EP1805341B1 - Hot-dip coating method in a zinc bath for strips of iron/carbon/manganese steel - Google Patents

Hot-dip coating method in a zinc bath for strips of iron/carbon/manganese steel Download PDF

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Publication number
EP1805341B1
EP1805341B1 EP05809221A EP05809221A EP1805341B1 EP 1805341 B1 EP1805341 B1 EP 1805341B1 EP 05809221 A EP05809221 A EP 05809221A EP 05809221 A EP05809221 A EP 05809221A EP 1805341 B1 EP1805341 B1 EP 1805341B1
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Prior art keywords
iron
manganese
zinc
strip
layer
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EP05809221A
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German (de)
French (fr)
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EP1805341A1 (en
Inventor
Pascal Drillet
Daniel Bouleau
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ArcelorMittal France SA
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ArcelorMittal France SA
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    • 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
    • C23C2/02—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
    • C23C2/0222—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
    • C23C2/34—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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S428/00—Stock material or miscellaneous articles
    • Y10S428/922—Static electricity metal bleed-off metallic stock
    • Y10S428/9335—Product by special process
    • Y10S428/939—Molten or fused coating
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/12—All metal or with adjacent metals
    • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
    • Y10T428/12771—Transition metal-base component
    • Y10T428/12785—Group IIB metal-base component
    • Y10T428/12792—Zn-base component
    • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

  • the present invention relates to a method of hot dip coating in a zinc-based liquid bath comprising aluminum, a strip of austenitic iron-carbon-manganese steel in scrolling.
  • the steel belts conventionally used in the automotive field such as for example the dual-phase steel belts, are coated with a zinc-based coating to protect them against corrosion before they are shaped or delivered.
  • This zinc layer is generally applied continuously either by electrodeposition in an electrolytic bath containing zinc salts, or by vacuum deposition, or by hot quenching of the high speed band in a bath of molten zinc.
  • the steel strips Before being coated with a zinc layer by hot dipping in a zinc bath, the steel strips undergo a recrystallization annealing in a reducing atmosphere in order to give the steel a homogeneous microstructure and to improve its properties. mechanical characteristics. Under industrial conditions, this recrystallization annealing is carried out in an oven in which there is a reducing atmosphere.
  • the strips run in the oven consisting of an enclosure completely isolated from the outside atmosphere, comprising three zones, a first heating zone, a second temperature holding zone, and a third cooling zone, in which there is an atmosphere composed of a reducing gas with respect to iron.
  • This gas may be chosen for example from hydrogen, and mixtures of nitrogen and hydrogen, and has a dew point between -40 ° C and -15 ° C.
  • JP-A-07278772 discloses the manufacture of a manganese-containing galvanized steel strip of hot dipping the steel strip in a zinc-based liquid bath comprising specific Al and min contents.
  • the present invention therefore aims to provide a method for coating by hot dipping in a zinc-based liquid bath, an iron-carbon-manganese steel strip running through a coating based on zinc.
  • the invention also relates to the austenitic iron-carbon-manganese steel strip coated with a zinc-based coating obtainable by this method.
  • the inventors have thus demonstrated that by creating favorable conditions for the bi-layer of mixed oxide (Fe, Mn) O and of manganese oxide forming on the surface of the iron-carbon steel strip.
  • manganese being reduced by the aluminum contained in the zinc-based liquid bath, the surface of the strip became wetting with respect to the zinc, which allowed to coat it with a coating based on zinc.
  • This steel strip is typically between 0.2 and 6 mm, and may be issued from either the hot band or the cold band train.
  • the austenitic iron-carbon-manganese steel used according to the invention comprises, in% by weight: 0.30% ⁇ C ⁇ 1.05%, 16% ⁇ Mn ⁇ 26%, Si ⁇ 1%, Al ⁇ 0.050%, S ⁇ 0.030%, P ⁇ 0.080%, N ⁇ 0.1%, and optionally, one or more elements such as: Cr ⁇ 1%, Mo ⁇ 0.40%, Ni ⁇ 1%, Cu ⁇ 5%, Ti ⁇ 0.50%, Nb ⁇ 0.50%, V ⁇ 0.50%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation.
  • the carbon content is between 0.40 and 0.70% by weight.
  • the carbon content is between 0.40% and 0.70%, the stability of the austenite is increased and the strength is increased.
  • the manganese content in the steel according to the invention is between 20 and 25% by weight.
  • Silicon is an effective element for deoxidizing steel as well as for hardening in the solid phase. However, beyond a content of 1%, Mn 2 SiO 4 and SiO 2 layers are formed on the surface of the steel, which show a reduction ability of the aluminum contained in the water-based bath. zinc significantly lower than the mixed oxide (Fe, Mn) O and manganese oxide MnO layers.
  • the silicon content in the steel is less than 0.5% by weight.
  • Aluminum is also a particularly effective element for the deoxidation of steel. Like carbon, it increases the stacking fault energy. However, its excessive presence in steels with a high manganese content has a disadvantage: In fact, manganese increases the solubility of nitrogen in the liquid iron, and if too much aluminum is present in the steel, Nitrogen combined with aluminum precipitates in the form of aluminum nitrides hindering the migration of grain boundaries during hot processing and greatly increases the risk of crack appearances. An Al content less than or equal to 0.050% makes it possible to avoid a precipitation of AlN. Correlatively, the nitrogen content must be less than or equal to 0.1% in order to prevent this precipitation and the formation of volume defects (blowholes) during solidification.
  • oxides such as MnAl 2 O 4 , MnO.Al 2 O 3, which are more difficult to reduce, are formed during the recrystallization annealing of the steel.
  • aluminum contained in the zinc-based coating bath as oxides (Fe, Mn) O and MnO.
  • oxides (Fe, Mn) O and MnO are much more stable than oxides (Fe, Mn) O and MnO. Therefore, even if it is possible to form on the surface of the steel a zinc-based coating, it will in any case little adherent because of the presence of alumina.
  • it is essential that the aluminum content in the steel is less than 0.050% by weight.
  • Sulfur and phosphorus are impurities that weaken the grain boundaries. Their respective content must be less than or equal to 0.030 and 0.080% in order to maintain sufficient hot ductility.
  • Chromium and nickel can be used as an option to increase the strength of the steel by hardening in solid solution.
  • chromium decreases the stacking fault energy, its content must be less than or equal to 1%.
  • Nickel contributes to a significant elongation rupture, and in particular increases the toughness.
  • the molybdenum may be added in an amount less than or equal to 0.40%.
  • addition of copper to a content of less than or equal to 5% is a means of hardening the steel by precipitation of metallic copper. However, beyond this content, copper is responsible for the appearance of surface defects hot sheet.
  • Titanium, niobium and vanadium are also elements that can optionally be used to obtain precipitation hardening of carbonitrides.
  • Nb or V, or Ti content is greater than 0.50%, excessive precipitation of carbonitrides can cause a reduction in toughness, which should be avoided.
  • the austenitic iron-carbon-manganese steel strip undergoes heat treatment to recrystallize the steel.
  • the recrystallization annealing makes it possible to give the steel a homogeneous microstructure, to improve its mechanical characteristics, and in particular to give it ductility to allow its use in stamping.
  • This heat treatment is performed in an oven inside which there is an atmosphere composed of a reducing gas vis-à-vis the iron, to avoid excessive oxidation of the surface of the strip, and allow good adhesion of zinc.
  • This gas is selected from hydrogen, and nitrogen-hydrogen mixtures.
  • the gaseous mixtures comprising between 20 and 97% by volume of nitrogen and between 3 and 80% by volume of hydrogen, and more preferably between 85 and 95% by volume of nitrogen and between 5 and 15%, are chosen. in volume of hydrogen.
  • hydrogen is an excellent iron reducing agent, it is preferred to limit its concentration because of its high cost relative to nitrogen.
  • calamine is a layer of iron oxide comprising a small proportion of manganese. Gold not only this calamine layer prevents any adhesion of zinc on steel, but also it is a layer that tends to crack easily which makes it all the more undesirable.
  • the atmosphere prevailing in the furnace is certainly reducing with respect to iron, but not for elements such as manganese.
  • the gas constituting the atmosphere in the furnace comprises traces of moisture and / or oxygen that can not be avoided, but it is possible to control by imposing the dew point of said gas.
  • the inventors have observed that, according to the invention, at the end of the recrystallization annealing, the lower the dew point in the oven, or in other words the lower the oxygen partial pressure, the lower the The manganese oxide formed on the surface of the iron-carbon-manganese steel strip is thin. This observation may seem at odds with Wagner's theory that the lower the dew point, the higher the density of oxides formed on the surface of a carbon steel strip. Indeed, when the amount of oxygen decreases on the surface of the carbon steel, the migration of the oxidizable elements contained in the steel to the surface accelerates, which promotes the oxidation of the surface.
  • the inventors believe that in the case of the invention, the amorphous oxide layer (Fe, Mn) O becomes rapidly continuous. It therefore constitutes a barrier for the oxygen of the atmosphere in the furnace, which is no longer in direct contact with the steel. An increase in the oxygen partial pressure in the furnace therefore leads to an increase in the thickness of the manganese oxide and does not cause internal oxidation, ie no layer is observed. additional oxide between the surface of the austenitic iron-carbon-manganese steel and the amorphous oxide layer (Fe, Mn) O.
  • the recrystallization annealing carried out under the conditions of the invention thus makes it possible to form on both sides of the strip a continuous sub-layer of mixed oxide of iron and manganese (Fe, Mn) O amorphous, the thickness of which is preferably between 5 and 10 nm, and a continuous or discontinuous outer layer of crystalline MnO manganese oxide whose thickness is preferably between 5 and 90 nm, preferably between 5 and 50 nm, and more preferably between 10 and 40 nm.
  • the MnO outer layer has a granular appearance, and the size of the MnO crystals increases sharply as the dew point also increases.
  • the inventors have demonstrated that, when the content by weight of aluminum in the zinc-based liquid bath is less than 0.18% and when the manganese oxide layer MnO is greater than 100 nm, the latter It is not reduced by the aluminum contained in the bath, and the zinc-based coating is not obtained due to the non-wetting effect of MnO with respect to the zinc.
  • the dew point according to the invention at least in the zone for maintaining the temperature of the oven, and preferably in the entire enclosure of the oven, is preferably between -80 and 20 ° C, advantageously between - 80 and -40 ° C and more preferably between -60 and -40 ° C.
  • the thickness of the manganese oxide layer becomes too great to be reduced by the aluminum contained in the zinc-based liquid bath under industrial conditions, that is to say during a period of time. time less than 10 seconds.
  • the range -60 to -40 ° C is advantageous because it allows to form a bi-oxide layer of relatively reduced thickness which will be easily reduced by the aluminum contained in the zinc-based bath.
  • the heat treatment comprises a heating phase at a heating rate V1, a holding phase at a temperature T1 and during a holding time M, followed by a cooling phase at a cooling rate V2.
  • the heat treatment is preferably carried out at a heating rate V1 greater than or equal to 6 ° C / s, because below this value the holding time M of the strip in the oven is too long and does not correspond to the industrial requirements. of productivity.
  • the temperature T1 is preferably between 600 and 900 ° C. Indeed, below 600 ° C, the steel will not be completely recrystallized and its mechanical characteristics will be insufficient. Beyond 900 ° C, not only the grain size of the steel increases which is harmful for obtaining good mechanical characteristics, but also the thickness of the manganese oxide layer MnO strongly increases and makes difficult, if not impossible, the subsequent deposition of a coating based on zinc, because the aluminum contained in the bath will not have completely reduced the MnO.
  • T1 is preferably between 600 and 820 ° C., advantageously less than or equal to 750 ° C, and more preferably between 650 and 750 ° C.
  • the holding time M is preferably between 20 s and 60 s, and advantageously between 20 and 40 s.
  • the recrystallization annealing is generally carried out by a radiant tube heater.
  • the strip is cooled to an immersion temperature of the T3 band between (T2 - 10 ° C) and (T2 + 30 ° C), T2 being defined as the temperature of the liquid bath based on zinc.
  • T3 an immersion temperature of the T3 band between (T2 - 10 ° C) and (T2 + 30 ° C)
  • T2 being defined as the temperature of the liquid bath based on zinc.
  • the strip is preferably cooled at a cooling rate V2 of greater than or equal to 3 ° C./s, advantageously greater than 10 ° C./s, so as to avoid the enlargement of the grains and to obtain a steel strip having good mechanical characteristics.
  • V2 a cooling rate of greater than or equal to 3 ° C./s, advantageously greater than 10 ° C./s, so as to avoid the enlargement of the grains and to obtain a steel strip having good mechanical characteristics.
  • the strip is generally cooled by injection of an air flow on both sides.
  • the austenitic iron-carbon-manganese steel strip When, after undergoing recrystallization annealing, the austenitic iron-carbon-manganese steel strip is covered on both sides by the two-layer oxide, it is passed through the zinc-based liquid bath containing 'aluminum.
  • the aluminum contained in the zinc bath contributes not only to the at least partial reduction of the two-layer oxide, but also to obtaining a coating having a homogeneous surface appearance.
  • a homogeneous surface appearance is characterized by a uniform thickness, whereas a heterogeneous appearance is characterized by strong thickness heterogeneities.
  • Fe 2 Al 5 and / or FeAl 3 inter-facial layer is not formed on the surface of iron-carbon-manganese steel, or if it is formed, it is immediately destroyed by the formation of the phases (Fe, Mn) Zn.
  • matts of Fe 2 Al 5 and / or FeAl 3 type are found in the bath.
  • the aluminum content in the bath is adjusted to a value at least equal to the content necessary for the aluminum to completely reduce the crystalline MnO manganese oxide layer and at least partially the oxide (Fe, Mn) O layer. amorphous.
  • the weight content of aluminum in the bath is between 0.15 and 5%. Below 0.15%, the aluminum content will be insufficient to completely reduce the manganese oxide layer MnO and at least partially the layer of (Fe, Mn) O, and the surface of the steel strip will not present sufficient wettability with respect to zinc. Above 5% of aluminum in the bath, a coating of a type different from that obtained by the invention will form on the surface of the steel strip. This coating will include an increasing proportion of aluminum as the aluminum content in the bath increases.
  • the zinc-based bath may also contain iron, preferably at a content such that it is supersaturation with respect to Fe 2 Al 5 and / or FeAl 3 .
  • T2 a temperature preferably greater than or equal to 430 ° C, but to avoid excessive evaporation of zinc, T2 is less than or equal to 480 ° C.
  • the strip is in contact with the bath for a contact time C preferably between 2 and 10 seconds, and more preferably between 3 and 5 seconds.
  • the aluminum does not have enough time to completely reduce the MnO layer of manganese oxide and at least partially the mixed oxide layer (Fe, Mn) O, and thus to make the surface of wetting steel vis-à-vis zinc. Above 10 seconds, the two-layer oxides will certainly be completely reduced, however the line speed may be industrially too low, and the coating too alloyed and then difficult to adjust in thickness.
  • a zinc-based coating comprising, in order from the steel / coating interface, a layer of iron-manganese-zinc alloy composed of two cube and cubic phases. with centered face ⁇ 1, a layer of iron-manganese-zinc alloy ⁇ 1 of hexagonal structure, a layer of iron-manganese-zinc alloy ⁇ of monoclinic structure, and a surface layer of zinc.
  • the inventors have thus verified that according to the invention, and contrary to what happens in the case of a coating of a carbon steel strip in a zinc bath containing aluminum, it is not formed. no Fe 2 Al 5 layer at the steel / coating interface.
  • the aluminum of the bath reduces the bi-oxide layer.
  • the MnO layer is more easily reducible by the aluminum of the bath than the oxide layers based on silicon. This results in a local depletion of aluminum which leads to the formation of a coating comprising FeZn phases instead of the expected Fe 2 Al 5 (Zn) coating, which is formed in the case of carbon steels.
  • a strip coated on both sides by a zinc-based coating comprising in order from the steel / coating interface a layer of iron-manganese-zinc alloy composed of two cubic phase ⁇ and cubic to centered face ⁇ 1, an iron-manganese-zinc alloy layer ⁇ 1 of hexagonal structure, and possibly a layer of iron-manganese-zinc alloy ⁇ of monoclinic structure.
  • the alloying heat treatment is preferably carried out directly at the outlet of the zinc bath, at a temperature of between 490 and 540 ° C., for a duration of between 2 and 10 seconds.
  • Table 2 shows the characteristics of the oxide bi-layer comprising an amorphous continuous lower layer (Fe, Mn) O, and an upper layer MnO, formed on the samples after annealing according to the dew point.
  • Table 2 RT -80 ° C PR -45 ° C PR + 10 ° C Surface color the band yellow green blue Mean diameter of the crystals MnO (nm) 50 discontinuous layer 100 continuous layer 300 continuous layer Thickness of the bilayer (nm) 10 110 1500
  • the samples After being recrystallized, the samples are cooled to a temperature T3 of 480 ° C. and are immersed in a zinc bath comprising, by weight, 0.18% of aluminum and 0.02% of iron, the temperature of which T2 is 460 ° C. The samples remain in contact with the bath for a period of contact C of 3 seconds. After immersion, the samples are examined to see if a zinc-based coating is present on the surface of the sample. Table 3 shows the result obtained as a function of the dew point. Table 3 PR-80 ° C PR -45 ° C PR + 10 ° C Presence of zinc-based coating Yes no no no
  • the inventors have demonstrated that if the oxide bilayer formed on the iron-carbon-manganese austenitic steel strip after recrystallization annealing was greater than 110 nm, the presence in the bath of 0.18% by weight of Aluminum was insufficient to reduce the bi-oxide layer and give the strip sufficient wettability of the zinc to the steel to form a zinc-based coating.
  • Table 5 shows the structures of the various oxide films that formed on the surface of the steel after annealing in function.
  • Table 5 Oxide films Steel A * Steel B Undercoat MnAl 2 O 4 (Fe, Mn) O Upper layer MnO.Al 2 O 3 MnO * according to the invention
  • the samples After having been recrystallized, the samples are cooled to a temperature T3 of 480 ° C. and are immersed in a zinc bath comprising 0.18% of aluminum and 0.02% of iron, whose T2 temperature is 460 ° C. vs. The samples remain in contact with the bath for a contact time C of 3 seconds. After immersion, the samples are coated with a zinc coating.

Abstract

The subject of the invention is a method for the hot-dip coating, in a liquid bath based on zinc containing aluminum, of a running strip of iron-carbon-manganese austenitic steel, in which said strip is subjected to a heat treatment in a furnace in which an atmosphere that is reducing with respect to iron prevails, in order to obtain a strip covered with a thin manganese oxide layer, and then the strip covered with the thin manganese oxide layer is made to run through said bath, the aluminum content in the bath being adjusted to a value at least equal to the content needed for the aluminum to completely reduce the manganese oxide layer, so as to form, on the surface of the strip, a coating comprising an iron-manganese-zinc alloy layer and a zinc surface layer.

Description

La présente invention concerne un procédé de revêtement au trempé à chaud dans un bain liquide à base de zinc comprenant de l'aluminium, d'une bande en acier austénitique fer-carbone-manganèse en défilement.The present invention relates to a method of hot dip coating in a zinc-based liquid bath comprising aluminum, a strip of austenitic iron-carbon-manganese steel in scrolling.

Les bandes en acier classiquement utilisées dans le domaine automobile, comme par exemple les bandes en acier dual-phase, sont revêtues d'un revêtement à base de zinc pour les protéger contre la corrosion avant leur mise en forme ou leur livraison. Cette couche de zinc est généralement appliquée en continu soit par électrodéposition dans un bain électrolytique contenant des sels de zinc, soit par dépôt sous vide, soit encore par trempé à chaud de la bande défilant à grande vitesse dans un bain de zinc fondu.The steel belts conventionally used in the automotive field, such as for example the dual-phase steel belts, are coated with a zinc-based coating to protect them against corrosion before they are shaped or delivered. This zinc layer is generally applied continuously either by electrodeposition in an electrolytic bath containing zinc salts, or by vacuum deposition, or by hot quenching of the high speed band in a bath of molten zinc.

Avant d'être revêtues d'une couche de zinc par trempé à chaud dans un bain de zinc, les bandes en acier subissent un recuit de recristallisation dans une atmosphère réductrice en vue de conférer à l'acier une microstructure homogène et d'améliorer ses caractéristiques mécaniques. Dans les conditions industrielles, ce recuit de recristallisation est réalisé dans un four dans lequel règne une atmosphère réductrice. A cet effet, les bandes défilent dans le four constitué d'une enceinte complètement isolée de l'atmosphère extérieure, comprenant trois zones, une première zone de chauffage, une deuxième zone de maintien en température, et une troisième zone de refroidissement, dans lesquelles règne une atmosphère composée d'un gaz réducteur vis à vis du fer. Ce gaz peut être choisi par exemple parmi l'hydrogène, et les mélanges d'azote et d'hydrogène, et présente un point de rosée compris entre -40°C et -15°C. Ainsi, outre l'amélioration des caractéristiques mécaniques de l'acier, le recuit de recristallisation des bandes en acier sous atmosphère réductrice permet un bon accrochage de la couche de zinc sur l'acier, car les oxydes de fer présents à la surface de la bande sont réduits par le gaz réducteur.Before being coated with a zinc layer by hot dipping in a zinc bath, the steel strips undergo a recrystallization annealing in a reducing atmosphere in order to give the steel a homogeneous microstructure and to improve its properties. mechanical characteristics. Under industrial conditions, this recrystallization annealing is carried out in an oven in which there is a reducing atmosphere. For this purpose, the strips run in the oven consisting of an enclosure completely isolated from the outside atmosphere, comprising three zones, a first heating zone, a second temperature holding zone, and a third cooling zone, in which there is an atmosphere composed of a reducing gas with respect to iron. This gas may be chosen for example from hydrogen, and mixtures of nitrogen and hydrogen, and has a dew point between -40 ° C and -15 ° C. Thus, in addition to the improvement of the mechanical properties of the steel, the recrystallization annealing of the steel strips under a reducing atmosphere makes it possible to bond the zinc layer to the steel, because the iron oxides present on the surface of the steel band are reduced by the reducing gas.

Pour certaines applications automobiles qui requièrent un allégement et une résistance accrus des structures métalliques en cas de choc, on commence à remplacer les nuances d'acier conventionnelles par des aciers austénitiques fer-carbone-manganèse qui présentent des caractéristiques mécaniques supérieures, et notamment une combinaison de résistance mécanique et d'allongement à la rupture particulièrement avantageuse, une excellente aptitude à la mise en forme et une résistance élevée à la rupture en présence de défauts ou de concentration de contraintes. Les applications concernent par exemple des pièces participant à la sécurité et à la durabilité des véhicules automobiles ou encore des pièces de peau.For certain automotive applications that require increased lightening and strength of metal structures in the event of impact, is beginning to replace conventional steel grades with austenitic iron-carbon-manganese steels which have superior mechanical properties, including a particularly advantageous combination of strength and elongation at break, excellent workability and high resistance to fracture in the presence of defects or concentration of stresses. The applications concern, for example, parts that contribute to the safety and durability of motor vehicles or even pieces of skin.

Ces aciers doivent également, après recuit de recristallisation, être protégés contre la corrosion par une couche de zinc. Cependant, les inventeurs ont mis en évidence qu'il était impossible, dans les conditions usuelles, de revêtir une bande en acier fer-carbone-manganèse défilant à une vitesse élevée (supérieure à 40 m/s) par une couche de zinc en mettant en oeuvre un procédé de revêtement au trempé à chaud dans un bain de zinc. En effet, les oxydes de type MnO et (Mn,Fe)O qui se forment lors du traitement thermique que subit la bande avant d'être revêtue, rendent la surface de la bande non mouillante pour le zinc liquide.These steels must also, after recrystallization annealing, be protected against corrosion by a layer of zinc. However, the inventors have shown that it was impossible, under the usual conditions, to coat a steel-carbon-manganese steel strip traveling at a high speed (greater than 40 m / s) by a layer of zinc, a hot dipping coating process in a zinc bath. In fact, the oxides of the MnO and (Mn, Fe) O type which are formed during the heat treatment which the strip undergoes before being coated render the surface of the strip non-wetting for liquid zinc.

JP-A-07278772 décrit la fabrication d'une bande d'acier galvanisé contenant du manganèse consistant à tremper à chaud la bande d'acier dans un bain liquide à base de zinc comprenant des teneurs spécifiques en Al et mn. JP-A-07278772 discloses the manufacture of a manganese-containing galvanized steel strip of hot dipping the steel strip in a zinc-based liquid bath comprising specific Al and min contents.

La présente invention a donc pour but de proposer un procédé permettant de revêtir au trempé à chaud dans un bain liquide à base de zinc, une bande en acier fer-carbone-manganèse en défilement par un revêtement à base de zinc.The present invention therefore aims to provide a method for coating by hot dipping in a zinc-based liquid bath, an iron-carbon-manganese steel strip running through a coating based on zinc.

A cet effet, l'invention a pour objet un procédé de revêtement au trempé à chaud dans un bain liquide à base de zinc comprenant de l'aluminium, ledit bain ayant une température T2, d'une bande en acier austénitique fer-carbone-manganèse comprenant : 0,30% ≤ C ≤ 1,05%, 16%≤ Mn ≤ 26%, Si ≤ 1%, et AI ≤ 0,050%, les teneurs étant exprimées en poids, ledit procédé comprenant les étapes consistant à :

  • faire subir à ladite bande un traitement thermique dans un four à l'intérieur duquel règne une atmosphère réductrice vis-à-vis du fer, ledit traitement thermique comprenant une phase de chauffage à une vitesse de chauffage V1, une phase de maintien à une température T1 et pendant un temps de maintien M, suivi d'une phase de refroidissement à une vitesse de refroidissement V2, pour obtenir une bande couverte sur ses deux faces d'une sous-couche continue d'oxyde mixte de fer et de manganèse (Fe, Mn)O amorphe, et d'une couche externe continue ou discontinue d'oxyde de manganèse MnO cristallin, puis
  • faire défiler ladite bande couverte des couches d'oxyde dans ledit bain pour la revêtir par un revêtement à base de zinc, la teneur en aluminium dans ledit bain étant ajustée à une valeur au moins égale à la teneur nécessaire pour que l'aluminium réduise complètement la couche d'oxyde de manganèse MnO cristallin et au moins partiellement la couche d'oxyde (Fe,Mn)O amorphe, de manière à former à la surface de la bande ledit revêtement comprenant trois couches d'alliage fer-manganèse-zinc et une couche superficielle de zinc.
To this end, the subject of the invention is a method of coating by hot quenching in a zinc-based liquid bath comprising aluminum, said bath having a temperature T2, of an austenitic steel-carbon steel strip. manganese comprising: 0.30% ≤ C ≤ 1.05%, 16% ≤ Mn ≤ 26%, Si ≤ 1%, and AI ≤ 0.050%, the contents being expressed by weight, said process comprising the steps of:
  • subjecting said strip to heat treatment in an oven inside which there is a reducing atmosphere with respect to the iron, said heat treatment comprising a heating phase at a heating rate V1, a holding phase at a temperature T1 and during a holding time M, followed by a cooling phase at a cooling rate V2, to obtain a strip covered on both sides with a continuous undercoat of mixed iron oxide and manganese (Fe, Mn) O amorphous, and a continuous or discontinuous outer layer of crystalline MnO manganese oxide, and then
  • scrolling said covered strip of oxide layers in said bath to coat it with a zinc-based coating, the aluminum content in said bath being adjusted to a value at least equal to the amount necessary for the aluminum to reduce completely the crystalline MnO manganese oxide layer and at least partially the amorphous oxide (Fe, Mn) O layer, so as to form on the surface of the strip said coating comprising three layers of iron-manganese-zinc alloy and a superficial layer of zinc.

L'invention a également pour objet la bande en acier austénitique fer-carbone-manganèse revêtue par un revêtement à base de zinc pouvant être obtenue par ce procédé.The invention also relates to the austenitic iron-carbon-manganese steel strip coated with a zinc-based coating obtainable by this method.

Les caractéristiques et avantages de la présente invention apparaîtront mieux au cours de la description qui va suivre, donnée à titre d'exemple non limitatif.The features and advantages of the present invention will appear better in the following description given by way of non-limiting example.

Les inventeurs ont ainsi mis en évidence qu'en créant des conditions favorables pour que la bi-couche d'oxyde mixte (Fe,Mn)O et d'oxyde de manganèse se formant à la surface de la bande en acier fer-carbone-manganèse, soit réduite par l'aluminium contenu dans le bain liquide à base de zinc, la surface de la bande devenait mouillante vis à vis du zinc, ce qui permettait de la revêtir par un revêtement à base de zinc.The inventors have thus demonstrated that by creating favorable conditions for the bi-layer of mixed oxide (Fe, Mn) O and of manganese oxide forming on the surface of the iron-carbon steel strip. manganese, being reduced by the aluminum contained in the zinc-based liquid bath, the surface of the strip became wetting with respect to the zinc, which allowed to coat it with a coating based on zinc.

L'épaisseur de cette bande en acier est typiquement comprise entre 0,2 et 6 mm, et peut être issue soit du train à bandes à chaud, soit du train à bandes à froid.The thickness of this steel strip is typically between 0.2 and 6 mm, and may be issued from either the hot band or the cold band train.

L'acier austénitique fer-carbone-manganèse mis en oeuvre selon l'invention comprend, en % en poids : 0,30% ≤ C ≤ 1,05%, 16%≤ Mn ≤ 26%, Si ≤ 1%, Al ≤ 0,050%, S ≤ 0,030%, P≤ 0,080%, N ≤ 0,1%, et à titre optionnel, un ou plusieurs éléments tels que : Cr ≤ 1%, Mo ≤ 0,40%, Ni ≤ 1%, Cu ≤ 5%, Ti ≤ 0,50%, Nb ≤ 0,50%, V ≤ 0,50%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration.The austenitic iron-carbon-manganese steel used according to the invention comprises, in% by weight: 0.30% ≤ C ≤ 1.05%, 16% ≤ Mn ≤ 26%, Si ≤ 1%, Al ≤ 0.050%, S ≤ 0.030%, P ≤ 0.080%, N ≤ 0.1%, and optionally, one or more elements such as: Cr ≤ 1%, Mo ≤ 0.40%, Ni ≤ 1%, Cu ≤ 5%, Ti ≤ 0.50%, Nb ≤ 0.50%, V ≤ 0.50%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation.

Le carbone joue un rôle très important sur la formation de la microstructure : il augmente l'énergie de défaut d'empilement et favorise la stabilité de la phase austénitique. En combinaison avec une teneur en manganèse allant de 16 à 26% en poids, cette stabilité est obtenue pour une teneur en carbone supérieure ou égale à 0,30%. Cependant, pour une teneur en carbone supérieure à 1,05% il devient difficile d'éviter une précipitation de carbures qui intervient au cours de certains cycles thermiques de fabrication industrielle, en particulier lors du refroidissement au bobinage, et qui dégrade la ductilité et la ténacité.Carbon plays a very important role in the formation of the microstructure: it increases the stacking fault energy and promotes the stability of the austenitic phase. In combination with a manganese content ranging from 16 to 26% by weight, this stability is obtained for a carbon content greater than or equal to 0.30%. However, for a carbon content greater than 1.05%, it becomes difficult to avoid a precipitation of carbides which occurs during certain thermal cycles of industrial manufacture, in particular during winding cooling, and which degrades the ductility and tenacity.

De préférence, la teneur en carbone est comprise entre 0,40 et 0,70% en poids. En effet, lorsque la teneur en carbone est comprise entre 0,40% et 0,70%, la stabilité de l'austénite est accrue et la résistance est augmentée.Preferably, the carbon content is between 0.40 and 0.70% by weight. Indeed, when the carbon content is between 0.40% and 0.70%, the stability of the austenite is increased and the strength is increased.

Le manganèse est également un élément indispensable pour accroître la résistance, augmenter l'énergie de défaut d'empilement et stabiliser la phase austénitique. Si sa teneur est inférieure à 16%, il existe un risque de formation de phases martensitiques qui diminuent très notablement l'aptitude à la déformation. Par ailleurs, lorsque la teneur en manganèse est supérieure à 26%, la ductilité à température ambiante est dégradée. De plus, pour des questions de coût, il n'est pas souhaitable que la teneur en manganèse soit élevée.Manganese is also an essential element for increasing strength, increasing stacking fault energy and stabilizing the austenitic phase. If its content is less than 16%, there is a risk of formation of martensitic phases which significantly reduce the ability to deform. On the other hand, when the manganese content is greater than 26%, the ductility at room temperature is degraded. In addition, for cost reasons, it is not desirable for the manganese content to be high.

De préférence, la teneur en manganèse dans l'acier selon l'invention est comprise entre 20 et 25% en poids.Preferably, the manganese content in the steel according to the invention is between 20 and 25% by weight.

Le silicium est un élément efficace pour désoxyder l'acier ainsi que pour durcir en phase solide. Cependant, au-delà d'une teneur de 1 %, il se forme à la surface de l'acier des couches Mn2SiO4 et SiO2 qui montrent une aptitude à la réduction par l'aluminium contenu dans le bain à base de zinc nettement inférieure aux couches d'oxyde mixte (Fe,Mn)O et d'oxyde de manganèse MnO.Silicon is an effective element for deoxidizing steel as well as for hardening in the solid phase. However, beyond a content of 1%, Mn 2 SiO 4 and SiO 2 layers are formed on the surface of the steel, which show a reduction ability of the aluminum contained in the water-based bath. zinc significantly lower than the mixed oxide (Fe, Mn) O and manganese oxide MnO layers.

De préférence, la teneur en silicium dans l'acier est inférieure à 0,5 % en poids.Preferably, the silicon content in the steel is less than 0.5% by weight.

L'aluminium est également un élément particulièrement efficace pour la désoxydation de l'acier. Comme le carbone, il augmente l'énergie de défaut d'empilement. Cependant, sa présence excessive dans des aciers à forte teneur en manganèse présente un inconvénient: En effet, le manganèse augmente la solubilité de l'azote dans le fer liquide, et si une quantité d'aluminium trop importante est présente dans l'acier, l'azote se combinant avec l'aluminium précipite sous forme de nitrures d'aluminium gênant la migration des joints de grains lors de la transformation à chaud et augmente très notablement le risque d'apparitions de fissures. Une teneur en Al inférieure ou égale à 0,050 % permet d'éviter une précipitation d'AIN. Corrélativement, la teneur en azote doit être inférieure ou égale à 0,1% afin d'éviter cette précipitation et la formation de défauts volumiques (soufflures) lors de la solidification.Aluminum is also a particularly effective element for the deoxidation of steel. Like carbon, it increases the stacking fault energy. However, its excessive presence in steels with a high manganese content has a disadvantage: In fact, manganese increases the solubility of nitrogen in the liquid iron, and if too much aluminum is present in the steel, Nitrogen combined with aluminum precipitates in the form of aluminum nitrides hindering the migration of grain boundaries during hot processing and greatly increases the risk of crack appearances. An Al content less than or equal to 0.050% makes it possible to avoid a precipitation of AlN. Correlatively, the nitrogen content must be less than or equal to 0.1% in order to prevent this precipitation and the formation of volume defects (blowholes) during solidification.

En outre au-delà de 0,050% en poids d'aluminium, on commence à former, lors du recuit de recristallisation de l'acier, des oxydes tels que MnAl2O4, MnO.Al2O3 qui sont plus difficilement réduits par l'aluminium contenu dans le bain de revêtement à base de zinc, que les oxydes (Fe,Mn)O et MnO. En effet, ces oxydes comprenant de l'aluminium sont beaucoup plus stables que les oxydes (Fe,Mn)O et MnO. Par conséquent, même si on parvient à former à la surface de l'acier un revêtement à base de zinc, celui-ci sera de toute façon peu adhérent à cause de la présence d'alumine. Ainsi, pour obtenir une bonne adhérence du revêtement à base de zinc, il est essentiel que la teneur en aluminium dans l'acier soit inférieure à 0,050% en poids.In addition, in excess of 0.050% by weight of aluminum, oxides such as MnAl 2 O 4 , MnO.Al 2 O 3, which are more difficult to reduce, are formed during the recrystallization annealing of the steel. aluminum contained in the zinc-based coating bath, as oxides (Fe, Mn) O and MnO. Indeed, these oxides comprising aluminum are much more stable than oxides (Fe, Mn) O and MnO. Therefore, even if it is possible to form on the surface of the steel a zinc-based coating, it will in any case little adherent because of the presence of alumina. Thus, to achieve good adhesion of the zinc-based coating, it is essential that the aluminum content in the steel is less than 0.050% by weight.

Le soufre et le phosphore sont des impuretés fragilisant les joints de grains. Leur teneur respective doit être inférieure ou égale à 0,030 et 0,080% afin de maintenir une ductilité à chaud suffisante.Sulfur and phosphorus are impurities that weaken the grain boundaries. Their respective content must be less than or equal to 0.030 and 0.080% in order to maintain sufficient hot ductility.

Le chrome et le nickel peuvent être utilisés à titre optionnel pour augmenter la résistance de l'acier par durcissement en solution solide. Cependant, le chrome diminuant l'énergie de défaut d'empilement, sa teneur doit être inférieure ou égale à 1 %. Le nickel contribue à obtenir un allongement à rupture important, et augmente en particulier la ténacité. Cependant, il est également souhaitable, pour des questions de coûts, de limiter la teneur en nickel à une teneur maximale inférieure ou égale à 1 %. Pour des raisons similaires, le molybdène peut être ajouté en quantité inférieure ou égale à 0,40%.Chromium and nickel can be used as an option to increase the strength of the steel by hardening in solid solution. However, since chromium decreases the stacking fault energy, its content must be less than or equal to 1%. Nickel contributes to a significant elongation rupture, and in particular increases the toughness. However, it is also desirable, for cost reasons, to limit the nickel content to a maximum content of less than or equal to 1%. For reasons similar, the molybdenum may be added in an amount less than or equal to 0.40%.

De même, à titre optionnel, une addition de cuivre jusqu'à une teneur inférieure ou égale à 5% est un moyen de durcir l'acier par précipitation de cuivre métallique. Cependant, au-delà de cette teneur, le cuivre est responsable de l'apparition de défauts de surface en tôle à chaud.Similarly, optionally, addition of copper to a content of less than or equal to 5% is a means of hardening the steel by precipitation of metallic copper. However, beyond this content, copper is responsible for the appearance of surface defects hot sheet.

Le titane, le niobium et le vanadium sont également des éléments pouvant être utilisés optionnellement pour obtenir un durcissement par précipitation de carbonitrures. Cependant, lorsque la teneur en Nb ou en V, ou en Ti est supérieure à 0,50%, une précipitation excessive de carbonitrures peut provoquer une réduction de la ténacité, ce qui doit être évité.Titanium, niobium and vanadium are also elements that can optionally be used to obtain precipitation hardening of carbonitrides. However, when the Nb or V, or Ti content is greater than 0.50%, excessive precipitation of carbonitrides can cause a reduction in toughness, which should be avoided.

Après avoir été laminée à froid, la bande en acier austénitique fer-carbone-manganèse subit un traitement thermique afin de recristalliser l'acier. Le recuit de recristallisation permet de conférer à l'acier une microstructure homogène, d'améliorer ses caractéristiques mécaniques, et en particulier de lui redonner de la ductilité pour permettre son utilisation en emboutissage.After being cold rolled, the austenitic iron-carbon-manganese steel strip undergoes heat treatment to recrystallize the steel. The recrystallization annealing makes it possible to give the steel a homogeneous microstructure, to improve its mechanical characteristics, and in particular to give it ductility to allow its use in stamping.

Ce traitement thermique est réalisé dans un four à l'intérieur duquel règne une atmosphère composée d'un gaz réducteur vis-à-vis du fer, pour éviter toute oxydation excessive de la surface de la bande, et permettre un bon accrochage du zinc. Ce gaz est choisi parmi l'hydrogène, et les mélanges azote - hydrogène. De préférence, on choisit les mélange gazeux comprenant entre 20 et 97% en volume d'azote et entre 3 et 80% en volume d'hydrogène, et plus préférentiellement entre 85 et 95% en volume d'azote et entre 5 et 15% en volume d'hydrogène. En effet, bien que l'hydrogène soit un excellent agent réducteur du fer, on préfère limiter sa concentration en raison de son coût élevé par rapport à l'azote. En faisant régner dans l'enceinte du four une atmosphère réductrice vis-à-vis du fer, on évite ainsi la formation d'une couche épaisse de calamine, c'est à dire dont l'épaisseur est largement supérieure à 100 nm. Dans le cas des aciers fer-carbone-manganèse, la calamine est une couche d'oxyde de fer comprenant une faible proportion de manganèse. Or non seulement cette couche de calamine empêche toute adhérence du zinc sur l'acier, mais aussi c'est une couche qui a tendance à se fissurer facilement ce qui la rend d'autant plus indésirable.This heat treatment is performed in an oven inside which there is an atmosphere composed of a reducing gas vis-à-vis the iron, to avoid excessive oxidation of the surface of the strip, and allow good adhesion of zinc. This gas is selected from hydrogen, and nitrogen-hydrogen mixtures. Preferably, the gaseous mixtures comprising between 20 and 97% by volume of nitrogen and between 3 and 80% by volume of hydrogen, and more preferably between 85 and 95% by volume of nitrogen and between 5 and 15%, are chosen. in volume of hydrogen. Indeed, although hydrogen is an excellent iron reducing agent, it is preferred to limit its concentration because of its high cost relative to nitrogen. By making prevail in the enclosure of the furnace a reducing atmosphere vis-à-vis the iron, thus prevents the formation of a thick layer of calamine, that is to say whose thickness is much greater than 100 nm. In the case of iron-carbon-manganese steels, calamine is a layer of iron oxide comprising a small proportion of manganese. Gold not only this calamine layer prevents any adhesion of zinc on steel, but also it is a layer that tends to crack easily which makes it all the more undesirable.

Dans les conditions industrielles, l'atmosphère régnant dans le four est certes réductrice vis-à-vis du fer, mais pas pour des éléments comme le manganèse. En effet, le gaz constituant l'atmosphère régnant dans le four comprend des traces d'humidité et/ou d'oxygène qui ne peuvent être évitées, mais qu'il est possible de contrôler en imposant le point de rosée dudit gaz.Under industrial conditions, the atmosphere prevailing in the furnace is certainly reducing with respect to iron, but not for elements such as manganese. Indeed, the gas constituting the atmosphere in the furnace comprises traces of moisture and / or oxygen that can not be avoided, but it is possible to control by imposing the dew point of said gas.

Ainsi, les inventeurs ont observé, que selon l'invention, à l'issue du recuit de recristallisation, plus le point de rosée dans le four est bas, ou autrement dit plus la pression partielle d'oxygène est basse, plus la couche d'oxyde de manganèse formée à la surface de la bande en acier fer-carbone-manganèse est fine. Cette observation peut paraître en désaccord avec la théorie de Wagner selon laquelle plus le point de rosée est bas, plus la densité d'oxydes formée à la surface d'une bande en acier au carbone est élevée. En effet, lorsque la quantité d'oxygène diminue à la surface de l'acier au carbone, la migration des éléments oxydables contenus dans l'acier vers la surface s'accélère, ce qui favorise l'oxydation de la surface. Sans vouloir être lié par une quelconque théorie, les inventeurs pensent que dans le cas de l'invention, la couche d'oxyde (Fe,Mn)O amorphe devient rapidement continue. Elle constitue donc une barrière pour l'oxygène de l'atmosphère régnant dans le four, qui n'est plus en contact direct avec l'acier. Une augmentation de la pression partielle d'oxygène dans le four conduit donc à une augmentation de l'épaisseur de l'oxyde de manganèse et ne provoque pas d'oxydation interne, c'est à dire qu'on n'observe pas de couche d'oxyde supplémentaire entre la surface de l'acier austénitique fer-carbone-manganèse et la couche d'oxyde amorphe (Fe,Mn)O.Thus, the inventors have observed that, according to the invention, at the end of the recrystallization annealing, the lower the dew point in the oven, or in other words the lower the oxygen partial pressure, the lower the The manganese oxide formed on the surface of the iron-carbon-manganese steel strip is thin. This observation may seem at odds with Wagner's theory that the lower the dew point, the higher the density of oxides formed on the surface of a carbon steel strip. Indeed, when the amount of oxygen decreases on the surface of the carbon steel, the migration of the oxidizable elements contained in the steel to the surface accelerates, which promotes the oxidation of the surface. Without wishing to be bound by any theory, the inventors believe that in the case of the invention, the amorphous oxide layer (Fe, Mn) O becomes rapidly continuous. It therefore constitutes a barrier for the oxygen of the atmosphere in the furnace, which is no longer in direct contact with the steel. An increase in the oxygen partial pressure in the furnace therefore leads to an increase in the thickness of the manganese oxide and does not cause internal oxidation, ie no layer is observed. additional oxide between the surface of the austenitic iron-carbon-manganese steel and the amorphous oxide layer (Fe, Mn) O.

Le recuit de recristallisation réalisé dans les conditions de l'invention permet ainsi de former sur les deux faces de la bande une sous-couche continue d'oxyde mixte de fer et de manganèse (Fe,Mn)O amorphe dont l'épaisseur est de préférence comprise entre 5 et 10 nm, et une couche externe continue ou discontinue d'oxyde de manganèse MnO cristallin dont l'épaisseur est de préférence comprise entre 5 et 90 nm, avantageusement entre 5 et 50 nm, et plus préférentiellement entre 10 et 40 nm. La couche externe MnO présente un aspect granuleux, et la taille des cristaux de MnO augmente fortement lorsque le point de rosée augmente également. En effet, leur diamètre moyen varie d'environ 50 nm pour un point de rosée de -80°C, la couche de MnO étant alors discontinue, jusqu'à 300 nm pour un point de rosée de +10°C, la couche de MnO étant dans ce cas continue.The recrystallization annealing carried out under the conditions of the invention thus makes it possible to form on both sides of the strip a continuous sub-layer of mixed oxide of iron and manganese (Fe, Mn) O amorphous, the thickness of which is preferably between 5 and 10 nm, and a continuous or discontinuous outer layer of crystalline MnO manganese oxide whose thickness is preferably between 5 and 90 nm, preferably between 5 and 50 nm, and more preferably between 10 and 40 nm. The MnO outer layer has a granular appearance, and the size of the MnO crystals increases sharply as the dew point also increases. Indeed, their mean diameter varies from approximately 50 nm for a dew point of -80 ° C, the MnO layer being then discontinuous, up to 300 nm for a dew point of + 10 ° C, the MnO layer being in this case continues.

Les inventeurs ont mis en évidence que, lorsque la teneur en poids en aluminium dans le bain liquide à base de zinc est inférieure à 0,18% et lorsque la couche d'oxyde de manganèse MnO est supérieure à 100 nm, celle-ci n'est pas réduite par l'aluminium contenu dans le bain, et le revêtement à base de zinc ne s'obtient pas en raison de l'effet non mouillant de MnO vis-à-vis du zinc.The inventors have demonstrated that, when the content by weight of aluminum in the zinc-based liquid bath is less than 0.18% and when the manganese oxide layer MnO is greater than 100 nm, the latter It is not reduced by the aluminum contained in the bath, and the zinc-based coating is not obtained due to the non-wetting effect of MnO with respect to the zinc.

A cet effet, le point de rosée selon l'invention, au moins dans la zone de maintien en température du four, et préférentiellement dans toute l'enceinte du four, est de préférence compris entre -80 et 20°C, avantageusement entre - 80 et -40°C et plus préférentiellement entre -60 et -40°C.For this purpose, the dew point according to the invention, at least in the zone for maintaining the temperature of the oven, and preferably in the entire enclosure of the oven, is preferably between -80 and 20 ° C, advantageously between - 80 and -40 ° C and more preferably between -60 and -40 ° C.

En effet, dans les conditions industrielles usuelles on parvient dans des conditions particulières à baisser le point de rosée d'un four de recuit de recristallisation à une valeur inférieure à -60°C, mais pas en dessous de - 80°C.Indeed, under the usual industrial conditions it is possible in particular conditions to lower the dew point of a recrystallization annealing furnace to a value below -60 ° C., but not below -80 ° C.

Au-delà de 20°C, l'épaisseur de la couche d'oxyde de manganèse devient trop importante pour être réduite par l'aluminium contenu dans le bain liquide à base de zinc dans les conditions industrielles, c'est à dire pendant un temps inférieur à 10 secondes.Above 20 ° C., the thickness of the manganese oxide layer becomes too great to be reduced by the aluminum contained in the zinc-based liquid bath under industrial conditions, that is to say during a period of time. time less than 10 seconds.

La fourchette -60 à -40°C est avantageuse, car elle permet de former une bi-couche d'oxydes d'épaisseur relativement réduite qui sera facilement réduite par l'aluminium contenu dans le bain à base de zinc.The range -60 to -40 ° C is advantageous because it allows to form a bi-oxide layer of relatively reduced thickness which will be easily reduced by the aluminum contained in the zinc-based bath.

Le traitement thermique comprend une phase de chauffage à une vitesse de chauffage V1, une phase de maintien à une température T1 et pendant un temps de maintien M, suivi d'une phase de refroidissement à une vitesse de refroidissement V2.The heat treatment comprises a heating phase at a heating rate V1, a holding phase at a temperature T1 and during a holding time M, followed by a cooling phase at a cooling rate V2.

Le traitement thermique est réalisé de préférence à une vitesse de chauffage V1 supérieure ou égale à 6°C/s, car en dessous de cette valeur le temps de maintien M de la bande dans le four est trop long et ne correspond pas aux exigences industrielles de productivité.The heat treatment is preferably carried out at a heating rate V1 greater than or equal to 6 ° C / s, because below this value the holding time M of the strip in the oven is too long and does not correspond to the industrial requirements. of productivity.

La température T1 est de préférence comprise entre 600 et 900°C. En effet, en dessous de 600°C, l'acier ne sera pas complètement recristallisé et ses caractéristiques mécaniques seront insuffisantes. Au-delà de 900°C, non seulement la taille des grains de l'acier augmente ce qui est néfaste pour l'obtention de bonnes caractéristiques mécaniques, mais aussi l'épaisseur de la couche d'oxyde de manganèse MnO augmente fortement et rend difficile, voir impossible, le dépôt ultérieur d'un revêtement à base de zinc, car l'aluminium contenu dans le bain n'aura pas complètement réduit le MnO. Plus la température T1 est basse, plus la quantité de MnO formée sera faible, et plus il sera facile de le réduire par l'aluminium, c'est pourquoi T1 est de préférence comprise entre 600 et 820°C, avantageusement inférieure ou égale à 750°C, et plus préférentiellement comprise entre 650 et 750°C.The temperature T1 is preferably between 600 and 900 ° C. Indeed, below 600 ° C, the steel will not be completely recrystallized and its mechanical characteristics will be insufficient. Beyond 900 ° C, not only the grain size of the steel increases which is harmful for obtaining good mechanical characteristics, but also the thickness of the manganese oxide layer MnO strongly increases and makes difficult, if not impossible, the subsequent deposition of a coating based on zinc, because the aluminum contained in the bath will not have completely reduced the MnO. The lower the temperature T1, the lower the amount of MnO formed, and the easier it will be to reduce it with aluminum, that is why T1 is preferably between 600 and 820 ° C., advantageously less than or equal to 750 ° C, and more preferably between 650 and 750 ° C.

Le temps de maintien M est de préférence compris entre 20 s et 60 s, et avantageusement compris entre 20 et 40 s. Le recuit de recristallisation est généralement réalisé par un dispositif de chauffage à tubes radiants.The holding time M is preferably between 20 s and 60 s, and advantageously between 20 and 40 s. The recrystallization annealing is generally carried out by a radiant tube heater.

De préférence, la bande est refroidie jusqu'à une température d'immersion de la bande T3 comprise entre (T2 - 10°C) et (T2 + 30°C), T2 étant défini comme étant la température du bain liquide à base de zinc. En refroidissant la bande à une température T3 voisine de la température T2 du bain, on évite de refroidir ou de réchauffer le zinc liquide au voisinage de la bande en défilement dans le bain, ce qui permet de former sur la bande un revêtement à base de zinc ayant une structure homogène tout le long de la bande.Preferably, the strip is cooled to an immersion temperature of the T3 band between (T2 - 10 ° C) and (T2 + 30 ° C), T2 being defined as the temperature of the liquid bath based on zinc. By cooling the strip to a temperature T3 close to the temperature T2 of the bath, it is avoided to cool or heat the liquid zinc in the vicinity of the strip running in the bath, which allows to form on the strip a coating based on zinc having a homogeneous structure all along the strip.

La bande est préférentiellement refroidie à une vitesse de refroidissement V2 supérieure ou égale à 3°C/s, avantageusement supérieure à 10°C/s, de manière à éviter le grossissement des grains et à obtenir une bande en acier présentant de bonnes caractéristiques mécaniques. Ainsi, la bande est généralement refroidie par injection d'un flux d'air sur ses deux faces.The strip is preferably cooled at a cooling rate V2 of greater than or equal to 3 ° C./s, advantageously greater than 10 ° C./s, so as to avoid the enlargement of the grains and to obtain a steel strip having good mechanical characteristics. . Thus, the strip is generally cooled by injection of an air flow on both sides.

Lorsque, après avoir subi le recuit de recristallisation, la bande en acier austénitique fer-carbone-manganèse est couverte sur ses deux faces par la bi-couche d'oxydes, on la fait défiler dans le bain liquide à base de zinc contenant de l'aluminium.When, after undergoing recrystallization annealing, the austenitic iron-carbon-manganese steel strip is covered on both sides by the two-layer oxide, it is passed through the zinc-based liquid bath containing 'aluminum.

L'aluminium contenu dans le bain de zinc contribue non seulement à la réduction au moins partielle la bi-couche d'oxydes, mais aussi à l'obtention d'un revêtement présentant un aspect de surface homogène.The aluminum contained in the zinc bath contributes not only to the at least partial reduction of the two-layer oxide, but also to obtaining a coating having a homogeneous surface appearance.

Un aspect de surface homogène est caractérisé par une épaisseur uniforme, alors qu'un aspect hétérogène est caractérisé par de fortes hétérogénéités d'épaisseur. Contrairement à ce qui se passe pour les aciers au carbone, il ne se forme pas de couche inter-faciale de type Fe2Al5 et/ou FeAl3 à la surface de l'acier fer-carbone-manganèse, ou si celle-ci se forme, elle est immédiatement détruite par la formation des phases (Fe,Mn) Zn. Cependant, on retrouve des mattes de type Fe2Al5 et/ou FeAl3 dans le bain.A homogeneous surface appearance is characterized by a uniform thickness, whereas a heterogeneous appearance is characterized by strong thickness heterogeneities. In contrast to carbon steels, Fe 2 Al 5 and / or FeAl 3 inter-facial layer is not formed on the surface of iron-carbon-manganese steel, or if it is formed, it is immediately destroyed by the formation of the phases (Fe, Mn) Zn. However, matts of Fe 2 Al 5 and / or FeAl 3 type are found in the bath.

La teneur en aluminium dans le bain est ajustée à une valeur au moins égale à la teneur nécessaire pour que l'aluminium réduise complètement la couche d'oxyde de manganèse MnO cristallin et au moins partiellement la couche d'oxyde (Fe,Mn)O amorphe.The aluminum content in the bath is adjusted to a value at least equal to the content necessary for the aluminum to completely reduce the crystalline MnO manganese oxide layer and at least partially the oxide (Fe, Mn) O layer. amorphous.

A cet effet, la teneur en poids de l'aluminium dans le bain est comprise entre 0,15 et 5%. En dessous de 0,15%, la teneur en aluminium sera insuffisante pour réduire complètement la couche d'oxyde de manganèse MnO et au moins partiellement la couche de (Fe,Mn)O, et la surface de la bande en acier ne présentera pas une mouillabilité suffisante vis à vis du zinc. Au-delà de 5% d'aluminium dans le bain, il se formera à la surface de la bande en acier un revêtement d'un type différent de celui qui est obtenu par l'invention. Ce revêtement comprendra une proportion croissante d'aluminium à mesure que la teneur en aluminium dans le bain augmente.For this purpose, the weight content of aluminum in the bath is between 0.15 and 5%. Below 0.15%, the aluminum content will be insufficient to completely reduce the manganese oxide layer MnO and at least partially the layer of (Fe, Mn) O, and the surface of the steel strip will not present sufficient wettability with respect to zinc. Above 5% of aluminum in the bath, a coating of a type different from that obtained by the invention will form on the surface of the steel strip. This coating will include an increasing proportion of aluminum as the aluminum content in the bath increases.

Outre l'aluminium, le bain à base de zinc peut également contenir du fer, de préférence à une teneur telle qu'il soit en sursaturation vis-à vis de Fe2Al5 et/ou FeAl3.In addition to aluminum, the zinc-based bath may also contain iron, preferably at a content such that it is supersaturation with respect to Fe 2 Al 5 and / or FeAl 3 .

Pour maintenir le bain à l'état liquide, il est porté à une température T2 de préférence supérieure ou égale à 430°C, mais pour éviter toute évaporation excessive de zinc, T2 est inférieure ou égale à 480°C.To maintain the bath in the liquid state, it is brought to a temperature T2 preferably greater than or equal to 430 ° C, but to avoid excessive evaporation of zinc, T2 is less than or equal to 480 ° C.

La bande est en contact avec le bain pendant un temps de contact C compris de préférence entre 2 et 10 secondes, et plus préférentiellement entre 3 et 5 secondes.The strip is in contact with the bath for a contact time C preferably between 2 and 10 seconds, and more preferably between 3 and 5 seconds.

En dessous de 2 secondes, l'aluminium n'a pas suffisamment de temps pour réduire complètement la couche d'oxyde de manganèse MnO et au moins partiellement la couche d'oxyde mixte (Fe,Mn)O, et rendre ainsi la surface de l'acier mouillante vis-à-vis du zinc. Au-dessus de 10 secondes, la bi-couche d'oxydes sera certes complètement réduite, cependant la vitesse de ligne risque d'être industriellement trop basse, et le revêtement trop allié et ensuite difficile à ajuster en épaisseur.Below 2 seconds, the aluminum does not have enough time to completely reduce the MnO layer of manganese oxide and at least partially the mixed oxide layer (Fe, Mn) O, and thus to make the surface of wetting steel vis-à-vis zinc. Above 10 seconds, the two-layer oxides will certainly be completely reduced, however the line speed may be industrially too low, and the coating too alloyed and then difficult to adjust in thickness.

Ces conditions permettent de revêtir la bande sur ses deux faces par un revêtement à base de zinc comportant dans l'ordre à partir de l'interface acier/revêtement une couche d'alliage fer-manganèse-zinc composée de deux phases cubique Γ et cubique à face centrée Γ1, une couche d'alliage fer-manganèse-zinc δ 1 de structure hexagonale, une couche d'alliage fer-manganèse-zinc ζ de structure monoclinique, et une couche superficielle de zinc.These conditions make it possible to coat the strip on both sides with a zinc-based coating comprising, in order from the steel / coating interface, a layer of iron-manganese-zinc alloy composed of two cube and cubic phases. with centered face Γ1, a layer of iron-manganese-zinc alloy δ 1 of hexagonal structure, a layer of iron-manganese-zinc alloy ζ of monoclinic structure, and a surface layer of zinc.

Les inventeurs ont ainsi vérifié que selon l'invention, et contrairement à ce qui se passe dans le cas d'un revêtement d'une bande en acier au carbone dans un bain à base de zinc contenant de l'aluminium, il ne se forme pas de couche Fe2Al5 à l'interface acier/revêtement. Selon l'invention, l'aluminium du bain réduit la bi-couche d'oxyde. Or la couche de MnO est plus facilement réductible par l'aluminium du bain que les couches d'oxydes à base de silicium. Il en résulte un appauvrissement local en aluminium qui conduit à la formation d'un revêtement comprenant des phases FeZn au lieu et place du revêtement Fe2Al5(Zn) attendu, et qui se forme dans le cas des aciers au carbone.The inventors have thus verified that according to the invention, and contrary to what happens in the case of a coating of a carbon steel strip in a zinc bath containing aluminum, it is not formed. no Fe 2 Al 5 layer at the steel / coating interface. According to the invention, the aluminum of the bath reduces the bi-oxide layer. However, the MnO layer is more easily reducible by the aluminum of the bath than the oxide layers based on silicon. This results in a local depletion of aluminum which leads to the formation of a coating comprising FeZn phases instead of the expected Fe 2 Al 5 (Zn) coating, which is formed in the case of carbon steels.

Pour améliorer la soudabilité de la bande revêtue par le revêtement à base de zinc comprenant trois couches d'alliage fer-manganèse-zinc et une couche superficielle de zinc selon l'invention, on la soumet à un traitement thermique d'alliation de manière à allier complètement ledit revêtement. On obtient ainsi une bande revêtue sur ses deux faces par un revêtement à base de zinc comportant dans l'ordre à partir de l'interface acier/revêtement une couche d'alliage fer-manganèse-zinc composée de deux phases cubique Γ et cubique à face centrée Γ1, une couche d'alliage fer-manganèse-zinc δ 1 de structure hexagonale, et éventuellement une couche d'alliage fer-manganèse-zinc ζ de structure monoclinique.To improve the weldability of the coated strip by the zinc-based coating comprising three layers of iron-manganese-zinc alloy and a surface layer of zinc according to the invention, it is subjected to a heat treatment of alloying so as to completely combine said coating. Thus, a strip coated on both sides by a zinc-based coating comprising in order from the steel / coating interface a layer of iron-manganese-zinc alloy composed of two cubic phase Γ and cubic to centered face Γ1, an iron-manganese-zinc alloy layer δ 1 of hexagonal structure, and possibly a layer of iron-manganese-zinc alloy ζ of monoclinic structure.

En outre, les inventeurs ont mis en évidence que ces composés (Fe,Mn)Zn sont favorables à l'adhérence de la peinture.In addition, the inventors have demonstrated that these compounds (Fe, Mn) Zn are favorable to the adhesion of the paint.

Le traitement thermique d'alliation est de préférence réalisé directement à la sortie du bain de zinc, à une température comprise entre 490 et 540°C, pendant une durée comprise entre 2 et 10 secondes.The alloying heat treatment is preferably carried out directly at the outlet of the zinc bath, at a temperature of between 490 and 540 ° C., for a duration of between 2 and 10 seconds.

L'invention va à présent être illustrée par des exemples donnés à titre indicatif, et non limitatif, et en référence aux figures annexées sur lesquelles :

  • les figures 1, 2 et 3 sont des photographies de la surface d'une bande en acier austénitique fer-carbone-manganèse ayant subi un recuit avec respectivement un point de rosée de -80°C, de -45°C et de +10°C, dans les conditions décrites ci-dessous,
  • la figure 4 est une micrographie MEB montrant en coupe transversale la bi-couche d'oxyde formée sur un acier austénitique fer-carbone-manganèse après recuit de recristallisé avec un point de rosée +10°C, dans les conditions décrites ci-dessous,
  • la figure 5 est une micrographie MEB montrant en coupe transversale le revêtement à base de zinc formé après immersion dans un bain de zinc comprenant 0,18% en poids d'aluminium, sur un acier austénitique fer-carbone-manganèse recuit avec un point de rosée -80°C, dans les conditions décrites ci-dessous.
The invention will now be illustrated by examples given for information only, and not limiting, and with reference to the appended figures in which:
  • the Figures 1, 2 and 3 are photographs of the surface of an annealed iron-carbon-manganese austenitic steel strip with respectively a dew point of -80 ° C, -45 ° C and + 10 ° C, under the conditions described below,
  • the figure 4 is an SEM micrograph showing in cross-section the oxide bi-layer formed on an austenitic iron-carbon-manganese steel after recrystallized annealing with a dew point of + 10 ° C, under the conditions described below,
  • the figure 5 is a SEM micrograph showing in cross-section the zinc-based coating formed after immersion in a zinc bath comprising 0.18% by weight of aluminum, on an austenitic iron-carbon-manganese steel annealed with a dew point -80 ° C under the conditions described below.

1) Influence du point de rosée sur l'aptitude à la revêtabilité1) Influence of the dew point on the suitability for coating

Les essais ont été réalisés en utilisant des échantillons découpés dans une bande en acier austénitique fer-carbone-manganèse, qui après laminage à chaud et laminage à froid, présente une épaisseur de 0,7 mm. La composition chimique de cet acier est présentée dans le tableau 1, la teneur étant exprimé en % en poids. Tableau 1 Mn C Si Al S P Mo Cr 20,77 0,57 0,009 traces 0,008 0,001 0,001 0,049 The tests were carried out using samples cut from an iron-carbon-manganese austenitic steel strip, which after hot rolling and cold rolling has a thickness of 0.7 mm. The chemical composition of this steel is shown in Table 1, the content being expressed in% by weight. Table 1 mn VS Yes al S P MB Cr 20.77 0.57 0,009 footsteps 0,008 0,001 0,001 0.049

Les échantillons ont subi un recuit de recristallisation dans un four Infra Rouge dont on a fait varier le point de rosée (PR) de -80°C à +10°C, dans les conditions suivantes :

  • atmosphère gazeuse : azote + 15% en volume d'hydrogène
  • vitesse de chauffage V1 : 6°C/s
  • température de chauffage T1 : 810 °C
  • temps de maintien M : 42 s
  • vitesse de refroidissement V2 : 3°C/s
  • température d'immersion T3 : 480°C
The samples were recrystallized in an Infra Red oven whose dew point (PR) was varied from -80 ° C to + 10 ° C under the following conditions:
  • gaseous atmosphere: nitrogen + 15% by volume of hydrogen
  • heating rate V1: 6 ° C / s
  • heating temperature T1: 810 ° C
  • hold time M: 42 s
  • cooling rate V2: 3 ° C / sec
  • immersion temperature T3: 480 ° C

Dans ces conditions, l'acier est complètement recristallisé, et le tableau 2 présente les caractéristiques de la bi-couche d'oxyde comprenant une couche inférieure continue amorphe (Fe,Mn)O, et une couche supérieure MnO, formée sur les échantillons après le recuit en fonction du point de rosée. Tableau 2 PR -80°C PR -45°C PR +10°C Couleur de la surface la bande jaune vert bleu Diamètre moyen des cristaux MnO (nm) 50 couche discontinue 100 couche continue 300 couche continue Epaisseur de la bicouche (nm) 10 110 1500 Under these conditions, the steel is completely recrystallized, and Table 2 shows the characteristics of the oxide bi-layer comprising an amorphous continuous lower layer (Fe, Mn) O, and an upper layer MnO, formed on the samples after annealing according to the dew point. Table 2 RT -80 ° C PR -45 ° C PR + 10 ° C Surface color the band yellow green blue Mean diameter of the crystals MnO (nm) 50 discontinuous layer 100 continuous layer 300 continuous layer Thickness of the bilayer (nm) 10 110 1500

Après avoir été recristallisés, les échantillons sont refroidis jusqu'à une température T3 de 480°C et sont immergés dans un bain de zinc comprenant, en poids, 0,18% d'aluminium et 0,02% de fer, dont la température T2 est 460°C. Les échantillons restent en contact avec le bain pendant un temps de contact C de 3 secondes. Après immersion, les échantillons sont examinés pour vérifier si un revêtement à base de zinc est présent à la surface de l'échantillon. On a indiqué au tableau 3, le résultat obtenu en fonction du point de rosée. Tableau 3 PR-80°C PR -45°C PR +10°C Présence du revêtement à base de zinc oui non non After being recrystallized, the samples are cooled to a temperature T3 of 480 ° C. and are immersed in a zinc bath comprising, by weight, 0.18% of aluminum and 0.02% of iron, the temperature of which T2 is 460 ° C. The samples remain in contact with the bath for a period of contact C of 3 seconds. After immersion, the samples are examined to see if a zinc-based coating is present on the surface of the sample. Table 3 shows the result obtained as a function of the dew point. Table 3 PR-80 ° C PR -45 ° C PR + 10 ° C Presence of zinc-based coating Yes no no

Les inventeurs ont mis en évidence que si la bi-couche d'oxyde formée sur la bande en acier austénitique fer-carbone-manganèse après recuit de recristallisation était supérieure à 110 nm, la présence dans le bain de 0,18% en poids d'aluminium était insuffisante pour réduire la bi-couche d'oxyde et conférer à la bande une mouillabilité suffisante du zinc vis-à-vis de l'acier pour former un revêtement à base de zinc.The inventors have demonstrated that if the oxide bilayer formed on the iron-carbon-manganese austenitic steel strip after recrystallization annealing was greater than 110 nm, the presence in the bath of 0.18% by weight of Aluminum was insufficient to reduce the bi-oxide layer and give the strip sufficient wettability of the zinc to the steel to form a zinc-based coating.

2) Influence de la teneur en aluminium dans l'acier2) Influence of aluminum content in steel

Les essais ont été réalisés en utilisant des échantillons découpés dans une bande en acier austénitique fer-carbone-manganèse, qui après laminage à chaud et laminage à froid, présente une épaisseur de 0,7 mm. La composition chimique des aciers mis en oeuvre est présentée dans le tableau 4, la teneur étant exprimé en % en poids. Tableau 4 Mn C Si Al Acier A 25,10 0,50 0,009 1,27 *Acier B 24,75 0,41 0,009 traces * selon l'invention The tests were carried out using samples cut from an iron-carbon-manganese austenitic steel strip, which after hot rolling and cold rolling has a thickness of 0.7 mm. The chemical composition of the steels used is shown in Table 4, the content being expressed in% by weight. Table 4 mn VS Yes al Steel A 25,10 0.50 0,009 1.27 * Steel B 24.75 0.41 0,009 footsteps * according to the invention

Les échantillons ont subi un recuit de recristallisation dans un four Infrarouge dont le point de rosée (PR) est de -80°C, dans les conditions suivantes :

  • atmosphère gazeuse : azote + 15% en volume d'hydrogène
  • vitesse de chauffage V1 : 6°C/s
  • température de chauffage T1 : 810 °C
  • temps de maintien M : 42 s
  • vitesse de refroidissement V2 : 3°C/s
  • température d'immersion T3 : 480°C
The samples were recrystallized in an infrared oven with a dew point (PR) of -80 ° C under the following conditions:
  • gaseous atmosphere: nitrogen + 15% by volume of hydrogen
  • heating rate V1: 6 ° C / s
  • heating temperature T1: 810 ° C
  • hold time M: 42 s
  • cooling rate V2: 3 ° C / sec
  • immersion temperature T3: 480 ° C

Dans ces conditions, l'acier est complètement recristallisé, et le tableau 5 présente les structures des différents films d'oxydes qui se sont formés à la surface de l'acier après le recuit en fonction. Tableau 5 Films d'oxydes Acier A *Acier B Sous-couche MnAl2O4 (Fe,Mn)O Couche supérieure MnO.Al2O3 MnO * selon l'invention Under these conditions, the steel is completely recrystallized, and Table 5 shows the structures of the various oxide films that formed on the surface of the steel after annealing in function. Table 5 Oxide films Steel A * Steel B Undercoat MnAl 2 O 4 (Fe, Mn) O Upper layer MnO.Al 2 O 3 MnO * according to the invention

Après avoir été recristallisés, les échantillons sont refroidis jusqu'à une température T3 de 480°C et sont immergés dans un bain de zinc comprenant 0,18% d'aluminium et 0,02% de fer, dont la température T2 est 460°C. Les échantillons restent en contact avec le bain pendant un temps de contact C de 3 secondes. Après immersion, les échantillons sont revêtus par un revêtement à base de zinc.After having been recrystallized, the samples are cooled to a temperature T3 of 480 ° C. and are immersed in a zinc bath comprising 0.18% of aluminum and 0.02% of iron, whose T2 temperature is 460 ° C. vs. The samples remain in contact with the bath for a contact time C of 3 seconds. After immersion, the samples are coated with a zinc coating.

Pour caractériser l'adhésion de ce revêtement à base de zinc formé sur les échantillons d'acier A et d'acier B, un ruban adhésif a été appliqué sur l'acier revêtu, puis arraché. Le tableau 6 reprend les résultats après arrachage du ruban adhésif de ce test d'adhésion. L'adhésion est qualifiée par cotation des niveaux de gris sur le scotch, en partant de 0 pour lequel le scotch est resté propre après arrachage, jusqu'au niveau 3 présentant le niveau de gris le plus intense. Tableau 6 Acier A Mauvaise adhésion, niveau de gris : 3 *Acier B Bonne adhésion, niveau de gris : 0, pas de trace du revêtement à base de zinc sur le ruban adhésif * selon l'invention To characterize the adhesion of this zinc-based coating formed on the steel A and steel B samples, an adhesive tape was applied to the coated steel and then torn off. Table 6 shows the results after tearing off the adhesive tape of this adhesion test. The adhesion is qualified by grading the gray levels on the tape, starting from 0 for which the tape has remained clean after tearing up to level 3 with the highest level of gray. Table 6 Steel A Bad adhesion, gray level: 3 * Steel B Good adhesion, gray level: 0, no trace of the zinc-based coating on the adhesive tape * according to the invention

Claims (26)

  1. Method for the hot-dip coating, in a liquid bath based on zinc containing aluminum, said bath having a temperature T2, of a strip of iron-carbon-manganese austenitic steel comprising: 0.30% ≤ C ≤ 1.05%, 16% ≤ Mn ≤ 26%, Si ≤ 1 %, and Al ≤ 0.050%, the contents being expressed by weight, said method comprising the steps consisting in:
    - subjecting said strip to a heat treatment in a furnace in which an atmosphere that is reducing with respect to iron prevails, said heat treatment comprising a heating phase at a heating rate V1, a soak phase at a temperature T1 for a soak time M, followed by a cooling phase at a cooling rate V2, in order to obtain a strip covered on both its sides with a continuous sublayer of an amorphous iron manganese mixed oxide (Fe,Mn)O and with a continuous or discontinuous external layer of crystalline MnO manganese oxide; and then
    - making said strip covered with said oxide layers run through said bath in order to coat the strip with a zinc-based coating, the aluminum content in said bath being adjusted to a value at least equal to the content needed for the aluminum to completely reduce the crystalline MnO manganese oxide layer and at least partially reduce the amorphous (Fe,Mn)O oxide layer so as to form, on the surface of the strip, said coating comprising three iron-manganese-zinc alloy layers and one surface zinc layer.
  2. Method according to Claim 1, characterized in that said atmosphere reducing with respect to iron is composed of a gas chosen from hydrogen and nitrogen - hydrogen mixtures.
  3. Method according to Claim 2, characterized in that said gas comprises between 20 and 97% nitrogen by volume and between 3 and 80% hydrogen by volume.
  4. Method according to Claim 3, characterized in that said gas comprises between 85 and 95% nitrogen by volume and between 5 and 15% hydrogen by volume.
  5. Method according to any one of Claims 1 to 4, characterized in that said gas has a dew point between -80 and 20°C.
  6. Method according to Claim 5, characterized in that said gas has a dew point between -80 and -40°C.
  7. Method according to Claim 6, characterized in that said gas has a dew point between -60 and -40°C.
  8. Method according to any one of Claims 1 to 7, characterized in that the heat treatment of the strip is carried out at a heating rate V1 of 6°C/s or higher, at a temperature T1 between 600 and 900°C for a soak time M between 20 s and 60 s, and with a cooling rate V2 of 3°C/s or higher down to a strip immersion temperature T3 between (T2 - 10°C) and (T2 + 30°C).
  9. Method according to Claim 8, characterized in that the temperature T1 is between 650 and 820°C.
  10. Method according to Claim 9, characterized in that the temperature T1 does not exceed 750°C.
  11. Method according to one of Claims 8 to 10, characterized in that the soak time M is between 20 and 40 s.
  12. Method according to any one of Claims 1 to 11, characterized in that the heat treatment is carried out in a reducing atmosphere in such a way that an amorphous (Fe,Mn)O mixed oxide layer is formed with a thickness of between 5 and 10 nm, together with a crystalline MnO manganese oxide layer having a thickness between 5 and 90 nm, before the MnO layer is completely reduced by the aluminum of the bath.
  13. Method according to Claim 12, characterized in that the crystalline MnO manganese oxide layer has a thickness between 5 and 50 nm.
  14. Method according to Claim 13, characterized in that the crystalline MnO manganese oxide layer has a thickness between 10 and 40 nm.
  15. Method according to any one of Claims 1 to 14, characterized in that the liquid zinc-based bath contains between 0.15 and 5% aluminum by weight.
  16. Method according to any one of Claims 1 to 15, characterized in that the temperature T2 of the liquid zinc-based bath is between 430 and 480°C.
  17. Method according to any one of Claims 1 to 16, characterized in that the strip is in contact with the liquid zinc-based bath for a contact time C between 2 and 10 s.
  18. Method according to Claim 17, characterized in that the contact time C is between 3 and 5 s.
  19. Method according to any one of Claims 1 to 18, characterized in that the carbon content of the steel is between 0.40 and 0.70% by weight.
  20. Method according to any one of Claims 1 to 19, characterized in that the manganese content of the steel is between 20 and 25% by weight.
  21. Method according to any one of Claims 1 to 20, characterized in that, after the austenitic steel strip has been coated with the coating comprising three iron-manganese-zinc alloy layers and surface zinc layer, said coated strip is subjected to a heat treatment so as to completely alloy said coating.
  22. Iron-carbon-manganese austenitic steel strip that can be obtained according to any one of Claims 1 to 20, the chemical composition of which comprises, the contents being expressed by weight:
    0.30% ≤C≤ 1.05%
    16% ≤ Mn ≤ 26%
    Si ≤ 1%
    Al ≤ 0.050%
    S ≤ 0.030%
    P ≤ 0.080%
    N ≤ 0.1 %,
    and, optionally, one or more elements such as
    Cr ≤ 1%
    Mo ≤ 0.40%
    Ni ≤ 1%
    Cu ≤ 5%
    Ti ≤ 0.50%
    Nb ≤ 0.50%
    V ≤ 0.50%,
    the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, said strip being coated on both sides with a zinc-based coating comprising, in order starting from the steel/coating interface, a layer of iron-manganese-zinc alloy composed of two phases, namely a cubic phase Γ and a face-centered cubic phase Γ1, a layer of iron-manganese-zinc alloy δ1 of hexagonal structure, a layer of iron-manganese-zinc alloy ζ of monoclinic structure, and a zinc surface layer.
  23. Iron-carbon-manganese austenitic steel strip that may be obtained according to Claim 21, the chemical composition of which comprises, the contents being expressed by weight:
    0.30% ≤ C ≤ 1.05%
    16% ≤ Mn ≤ 26%
    Si ≤ 1%
    Al ≤ 0.050%
    S ≤ 0.030%
    P ≤ 0.080%
    N ≤ 0.1 %,
    and, optionally, one or more elements such as
    Cr ≤ 1 %
    Mo ≤ 0.40%
    Ni ≤1%
    Cu ≤ 5%
    Ti ≤ 0.50%
    Nb ≤ 0.50%
    V ≤ 0.50%,
    the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, said strip being coated on at least one of its sides with a zinc-based coating comprising, in order starting from the steel/coating interface, a layer of iron-manganese-zinc alloy composed of two phases, namely a cubic phase Γ and a face-centered cubic phase Γ1, a layer of iron-manganese-zinc alloy δ1 of hexagonal structure, and optionally a surface layer of iron-manganese-zinc alloy ζ of monoclinic structure.
  24. Steel strip according to either of Claims 22 and 23, characterized in that the silicon content is less than 0.5% by weight.
  25. Steel strip according to any one of Claims 22 to 24, characterized in that the carbon content is between 0.40 and 0.70% by weight.
  26. Steel strip according to any one of Claims 22 to 25, characterized in that the manganese content is between 20 and 25% by weight.
EP05809221A 2004-10-20 2005-10-10 Hot-dip coating method in a zinc bath for strips of iron/carbon/manganese steel Active EP1805341B1 (en)

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FR0411190A FR2876711B1 (en) 2004-10-20 2004-10-20 HOT-TEMPERATURE COATING PROCESS IN ZINC BATH OF CARBON-MANGANESE STEEL BANDS
PCT/FR2005/002491 WO2006042930A1 (en) 2004-10-20 2005-10-10 Hot-dip coating method in a zinc bath for strips of iron/carbon/manganese steel

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KR100911639B1 (en) 2009-08-12
FR2876711A1 (en) 2006-04-21

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