Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0222—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating in a reactive atmosphere, e.g. oxidising or reducing atmosphere
• • 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
  • C23C2/04—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

Definitions

• the present invention relates to a new annealing process and continuous preparation of a high-strength steel strip for its coating by hot quenching in a bath of liquid metal, preferably a galvanization or a. so-called "galvannealing" treatment.
• the technical field considered here is that of galvanization by continuous scrolling in a coating bath composed of zinc or zinc alloy strips of steel. heavily loaded with alloying elements, especially HSS steels (high strength steels).
• HSS steels high strength steels.
• These special steels known to be difficult to galvanize are, for example, steels which may contain levels of alloying elements (aluminum, manganese, silicon, chromium, etc.) up to .
• the water vapor is generated here by the reduction of the iron oxide, always present on the cold-rolled sheet, by the hydrogen contained in the atmosphere of the annealing furnaces. [0004] Therefore, it has been sought to eliminate the selective oxidation in external mode or to make it migrate inside the steel, at 1 or 2 ⁇ m under the outer layer of the surface, to make it possible to present the zinc liquid a layer of substantially pure metal iron, regardless of the alloy composition and promoting the attachment of zinc coating or zinc alloy.
• pre-deposition of iron or nickel for example JP-A-04 280925, JP-A-2005/105399.
• an annealing installation and preparation of a steel strip for galvanizing typically comprises, in the direction of progression of the strip:
• a first (pre) heating section heating the strip to a temperature allowing the formation of an oxide film of adequate thickness (about 50 nanometers) for its subsequent reduction;
• this section is under an oxidizing atmosphere by adding air or oxygen, for example in the form of an air / fuel mixture gas in the case of a direct flame furnace or air alone in le case of a radiant furnace;
• a second annealing section separated from the heating section by a conventional airlock, where the strip is kept at the high annealing temperature and which is under an inert atmosphere at overpressure, to prevent the entry of gases from the heating section;
• a third reduction section also separated from the second section by a conventional airlock, under an atmosphere slightly depressed with respect thereto but at a slight overpressure with respect to the ambient; this section is intended to finish the annealing cycle
• the oxide layer created in the first section is ideally reduced completely by a hydrogen / inert gas atmosphere with a very low dew point.
• the present invention aims to provide a solution that makes it possible to overcome the disadvantages of the state of the art.
• the invention aims to provide a method of annealing and preparation for galvanizing high strength steels which is more economical, the latter being carried out with or without accompanying heat treatment galvannealing type.
• the invention also aims to allow a preparation of high strength steels for galvanizing, which are free of brittleness defects.
• the invention aims to provide a confined atmosphere annealing process free of added hydrogen.
• An additional object of the invention is to prevent the selective oxidation of alloying elements in the outermost layer of the surface of the strip during the total oxidation step during continuous annealing. previous cooling and immersion in the zinc bath.
• the present invention relates to a method of continuous annealing and preparation of a high strength steel strip for hot dip coating in a bath of molten metal.
• said steel strip is treated in at least two sections, comprising successively, considering the direction of progression of the strip: a so-called heating and holding section, in which a heating of the strip is achieved; strip followed by maintaining at a given annealing temperature under an oxidizing atmosphere comprising an air (or oxygen) / non-oxidizing or inert gas mixture, in order to form on the surface of the strip a thin oxide film of which the thickness, preferably between 0.02 and 0.
• a so-called cooling and transfer section in which, before it is transferred to the coating bath, at least the annealed strip is cooled and undergoes a complete reduction in iron metal of the iron oxide present in the oxide layer; formed in the heating and holding section, under a reducing atmosphere comprising a mixture of low hydrogen content and inert gas, the said two sections being separated from each other by a conventional airlock; characterized in that the oxidizing atmosphere is at least partially separated from the reducing atmosphere by maintaining a controlled oxygen content in the heating and holding section between 50 and 1000 ppm and in that maintains a controlled hydrogen content in the cooling section and transfer to a value of less than 4% and preferably less than 0.5%.
• the controlled oxygen content is maintained in the heating and holding section between 50 and 400 ppm.
• the separation of the oxidizing atmosphere from the reducing atmosphere is carried out by an overpressure of the oxidizing atmosphere, so that the oxygen entrained by the strip in the cooling zone and transfer through the airlock, following this overpressure, react completely with the hydrogen contained in the cooling atmosphere by forming steam.
• the hydrogen is allowed to react, present in the cooling and transfer section, entrained in the hot gas stream directed upstream, with oxygen from the heating and holding section to form water vapor.
• the cooling and transfer section is maintained in overpressure with respect to the heating and holding section.
• the control of the oxygen content of the oxide layer formed in the heating and holding section is obtained either by modifying the gaseous mixture containing combustion air supplying the flame heating means. direct, or by controlled injection of the mixture air (or oxygen) / inert gas in the case of radiation heating or induction.
• the non-oxidizing or inert gas is nitrogen or argon.
• the liquid metal is zinc or one of its alloys.
• the heating zone and maintenance is devoid of reducing atmosphere.
• the hot dip coating process is a galvanization or a galvannealing treatment.
• the atmosphere both in the heating and holding section and in the cooling and transfer section has a dew point less than or equal to -10 0 C, preferably -20 0 C.
• the strip is heated to a temperature between -10 0 C, preferably -20 0 C.
• the band is then cooled down to a maximum of ⁇
• An economical method proposed according to the invention, aims at carrying out the annealing step preparatory to galvanizing, without the addition of hydrogen, gas which is ten times more expensive than a more common gas such as nitrogen and which is also causing serious fragility defects of resistance steels.
• the invention aims to obtain a perfect galvanization for all grades of steel resistance. To avoid oxidation of the alloy elements at the extreme surface, it is proposed to inject an air / nitrogen mixture into the oven during the entire cycle of (pre-) heating and holding the sheet at high temperature.
• This method therefore does not require atmospheric separation throughout the heating / maintenance portion as is the case in other processes (eg JP-A-2003/342645) where reactive zones in depression are included at this part of the. oven.
• the oxygen contained in the air / nitrogen mixture will have the effect of creating in the annealing section two simultaneous and competitive reactions:
• the alloying elements also contribute to the reduction of iron oxide when they migrate to the steel / iron oxide interface.
• the air / nitrogen atmosphere of the heating / holding portion will have to be separated and partially isolated from the non-oxidizing atmosphere of the cooling and transfer stages of the strip into the zinc bath.
• the oxidizing atmosphere will preferably be maintained at an excess pressure relative to the non-oxidizing atmosphere such that the oxygen entrained by the sheet reacts completely with the hydrogen contained in the atmosphere of the cooling.
• a steel containing, for example, 1.2% of aluminum will for example be heated and annealed to a temperature of 800 ° C. in an atmosphere containing 100 ppm of oxygen in nitrogen.
• the sheet is cooled to 500 ° C. at a speed of 50 ° C./s in an atmosphere containing 4% of hydrogen and 0.1% of water vapor. which corresponds to a dew point of -2O 0 C.
• This sheet is then introduced at the temperature of 470 0 C in a zinc bath, containing 0.2% of aluminum, which is maintained at 46O 0 C. After immersion of 3 seconds, the coating is wrung out so as to keep a zinc layer of 8 ⁇ m. Such a zinc deposit is then perfectly wetting and has adhesion qualities comparable to that obtained for ordinary low-carbon steel.
• Another way to proceed is to allow the usual flow to establish from the zinc bath to the heating section and to leave the very low hydrogen content ( ⁇ 0.5%), contained in the transfer section. / cooling, react with the oxygen of the heating / holding portion to form water vapor.
• An additional supply of oxygen, at the exit of the holding section, can be made to neutralize the entry of hydrogen, the contents used being always located very far from the dangerous, that is to say, explosive ( 4% H 2 in air).
• a high hydrogen content is indeed not necessary in the cooling section because the carbon steel will be sufficient to reduce the thin layer of iron oxide created in the heating / holding part and the metal iron thus prepared will ensure good wettability by the zinc during the immersion of the sheet in the bath.
• this method should provide for controlling the oxygen content in the oven within the range of between 50 and 1000 ppm.
• a content that is too low will not make it possible to produce a layer of iron oxide sufficiently impervious to the diffusion of the alloying elements towards the extreme surface, and a content that is too high in oxygen will produce a layer of iron oxide that is too thick. , which can not be reduced during the cooling and transfer steps to the zinc bath.
• This oxygen content will preferably be in the range of 50 to 400 ppm.
• the invention has a number of advantages, including the fact that: - that hydrogen addition is much lower than in the state of the art, or even zero, in the zone of maintenance-heating, which constitutes an important economy of exploitation and guarantees the obtaining of high strength steel with fewer frailty defects;
• the gaseous atmosphere used is less fragile for the equipment (for example the radiant tubes), in particular following the reduction of the content thereof in hydrogen.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Coating With Molten Metal (AREA)
• Heat Treatment Of Strip Materials And Filament Materials (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Pressure Welding/Diffusion-Bonding (AREA)

Priority Applications (12)

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Publications (1)

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Cited By (9)

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• 2006
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• 2007
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