EP3658692B1 - Bande, feuille ou ébauche d'acier pour produire une pièce formée à chaud, pièce et procédé de formage à chaud d'une ébauche en une pièce - Google Patents

Bande, feuille ou ébauche d'acier pour produire une pièce formée à chaud, pièce et procédé de formage à chaud d'une ébauche en une pièce Download PDF

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EP3658692B1
EP3658692B1 EP18740258.1A EP18740258A EP3658692B1 EP 3658692 B1 EP3658692 B1 EP 3658692B1 EP 18740258 A EP18740258 A EP 18740258A EP 3658692 B1 EP3658692 B1 EP 3658692B1
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blank
steel
hot
temperature
sheet
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EP3658692A1 (fr
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Radhakanta RANA
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Tata Steel Ijmuiden BV
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Tata Steel Ijmuiden BV
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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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with 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
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • C22C38/04—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • 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
    • 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
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • 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

Definitions

  • the present invention relates to a steel strip, sheet or blank for producing a hot formed part; a hot formed part; and a method for producing a hot formed part.
  • WO2015/144318 discloses a method for hot forming a part using a coated steel blank used in the automotive industry e.g. body-in-white of automotive vehicles.
  • a steel typically used for hot-forming is 22MnB5 steel.
  • This boron steel can be furnace-heated and is usually austenitized between 870-940 °C, transferred from furnace to forming tool, and stamped into the desired part geometry, while the part is at the same time cooled.
  • the advantage of such boron steel parts produced this way is that they display a high ultimate tensile strength for anti-intrusive crashworthiness due to their fully martensitic microstructure, but at the same time they display a low ductility and bendability which in turn results in a limited toughness and thus a poor impact-energy absorptive crashworthiness.
  • Fracture toughness measurement is an useful tool to indicate the crash energy absorption of steels. When the fracture toughness parameters are high, generally a good crash behavior is obtained.
  • Yet another object of the present invention is to provide a method for hot-forming a steel blank into a part.
  • the present invention relates to a steel strip, sheet or blank for producing hot formed parts as disclosed in appended claim 1.
  • the hot formed part produced from the steel strip, sheet or blank in accordance with the present invention displays an improved combination of tensile strength, ductility and bendability, and thereby impact toughness when compared to conventional hot-formed boron steels.
  • the two steel blanks are joined by laser welding before hot stamping and then the hybrid blank is stamped into the B-pillar.
  • the invented higher strength steel can replace the lower strength steel of the lower part with a higher energy absorption capability.
  • the steel strip, sheet or blank for producing hot formed parts as described above has the following composition in weight%:
  • Carbon is added for securing good mechanical properties.
  • C is added in an amount of 0.03 wt% or more to achieve high strength and to increase the hardenability of the steel. When too much carbon is added there is the possibility that the toughness and weldability of the steel sheet will deteriorate.
  • the C amount used in accordance with the invention is therefore in the range of from 0.03 - 0.17 wt%, preferably in the range of from 0.05 - 0.17 wt%, and more preferably in the range of from 0.07 - 0.15 wt%.
  • Manganese is used because it promotes hardenability and gives solid solution strengthening.
  • the Mn content is at least 0.65 wt% to provide adequate substitutional solid solution strengthening and adequate quench hardenability, while minimising segregation of Mn during casting and while maintaining sufficiently low carbon equivalent for automotive resistance spot-welding techniques.
  • Mn is an element that is useful in lowering the Ac3 temperature. A higher Mn content is advantageous in lowering the temperature necessary for hot press forming. When the Mn content exceeds 2.5 wt%, the steel sheet may suffer from poor weldability and poor hot and cold rolling characteristics that affect the steel processability.
  • the Mn amount used in accordance with the invention is in the range of from 0.65 - 2.5 wt%, preferably in the range of from 1.0 - 2.1 wt%, and more preferably in the range of from 1.2 - 1.8 wt%.
  • Chromium improves the hardenability of the steel and facilitates avoiding the formation of ferrite and/or pearlite during press quenching. In this respect it is observed that the presence of ferrite and/or pearlite in the microstructure is detrimental to mechanical properties for the targeted microstructure in this invention.
  • the amount of Cr used in the invention is in the range of from 0.2 - 2.0 wt%, preferably in the range of from 0.5 - 1.7 wt%, more preferably in the range of 0.8 - 1.5 wt%.
  • manganese and chromium are used in such an amount that Mn + Cr ⁇ 2.7, preferably Mn + Cr is in the range of from 0.5 - 2.5, and more preferably Mn + Cr is in the range of from 2.0 - 2.5.
  • Titanium is added to form TiN precipitates to scavenge out N at high temperatures while the steel melt cools. Formation of TiN prohibits formation of B 3 N 4 at lower temperatures so that B, which is also an essential element for this invention, becomes more effective. Stoichiometrically, the ratio of Ti to N (Ti/N) addition should be > 3.42.
  • the amount of titanium is in the range of from 0.01 - 0.1 wt%, preferably in the range of from 0.015 - 0.07 wt%, and more preferably in the range of from 0.025 - 0.05 wt%.
  • Niobium has the effect of forming strengthening precipitates and refining microstructure.
  • Nb increases the strength by means of grain refinement and precipitation hardening. Grain refinement results in a more homogeneous microstructure improving the hot-forming behavior, in particular when high localized strains are being introduced. A fine, homogeneous microstructure also improves the bending behavior.
  • the amount of Nb used in the invention is in the range of from 0.01 - 0.1 wt%, preferably in the range of from 0.02 - 0.08 wt%, and more preferably in the range of from 0.03 - 0.07 wt%.
  • Boron is an important element for increasing the hardenability of steel sheets and for further increasing the effect of stably guaranteeing strength after quenching.
  • B is present in an amount in the range of from 0.0005 - 0.005 wt%, preferably in the range of from 0.0005 - 0.004 wt%, more preferably in the range of from 0.001 - 0.003 wt%.
  • N has an effect similar to C.
  • N is suitably combined with titanium to form TiN precipitates.
  • the amount of N according to the invention is at most 0.01 wt%.
  • the amount of N is in the range of 0.001 - 0.008 wt%.
  • N is present in an amount in the range of from 0.002 - 0.005 wt%.
  • Mn, Cr and B are used in such amounts that (B x 1000)/(Mn + Cr) is in the range of from 0.185 - 2.5, preferably in the range of from 0.2 - 2.0, and more preferably in the range of from 0.5 - 1.5.
  • the (B x 1000)/(Mn + Cr) ratio as applied in accordance with the present invention establishes an adequate hardenability of the steel.
  • Silicon is also added to promote hardenability and adequate substitutional solid solution strengthening.
  • the Si amount used in the invention is at most 0.1 wt%, preferably at most 0.5 wt%.
  • Aluminium is added to deoxidize the steel.
  • the Al amount is at most 0.1 wt%, preferably at most 0.05 wt%.
  • Molybdenum is added to improve the hardenability of the steel and facilitate the formation of bainite.
  • the Mo amount used in accordance with the invention is at most 0.1 wt%, preferably at most 0.05 wt%.
  • Copper is added to improve hardenability and increase strength of the steel. If present, Cu is used in accordance with the invention in an amount of at most 0.1 wt%, preferably at most 0.05 wt%.
  • P is known to widen the intercritical temperature range of a steel. P is also an element useful for maintaining desired retained austenite. However, P may deteriorate the workability of the steel. In accordance with the invention P should be present in an amount of at most 0.03 wt%, preferably at most 0.015 wt%.
  • the amount of sulphur needs to be minimised to reduce harmful non-metallic inclusions.
  • S forms a sulfide based inclusions such as MnS, which initiates crack, and deteriorates processability. Therefore, it is desirable to reduce the S amount as much as possible.
  • the amount of S is at most 0.025 wt%, preferably an amount of at most 0.01 wt%.
  • the amount of O is at most 0.01 wt%, preferably at most 0.005 wt%.
  • Vanadium may be added to form V(C, N) precipitates to strengthen the steel product.
  • the amount of vanadium, if any, is at most 0.15 wt%, preferably at most 0.05 wt%.
  • Nickel may be added in an amount of at most 0.15 wt%. Ni can be added to increase the strength and toughness of the steel.
  • Calcium may be present in an amount of up to 0.05 wt%, preferably up to 0.01 wt%. Ca is added to spheroidize the sulphur containing inclusions and to minimize the amount of elongated inclusions. However, the presence of CaS inclusions will still lead to inhomogeneities in the matrix; it is thus best to reduce the amount of S.
  • 1000*B divided by the sum of Mn and Cr has to be between 0.185 and 2.5, preferably between 0.5 and 1.5. This limitation improves the hardenability of the steel.
  • the steel strip, sheet or blank is provided with a zinc based coating, an aluminium based coating or an organic based coating.
  • a zinc based coating reduces oxidation and/or decarburization during a hot forming process.
  • the zinc based coating is a coating containing 0.2 - 5.0 wt% Al, 0.2 - 5.0 wt% Mg, optionally at most 0.3 wt% of one or more additional elements, the balance being zinc and unavoidable impurities.
  • the additional elements can be selected from the group comprising Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. Pb, Sn, Bi and Sb are usually added to form spangles.
  • the total amount of additional elements in the zinc alloy is at most 0.3 wt.%. These small amounts of an additional element do not alter the properties of the coating nor the bath to any significant extent for the usual applications.
  • each is preferably present in an amount of at most_0.03 wt%, preferably each is present in an amount of at most 0.01 wt%. Additional elements are usually only added to prevent dross forming in the bath with molten zinc alloy for the hot dip galvanizing, or to form spangles in the coating layer.
  • the hot formed part produced from a steel strip, sheet or blank in accordance with the present invention has a microstructure comprising at most 60% bainite, the remainder being martensite.
  • the microstructure comprises at most 50 vol. % of bainite, the remainder being martensite. More preferably , the microstructure comprises at most 40 vol. % of bainite, the remainder being martensite.
  • the martensite provides a high strength, whereas the softer bainite improves the ductility. The small strength difference between martensite and bainite helps in maintaining a high bendability due to lack of weak phase interfaces.
  • the hot formed part in accordance with the present invention displays excellent mechanical properties.
  • the part has a tensile strength (TS) of at least 750 MPa, preferably of at least 800 MPa, more preferably of at least 900 MPa, and further has a tensile strength of at most 1400 MPa.
  • TS tensile strength
  • the part suitably has a total elongation (TE) of at least 5%, preferably 5.5%, more preferably at least 6% and most preferably at least 7%, and/or a bending angle (BA) at 1.0 mm thickness of at least 100 °, preferably at least 115 °, more preferably at least 130 ° and most preferably at least 140 °.
  • TE total elongation
  • BA bending angle
  • the present invention also relates to the use of hot formed parts as described above, as structural part in the body-in-white of a vehicle.
  • Such parts are made of the present steel strip, sheet or blank. These parts have a high strength, high ductility and a high bendability.
  • parts in the form of structural parts of vehicles are very attractive since they exhibit excellent crash energy absorption and in turn, down-gauging and lightweighting opportunities based on crashworthiness compared to the use of conventional hot-formed boron steels and cold-formed multiphase steels.
  • the present invention also relates to a method for producing a part in accordance with the present invention.
  • the present invention also relates to a method for hot-forming a steel blank or a preformed part into an part as disclosed in appended claim 11.
  • the part After the cooling of the part to a temperature below the Mf temperature, the part can for instance be further cooled to room temperature in air, or can be forcibly cooled to room temperature.
  • the blank to be heated in step (a) is provided as an intermediate for the subsequent steps.
  • the steel strip or sheet from which the blank is produced can be obtained by standard casting processes.
  • the steel strip or sheet is cold-rolled.
  • the steel strip or sheet can suitably be cut to a steel blank.
  • a preformed steel part may also be used. The preformed part may be partially or entirely formed into the desired geometry, preferably at ambient temperature.
  • the steel blank is heated in step (a) to a temperature T1 for a time period t1.
  • the temperature T1 is 50-100 °C higher than the Ac3 temperature of the steel, and/or the temperature T2 is above the Ar3 temperature.
  • T1 is 50 - 100 °C above the Ac3 temperature
  • the steel is fully or almost fully austenitized within the time period t1, and the cooling during step (b) is easily possible.
  • the microstructure is a homogenous austenitic microstructure the formability is enhanced.
  • the time period t1 is at least 1 minute and at most 7 minutes. Too long a time period t1 may result in coarse austenitic grains, which will deteriorate the final mechanical properties
  • the heating apparatus to be used in step (a) may for instance be an electric or gas powered furnace, electrical resistance heating device, infra-red induction heating device.
  • step (b) the heated steel blank or preformed part is transferred to a hot-forming tool during a transport time t2 during which the temperature of the heated steel blank or preformed part decreases from temperature T1 to a temperature T2, wherein the transport time t2 is at most 20 seconds.
  • Time t2 is the time needed to transport the heated blank from the heating apparatus to the hot-forming tool (e.g. press) and till the hot-forming apparatus is closed.
  • the heated blank or preformed part may be transferred from the heating apparatus to the forming tool by an automated robotic system or any other transfer method.
  • Time t2 may also be chosen in combination with T1, t1 and T2 in order to control the microstructural evolution of steel at the commencement of forming and quenching.
  • t2 is equal or less than 12 seconds, preferably t2 is equal or less than 10 s, more preferably t2 is equal or less than 8s, and most preferably equal or less than 6s.
  • the blank or preformed part can be cooled from temperature T1 to a temperature at a cooling rate V2 of at least 10 °C/s.
  • V2 is preferably in the range of from 10 - 15 °C/s.
  • the cooling rate should be higher, for instance at least 20 °C/s, up to 50 °C/s or more.
  • step (c) a heated blank or preformed part is formed into a part having the desired geometry.
  • the formed part is preferably a structural part of a vehicle.
  • step (d) the formed part in the hot-forming tool is cooled to a temperature below the Mf temperature of the steel with a cooling rate V3 of at least 30 °C/s.
  • the cooling rate V3 in step (d) is in the range of from 30 - 150 °C/s, more preferably in the range of from 30 - 100 °C/s.
  • the present invention provides an improved method of introducing during hot-forming operation the desired bainitic phase into the steel microstructure.
  • the present method enables the production of hot formed steel parts displaying an excellent combination of high strength, high ductility and high bendability.
  • One or more steps of the method according to the present invention may be conducted in a controlled inert atmosphere of hydrogen, nitrogen, argon or any other inert gas in order to prevent oxidation and/or decarburisation of said steel.
  • the horizontal axis represents the time t
  • the vertical axis represents the temperature T.
  • the time t and temperature T are indicated diagrammatically in Figure 1 . No values can be derived from Figure 1 .
  • a steel blank or preformed part is (re)heated up to the austenitizing temperature above Ac1 at a particular (re)heating rate. Once the Ac1 has been exceeded the (re)heating rate is lowered until the blank or preformed part has reached a temperature higher than the Ac3. Then the strip, sheet or blank is held at this particular temperature for a period of time. Subsequently, the heated blank is transferred from the furnace to the hot forming tool, during which cooling of the blank by air occurs to some extent. The blank or preformed part is then hot-formed into a part and cooled down (or quenched) at a cooling rate of at least 30 °C/s. After reaching a temperature below the Mf temperature of the steel, the hot-forming tool is opened and the formed article is cooled down to room temperature.
  • Steel blanks with dimensions of 220 mm x 110 mm x 1.5 mm were prepared from a cold-rolled steel sheet having the composition as shown in Table 1. These steel blanks were subjected to hot forming thermal cycles in a hot dip annealing simulator (HDAS) and an SMG press. The HDAS was used for slower cooling rates (30-80°C/s) whereas the SMG press was used for fastest cooling rate (200 °C/s). The steel blanks were reheated to a T1 of respectively 900°C (36°C above Ac3) and 940°C (76°C above Ac3), soaked for 5 min. in nitrogen atmosphere to minimize surface degradation.
  • HDAS hot dip annealing simulator
  • SMG press was used for fastest cooling rate (200 °C/s).
  • the steel blanks were reheated to a T1 of respectively 900°C (36°C above Ac3) and 940°C (76°C above Ac3), soaked for 5 min. in nitrogen atmosphere to minimize surface degradation.
  • the blanks were then subjected to transfer cooling for a drop in temperature of 120°C in 10s, so at a cooling rate V2 of about 12°C/s and then subjected to cooling to 160°C at the following cooling rates V3: 30, 40, 50, 60, 80, 200°C/s.
  • longitudinal tensile specimens with 50 mm gauge length and 12.5 mm width (A50 specimen geometry) were prepared and tested with quasistatic strain rate. Microstructures were characterized from the RD-ND planes. Bending specimens (40 mm x 30 mm x 1.5 mm) from parallel and transverse to rolling directions were prepared from each of the conditions and tested till fracture by three-point bending test as described in the VDA 238-100 standard.
  • the samples with bending axis parallel to the rolling direction were identified as longitudinal (L) bending specimens whereas those with bending axis perpendicular to the rolling direction were denoted as perpendicular (T) bending specimens.
  • J-integral fracture toughness and drop tower axial crash tests were conducted.
  • Compact tension specimens according to NFMT76J standard were prepared from both longitudinal and transverse directions for fracture toughness tests.
  • the specimens were tested according to ASTM E1820-09 standard at room temperature.
  • the pre-cracks were introduced by fatigue loading.
  • the final tests were done with tensile loading with anti-buckle plates to keep the stress in plane for sheet material.
  • CTOD is the Crack Tip Opening Displacement and is a measure of how much the crack opens 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.
  • Drop tower axial crash tests were done in SMG-pressed condition with a load of 200 kg and a loading speed of 50 km/hour for the load to hit the crash boxes having a closed top hat geometry ( Figure 2 ) with 500 mm height (transverse to the rolling direction).
  • the back plates of 100 mm width were spot-welded to the profiles to prepare the crash boxes.
  • Table 3 the yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UE), and total elongation (TE) are shown for steel composition A after a variety of cooling rates V3.
  • Table 3 shows the microstructure in terms of martensite (M) and bainite (B). It will be clear from Table 3 that an ultimate tensile strength of greater than 800 MPa was achieved at the different cooling rates V3.
  • the high and improved crash behavior of hot formed steel composition A in accordance with the present invention when compared to the conventional steel products of similar strength is due to the higher bending angle and higher fracture toughness properties.
  • the steel component need to fold which is determined by its bendability, whereas on the other hand the energy absorption capability before failure is determined by its fracture toughness parameters.
  • Table 1 chemistry (wt%) Steel C Mn Si Nb B Cr Ti N Remainder A 0.075 1.48 - 0.05 0.0025 1.01 0.03 0.0045 Fe + impurities B 0.15 2.3 0.1 0.01 - - 0.015 0.0035 Fe + impurities C 0.23 1.25 0.2 - 0.003 - - 0.004 Fe + impurities
  • Table 2 Transformation temperatures steel composition A A c1 (°C) A c3 (°C) M s (°C) M f (°C) 770 864 486 287
  • Table 3 Mechanical properties and microstructures for steel composition A T1 (°C) V3 (°C/s) YS (MPa) UTS (MPa) UE (%) TE (%) Microstructure (vol.

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Claims (15)

  1. Bande, feuille ou ébauche d'acier pour la production de pièces thermoformées possédant la composition suivante en % en poids :
    C : 0,03 à 0,20,
    Mn : 0,65 à 2,50,
    Cr : 0,2 à 2,0,
    Ti : 0,01 à 0,10,
    Nb : 0,01 à 0,10,
    B : 0,0005 à 0,005,
    N : ≤ 0,01,
    avec Ti/N ≥ 3,42,
    et éventuellement un ou plusieurs des éléments choisis parmi :
    Si : ≤ 0,1,
    Mo : ≤ 0,1,
    Al : ≤ 0,1,
    Cu : ≤ 0,1,
    P : ≤ 0,03,
    S : ≤ 0,025,
    O : ≤ 0,01,
    V : ≤ 0,15,
    Ni : ≤ 0,15
    Ca : ≤ 0,15
    le reste étant du fer et des impuretés inévitables.
  2. Bande, feuille ou ébauche d'acier selon la revendication 1, dans laquelle :
    C : 0,05 à 0,17, préférablement 0,07 à 0,15, et/ou
    Mn : 1,00 à 2,10, préférablement 1,20 à 1,80, et/ou
    Cr : 0,5 à 1,7, préférablement 0,8 à 1,5, et/ou Ti : 0,015 à 0,07, préférablement 0,025 à 0,05, et/ou
    Nb : 0,02 à 0,08, préférablement 0,03 à 0,07, et/ou
    B : 0,0005 à 0,004, préférablement 0,001 à 0,003, et/ou
    N : 0,001 à 0,008, préférablement 0,002 à 0,005 Ca : ≤ 0,01.
  3. Bande, feuille ou ébauche d'acier selon la revendication 1 ou 2, la somme de la quantité de Mn et Cr étant inférieure à 2,7, préférablement entre 0,5 et 2,5 et plus préférablement entre 2,0 et 2,5.
  4. Bande, feuille ou ébauche d'acier selon la revendication 1, 2 ou 3, Mn, Cr et B étant utilisés en de telles quantités que (B x 1 000)/(Mn + Cr) soit dans la plage allant de 0,185 à 2,5, préférablement dans la plage allant de 0,2 à 2,0, et plus préférablement dans la plage allant de 0,5 à 1,5.
  5. Bande, feuille ou ébauche d'acier selon l'une quelconque des revendications 1 à 4, pourvue d'un revêtement à base de zinc ou d'un revêtement à base d'aluminium ou d'un revêtement à base organique.
  6. Bande, feuille ou ébauche d'acier selon la revendication 5, le revêtement à base de zinc étant un revêtement contenant 0,2 à 5,0 % en poids d'Al, 0,2 à 5,0 % en poids de Mg, éventuellement au plus 0,3 % en poids d'un ou plusieurs éléments supplémentaires, le reste étant du zinc et des impuretés inévitables.
  7. Pièce thermoformée produite à partir d'une bande, d'une feuille ou d'une ébauche d'acier selon l'une quelconque des revendications précédentes, la pièce possédant une résistance à la traction d'au moins 750 MPa, préférablement d'au moins 800 MPa, plus préférablement d'au moins 900 MPa, et en outre possédant une résistance à la traction d'au plus 1 400 MPa.
  8. Pièce thermoformée selon la revendication 7 possédant une élongation totale (ET) d'au moins 5 %, préférablement d'au moins 5,5 %, plus préférablement d'au moins 6 % et le plus préférablement d'au moins 7 % et/ou un angle de pliage (AP) à 1,0 mm d'épaisseur d'au moins 100°, préférablement d'au moins 115°, plus préférablement d'au moins 130° et le plus préférablement d'au moins 140°.
  9. Pièce thermoformée selon la revendication 7 ou 8, la pièce possédant une microstructure comprenant au plus 60 % de bainite, le reste étant de la martensite, la microstructure comprenant préférablement au plus 50 % de bainite, plus préférablement la microstructure comprenant au plus 40 % de bainite.
  10. Utilisation d'une pièce thermoformée selon la revendication 7, 8 ou 9 en tant que pièce structurale dans la carrosserie brute d'un véhicule.
  11. Procédé pour le thermoformage d'une ébauche d'acier ou d'une pièce pré-formée pour donner une pièce comprenant les étapes de :
    a. chauffage de l'ébauche, ou d'une pièce préformée produite à partir de l'ébauche, selon l'une quelconque des revendications 1 à 3 à une température T1 et maintien de l'ébauche chauffée à T1 pendant une période de temps t1, T1 étant supérieure à la température Ac3 de l'acier, et t1 étant d'au plus 10 minutes ;
    b. transfert de l'ébauche ou de la pièce préformée chauffée vers un outil de thermoformage pendant un temps de transport t2 pendant lequel la température de l'ébauche ou de la pièce préformée chauffée diminue de la température T1 à une température T2, le temps de transport t2 étant d'au plus 20 secondes ;
    c. thermoformage de l'ébauche ou de la pièce préformée chauffée pour donner une pièce ; et
    d. refroidissement de la pièce dans l'outil de thermoformage à une température inférieure à la température Mf de l'acier avec une vitesse de refroidissement d'au moins 30 °C/s.
  12. Procédé selon la revendication 11, la température T1 dans l'étape (a) étant 50 à 100 °C plus élevée que l'Ac3 et/ou la température T2 étant supérieure à Ar3.
  13. Procédé selon la revendication 11 ou 12, la période de temps t1 dans l'étape (a) étant d'au moins 1 minute et d'au plus 7 minutes et/ou la période de temps t2 dans l'étape (b) étant d'au plus 12 secondes, préférablement la période de temps t2 étant comprise entre 2 et 10 secondes.
  14. Procédé selon l'une quelconque des revendications 11 à 13, la pièce étant refroidie dans l'étape (d) avec une vitesse de refroidissement dans la plage de 30 à 150 °C/s, préférablement avec une vitesse de refroidissement de 30 à 100 °C/s.
  15. Véhicule comprenant au moins une pièce selon l'une quelconque des revendications 7 à 9 et/ou produite selon le procédé selon l'une quelconque des revendications 11 à 14.
EP18740258.1A 2017-07-25 2018-07-23 Bande, feuille ou ébauche d'acier pour produire une pièce formée à chaud, pièce et procédé de formage à chaud d'une ébauche en une pièce Active EP3658692B1 (fr)

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EP4092145A4 (fr) * 2020-01-16 2023-10-04 Nippon Steel Corporation Corps moulé par estampage à chaud
KR102345608B1 (ko) * 2020-12-23 2021-12-30 현대제철 주식회사 핫 스탬핑 부품, 및 이의 제조 방법

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WO2019020575A1 (fr) 2019-01-31
BR112020000917A2 (pt) 2020-07-21
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KR20200035259A (ko) 2020-04-02
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