EP3149217B1 - Steel sheet provided with a sacrificial cathodically protected coating comprising lanthane - Google Patents

Steel sheet provided with a sacrificial cathodically protected coating comprising lanthane Download PDF

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Publication number
EP3149217B1
EP3149217B1 EP15724718.0A EP15724718A EP3149217B1 EP 3149217 B1 EP3149217 B1 EP 3149217B1 EP 15724718 A EP15724718 A EP 15724718A EP 3149217 B1 EP3149217 B1 EP 3149217B1
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EP
European Patent Office
Prior art keywords
coating
weight
cathodic protection
steel
steel sheet
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EP15724718.0A
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German (de)
French (fr)
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EP3149217A1 (en
Inventor
Christian ALLÉLY
Jacques Petitjean
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ArcelorMittal SA
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ArcelorMittal SA
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Priority to PL15724718T priority Critical patent/PL3149217T3/en
Priority to MA39875A priority patent/MA39875B1/en
Publication of EP3149217A1 publication Critical patent/EP3149217A1/en
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20—Deep-drawing
    • B21D22/208—Deep-drawing by heating the blank or deep-drawing associated with heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D28/00—Shaping by press-cutting; Perforating
    • B21D28/02—Punching blanks or articles with or without obtaining scrap; Notching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C21/00—Alloys based on aluminium
    • C22C21/06—Alloys based on aluminium with magnesium as the next major constituent
    • C22C21/08—Alloys based on aluminium with magnesium as the next major constituent with silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C21/00—Alloys based on aluminium
    • C22C21/10—Alloys based on aluminium with zinc as the next major constituent
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/12—Aluminium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath

Definitions

  • the present invention relates to a steel sheet provided with a sacrificial cathodic protection coating, more particularly intended for the manufacture of automotive parts, without being limited thereby.
  • the other family of metal coatings commonly used for the production of automobile parts is the family of aluminum and silicon-based coatings. These coatings do not generate microcracking in the steel when deformed due to the presence of an Al-Si-Fe intermetallic layer and have good paintability. Although they provide barrier protection and are weldable, they do not provide cathodic protection.
  • Requirement EP 1 997 927 discloses corrosion-resistant steel sheets coated with more than 35% by weight of Zn and comprising a non-equilibrium phase whose specific heat is measured by differential scanning calorimetry is greater than or equal to 1 J / g, typically having an amorphous structure.
  • the coating comprises at least 40% by weight of zinc, 1 to 60% by weight of magnesium and 0.07 to 59% by weight of aluminum.
  • the coating may comprise from 0.1 to 10% lanthanum to improve the ductility and machinability of the coating.
  • CN 103045980 A discloses coatings of Al alloys with Zn and minor participation of rare earths.
  • One of the objectives of the present application is to overcome the disadvantages of the coatings of the prior art by providing coated steel sheets having a reinforced protection against corrosion, before and after implementation by stamping, in particular.
  • the sheets When the sheets are intended to be hardened in press, in particular hot-stamped, it also seeks a resistance to the propagation of microcracks in the steel and, preferably, a window of the widest possible use in time and temperature during the heat treatment preceding the curing in press.
  • the invention relates to a steel sheet defined according to claim 1 is provided with a sacrificial cathodic protection coating, the coating comprising from 1 to 40% by weight of zinc, from 0.01 to 0 , 4% by weight of lanthanum, and optionally up to 10% by weight of magnesium, optionally up to 15% by weight of silicon, and optionally up to 0.3% by weight, in cumulative concentrations, of any additional elements, the rest being made of aluminum and residual elements or unavoidable impurities.
  • a sacrificial cathodic protection coating comprising from 1 to 40% by weight of zinc, from 0.01 to 0 , 4% by weight of lanthanum, and optionally up to 10% by weight of magnesium, optionally up to 15% by weight of silicon, and optionally up to 0.3% by weight, in cumulative concentrations, of any additional elements, the rest being made of aluminum and residual elements or unavoidable impurities.
  • the expression “between X and Y%” implies that the values X and Y are excluded, whereas the expression “de X at Y% "(eg 1 to 40 wt.% zinc) implies that the X and Y values are included.
  • the coating of the sheet according to the invention may especially comprise from 1 to 34% by weight of zinc, from 0.05 to 0.4% by weight of lanthanum, from 0 to 5% by weight of magnesium, from 0.3 10% by weight of silicon, and up to 0.3% by weight, in cumulative contents, of additional elements, the remainder being made of aluminum and residual elements or unavoidable impurities.
  • the steel of the sheet comprises, in percent by weight, 0.15% ⁇ C ⁇ 0.5%, 0.5% ⁇ Mn ⁇ 3%, 0.1% ⁇ silicon ⁇ 0.5%, Cr ⁇ 1%, Ni ⁇ 0.1%, Cu ⁇ 0.1%, Ti ⁇ 0.2%, Al ⁇ 0.1%, P ⁇ 0.1%, S ⁇ 0.05%, 0.0005% ⁇ B ⁇ 0.08%, the remainder being iron and unavoidable impurities due to steel making.
  • Another object of the invention is constituted by a part provided with a sacrificial cathodic protection coating obtainable by the method according to the invention or by cold stamping of a sheet according to the invention, and which is more particularly intended for the automotive industry.
  • the invention relates to a steel sheet provided with a coating comprising in particular lanthanum. Without wishing to be bound by a particular theory, it seems that lanthanum acts as a protective element of the coating.
  • the coating comprises from 0.01 to 0.4% by weight of lanthanum, in particular from 0.05 to 0.4% by weight of lanthanum, typically from 0.1 to 0.3% by weight of lanthanum, preferably from 0 to When the lanthanum content is less than 0.01%, the effect of increased resistance against corrosion is not observed. The same applies when the lanthanum content exceeds 0.4%. Proportions of 0.1 to 0.3% by weight of lanthanum are particularly suitable to minimize the appearance of red rust and thus to protect against corrosion.
  • the coating of the sheet according to the invention comprises from 5 to 40% by weight of zinc and optionally up to 10% by weight of magnesium. Without wishing to be bound by a particular theory, it would seem that these elements make it possible, in combination with lanthanum, to reduce the electrochemical potential of the coating with respect to the steel, in media containing or not containing chloride ions.
  • the coatings according to the invention thus have sacrificial cathodic protection.
  • zinc whose protective effect is greater than that of magnesium and which is simpler to implement because less oxidizable.
  • the coatings of the sheets according to the invention also comprise up to 15% by weight of silicon, in particular from 0.1 to 15%, typically from 0.5 to 10% by weight of silicon, preferably from 0.5 to 5% by weight. by weight of silicon, for example from 1 to 3% of silicon.
  • silicon makes it possible to give the sheets high resistance to oxidation at high temperatures.
  • the presence of silicon allows their use up to 650 ° C without risk of flaking coating.
  • silicon can prevent the formation of a thick layer of iron-zinc intermetallic during a hot dip coating, intermetallic layer that reduces the adhesion and formability of the coating.
  • the presence of a silicon content greater than 0.5% by weight makes them more particularly able to be hardened in press and in particular to be shaped by hot stamping. It is preferred to use for this purpose an amount of 0.5 to 15% silicon.
  • a content greater than 15% by weight is undesirable since primary silicon is formed which could degrade the properties of the coating, in particular the properties of corrosion resistance.
  • the coatings of the sheets according to the invention can also comprise, in aggregated contents, up to 0.3% by weight, preferably up to 0.1% by weight, or even less than 0.05% by weight of elements. additional such as Sb, Pb, Ti, Ca, Mn, Cr, Ni, Zr, In, Sn, Hf or Bi. These various elements may allow, among other things, to improve the corrosion resistance of the coating or its fragility or adhesion, for example. Those skilled in the art who know their effects on the characteristics of the coating will be able to use them according to the complementary objective sought, in the proportion adapted for this purpose which will generally be from 20 ppm to 50 ppm. It was further verified that these elements did not interfere with the main properties sought in the context of the invention.
  • the coatings of the sheets according to the invention may also comprise unavoidable residual elements and impurities resulting, in particular, from the pollution of hot dip galvanizing baths by passage of the steel strips or impurities from the ingots of the same baths or ingots of vacuum deposition processes.
  • a residual element mention may be made of iron which may be present in amounts of up to 5% by weight and in general from 2 to 4% by weight in hot dip coating baths.
  • the coating may therefore comprise from 0 to 5% by weight of iron, for example from 2 to 4% by weight.
  • the sheet coatings according to the invention finally comprise aluminum, the content of which can range from about 29% to about 99% by weight.
  • This element makes it possible to provide protection against corrosion of the plates by a barrier effect. It increases the melting temperature and the evaporation temperature of the coating, thus making it possible to implement it more easily, in particular by hot stamping and in a wide range of time and temperature. This may be particularly interesting when the composition of the steel of the sheet and / or the final microstructure referred to for the part require to pass through austenitization at high temperature and / or for long periods.
  • the coating comprises more than 50%, especially more than 70%, preferably more than 80% by weight of aluminum.
  • the coatings of the sheets according to the invention do not comprise an amorphous phase.
  • the presence or absence of amorphous phase can in particular be verified by differential scanning calorimetry (DSC).
  • DSC differential scanning calorimetry
  • the amorphous phase is generally difficult to form. It is usually formed by considerably increasing the cooling rate.
  • the document EP 1 997 927 describes getting an amorphous phase by acting on the cooling rate, said speed being dependent on the cooling method and the thickness of the coating.
  • the upper layer may also contain Al-Zn binary.
  • the thickness of the coating is preferably from 10 to 50 ⁇ m. Indeed, below 10 microns, protection against corrosion of the band might be insufficient. Above 50 ⁇ m corrosion protection exceeds the required level, especially in the automotive field. In addition, if a coating of such thickness is subjected to a significant rise in temperature and / or for long periods, it may melt in the upper part and come to flow on the oven rolls or in the stamping tools , which would deteriorate them. A thickness of 27 to 50 microns is particularly suitable for the manufacture of press hardened parts, especially by hot stamping.
  • the nature of it is not critical as long as the coating can adhere sufficiently.
  • the steel has a composition that allows the part to reach a tensile strength of 500 to 1600 MPa, depending on the conditions. of use.
  • a steel composition comprising, in% by weight: 0.15% ⁇ C ⁇ 0.5%, 0.5% ⁇ Mn ⁇ 3%, 0.1% ⁇ If ⁇ 0.5%, Cr ⁇ 1%, Ni ⁇ 0.1%, Cu ⁇ 0.1%, Ti ⁇ 0.2%, Al ⁇ 0.1%, P ⁇ 0.1%, S ⁇ 0 , 05%, 0.0005% ⁇ B ⁇ 0.08%, the remainder being iron and unavoidable impurities from the elaboration of steel.
  • An example of a commercially available steel is 22MnB5.
  • a steel composition comprising: 0.040% ⁇ C ⁇ 0.100%, 0.80% ⁇ Mn ⁇ 2.00%, Si ⁇ 0.30 %, S ⁇ 0.005%, P ⁇ 0.030%, 0.010% ⁇ Al ⁇ 0.070%, 0.015% ⁇ Nb ⁇ 0.100%, 0.030% ⁇ Ti ⁇ 0.080%, N ⁇ 0.009%, Cu ⁇ 0.100%, Ni ⁇ 0.100%, Cr ⁇ 0.100%, Mo ⁇ 0.100%, Ca ⁇ 0.006%, the remainder being iron and unavoidable impurities resulting from the preparation of steel.
  • the steel sheets can be made by hot rolling and can optionally be cold rolled, depending on the final thickness referred to, which can vary, for example, from 0.7 to 3 mm.
  • the sheets may be coated by any suitable means such as an electrodeposition process or by a vacuum deposition method or under pressure close to atmospheric pressure, such as deposition by magnetron sputtering, by cold plasma or by evaporation under vacuum for example, but it will be preferred to obtain them by a hot dipping coating process in a molten metal bath. It is observed that the superficial cathodic protection is more important for coatings obtained by hot quenching than for coatings obtained by other coating processes.
  • said coating is cooled to complete solidification at a cooling rate advantageously between 5 and 30 ° C / s, preferably between 15 and 25 ° C / s, for example by blowing inert gas or air.
  • the cooling rate of the present invention does not allow to obtain an amorphous phase in the coating.
  • the sheets according to the invention can then be shaped by any method adapted to the structure and shape of the parts to be manufactured, such as for example cold stamping.
  • the sheets according to the invention are more particularly suitable for the manufacture of hardened parts in press, in particular by hot stamping.
  • This method consists of supplying a steel sheet according to the previously coated invention, then cutting the sheet to obtain a blank.
  • This blank is then heated in an oven under a non-protective atmosphere to an austenitization temperature Tm of 840 to 950 ° C, preferably of 880 to 930 ° C, and then to maintain the blank at this temperature Tm for a duration tm ranging from 1 to 8 minutes, preferably from 4 to 6 minutes.
  • the temperature Tm and the holding time tm depend on the nature of the steel but also on the thickness of the sheets to be stamped, which must be entirely in the austenitic field before they are shaped.
  • the speed of rise in temperature also affects these parameters, a high speed (greater than 30 ° C / s for example) to also reduce the holding time tm.
  • the blank is then transferred to a hot stamping tool and then stamped.
  • the resulting part is then cooled either in the stamping tool itself or after transfer to a specific cooling tool.
  • the cooling rate is in all cases controlled according to the composition of the steel, so that its final microstructure after the hot stamping comprises at least one component selected from martensite and bainite, in order to achieve the desired level of mechanical strength.
  • This upper portion has a thickness of at least 5 microns and generally less than 13 microns.
  • the proportion of iron can for example be measured by glow discharge spectrometry (LDS).
  • the temperature Tm and / or the holding time tm it is possible to limit the temperature Tm and / or the holding time tm. It is also possible to increase the thickness of the pre-coating to prevent the diffusion front of the iron from reaching the surface of the coating. In this respect, it is preferable to use a sheet having a pre-coating thickness greater than or equal to 27 ⁇ m, preferably greater than or equal to 30 ⁇ m or even 35 ⁇ m.
  • the contents of lanthanum and / or zinc and possibly magnesium of the previous coating may also be increased.
  • the figure shows the extent of red rust versus time per hour for each of the 6 coatings tested in the tests.
  • Tests were carried out with four trilayer samples, each consisting of a sheet of cold rolled 22MnB5 5 mm thick (1 st layer), provided with a coating obtained by hot dip a thickness of 1 mm and composition is specified below (2nd layer), itself covered with a second sheet of laminated 22MnB5 thickness cold 5 mm (3rd layer).
  • red rust and electrochemical extension measurements were performed and are provided in the tables below.
  • Al 2 Si-10Zn Al 2 Si-10Zn-0,2La Al 2 Si-10Zn-0,5La Al 2 Si-4Zn-2 Mg Al 2 Si-4Zn-2 Mg-0,2La Al 2 Si-4Zn-2 Mg-0,5La N-VDA test red rust No protection Partial protection No protection No protection Partial protection No protection Average area over which red rust has spread to static (%) 25 5 38 28 6 24 N-VDA, 35 ° C / 95% RH, medium Galvanic current (nA) -700 1862 240 N-VDA, 50 ° C / 90% RH, medium Galvanic current (nA) -120 1400 250
  • the figure also shows that the 0.2% lanthanum coating has a galvanic coupling current with steel much higher than the lanthanum or 0.5% La coating.

Description

La présente invention concerne une tôle d'acier munie d'un revêtement à protection cathodique sacrificielle, plus particulièrement destinée à la fabrication de pièces pour automobile, sans y être pour autant limitée.The present invention relates to a steel sheet provided with a sacrificial cathodic protection coating, more particularly intended for the manufacture of automotive parts, without being limited thereby.

En effet, à ce jour, seuls les revêtements de zinc ou d'alliages de zinc apportent une protection renforcée contre la corrosion en raison d'une double protection barrière et cathodique. L'effet barrière est obtenu par l'application du revêtement à la surface de l'acier, qui empêche ainsi tout contact entre l'acier et le milieu corrosif et est indépendant de la nature du revêtement et du substrat. Au contraire, la protection cathodique sacrificielle est basée sur le fait que le zinc est un métal moins noble que l'acier et, qu'en situation de corrosion, il se consomme préférentiellement à l'acier. Cette protection cathodique est en particulier essentielle dans les zones où l'acier est directement exposé à l'atmosphère corrosive, comme les bords découpés où les zones blessées où l'acier est à nu et où le zinc environnant va être consommé avant toute attaque de la zone non revêtue.Indeed, to date, only zinc coatings or zinc alloys provide enhanced protection against corrosion due to dual barrier and cathodic protection. The barrier effect is achieved by applying the coating to the surface of the steel, thereby preventing contact between the steel and the corrosive medium and is independent of the nature of the coating and the substrate. On the contrary, sacrificial cathodic protection is based on the fact that zinc is a less noble metal than steel and that, in a situation of corrosion, it is preferentially consumed with steel. This cathodic protection is particularly essential in areas where the steel is directly exposed to the corrosive atmosphere, such as the cut edges where the injured areas where the steel is exposed and where the surrounding zinc is going to be consumed before any attack of the uncoated area.

Cependant, du fait de son bas point de fusion, le zinc pose problème lorsqu'il faut souder les pièces, car on risque de le vaporiser. Pour pallier ce problème, une possibilité est de réduire l'épaisseur du revêtement, mais on limite alors la durée dans le temps de la protection contre la corrosion. En outre, lorsqu'on souhaite durcir la tôle sous presse, notamment par emboutissage à chaud, on observe la formation de microfissures dans l'acier qui se propagent depuis le revêtement. De même, la mise en peinture de certaines pièces revêtues préalablement de zinc et durcies sous presse nécessite une opération de sablage avant phosphatation en raison de la présence d'une couche d'oxyde fragile en surface de la pièce.However, because of its low melting point, zinc is a problem when welding parts, because it is likely to vaporize. To overcome this problem, one possibility is to reduce the thickness of the coating, but then the time duration of the protection against corrosion is limited. In addition, when it is desired to harden the sheet in press, particularly by hot stamping, the formation of microcracks in the steel which propagate from the coating is observed. Similarly, the painting of some parts previously coated with zinc and hardened in press requires a sanding operation before phosphating due to the presence of a fragile oxide layer on the surface of the workpiece.

L'autre famille de revêtements métalliques communément utilisés pour la production de pièces pour l'automobile est la famille des revêtements à base d'aluminium et de silicium. Ces revêtements ne génèrent pas de microfissuration dans l'acier lorsqu'on les déforme en raison de la présence d'une couche d'intermétallique Al-Si-Fe et présentent une bonne aptitude à la mise en peinture. S'ils permettent d'obtenir une protection par effet barrière et sont soudables, ils ne permettent cependant pas d'obtenir de protection cathodique.The other family of metal coatings commonly used for the production of automobile parts is the family of aluminum and silicon-based coatings. These coatings do not generate microcracking in the steel when deformed due to the presence of an Al-Si-Fe intermetallic layer and have good paintability. Although they provide barrier protection and are weldable, they do not provide cathodic protection.

La demande EP 1 997 927 décrit des tôles d'acier résistantes à la corrosion et revêtues d'un revêtement comprenant plus de 35% en poids de Zn et comprenant une phase en non-équilibre dont la chaleur spécifique mesurée par calorimétrie différentielle à balayage est supérieure ou égale à 1 J/g, typiquement ayant une structure amorphe. De préférence, le revêtement comprend au moins 40% en poids de zinc, de 1 à 60% en poids de magnésium et de 0,07 à 59% en poids d'aluminium. Le revêtement peut comprendre de 0,1 à 10% de lanthane pour améliorer la ductilité et l'usinabilité du revêtement. De plus, CN 103045980 A divulgue des revêtements des alliages de Al avec Zn et une participation mineure des terres rares. Un des objectifs de la présente demande est de remédier aux inconvénients des revêtements de l'art antérieur en mettant à disposition des tôles d'acier revêtues présentant une protection renforcée contre la corrosion, avant et après mise en oeuvre par emboutissage, notamment. Lorsque les tôles sont destinées à être durcies sous presse, en particulier embouties à chaud, on recherche en outre une résistance à la propagation de microfissures dans l'acier et, de préférence, une fenêtre d'utilisation la plus large possible en temps et température lors du traitement thermique précédant le durcissement sous presse.Requirement EP 1 997 927 discloses corrosion-resistant steel sheets coated with more than 35% by weight of Zn and comprising a non-equilibrium phase whose specific heat is measured by differential scanning calorimetry is greater than or equal to 1 J / g, typically having an amorphous structure. Preferably, the coating comprises at least 40% by weight of zinc, 1 to 60% by weight of magnesium and 0.07 to 59% by weight of aluminum. The coating may comprise from 0.1 to 10% lanthanum to improve the ductility and machinability of the coating. Moreover, CN 103045980 A discloses coatings of Al alloys with Zn and minor participation of rare earths. One of the objectives of the present application is to overcome the disadvantages of the coatings of the prior art by providing coated steel sheets having a reinforced protection against corrosion, before and after implementation by stamping, in particular. When the sheets are intended to be hardened in press, in particular hot-stamped, it also seeks a resistance to the propagation of microcracks in the steel and, preferably, a window of the widest possible use in time and temperature during the heat treatment preceding the curing in press.

En termes de protection cathodique sacrificielle, on cherche à atteindre un potentiel électrochimique au moins 50 mV plus négatif que celui de l'acier, soit une valeur minimale de -0,78 V par rapport à une électrode au calomel saturé (ECS). On ne souhaite cependant pas aller plus bas qu'une valeur de -1,4V, voire -1,25V qui entraînerait une consommation du revêtement trop rapide et diminuerait en final la durée de protection de l'acier.In terms of sacrificial cathodic protection, it is sought to achieve an electrochemical potential at least 50 mV more negative than that of steel, a minimum value of -0.78 V compared to a saturated calomel electrode (ECS). However, we do not want to go lower than a value of -1.4V or -1.25 V which would lead to a consumption of the coating too fast and ultimately reduce the protection time of the steel.

A cet effet, l'invention a pour objet une tôle d'acier définie selon la revendication 1 est munie d'un revêtement à protection cathodique sacrificielle, le revêtement comprenant de 1 à 40% en poids de zinc, de 0,01 à 0,4% en poids de lanthane, et éventuellement jusqu'à 10% en poids de magnésium, éventuellement jusqu'à 15% en poids de silicium, et éventuellement jusqu'à 0,3% en poids, en teneurs cumulées, d'éventuels éléments additionnels, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables.For this purpose, the invention relates to a steel sheet defined according to claim 1 is provided with a sacrificial cathodic protection coating, the coating comprising from 1 to 40% by weight of zinc, from 0.01 to 0 , 4% by weight of lanthanum, and optionally up to 10% by weight of magnesium, optionally up to 15% by weight of silicon, and optionally up to 0.3% by weight, in cumulative concentrations, of any additional elements, the rest being made of aluminum and residual elements or unavoidable impurities.

Le revêtement de la tôle selon l'invention peut en outre incorporer les caractéristiques suivantes, prises isolément ou en combinaison :

  • le revêtement comprend entre 1 et 40% en poids de zinc, notamment de 1 à 34% en poids de zinc, typiquement de 1 à 30% en poids de zinc, de préférence de 2 à 20% en poids de zinc,
  • le revêtement comprend de 0,05 à 0,4% en poids de lanthane, typiquement de 0,1 à 0,4% en poids de lanthane, de préférence de 0,1 à 0,3% en poids de lanthane, de préférence encore de 0,2 à 0,3% en poids de lanthane,
  • le revêtement comprend de 0 à 5% en poids de magnésium,
  • le revêtement comprend de 0,5 à 10% en poids de silicium, de préférence 0,5 à 5% en poids de silicium,
  • le revêtement présente une épaisseur de 10 à 50 µm, de préférence de 27 à 50 µm,
  • le revêtement est obtenu par trempé à chaud.
The coating of the sheet according to the invention may further incorporate the following features, taken separately or in combination:
  • the coating comprises between 1 and 40% by weight of zinc, in particular from 1 to 34% by weight of zinc, typically from 1 to 30% by weight of zinc, preferably from 2 to 20% by weight of zinc,
  • the coating comprises 0.05 to 0.4% by weight of lanthanum, typically 0.1 to 0.4% by weight of lanthanum, preferably 0.1 to 0.3% by weight of lanthanum, preferably still 0.2 to 0.3% by weight of lanthanum,
  • the coating comprises from 0 to 5% by weight of magnesium,
  • the coating comprises from 0.5 to 10% by weight of silicon, preferably 0.5 to 5% by weight of silicon,
  • the coating has a thickness of 10 to 50 μm, preferably 27 to 50 μm,
  • the coating is obtained by hot quenching.

Des revêtements comprenant, en poids :

  • 2% de silicium, 10% de zinc, 0,2% de lanthane, et jusqu'à 0,3% en poids, en teneurs cumulées, d'éléments additionnels, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables, ou
  • 2% de silicium, 4% de zinc, 2% de magnésium, 0,2% de lanthane, et jusqu'à 0,3% en poids, en teneurs cumulées, d'éléments additionnels, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables,
sont particulièrement préférés.Coatings comprising, by weight:
  • 2% of silicon, 10% of zinc, 0.2% of lanthanum, and up to 0.3% by weight, in cumulative contents, of additional elements, the remainder being composed of aluminum and residual elements or unavoidable impurities, or
  • 2% of silicon, 4% of zinc, 2% of magnesium, 0.2% of lanthanum, and up to 0.3% by weight, in cumulative contents, of additional elements, the remainder being composed of aluminum and residual elements or unavoidable impurities,
are particularly preferred.

Au sens de la présente demande, l'expression « entre X et Y% » (par exemple entre 1 et 40% en poids de zinc) sous-entend que les valeurs X et Y sont exclues, alors que l'expression « de X à Y% » (par exemple de 1 à 40% en poids de zinc) sous-entend que les valeurs X et Y sont inclues.For the purposes of the present application, the expression "between X and Y%" (for example between 1 and 40% by weight of zinc) implies that the values X and Y are excluded, whereas the expression "de X at Y% "(eg 1 to 40 wt.% zinc) implies that the X and Y values are included.

Le revêtement de la tôle selon l'invention peut notamment comprendre de 1 à 34% en poids de zinc, de 0,05 à 0,4% en poids de lanthane, de 0 à 5% en poids de magnésium, de 0,3 à 10% en poids de silicium, et jusqu'à 0,3% en poids, en teneurs cumulées, d'éléments additionnels, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables.The coating of the sheet according to the invention may especially comprise from 1 to 34% by weight of zinc, from 0.05 to 0.4% by weight of lanthanum, from 0 to 5% by weight of magnesium, from 0.3 10% by weight of silicon, and up to 0.3% by weight, in cumulative contents, of additional elements, the remainder being made of aluminum and residual elements or unavoidable impurities.

Généralement, l'acier de la tôle comprend, en pourcents en poids, 0,15%<C<0,5%, 0,5%<Mn<3%, 0,1%<silicium<0,5%, Cr<1%, Ni<0,1%, Cu<0,1%, Ti<0,2%, Al<0,1%, P<0,1%, S<0,05%, 0,0005%<B<0,08%, le reste étant constitué de fer et d'impuretés inévitables dues à l'élaboration de l'acier.Generally, the steel of the sheet comprises, in percent by weight, 0.15% <C <0.5%, 0.5% <Mn <3%, 0.1% <silicon <0.5%, Cr <1%, Ni <0.1%, Cu <0.1%, Ti <0.2%, Al <0.1%, P <0.1%, S <0.05%, 0.0005% <B <0.08%, the remainder being iron and unavoidable impurities due to steel making.

Un autre objet de l'invention est constitué par un procédé de fabrication d'une pièce en acier munie d'un revêtement à protection cathodique sacrificielle comprenant les étapes suivantes, prises dans cet ordre et consistant à:

  • approvisionner une tôle d'acier telle que définie ci-dessus revêtue préalablement, puis à
  • découper la tôle pour obtenir un flan, puis à
  • chauffer le flan sous une atmosphère non protectrice jusqu'à une température d'austénitisation Tm de 840 à 950°C, puis à
  • maintenir le flan à cette température Tm pendant une durée tm de 1 à 8 minutes, puis à
  • emboutir à chaud le flan pour obtenir une pièce que l'on refroidit à une vitesse telle que la microstructure de l'acier comprend au moins un constituant choisi parmi la martensite et la bainite pour obtenir une d'une pièce en acier munie d'un revêtement à protection cathodique sacrificielle,
  • la température Tm, le temps tm, l'épaisseur du revêtement préalable et ses teneurs en lanthane, en zinc et éventuellement en magnésium étant choisis de telle sorte que la teneur moyenne finale en fer dans une partie supérieure du revêtement de ladite pièce en acier munie d'un revêtement à protection cathodique sacrificielle soit inférieure à 75% en poids.
Another object of the invention is constituted by a method of manufacturing a steel part provided with a sacrificial cathodic protection coating comprising the following steps, taken in this order and consisting of:
  • supply a sheet of steel as defined above previously coated, then to
  • cut the sheet to obtain a blank, then to
  • heating the blank under a non-protective atmosphere to an austenitization temperature Tm of 840 to 950 ° C, then to
  • keep the blank at this temperature Tm for a duration tm of 1 to 8 minutes, then at
  • hot stamping the blank to obtain a part that is cooled at a speed such that the microstructure of the steel comprises at least one component selected from martensite and bainite to obtain a steel piece provided with a sacrificial cathodic protection coating,
  • the temperature Tm, the time tm, the thickness of the primer coating and its contents of lanthanum, zinc and optionally magnesium being chosen so that the final average iron content in an upper portion of the coating of said steel piece provided a sacrificial cathodic protection coating is less than 75% by weight.

Un autre objet de l'invention est constitué par une pièce munie d'un revêtement à protection cathodique sacrificielle pouvant être obtenue par le procédé selon l'invention ou par emboutissage à froid d'une tôle selon l'invention, et qui est plus particulièrement destinée à l'industrie automobile.Another object of the invention is constituted by a part provided with a sacrificial cathodic protection coating obtainable by the method according to the invention or by cold stamping of a sheet according to the invention, and which is more particularly intended for the automotive industry.

L'invention va à présent être décrite plus en détail en référence à des modes de réalisation particuliers donnés à titre d'exemples non limitatif.The invention will now be described in more detail with reference to particular embodiments given as non-limiting examples.

L'invention porte sur une tôle d'acier munie d'un revêtement comprenant notamment du lanthane. Sans vouloir être lié par une théorie particulière, il semblerait que le lanthane agisse comme élément de protection du revêtement.The invention relates to a steel sheet provided with a coating comprising in particular lanthanum. Without wishing to be bound by a particular theory, it seems that lanthanum acts as a protective element of the coating.

Le revêtement comprend de 0,01 à 0,4% en poids de lanthane, notamment 0,05 à 0,4% en poids de lanthane, typiquement de 0,1 à 0,3% en poids de lanthane, de préférence de 0,2 à 0,3% en poids de lanthane, Lorsque la teneur en lanthane est inférieure à 0,01%, l'effet de résistance accrue contre la corrosion n'est pas observé. Il en va de même lorsque la teneur en lanthane dépasse 0,4%. Des proportions de 0,1 à 0,3% en poids de lanthane sont particulièrement adaptées pour minimiser l'apparition de rouille rouge et donc pour protéger contre la corrosion.The coating comprises from 0.01 to 0.4% by weight of lanthanum, in particular from 0.05 to 0.4% by weight of lanthanum, typically from 0.1 to 0.3% by weight of lanthanum, preferably from 0 to When the lanthanum content is less than 0.01%, the effect of increased resistance against corrosion is not observed. The same applies when the lanthanum content exceeds 0.4%. Proportions of 0.1 to 0.3% by weight of lanthanum are particularly suitable to minimize the appearance of red rust and thus to protect against corrosion.

Le revêtement de la tôle selon l'invention comprend de 5 à 40% en poids de zinc et éventuellement jusqu'à 10% en poids de magnésium. Sans vouloir être lié par une théorie particulière, il semblerait que ces éléments permettent, en association avec le lanthane, de diminuer le potentiel électrochimique du revêtement par rapport à l'acier, dans des milieux contenant ou ne contenant pas d'ions chlorures. Les revêtements selon l'invention présentent ainsi une protection cathodique sacrificielle.The coating of the sheet according to the invention comprises from 5 to 40% by weight of zinc and optionally up to 10% by weight of magnesium. Without wishing to be bound by a particular theory, it would seem that these elements make it possible, in combination with lanthanum, to reduce the electrochemical potential of the coating with respect to the steel, in media containing or not containing chloride ions. The coatings according to the invention thus have sacrificial cathodic protection.

On préfère utiliser le zinc dont l'effet de protection est plus important que celui du magnésium et qui est plus simple à mettre en oeuvre car moins oxydable. Ainsi, on préfère utiliser entre 1 et 40% en poids de zinc, notamment de 1 à 34% en poids de zinc, de préférence de 2 à 20% en poids de zinc, associé ou non à 1 à 10%, voire 1 à 5% en poids de magnésium.It is preferred to use zinc whose protective effect is greater than that of magnesium and which is simpler to implement because less oxidizable. Thus, it is preferred to use between 1 and 40% by weight of zinc, in particular from 1 to 34% by weight of zinc, preferably from 2 to 20% by weight of zinc, with or without 1 to 10%, or even 1 to 5% by weight of magnesium.

Les revêtements des tôles selon l'invention comprennent également jusqu'à 15% en poids de silicium, notamment de 0,1 à 15%, typiquement de 0,5 à 10% en poids de silicium, de préférence 0,5 à 5% en poids de silicium, par exemple de 1 à 3% de silicium. Le silicium permet notamment de conférer aux tôles une grande résistance à l'oxydation à haute température. La présence de silicium permet ainsi leur utilisation jusqu'à 650°C sans risque d'écaillage du revêtement. Par ailleurs, le silicium permet de prévenir la formation d'une épaisse couche d'intermétallique fer-zinc lors d'un revêtement au trempé à chaud, couche d'intermétallique qui réduirait l'adhérence et la formabilité du revêtement. La présence d'une teneur en silicium supérieure à 0,5% en poids les rend ainsi plus particulièrement aptes à être durcies sous presse et en particulier à être mises en forme par emboutissage à chaud. On préfère utiliser à cette fin une quantité de 0,5 à 15% de silicium. Une teneur supérieure à 15% en poids n'est pas souhaitable car il se forme alors du silicium primaire qui pourrait dégrader les propriétés du revêtement, en particulier les propriétés de résistance à la corrosion.The coatings of the sheets according to the invention also comprise up to 15% by weight of silicon, in particular from 0.1 to 15%, typically from 0.5 to 10% by weight of silicon, preferably from 0.5 to 5% by weight. by weight of silicon, for example from 1 to 3% of silicon. In particular, silicon makes it possible to give the sheets high resistance to oxidation at high temperatures. The presence of silicon allows their use up to 650 ° C without risk of flaking coating. In addition, silicon can prevent the formation of a thick layer of iron-zinc intermetallic during a hot dip coating, intermetallic layer that reduces the adhesion and formability of the coating. The presence of a silicon content greater than 0.5% by weight makes them more particularly able to be hardened in press and in particular to be shaped by hot stamping. It is preferred to use for this purpose an amount of 0.5 to 15% silicon. A content greater than 15% by weight is undesirable since primary silicon is formed which could degrade the properties of the coating, in particular the properties of corrosion resistance.

Les revêtements des tôles selon l'invention peuvent également comprendre, en teneurs cumulées, jusqu'à 0,3% en poids, de préférence jusqu'à 0,1% en poids, voire moins de 0,05% en poids d'éléments additionnels tels que Sb, Pb, Ti, Ca, Mn, Cr, Ni, Zr, In, Sn, Hf ou Bi. Ces différents éléments peuvent permettre, entre autres, d'améliorer la résistance à la corrosion du revêtement ou bien sa fragilité ou son adhésion, par exemple. L'homme du métier qui connaît leurs effets sur les caractéristiques du revêtement saura les employer en fonction du but complémentaire recherché, dans la proportion adaptée à cet effet qui sera généralement de 20 ppm à 50 ppm. On a en outre vérifié que ces éléments n'interféraient pas avec les propriétés principales recherchées dans le cadre de l'invention.The coatings of the sheets according to the invention can also comprise, in aggregated contents, up to 0.3% by weight, preferably up to 0.1% by weight, or even less than 0.05% by weight of elements. additional such as Sb, Pb, Ti, Ca, Mn, Cr, Ni, Zr, In, Sn, Hf or Bi. These various elements may allow, among other things, to improve the corrosion resistance of the coating or its fragility or adhesion, for example. Those skilled in the art who know their effects on the characteristics of the coating will be able to use them according to the complementary objective sought, in the proportion adapted for this purpose which will generally be from 20 ppm to 50 ppm. It was further verified that these elements did not interfere with the main properties sought in the context of the invention.

Les revêtements des tôles selon l'invention peuvent aussi comprendre des éléments résiduels et impuretés inévitables provenant, notamment, de la pollution des bains de galvanisation au trempé à chaud par passage des bandes d'acier ou des impuretés provenant des lingots d'alimentation des mêmes bains ou des lingots d'alimentation des procédés de dépôt sous vide. On pourra notamment citer, comme élément résiduel, le fer qui peut être présent en des quantités allant jusqu'à 5% en poids et en général de 2 à 4% en poids dans les bains de revêtement au trempé à chaud. Le revêtement peut donc comprendre de 0 à 5% en poids de fer, par exemple de 2 à 4% en poids.The coatings of the sheets according to the invention may also comprise unavoidable residual elements and impurities resulting, in particular, from the pollution of hot dip galvanizing baths by passage of the steel strips or impurities from the ingots of the same baths or ingots of vacuum deposition processes. As a residual element, mention may be made of iron which may be present in amounts of up to 5% by weight and in general from 2 to 4% by weight in hot dip coating baths. The coating may therefore comprise from 0 to 5% by weight of iron, for example from 2 to 4% by weight.

Les revêtements des tôles selon l'invention comprennent finalement de l'aluminium dont la teneur peut aller d'environ 29% à près de 99% en poids. Cet élément permet d'assurer une protection contre la corrosion des tôles par effet barrière. Il augmente la température de fusion et la température d'évaporation du revêtement, permettant ainsi de pouvoir le mettre en oeuvre plus facilement, en particulier par emboutissage à chaud et ce dans une gamme étendue de temps et de température. Ceci peut être particulièrement intéressant lorsque la composition de l'acier de la tôle et/ou la microstructure finale visée pour la pièce imposent de passer par une austénitisation à haute température et/ou pendant des temps longs. Généralement, le revêtement comprend plus de 50%, notamment plus de 70%, de préférence plus de 80% en poids d'aluminium.The sheet coatings according to the invention finally comprise aluminum, the content of which can range from about 29% to about 99% by weight. This element makes it possible to provide protection against corrosion of the plates by a barrier effect. It increases the melting temperature and the evaporation temperature of the coating, thus making it possible to implement it more easily, in particular by hot stamping and in a wide range of time and temperature. This may be particularly interesting when the composition of the steel of the sheet and / or the final microstructure referred to for the part require to pass through austenitization at high temperature and / or for long periods. Generally, the coating comprises more than 50%, especially more than 70%, preferably more than 80% by weight of aluminum.

Les revêtements des tôles selon l'invention ne comprennent pas de phase amorphe. La présence ou l'absence de phase amorphe peut notamment être vérifiée par calorimétrie à balayage différentiel (« differential scanning calorimetry » (DSC) en anglais).La phase amorphe est généralement difficile à former. Elle est habituellement formée en augmentant considérablement la vitesse de refroidissement. Le document EP 1 997 927 décrit l'obtention d'une phase amorphe en jouant sur la vitesse de refroidissement, ladite vitesse étant dépendante de la méthode de refroidissement et de l'épaisseur du revêtement.The coatings of the sheets according to the invention do not comprise an amorphous phase. The presence or absence of amorphous phase can in particular be verified by differential scanning calorimetry (DSC). The amorphous phase is generally difficult to form. It is usually formed by considerably increasing the cooling rate. The document EP 1 997 927 describes getting an amorphous phase by acting on the cooling rate, said speed being dependent on the cooling method and the thickness of the coating.

De préférence, la microstructure du revêtement comprend:

  • une couche interfaciale comprenant deux couches:
    1. (i) une très fine couche de FeAl3/Fe2Al5 et
    2. (ii) une couche d'intermétallique FeSiAl, par exemple de 5 µm d'épaisseur,
  • une couche supérieure, constituée d'une solution solide Al-Zn et d'aiguilles riches en Si.
Preferably, the microstructure of the coating comprises:
  • an interfacial layer comprising two layers:
    1. (i) a very thin layer of FeAl 3 / Fe 2 Al 5 and
    2. (ii) a FeSiAl intermetallic layer, for example 5 μm thick,
  • an upper layer consisting of an Al-Zn solid solution and Si-rich needles.

Le lanthane est également présent dans la microstructure du revêtement.Lanthanum is also present in the microstructure of the coating.

Lorsque la teneur en zinc est supérieure à 20%, la couche supérieure peut également contenir du binaire Al-Zn.When the zinc content is greater than 20%, the upper layer may also contain Al-Zn binary.

L'épaisseur du revêtement est de préférence comprise de 10 à 50 µm. En effet, en dessous de 10 µm, la protection contre la corrosion de la bande risquerait d'être insuffisante. Au-delà de 50 µm, la protection contre la corrosion dépasse le niveau requis, en particulier dans le domaine de l'automobile. En outre, si un revêtement d'une telle épaisseur est soumis à une élévation de température importante et/ou pendant des durées longues, il risque de fondre en partie supérieure et de venir couler sur les rouleaux de four ou dans les outils d'emboutissage, ce qui les détériorerait. Une épaisseur de 27 à 50 µm est particulièrement adaptée à la fabrication de pièces durcies sous presse, notamment par emboutissage à chaud.The thickness of the coating is preferably from 10 to 50 μm. Indeed, below 10 microns, protection against corrosion of the band might be insufficient. Above 50 μm corrosion protection exceeds the required level, especially in the automotive field. In addition, if a coating of such thickness is subjected to a significant rise in temperature and / or for long periods, it may melt in the upper part and come to flow on the oven rolls or in the stamping tools , which would deteriorate them. A thickness of 27 to 50 microns is particularly suitable for the manufacture of press hardened parts, especially by hot stamping.

En ce qui concerne à présent l'acier employé pour la tôle selon l'invention, la nature de celui-ci n'est pas critique tant que le revêtement peut y adhérer de façon suffisante.With regard to now the steel used for the sheet according to the invention, the nature of it is not critical as long as the coating can adhere sufficiently.

Cependant, pour certaines applications nécessitant des résistances mécaniques élevées, comme pour les pièces de structure pour automobile, on préfère que l'acier présente une composition permettant à la pièce d'atteindre une résistance en traction de 500 à 1600 MPa, en fonction des conditions d'usage.However, for certain applications requiring high mechanical strengths, such as for automotive structural parts, it is preferred that the steel has a composition that allows the part to reach a tensile strength of 500 to 1600 MPa, depending on the conditions. of use.

Dans cette gamme de résistances, on préférera en particulier utiliser une composition d'acier comprenant, en % en poids : 0,15%<C<0,5%, 0,5%<Mn<3%, 0,1%<Si<0,5%, Cr<1%, Ni<0,1%, Cu<0,1%, Ti<0,2%, Al<0,1%, P<0,1%, S<0,05%, 0,0005%<B<0,08%, le reste étant du fer et des impuretés inévitables issues de l'élaboration de l'acier. Un exemple d'un acier disponible dans le commerce est le 22MnB5.In this range of resistors, it will be preferred in particular to use a steel composition comprising, in% by weight: 0.15% <C <0.5%, 0.5% <Mn <3%, 0.1% < If <0.5%, Cr <1%, Ni <0.1%, Cu <0.1%, Ti <0.2%, Al <0.1%, P <0.1%, S <0 , 05%, 0.0005% <B <0.08%, the remainder being iron and unavoidable impurities from the elaboration of steel. An example of a commercially available steel is 22MnB5.

Lorsque le niveau de résistance recherché est de l'ordre de 500 MPa, on préfère utiliser une composition d'acier comprenant : 0,040% ≤ C ≤ 0,100%, 0,80% ≤ Mn ≤ 2,00%, Si ≤ 0,30%, S ≤ 0,005%, P≤ 0,030%, 0,010% ≤ Al ≤ 0,070%, 0,015% ≤ Nb ≤ 0,100%, 0,030% ≤ Ti≤ 0,080%, N ≤ 0,009%, Cu ≤ 0,100%, Ni ≤ 0,100%, Cr ≤ 0,100%, Mo ≤ 0,100%, Ca ≤ 0,006%, le reste étant du fer et des impuretés inévitables issues de l'élaboration de l'acier.When the desired level of resistance is of the order of 500 MPa, it is preferred to use a steel composition comprising: 0.040% ≤ C ≤ 0.100%, 0.80% ≤ Mn ≤ 2.00%, Si ≤ 0.30 %, S ≤ 0.005%, P ≤ 0.030%, 0.010% ≤ Al ≤ 0.070%, 0.015% ≤ Nb ≤ 0.100%, 0.030% ≤ Ti ≤ 0.080%, N ≤ 0.009%, Cu ≤ 0.100%, Ni ≤ 0.100%, Cr ≤ 0.100%, Mo ≤ 0.100%, Ca ≤ 0.006%, the remainder being iron and unavoidable impurities resulting from the preparation of steel.

Les tôles d'acier peuvent être fabriquées par laminage à chaud et peuvent éventuellement être re-laminées à froid, en fonction de l'épaisseur finale visée, qui peut varier, par exemple, de 0,7 à 3 mm.The steel sheets can be made by hot rolling and can optionally be cold rolled, depending on the final thickness referred to, which can vary, for example, from 0.7 to 3 mm.

Les tôles peuvent être revêtues par tout moyen adapté tel qu'un procédé d'électrodéposition ou par un procédé de dépôt sous vide ou sous pression proche de la pression atmosphérique, tel que le dépôt par sputtering magnétron, par plasma froid ou par évaporation sous vide, par exemple, mais on préférera les obtenir par un procédé de revêtement au trempé à chaud dans un bain métallique fondu. On observe en effet que la protection cathodique superficielle est plus importante pour les revêtements obtenus par trempé à chaud que pour les revêtements obtenus par d'autres procédés de revêtement.The sheets may be coated by any suitable means such as an electrodeposition process or by a vacuum deposition method or under pressure close to atmospheric pressure, such as deposition by magnetron sputtering, by cold plasma or by evaporation under vacuum for example, but it will be preferred to obtain them by a hot dipping coating process in a molten metal bath. It is observed that the superficial cathodic protection is more important for coatings obtained by hot quenching than for coatings obtained by other coating processes.

Lorsque le procédé de revêtement au trempé à chaud est réalisé, après le dépôt du revêtement, ledit revêtement est refroidi jusqu'à sa solidification complète à une vitesse de refroidissement avantageusement comprise entre 5 et 30°C/s, de préférence entre 15 et 25°C/s, par exemple par soufflage de gaz inerte ou d'air. La vitesse de refroidissement de la présente invention ne permet pas l'obtention d'une phase amorphe dans le revêtement. Les tôles selon l'invention peuvent ensuite être mises en forme par tout procédé adapté à la structure et à la forme des pièces à fabriquer, tel que par exemple l'emboutissage à froid.When the hot dip coating process is carried out, after the deposition of the coating, said coating is cooled to complete solidification at a cooling rate advantageously between 5 and 30 ° C / s, preferably between 15 and 25 ° C / s, for example by blowing inert gas or air. The cooling rate of the present invention does not allow to obtain an amorphous phase in the coating. The sheets according to the invention can then be shaped by any method adapted to the structure and shape of the parts to be manufactured, such as for example cold stamping.

Cependant, les tôles selon l'invention sont plus particulièrement adaptées à la fabrication de pièces durcies sous presse, notamment par emboutissage à chaud.However, the sheets according to the invention are more particularly suitable for the manufacture of hardened parts in press, in particular by hot stamping.

Ce procédé consiste à approvisionner une tôle d'acier selon l'invention préalablement revêtue, puis à découper la tôle pour obtenir un flan. Ce flan est ensuite chauffé dans un four sous une atmosphère non protectrice jusqu'à une température d'austénitisation Tm comprise de 840 à 950°C, de préférence comprise de 880 à 930°C, puis à maintenir le flan à cette température Tm pendant une durée tm comprise de 1 à 8 minutes, de préférence comprise de 4 à 6 minutes.This method consists of supplying a steel sheet according to the previously coated invention, then cutting the sheet to obtain a blank. This blank is then heated in an oven under a non-protective atmosphere to an austenitization temperature Tm of 840 to 950 ° C, preferably of 880 to 930 ° C, and then to maintain the blank at this temperature Tm for a duration tm ranging from 1 to 8 minutes, preferably from 4 to 6 minutes.

La température Tm et le temps de maintien tm dépendent de la nature de l'acier mais aussi de l'épaisseur des tôles à emboutir qui doivent être entièrement dans le domaine austénitique avant leur mise en forme. Plus la température Tm est élevée, plus le temps de maintien tm sera court et vice-versa. En outre, la vitesse de montée en température influe également sur ces paramètres, une vitesse élevée (supérieure à 30°C/s par exemple) permettant de réduire également le temps de maintien tm.The temperature Tm and the holding time tm depend on the nature of the steel but also on the thickness of the sheets to be stamped, which must be entirely in the austenitic field before they are shaped. The higher the temperature Tm, the shorter the holding time tm and vice versa. In addition, the speed of rise in temperature also affects these parameters, a high speed (greater than 30 ° C / s for example) to also reduce the holding time tm.

Le flan est ensuite transféré vers un outil d'emboutissage à chaud puis embouti. La pièce obtenue est ensuite refroidie soit dans l'outil d'emboutissage lui-même, soit après transfert dans un outil de refroidissement spécifique.The blank is then transferred to a hot stamping tool and then stamped. The resulting part is then cooled either in the stamping tool itself or after transfer to a specific cooling tool.

La vitesse de refroidissement est dans tous les cas contrôlée en fonction de la composition de l'acier, afin que sa microstructure finale à l'issue de l'emboutissage à chaud comprenne au moins un constituant choisi parmi la martensite et la bainite, afin d'atteindre le niveau de résistance mécanique recherché.The cooling rate is in all cases controlled according to the composition of the steel, so that its final microstructure after the hot stamping comprises at least one component selected from martensite and bainite, in order to achieve the desired level of mechanical strength.

Le contrôle de la température Tm, du temps tm, de l'épaisseur du revêtement préalable et/ou de sa teneur en lanthane, en zinc et éventuellement en magnésium de telle sorte que la teneur moyenne finale en fer dans la partie supérieure du revêtement de la pièce soit inférieure à 75% en poids, de préférence inférieure à 50% en poids voire inférieure à 30% en poids, permet généralement que la pièce revêtue et emboutie à chaud présente une protection cathodique sacrificielle. Cette partie supérieure présente une épaisseur au moins égale à 5 µm et généralement inférieure à 13 µm. La proportion en fer peut par exemple être mesurée par spectrométrie de décharge luminescente (SDL).Controlling the temperature Tm, the time tm, the thickness of the primer coating and / or its content of lanthanum, zinc and optionally magnesium such that the final average iron content in the upper part of the coating of the part is less than 75% by weight, preferably less than 50% by weight or even less than 30% by weight, generally allows the hot-stamped and coated part to have sacrificial cathodic protection. This upper portion has a thickness of at least 5 microns and generally less than 13 microns. The proportion of iron can for example be measured by glow discharge spectrometry (LDS).

En effet, sous l'effet du chauffage jusqu'à la température d'austénitisation Tm, du fer issu du substrat diffuse dans le revêtement préalable et augmente son potentiel électrochimique. Pour maintenir une protection cathodique satisfaisante, il est donc nécessaire de limiter la teneur moyenne en fer dans la partie supérieure du revêtement final de la pièce.Indeed, under the effect of heating up to the austenitization temperature Tm, iron from the substrate diffuses into the primer coating and increases its electrochemical potential. To maintain a satisfactory cathodic protection, it is therefore necessary to limit the average iron content in the upper part of the final coating of the room.

Pour cela, il est possible de limiter la température Tm et/ou le temps de maintien tm. Il est également possible d'augmenter l'épaisseur du revêtement préalable pour empêcher le front de diffusion du fer d'aller jusqu'en surface du revêtement. On préférera à cet égard utiliser une tôle présentant une épaisseur de revêtement préalable supérieure ou égale à 27 µm, de préférence supérieure ou égale à 30 µm voire à 35 µm.For this, it is possible to limit the temperature Tm and / or the holding time tm. It is also possible to increase the thickness of the pre-coating to prevent the diffusion front of the iron from reaching the surface of the coating. In this respect, it is preferable to use a sheet having a pre-coating thickness greater than or equal to 27 μm, preferably greater than or equal to 30 μm or even 35 μm.

Pour limiter la perte de pouvoir cathodique du revêtement final, on pourra également augmenter les teneurs en lanthane et/ou en zinc et éventuellement en magnésium du revêtement préalable.In order to limit the loss of cathodic power of the final coating, the contents of lanthanum and / or zinc and possibly magnesium of the previous coating may also be increased.

L'homme du métier est en tout cas à même de jouer sur ces différents paramètres, en tenant également compte de la nature de l'acier, pour obtenir une pièce d'acier revêtu durcie sous presse, et en particulier, emboutie à chaud présentant les qualités requises par l'invention.The skilled person is in any case able to play on these different parameters, also taking into account the nature of the steel, to obtain a press-hardened coated steel part, and in particular, hot stamped with the qualities required by the invention.

Les exemples et figure qui suivent illustrent l'invention.The following examples and figure illustrate the invention.

La figure représente l'extension de la rouille rouge en fonction du temps en heure pour chacun des 6 revêtements testés dans les essais.The figure shows the extent of red rust versus time per hour for each of the 6 coatings tested in the tests.

Des essais de mise en oeuvre ont été réalisés pour illustrer certains modes de réalisation de l'invention.Implementation tests have been carried out to illustrate certain embodiments of the invention.

Essaistesting

Des essais ont été réalisés avec 4 échantillons tricouches, chacun étant constitué d'une tôle de 22MnB5 laminée à froid d'épaisseur 5 mm (1ère couche), munie d'un revêtement obtenu au trempé à chaud d'épaisseur 1 mm et dont la composition est précisée ci-dessous (2ème couche), lui-même recouvert d'une seconde tôle de 22MnB5 laminée à froid d'épaisseur 5 mm (3ème couche).Tests were carried out with four trilayer samples, each consisting of a sheet of cold rolled 22MnB5 5 mm thick (1 st layer), provided with a coating obtained by hot dip a thickness of 1 mm and composition is specified below (2nd layer), itself covered with a second sheet of laminated 22MnB5 thickness cold 5 mm (3rd layer).

Les 6 revêtements testés et comprenaient en % en poids :

  • 2% de silicium, 10% de zinc, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables,
  • 2% de silicium, 10% de zinc, 0,2% de lanthane, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables,
  • 2% de silicium, 10% de zinc, 0,5% de lanthane, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables,
  • 2% de silicium, 4% de zinc, 2% de magnésium, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables, et
  • 2% de silicium, 4% de zinc, 2% de magnésium, 0,2% de lanthane, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables.
  • 2% de silicium, 4% de zinc, 2% de magnésium, 0,5% de lanthane, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables.
The 6 coatings tested and included in% by weight:
  • 2% silicon, 10% zinc, the rest being made of aluminum and residual elements or unavoidable impurities,
  • 2% silicon, 10% zinc, 0.2% lanthanum, the rest being made of aluminum and residual elements or unavoidable impurities,
  • 2% silicon, 10% zinc, 0.5% lanthanum, the rest being made of aluminum and residual elements or unavoidable impurities,
  • 2% silicon, 4% zinc, 2% magnesium, the remainder being made of aluminum and residual elements or unavoidable impurities, and
  • 2% silicon, 4% zinc, 2% magnesium, 0.2% lanthanum, the rest being made of aluminum and residual elements or unavoidable impurities.
  • 2% silicon, 4% zinc, 2% magnesium, 0.5% lanthanum, the rest consisting of aluminum and residual elements or unavoidable impurities.

Différents tests de corrosion ont été effectués sur ce lot d'échantillons :

  • un test de corrosion accéléré, permettant de simuler la corrosion atmosphérique (test de corrosion cyclique VDA 233-102) ;
  • des tests statiques en chambre climatique à 35°C ou 50°C et 90% ou 95% d'humidité relative (RH). Les échantillons étaient aspergés de solution NaCl 1% (pH 7) une fois par jour, et ce sur une durée totale de 15 jours.
Various corrosion tests were carried out on this batch of samples:
  • an accelerated corrosion test to simulate atmospheric corrosion (cyclic corrosion test VDA 233-102);
  • static tests in climatic chamber at 35 ° C or 50 ° C and 90% or 95% relative humidity (RH). The samples were sprayed with 1% NaCl solution (pH 7) once a day for a total of 15 days.

Pour chacun de ces tests, des mesures d'extension de rouille rouge et électrochimiques ont été effectuées et sont fournies dans les tableaux ci-dessous. Al-2Si-10Zn Al-2Si-10Zn-0,2La Al-2Si-10Zn-0,5La Al-2Si-4Zn-2Mg Al-2Si-4Zn-2Mg-0,2La Al-2Si-4Zn-2Mg-0,5La Test N-VDA, rouille rouge Pas de protection Protection partielle Pas de protection Pas de protection Protection partielle Pas de protection Surface moyenne sur laquelle la rouille rouge s'est étendue en statique (%) 25 5 38 28 6 24 N-VDA, 35°C/95% RH, moyenne Courant galvanique (nA) -700 1862 240 N-VDA, 50°C/90% RH, moyenne Courant galvanique (nA) -120 1400 250 For each of these tests, red rust and electrochemical extension measurements were performed and are provided in the tables below. Al 2 Si-10Zn Al 2 Si-10Zn-0,2La Al 2 Si-10Zn-0,5La Al 2 Si-4Zn-2 Mg Al 2 Si-4Zn-2 Mg-0,2La Al 2 Si-4Zn-2 Mg-0,5La N-VDA test, red rust No protection Partial protection No protection No protection Partial protection No protection Average area over which red rust has spread to static (%) 25 5 38 28 6 24 N-VDA, 35 ° C / 95% RH, medium Galvanic current (nA) -700 1862 240 N-VDA, 50 ° C / 90% RH, medium Galvanic current (nA) -120 1400 250

La figure montre que l'extension de la rouille rouge est plus faible :

  • avec un revêtement 2% de silicium, 10% de zinc, 0,2% de lanthane, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables par rapport :
    • à un revêtement 2% de silicium, 10% de zinc, 0,5% de lanthane, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables, ou
    • à un revêtement 2% de silicium, 10% de zinc, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables,
  • avec un revêtement 2% de silicium, 4% de zinc, 2% de magnésium, 0,2% de lanthane, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables par rapport :
    • à un revêtement 2% de silicium, 4% de zinc, 2% de magnésium, 0,5% de lanthane, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables, ou
    • à un revêtement 2% de silicium, 4% de zinc, 2% de magnésium, le reste étant constitué d'aluminium et d'éléments résiduels ou d'impuretés inévitables.
The figure shows that the extent of red rust is lower:
  • with a coating of 2% silicon, 10% zinc, 0.2% lanthanum, the remainder being made of aluminum and residual elements or unavoidable impurities with respect to:
    • a coating of 2% silicon, 10% zinc, 0.5% lanthanum, the remainder consisting of aluminum and residual elements or unavoidable impurities, or
    • a coating of 2% silicon, 10% zinc, the remainder being made of aluminum and residual elements or unavoidable impurities,
  • with a coating of 2% silicon, 4% zinc, 2% magnesium, 0.2% lanthanum, the remainder being composed of aluminum and residual elements or unavoidable impurities with respect to:
    • a coating of 2% silicon, 4% zinc, 2% magnesium, 0.5% lanthanum, the remainder consisting of aluminum and residual elements or unavoidable impurities, or
    • 2% silicon, 4% zinc, 2% magnesium, the remainder being aluminum and residual elements or unavoidable impurities.

La figure montre également que le revêtement à 0,2% de lanthane présente un courant de couplage galvanique avec l'acier beaucoup plus élevé que le revêtement sans lanthane ou à 0,5% de La. Ces résultats indiquent que le revêtement à 0,2% de lanthane est actif et sacrificiel, et apporte en conséquence une meilleure protection cathodique à l'acier.The figure also shows that the 0.2% lanthanum coating has a galvanic coupling current with steel much higher than the lanthanum or 0.5% La coating. These results indicate that the coating at 0, 2% of lanthanum is active and sacrificial, and consequently provides better cathodic protection to steel.

Claims (14)

  1. Steel sheet provided with a coating providing sacrificial cathodic protection, the coating comprising 1 to 40 weight % zinc, from 0.01 to 0.4 weight % lanthanum, and optionally up to 10 weight % magnesium, optionally up to 15 weight % silicon and optionally up to 0.3 weight %, in accumulated weight, of possible additional elements selected from among Sb, Pb, Ti, Ca, Mn, Cr, Ni, Zr, In, Sn, Hf and Bi, the remainder being formed of aluminium and residual elements, among which iron at a content of 0 to 5 weight %, or unavoidable impurities derived in particular from pollution of hot dip galvanising baths through the passing of steel strips, or impurities derived from the ingots feedings these same baths or from ingots feeding vacuum deposit processes.
  2. The steel sheet provided with a coating providing sacrificial cathodic protection according to claim 1, the coating comprising 1 to 34 weight % of zinc.
  3. The steel sheet provided with a coating providing sacrificial cathodic protection according to claim 2, the coating comprising 2 to 20 weight % of zinc.
  4. The steel sheet provided with a coating providing sacrificial cathodic protection according to any one of claims 1 to 3, the coating comprising 0.1 to 0.3 weight % of lanthanum.
  5. The steel sheet provided with a coating providing sacrificial cathodic protection according to any one of claims 1 to 4, the coating comprising 0.2 to 0.3 weight % of lanthanum.
  6. The steel sheet provided with a coating providing sacrificial cathodic protection according to any one of claims 1 to 5, the coating comprising from 0 to 5 weight % of magnesium.
  7. The steel sheet provided with a coating providing sacrificial cathodic protection according to any one of claims 1 to 6, the coating comprising from 0.5 to 10 weight % of silicon.
  8. The steel sheet provided with a coating providing sacrificial cathodic protection according to any one of claims 1 to 7, the steel having a weight content of 0.15%<C<0.5%, 0.5%<Mn<3%, 0.1%<silicon<0.5%, Cr<1%, Ni<0.1%, Cu<0.1%, Ti<0.2%, Al<0.1%, P<0.1%, S<0.05%, 0.0005%<B<0.08%, the remainder being formed of iron and unavoidable impurities due to steel processing.
  9. The steel sheet provided with a coating providing sacrificial cathodic protection according to any one of claims 1 to 8, wherein said coating has a thickness of 10 to 50 µm.
  10. The steel sheet provided with a coating providing sacrificial cathodic protection according to claim 9, wherein the said coating has a thickness of 27 to 50 µm.
  11. The steel sheet provided with a coating providing sacrificial cathodic protection according to any one of claims 1 to 10, the coating being obtained by hot dip.
  12. A process to manufacture a part in steel provided with a coating providing sacrificial cathodic protection comprising the following steps taken in this order and consisting of:
    - Providing a steel sheet according to any one of claims 1 to 11 previously coated , then
    - Cutting said sheet to obtain a blank, then
    - Heating said blank in a non-protective atmosphere up to an austenitisation temperature Tm of 840 to 950°C, then
    - Holding said blank at this temperature Tm for a time tm of 1 to 8 minutes, then
    - Hot drawing said blank to obtain a part that is cooled at a rate such that the microstructure of said steel comprises at least one constituent selected from among martensite and bainite to obtain a steel part provided with a coating providing sacrificial cathodic protection,
    - the temperature Tm, time tm, thickness of the prior coating and the lanthanum, zinc and optionally magnesium contents thereof being selected so that the final mean iron content in an upper portion of the coating of said steel part provided with a coating providing sacrificial cathodic protection is lower than 75 weight %.
  13. A steel part provided with a coating providing sacrificial cathodic protection obtained using the hot drawing process according to claim 12.
  14. The steel part provided with a coating providing sacrificial cathodic protection obtained by cold drawing a sheet according to any one of claims 1 to 11.
EP15724718.0A 2014-05-28 2015-05-28 Steel sheet provided with a sacrificial cathodically protected coating comprising lanthane Active EP3149217B1 (en)

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CN106460138A (en) 2017-02-22
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US10676804B2 (en) 2020-06-09
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MA39875A (en) 2017-04-05
CN114717502A (en) 2022-07-08
PL3149217T3 (en) 2019-01-31
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