Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
  • B21J1/00—Preparing metal stock or similar ancillary operations prior, during or post forging, e.g. heating or cooling
  • B21J1/06—Heating or cooling methods or arrangements specially adapted for performing forging or pressing operations
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
  • B21J3/00—Lubricating during forging or pressing
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • 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

Definitions

• the invention relates to a method for manufacturing a coated steel part having very high mechanical properties using hot forming techniques, and a part thus formed.
• Hot forming of parts is well-known in the art and is used in the automotive industry to produce complex parts with a reduced weight.
• the heated blank has a temperature of at most 780° C when placed in the hot forming press.
• micro-cracks is strongly influenced by the temperature of the blank when it is placed in the hot forming press and the temperature at which the hot formed part is formed in the hot forming press.
• high temperatures such as used in the art, many micro-cracks are formed in the substrate, which often have a depth of more than 10 ⁇ .
• Lowering the temperature at which the coated blank is placed in the hot forming press reduced the micro-cracks in number and depth. It has been found that cooling the heated blank to a temperature of at most 780° C when placed in the hot forming press already reduces the number and depth of the micro-cracks to a considerable extent.
• the blank is heated to a temperature of 850° C or more, more preferably a temperature of 880° C or more, and still more preferably to a temperature of 900° C or more.
• the coated blank is heated to a temperature such that the substrate is fully and homogenously austenitised before it is placed in the hot forming press.
• the heated blank has a temperature of at least 500° C when placed in the hot forming press, more preferably a temperature of at least 600° C, and still more preferably a temperature of at least 650° C.
• the temperature of the blank can be below the Arl temperature when placed into the blank, but preferably the blank is not cooled too far because the cooling rate is very critical at low temperatures.
• the heated blank has a temperature of at most 740° C when placed in the hot forming press, and more preferably a temperature of at most 700° C.
• the temperature of the blank should be as low as possible to prevent (large) micro-cracks. Thus, it is optimal if the temperature of the blank is between 650° C and 700° C when the blank is placed in the hot forming press.
• the heated blank is at least partially cooled between the furnace and the hot forming press by forced cooling, preferably with an average cooling velocity of at least 30° C/sec, more preferably with an average cooling velocity of at least 40° C/sec, still more preferably with an average cooling velocity of at least 60° C/sec, most preferably with an average cooling velocity of at least 80° C/sec.
• Forced cooling effectively reduces the blank temperature at a cooling rate that is higher than the cooling rate obtainable when the blank is cooled in open air.
• a high cooling velocity is preferred, so the time in which the blank is kept between the furnace and the hot forming press can be as short as possible. This is advantageous so as to realise desired high strength of the formed part and so as to reduce oxidation of the coating on the blank.
• the heated steel blank is transported between the furnace and the hot forming press in a time span of at most 10 seconds, more preferably in a time span of at most 8 seconds, and still more preferably in a time span of at most 6 seconds.
• a short transportation time is also advantageous to reduce the cycle time to produce a hot formed part.
• the heated blank is cooled between the furnace and the hot forming press using accelerated cooling making use of gas, such as cooling with forced air flow, cooling with forced N 2 flow, or cooling with forced C0 2 flow or cooled C0 2 using solid C0 2.
• gas such as cooling with forced air flow, cooling with forced N 2 flow, or cooling with forced C0 2 flow or cooled C0 2 using solid C0 2.
• the heated blank is cooled between the furnace and the hot forming press using liquid or solid material, such as liquid or solid C0 2 particles or metal cooling plates.
• liquid or solid material such as liquid or solid C0 2 particles or metal cooling plates.
• Liquid or solid material will provide a fast cooling rate, but is more difficult to control.
• Solid C0 2 particles for instance have the advantage of a fast cooling capacity and the additional advantage that oxide layers on the coating are at least partially removed at the same time, but solid C0 2 particles are more difficult to supply at the right time period.
• Metal cooling plates that are brought into contact with the heated blank provide a very fast cooling rate, but a tight control of the cooling time is required.
• the coating on the blank is a metallic coating, preferably a coating layer of zinc or a zinc alloy.
• Zinc or zinc alloy can provide an active corrosion protection.
• the zinc alloy consists of 0.3 - 4.0% Mg and 0.05 - 6.0% Al, optionally at most 0.2% of one or more additional elements, unavoidable impurities, the remainder being zinc. It has been found that such a zinc alloy provides a better corrosion resistance to the hot formed part.
• the zinc or zinc alloy layer is pre-diffused in the substrate.
• the pre-diffused coating makes it possible to heat the blank fast to a temperature above the Ac3 temperature.
• the zinc or zinc alloy coated steel blank contains an average of 10 - 75 % iron in the coating after heating the blank in the furnace, preferably an average of 25 - 55 %.
• This amount of iron in the coating ensures that the (initial) zinc or zinc alloy coating does not evaporate at temperatures above the Ac3 temperature.
• the hot forming press is internally cooled such that the blank is cooled with a cooling velocity of at least 50° C/sec during the first second after the press is fully closed, preferably with a cooling velocity of at least 100° C/sec during the first second.
• This very fast cooling rate after the hot forming press is fully closed makes that the austenitic structure of the heated blank is transformed fully or almost fully to a martensitic structure.
• the hot forming press is fully closed in at most one second. This fast closing of the hot forming press is advantageous so as to fast cool also the portions of the blank that are not in immediate contact with the press.
• a hot formed part manufactured according to the above method, wherein the substrate of the part is free of micro-cracks having a depth of more than 15 ⁇ , and preferably the substrate is free of micro-cracks having a depth of more than 10 ⁇ . Parts having less deep micro-cracks are preferred over parts having deeper micro-cracks.
• a hot formed part manufactured according to the above method, wherein the micro-cracks in the substrate have an average depth of at most 3 ⁇ in five surface area's of the blank with a size of 1 cm 2 . Parts having on average shallow micro-cracks are preferred over parts having deep micro-cracks on average.

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• 2012
  • 2012-01-17 EP EP12700452.1A patent/EP2665837A1/de not_active Ceased
  • 2012-01-17 CN CN201280005548.4A patent/CN103403195B/zh active Active
  • 2012-01-17 EP EP14163594.6A patent/EP2754723A3/de not_active Ceased
  • 2012-01-17 WO PCT/EP2012/000186 patent/WO2012097976A1/en active Application Filing

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