% (i.e. 5 ppm).
  • the niobium content in the steel is lower than 0.0001 wt %, preferably there is no niobium present in the steel according to the invention.
  • the steel strip, sheet or blank which comprises the following composition in wt.%:
  • the carbon content is at least 0.10 wt.%, preferably at least 0.12 wt.%.
  • the Mo content is 0.21 wt.% or higher and/or 0.39 wt.% or lower. Preferably the Mo content is 0.29 wt.% or lower.
  • the production of a hot-formed part or a heat-treated pre-formed part can be done with or without a protective metallic coating layer present on the steel strip, sheet or blank. To prevent piece-wise coating of the finished part it is preferable to provide the steel strip, sheet or blank with the protective metallic coating layer prior to the forming operation. Providing a strip with such a metallic coating is preferably performed in a hot-dip coating process. The ultimate goal of using a coating on the steel strip is to enhance the corrosion resistance of the formed article in service.
  • the steel strip, sheet or blank according to the invention is provided with a metallic coating layer such as zinc or a zinc alloy coating layer, such as a zinc-alloy coating layer comprising 0.3-4.0 wt.% Mg and 0.3-6.0 wt.% Al; optionally at most 0.2 wt.% of one or more additional elements, unavoidable impurities; the remainder being zinc.
  • a metallic coating layer such as zinc or a zinc alloy coating layer
  • a zinc-alloy coating layer comprising 0.3-4.0 wt.% Mg and 0.3-6.0 wt.% Al; optionally at most 0.2 wt.% of one or more additional elements, unavoidable impurities; the remainder being zinc.
  • the alloying element contents in the zinc-alloy coating layer shall be 1.0 - 2.0 % Magnesium and 1.0 -3.0 %. Aluminium, optionally at most 0.2% of one or more additional elements, unavoidable impurities and the remainder being zinc.
  • the zinc alloy coating
  • the steel strip, sheet or blank is provided with a metallic coating layer such as a (commercially pure) aluminium layer or an aluminium alloy layer.
  • a typical metal bath for a hot dip coating such an aluminium layer comprises of aluminium alloyed with silicon e.g. aluminium alloyed with 8 to 11 wt.% of silicon and at most 4 wt.% of iron, optionally at most 0.2 wt.% of one or more additional elements such as calcium, unavoidable impurities, the remainder being aluminium.
  • Silicon is present in order to prevent the formation of a thick iron-intermetallic layer which reduces adherence and formability.
  • Iron is preferably present in amounts between 1 and 4 wt.%, more preferably at least 2 wt.%.
  • the hot-formed part or heat- treated pre-formed part produced from the steel strip, sheet or blank has a martensitic microstructure comprising from 0 to at most 70 vol.% bainite, and preferably at most 60 vol.% bainite.
  • the martensitic microstructure is defined as a microstructure comprising 0 to at most 70 vol.% bainite, preferably at most 60 vol.% bainite.
  • the hot-formed part or heat- treated pre-formed part produced from a steel strip, sheet or blank according to the invention has a martensitic microstructure comprising at most 15 vol.% ferrite, preferably at most 10 vol.% ferrite, preferably at most 5 vol.% ferrite, more preferably only trace amounts or no ferrite at all.
  • the term 'ferrite' as used herein above refers to the BCC ferrite (a) structure, and does not refer to acicular ferrite or ferritic bainite or upper bainite, as these are considered to be bainitic microstructures in the context of this invention.
  • the ferrite occurs if T1 is chosen in the intercritical range.
  • the hot-formed part or heat- treated pre-formed part produced from a steel strip, sheet or blank has a martensitic microstructure comprising at most 70 vol.% bainite, preferably at most 60 vol.% bainite and at most 15 vol.% ferrite, preferably at most 10 vol.% ferrite, preferably at most 5 vol.% ferrite.
  • the hot-formed part or heat- treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims, wherein the hot-formed part has a tensile strength of at least 700 MPa, preferably of at least 750 MPa.
  • the hot-formed part or heat- treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims, wherein the hot-formed part having a total elongation, TE, of at least 6% with respect to a gauge length of 50 millimetre (as per the EN 10002 standard) and a bending angle, BA, at 1.0 mm thickness (as per the VDA 238-100 standard) of at least 100°, preferably at least 115°.
  • TE total elongation
  • BA a bending angle
  • the finished article optionally after some post-processing to make the article suitable for it, is subjected to a paint baking treatment.
  • a method for hot-forming a steel blank or heat-treating a pre-formed part into an article comprising the steps of:
  • Arl is the temperature at which the austenite to ferrite transformation completes during cooling of the steel and Mf is the martensite transformation finish temperature.
  • thermo-mechanical routes A, B or C which all benefit from the specific chemical composition designed in this invention.
  • the difference between the three different thermo-mechanical routes lies in the combination or separation of the mechanical and the thermal part of the thermo-mechanical route.
  • Route A heating a steel blank to achieve the austenite or austenite + ferrite state, deform it to its final shape while it is in the defined state (> Arl) and cool it to transform the austenite into the desired microstructure (direct hot-forming of the article), or
  • Route B cold deform a steel blank to an intermediate shape, heating the intermediate shape to achieve the austenite or austenite + ferrite state, deform it to its final shape while it is in the defined state (hot calibration step) and cool it to transform the austenite into the desired microstructure (indirect hot-forming with hot calibration step of the article), or
  • Route C cold deform a steel blank to a pre-formed part having its final shape, heat the pre-formed part to achieve the austenite or austenite + ferrite state and cool it to transform the austenite into the desired microstructure (indirect hot-forming of the article)
  • Route A is very suitable for use in combination with an aluminium or aluminium-silicon coated steel substrate
  • Route B and C are more suitable if a zinc or zinc-alloy coated steel substrate is used.
  • thermo-mechanical methods A, B and C
  • Coating (optional - i.e. the steel strip can be coated &. uncoated): hot dip galvanizing, electrogalvanizing, aluminizing (Al or Al alloy) or chemical conversion treatment
  • the blanks for carrying out the hot-forming process can be prepared from any of the following states of the steel:
  • T1 is in the range of Ac3 - 20°C to Ac3 + 100°C.
  • T1 is preferably Ac3 +50°C or lower.
  • T1 is Ac3 or higher, and more preferably T1 is Ac3 + 20°C or higher.
  • the increasingly tighter temperature ranges allow a more controlled conditioning of the austenitic microstructure prior to the deformation or the start of the cooling, should this be required. E.g. excessive or abnormal austenite grain growth is prevented.
  • tl is in the range of 3 minutes to 12 minutes to provide optimal microstructure following cooling. If tl is less than 3 minutes, then the microstructure during reheating at Tl does not reach equilibrium and dissolution of all the alloying elements in the matrix is not complete. This can result in poor mechanical properties due to inhomogeneous microstructure after cooling in the hot-forming press. On the other hand, when tl is longer than 12 minutes, excessive grain growth during reheating at Tl will occur resulting in a coarse final microstructure which will deteriorate the final mechanical properties.
  • the cooling rate CR of the article is in the range of 30-200°C/s to provide optimal microstructure.
  • the cooling rate CR of the article is at least 40°C/s, preferably at least 50 °C/s, more preferably at least 60 °C/s and even more preferably at least 80 °C/s.
  • the higher the cooling rate the microstructure of the steel contains more martensitic phase and lesser bainitic phase, optionally together with the small amount of ferrite.
  • a cooling rate above 200°C/s is not necessary from a metallurgical viewpoint of achieving the microstructure (i.e. martensite from austenite), which is also practically always not possible to achieve.
  • the method according to the invention includes a paint baking treatment, used after painting hot-formed component, for instance an additional ageing treatment at about 170-200°C for 20 minutes.
  • the invention is also embodied in the use of an article manufactured according to the method of the invention, wherein the resulting article preferably is an automotive body or chassis part.
  • the invention is also embodied in a hot-formed part produced according to the invention and/or an article manufactured according to the method of the invention.
  • Example 1 As substrates the materials according to table 1 were used.
  • the inventive steel was cast and processed into cold-rolled strips with a gauge of 1.5 mm through reheating the cast steel to 1200°C, hot-rolling (Finish Rolling Temperature 900°C) to a final hot- rolled thickness of 4 mm. After finish rolling the steel was cooled on the run-out table at 25 °C/s to 700°C and simulated for coil cooling (i.e. coiling). After pickling the hot- rolled strips were cold-rolled to 1.5 mm.
  • the comparative steels of DP1000 and DP800 which are two cold-formable commercial steels, were also processed to the same condition (cold-rolled 1.5 mm gauge) and continuously annealed to achieve their mechanical properties as specified in the well-known industry specifications such as in VDA 239-100.
  • Table 1 Steel chemistry in wt.% (balance Fe and inevitable impurities).
  • CCT continuous cooling transformation
  • Blanks of dimensions 220 mm x 110 mm x 1.5 mm, were prepared from the cold- rolled material and were subjected to reheating to 900°C (10°C below Ac3) and 940°C (30°C above Ac3) and were soaked for 5 min. in a nitrogen atmosphere to minimize surface degradation, transferred resulting in a temperature drop of 120°C in 10s and then subjected to cooling to about 160°C in the following rates: 30, 40, 50, 60, 80, 200°C/s. From the heat-treated samples, A50 tensile specimens along the rolling direction were prepared and tested with quasi-static strain rate (EN10002 standard). Microstructures were characterized from the RD-ND planes (RD and ND stand for rolling direction and normal direction respectively).
  • Microstructures were quantified by an Image Analysis software after etching the samples with different etchants: 2 vol.% nital, 10 vol.% sodium metabisulphite and Le Pera reagents.
  • Bending specimens 40 mm x 30 mm x 1.5 mm
  • the samples with bending axis parallel to the rolling direction are identified as longitudinal (L) bending specimens whereas those with bending axis perpendicular to the rolling direction are denoted as perpendicular (T) bending specimens.
  • CTOD is the Crack Tip Opening Displacement and is a measure of how much the crack opens by at either failure (if brittle) or maximum load.
  • J is the J-integral and is a measure of toughness that takes account of the energy, so it is calculated from the area under the curve up to failure or maximum load.
  • K q is the value of stress intensity factor measured at load P q , where P q is determined by taking the elastic slope of the loading line, then taking a line with 5% less slope and defining P q as the load where this straight line intersects the loading line.
  • Table 3 shows that the ultimate tensile strength (UTS) greater than 700 MPa was achieved for all the cooling rates.
  • the yield strength (YS) increases with increasing cooling rate for both reheating temperatures since the amount of martensite increased in the microstructure with cooling rate.
  • Microstructures are either fully martensitic or a mixture of martensite, bainite and small amount of ferrite. High bending angles, greater than at least 115° at 1 mm thickness are also achieved as shown in Table 4 for both the specimen orientations.
  • the high performance of the invented steel in comparison with the available steels of similar strength is due to the higher bending angle and higher fracture toughness properties.
  • the component needs to fold, to be able to absorb energy without fracture, which is determined by its bendability.
  • the energy absorption capability before failure is determined by its fracture toughness parameters.
  • the improvements in these properties of the invented steel have been possible by virtue of the inventive steel chemistry design that provided the suitable microstructures, as defined in this invention, through the defined hot-forming processes.
  • Table 5 Tensile properties and bendability. hot pressed (900°C reheating with 200°C/s cooling rate) and baked.
  • Blanks of dimensions 220 mm x 110 mm x 1.5 mm, were prepared from the cold- rolled material and were first annealed continuously at 720, 750 and 780 °C with total times of 458, 250 and 172 seconds in HNx atmosphere in a hot-dip annealing simulator (HDAS). Then, the annealed blanks were subjected to a hot-forming thermal cycle in the HDAS apparatus, reheating to 910°C (above Ac3) at a rate of 15°C/s and were soaked for 5 min., simulated for transfer cooling resulting in a temperature drop of 120°C in 10s and then subjected to cooling to room temperature with a cooling rate of 30°C/s. The hot-forming thermal cycles were applied in a nitrogen atmosphere to minimize surface degradation of the samples.
  • HDAS hot-dip annealing simulator
  • A50 tensile specimens with 190 mm total length, 20 mm width, 60 mm parallel length and 50 mm gauge length were prepared. The length of the specimens were along the rolling direction. The tensile tests were done at a quasi-static strain rate following EN 10002 standard. Microstructures were characterized from the RD-ND planes (RD and ND stand for rolling direction and normal direction respectively). Microstructures were quantified by an Image Analysis software after etching the samples with different etchants: 2 vol.% nital, 10 vol.% sodium metabisulphite and Le Pera reagents.
  • Bending specimens (40 mm x 30 mm x 1.5 mm) from both parallel to rolling direction and perpendicular to rolling direction were prepared from each of the conditions and tested till fracture by three-point bending test according to the VDA 238-100 standard. These samples with bending axis parallel to the rolling direction are identified as longitudinal (L) bending specimens.
  • the tensile properties of the steels after various annealing heat treatments are presented in Table 10 and 11. After annealing the steels achieved a mixed microstructure of ferrite matrix with pearlite. This combination of phases led to a soft condition before hot forming.
  • the soft condition is characterized by low yield strength, low ultimate tensile strength and high total elongation. This soft condition is suitable for blanking od the sheets before hot forming.
  • Steel A had the following range of tensile properties after annealing : yield strength of 361 to 420 MPa; ultimate tensile strength of 579 to 942 MPa; total elongation A50 of 11.0 to 23.7%.
  • Steel B possessed the following range of tensile properties after hot forming : yield strength of 334 to 449 MPa; ultimate tensile strength of 541 to 902 MPa; total elongation A50 of 11.8 to 30.5%.
  • the tensile properties of steel A and steel B after hot forming are presented in Table 12 and Table 13 respectively, and bendability results are in Table 14 and Table 15 respectively for steel A and steel B. It is clear from Table 12 and Table 13 that the ultimate tensile strength (UTS) after hot forming is greater than 700 MPa for both the steels after all the annealing conditions (in the range of 1072 to 1280 MPa for steel A and 850 to 1149 MPa for steel B). The yield strength (YS) values are also high (719 to 883 MPa for steel A and 559 to 783 MPa for steel B). The total elongation values are also above 6% (6.4 to 10% for steel A and 7.3 to 14.7% for steel B).
  • the bending angles at 1 mm thickness in Table 14 and Table 15 are also higher than 100° for most of the conditions (101.0 to 137.4° for steel A and 112.6 to 140.2° for steel B).
  • the high UTS values were caused by the predominantly martensitic matrix after hot forming in both the steels.
  • the predominantly single phase martensite gave high bending angles because of absence of any substantial weak interfaces from other phases.
  • the minimum values of total elongation of 6% was guaranteed due to small specified amounts of bainite present in the microstructures.
  • Steel B showed higher total elongation and bendability because of slightly higher fractions of bainite in the martensitic matrix.
  • Table 11 Tensile properties after annealing of the cold-rolled Steel B.
  • Table 12 Tensile properties after annealing and hot forming of Steel A for different annealing conditions.
  • Table 13 Tensile properties after annealing and hot forming of Steel B for different annealing conditions.
  • Table 14 Bendability after annealing and hot forming of Steel A for different annealing conditions.
  • Table 15 Bendability after annealing and hot forming of Steel B for different annealing conditions.
  • Figure 2 CCT-diagram of the inventive steel in Table 2.
  • FIG. 3 Experimental details of the direct hot-forming experiments. Same temperature schedule is used for indirect hot-forming at a different timescale. Some critical temperatures, cooling rates, durations and process stages are indicated.
  • Figure 4 Graphical presentation of the mechanical property data in Table 3 after annealing at (a) 900 or (b) 940°C.
  • Left Y-axis - Open square tensile strength (Rm), open circles: yield strength (Rp);
  • FIG. 6 Schematic drawing of the three thermo-mechanical treatment routes A, B and C.

Abstract

La présente invention concerne une bande, feuille ou ébauche d'acier pour produire des pièces formées à chaud ou une pièce préformée traitée à chaud, et un procédé de formage à chaud d'une ébauche d'acier ou de traitement à chaud d'une pièce préformée en un article et une utilisation associée, la bande, la feuille ou l'ébauche d'acier ayant la composition suivante, en % en poids : - C : de 0,07 à 0,20; - Mn : de 0,5 à 2,0; - Si : de 0,3 à 1,5; - Mo : de 0,1 à 1,0; et éventuellement un ou plusieurs des éléments choisis parmi : - Al : < 0,1; - Cr : au plus 0,050; - Cu : < 0,2; - N : < 0,01; - P : < 0,04; - S : < 0,025; - O : < 0.01; - Ti : < 0,10; - V : < 0,15; le reste étant du fer et des impuretés inévitables.
EP22718071.8A 2021-03-17 2022-03-17 Bande, feuille ou ébauche d'acier et procédé de production d'une pièce formée à chaud ou d'une pièce préformée traitée à chaud Pending EP4308736A1 (fr)

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SE435527B (sv) 1973-11-06 1984-10-01 Plannja Ab Forfarande for framstellning av en detalj av herdat stal
JP5556961B2 (ja) * 2011-05-13 2014-07-23 新日鐵住金株式会社 ホットスタンプ成形品、ホットスタンプ成形品の製造方法、エネルギ吸収部材、及びエネルギ吸収部材の製造方法
EP2719786B1 (fr) * 2011-06-10 2016-09-14 Kabushiki Kaisha Kobe Seiko Sho Article moulé par pressage à chaud, procédé pour produire celui-ci, et tôle d'acier mince pour moulage à la presse à chaud
US20140083574A1 (en) * 2011-06-30 2014-03-27 Hyundai Hysco Co.,Ltd. Heat-hardened steel with excellent crashworthiness and method for manufacturing heat-hardenable parts using same
JP5505574B1 (ja) * 2012-08-15 2014-05-28 新日鐵住金株式会社 熱間プレス用鋼板、その製造方法、及び熱間プレス鋼板部材
CN104513937A (zh) * 2014-12-19 2015-04-15 宝山钢铁股份有限公司 一种屈服强度800MPa级别高强钢及其生产方法
CN108588612B (zh) * 2018-04-28 2019-09-20 育材堂（苏州）材料科技有限公司 热冲压成形构件、热冲压成形用预涂镀钢板及热冲压成形工艺
US20210189517A1 (en) * 2018-05-22 2021-06-24 Thyssenkrupp Steel Europe Ag Sheet Metal Part Formed from a Steel Having a High Tensile Strength and Method for Manufacturing Said Sheet Metal Part
EP3976846A1 (fr) * 2019-05-28 2022-04-06 Tata Steel IJmuiden B.V. Bande, tôle ou ébauche en acier pour la fabrication d'une pièce estampée à chaud, et procédé d'estampage à chaud d'une ébauche pour former une pièce

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