EP0939141A1 - Steel plate with aluminum coating having stress cracking resistance - Google Patents

Abstract

The steel sheet is coated with aluminum based alloy. After solidification of the applied coating, the sheet is subjected to a heat treatment that carries at least the outer layer (8) to a temperature greater than 570oC and lower than 660oC. Fabrication of a steel sheet (2) coated with an aluminum based metallic coating, principally subdivided into two layers, an inner layer (7) made up of one or several alloys with an iron, aluminum and/or silicon base and an outer layer (8) made up essentially of an aluminum based alloy. Other phases in the form of needles or elongated platelets (9) are distributed throughout the aluminum based alloy, which is applied by soaking in a bath of an aluminum based alloys. After solidification of the applied coating, the sheet is subjected to a heat treatment that carries at least the outer layer (8) to a temperature greater than 570oC and lower than 660oC. Sheet steel coated with an aluminum based alloy in accordance with this method is also claimed.

Description

The invention relates to aluminized sheets.

The application of a metallic coating based on aluminum on a sheet is a commonly used means of protecting a steel sheet against corrosion, in particular in the case where the temperature of use of this sheet exceeds about 400 ° C.

The thickness of the metal coating in question is generally between 5 and 100 µm.

We know several methods for applying a metallic coating on a sheet.

One can for example proceed by plating an aluminum film on the sheet to be coated, but this process is expensive.

We can for example proceed by soaking the sheet in a bath aluminum-based liquid.

• une couche interfaciale ou interne essentiellement composée d'un ou plusieurs alliages à base de fer et d'aluminium,
• et une couche externe comprenant essentiellement une phase principale à base d'aluminium et, accessoirement, d'autres phases sous formes d'aiguilles ou de lamelles allongées dispersées dans ladite phase principale ; l'article cite la présence de phases eutectiques entre les dendrites d'aluminium solidifiées.

When this dipping process is used, as described in the article in the journal STAHL und EISEN, vol. 111, no. 12, 12/12/91, pp. 111-116 (THYSSEN Forschung, Duisburg), in particular at FIG. 4 and in the middle of page 112, the covering comprises:

• an interfacial or internal layer essentially composed of one or more alloys based on iron and aluminum,
• and an outer layer essentially comprising a main phase based on aluminum and, incidentally, other phases in the form of needles or elongated lamellae dispersed in said main phase; the article cites the presence of eutectic phases between the solidified aluminum dendrites.

As seen in section, the slats are in the form of needles, it is therefore difficult to distinguish, in practice, needles and coverslips.

The internal layer of alloy having a fragile behavior, one seeks generally to limit its thickness.

To limit the thickness of this alloy layer, we generally use soaking baths containing an alloying inhibitor between aluminum and steel.

Silicon is the most commonly used alloying inhibitor; to be effective, its weight concentration must generally be greater than 6% in the soaking bath.

There are other known means for limiting the thickness of this layer of alloy, as to proceed, before coating, to a light nitriding of the surface to be coated, for example by performing recrystallization annealing of the steel to be coated in an atmosphere containing traces of ammonia.

Certain aluminized sheets can then be subjected to heat treatments, either to modify their properties, or even normal use (example: thermal screens); so it is important not to not significantly increase the thickness of the internal alloy layer.

• T.Yamada et H.Kawase, présenté au 5ème « IAVD Meeting » en 1989 (IAVD : « International Association for Vehicle Design »).
• Y.Hirose et Y.Uchida, dans le Supplément de la revue du « Japan Institute of Metals », n°3, 1983.

To limit this risk of growth of the internal layer of alloy during subsequent heat treatments, it is known to use steel grades containing sufficient contents of free nitrogen (for example ≥ 10 ^-2 % by weight); these steels can be renitrided steels; we will refer to this subject in the following articles:

• T.Yamada and H.Kawase, presented at the 5th "IAVD Meeting" in 1989 (IAVD: "International Association for Vehicle Design").
• Y. Hirose and Y. Uchida, in the Supplement to the review of the "Japan Institute of Metals", n ° 3, 1983.

• une couche interne 1, appliquée sur l'acier 2, essentiellement composée d'un ou plusieurs alliages à base de fer et d'aluminium, voire de silicium, notamment d'une phase dite τ5 et/ou d'une phase dite τ6.
• une couche externe 3 essentiellement composée d'aluminium sous forme de larges dendrites ; ces dendrites sont souvent (pas toujours) saturées en fer, et, le cas échéant, en silicium en solution solide.

As shown diagrammatically in FIG. 1, when the coating is applied by dipping, the coating which is obtained is then subdivided into two main superposed layers:

• an internal layer 1, applied to the steel 2, essentially composed of one or more alloys based on iron and aluminum, or even silicon, in particular of a phase called τ5 and / or a phase called τ6.
• an outer layer 3 essentially composed of aluminum in the form of large dendrites; these dendrites are often (not always) saturated with iron, and, where appropriate, with silicon in solid solution.

The inner layer can be subdivided into several sublayers comprising still other phases; at the interface between inner layer 1 and steel 2, we can sometimes find an underlay comprising the phases following: a phase called η (Fe2Al5), a phase called  (FeAl3), one or several phases based on aluminum nitride; the thickness of this underlay generally does not exceed 1 µm.

At the outer layer 3, when a bath containing silicon, we generally observe phases richer in silicon and / or iron than aluminum dendrites; these phases often have a form elongated coverslip or needles.

• des lamelles essentiellement composées de silicium,
• des aiguilles essentiellement composées d'une phase intermétallique τ6.

As phases 4 of elongated forms, we have for example identified:

• lamellae essentially composed of silicon,
• needles essentially composed of an intermetallic phase τ6.

This outer layer may also include alloying phases to based on aluminum, silicon and iron, in particular of eutectic composition with low melting point.

The τ5 phase has a hexagonal structure; it is sometimes called α _H or H; the iron content of this phase is generally between 29 and 36% by weight; the silicon content of this phase is generally between 6 and 12% by weight; the balance consists mainly of aluminum.

The τ6 phase has a monoclinic structure; it is sometimes called β or M; the iron content of this phase is generally between 26 and 29% in weight ; the silicon content of this phase is generally between 13 and 16% by weight; the balance consists mainly of aluminum.

Table I below summarizes the possible composition and the temperature of fusion of the phases present in the coatings obtained after soaked in an aluminizing bath (the composition and temperature of fusion are specified in this same table).

The τ6 phase is predominant when the bath contains more than 8% weight of silicon; phase inclusions τ6 have an elongated shape whereas the inclusions of phase τ5 generally have a globular shape.

It has been found that the steel sheets coated with an internal layer of alloy based on iron, aluminum and / or silicon and an outer layer composed mainly of aluminum did not resist corrosion well after deformation.

Indeed, a deformation such as a folding generally causes cracks which open onto the surface of the metallic coating; these cracks reduce the corrosion resistance of the steel. Composition of the coating phases Composition:% by mass Al Yes Fe Fusion temperature Bath <91 > 6 3
(saturation) 675 ° C
(T ° C hardened) Eutectic 87 12.2 0.8 ≈ 577 ° C Dendrites Al ≥98 ≤1.5 ≤0.5 ≈ 660 ° C Slats Si Majority silicon 1412 ° C Τ6 needles 55 14 31 > 577 ° C Phase τ5 55 to 62 6 to 12 31 to 36 > 577 ° C

The object of the invention is to offer a sheet with a coating based on of aluminum is more resistant to cracking by deformation, i.e. a sheet which is more resistant to corrosion after shaping.

• une couche interne composée essentiellement d'un ou plusieurs alliages à base de fer, aluminium et/ou silicium,
• une couche externe qui est composée essentiellement d'une phase à base d'aluminium et accessoirement d'autres phases sous forme d'aiguilles ou de lamelles allongées réparties dans ladite phase à base d'aluminium, et dont l'épaisseur est supérieure ou égale à celle de la dite couche interne d'alliage,

   dans lequel on applique ledit revêtement métallique à base d'aluminium par trempé dans un bain liquide à base d'aluminium,
   caractérisé en ce que, après solidification de ladite couche appliquée, on soumet ladite tôle à un traitement thermique adapté pour porter au moins ladite couche externe à une température supérieure à 570°C et inférieure à 660°C, dans des conditions notamment de durée, vitesses de chauffage et de refroidissement, adaptées :

• pour que l'épaisseur de ladite couche externe reste supérieure ou égale à celle de la dite couche interne d'alliage,
• et pour que la projection de la longueur de toutes lesdites aiguilles ou lamelles sur une direction perpendiculaire au plan de ladite couche externe soit strictement inférieure à l'épaisseur de cette couche.

To this end, the subject of the invention is a process for manufacturing a steel sheet coated with a metallic coating based on aluminum, which is mainly subdivided into two layers:

• an internal layer essentially composed of one or more alloys based on iron, aluminum and / or silicon,
• an outer layer which is essentially composed of an aluminum-based phase and incidentally other phases in the form of needles or elongated lamellae distributed in said aluminum-based phase, and whose thickness is greater than or equal to that of said internal alloy layer,

in which said metallic aluminum-based coating is applied by dipping in a liquid aluminum-based bath,
characterized in that, after solidification of said applied layer, said sheet is subjected to a heat treatment suitable for bringing at least said external layer to a temperature above 570 ° C and below 660 ° C, under conditions notably of duration, suitable heating and cooling speeds:

• so that the thickness of said external layer remains greater than or equal to that of said internal alloy layer,
• and so that the projection of the length of all said needles or strips on a direction perpendicular to the plane of said outer layer is strictly less than the thickness of this layer.

In this temperature range, greater than 570 ° C and less than 660 ° C, we are assured of the fusion of the eutectic phases of the outer layer (see the eutectic melting temperature in Table I ≈ 577 ° C) and the maintenance in the solid state of aluminum dendrites (cf. melting temperature of these dendrites in table I ≈ 660 ° C).

• ledit bain à base d'aluminium contient au moins 6 % en poids de silicium.
• ledit bain à base d'aluminium contient au moins 8 % en poids de silicium, auquel cas la proportion de phase τ6 est plus importante dans le revêtement, au dépens de celle de phase τ5.
• la durée du traitement thermique, dans la phase où ladite température est supérieure à 570°C, est inférieure ou égale à 15 secondes.

The invention may also have one or more of the following characteristics:

• said aluminum-based bath contains at least 6% by weight of silicon.
• said aluminum-based bath contains at least 8% by weight of silicon, in which case the proportion of phase τ6 is greater in the coating, at the expense of that of phase τ5.
• the duration of the heat treatment, in the phase where said temperature is greater than 570 ° C., is less than or equal to 15 seconds.

• une couche interne composée essentiellement d'un ou plusieurs alliages à base de fer, aluminium et/ou silicium,
• une couche externe qui est composée essentiellement d'une phase à base d'aluminium et accessoirement d'autres phases sous forme d'aiguilles ou de lamelles allongées réparties dans ladite phase à base d'aluminium, et dont l'épaisseur est supérieure ou égale à celle de la dite couche interne d'alliage,

   susceptible d'être obtenue par un procédé selon l'une quelconque des revendications précédentes,
   caractérisée en ce que la projection de la longueur de toutes lesdites aiguilles ou lamelles sur une direction perpendiculaire au plan de ladite couche externe est strictement inférieure à l'épaisseur de cette couche à l'endroit desdites lamelles ou aiguilles considérées.The subject of the invention is also a steel sheet coated with a metallic coating based on aluminum which is mainly subdivided into two layers:

• an internal layer essentially composed of one or more alloys based on iron, aluminum and / or silicon,
• an outer layer which is essentially composed of an aluminum-based phase and incidentally other phases in the form of needles or elongated lamellae distributed in said aluminum-based phase, and whose thickness is greater than or equal to that of said internal alloy layer,

capable of being obtained by a process according to any one of the preceding claims,
characterized in that the projection of the length of all of said needles or strips on a direction perpendicular to the plane of said outer layer is strictly less than the thickness of this layer at the location of said considered strips or needles.

According to this characteristic, considering the coating of the sheet and regardless of the variations in the thickness of the outer layer of this coating, no needle or strip continuously passes through this layer external.

• l'épaisseur de ladite couche interne d'alliage est inférieure ou égale à 5 µm ; cette épaisseur plus faible permet de limiter les risques d'apparition de fissures.
• ledit revêtement comprend des composés à base de nitrures d'aluminium intercalés entre l'acier de ladite tôle et ladite couche interne.
• la teneur en azote libre dudit acier est supérieure ou égale à environ 10^-2 % en poids.

The invention may also have one or more of the following characteristics:

• the thickness of said internal alloy layer is less than or equal to 5 μm; this smaller thickness makes it possible to limit the risks of the appearance of cracks.
• said coating comprises compounds based on aluminum nitrides interposed between the steel of said sheet and said inner layer.
• the content of free nitrogen in said steel is greater than or equal to about 10 ^-2 % by weight.

The presence of nitride at the interface or free nitrogen in the steel blocks or limits the growth of the thickness of the internal layer of alloy.

• une couche interne composée essentiellement d'un ou plusieurs alliages à base de fer, aluminium et/ou silicium,
• une couche externe qui est composée essentiellement d'une phase à base d'aluminium et accessoirement d'autres phases sous forme d'aiguilles ou de lamelles allongées réparties dans ladite phase à base d'aluminium, et dont l'épaisseur est supérieure ou égale à celle de la dite couche interne d'alliage,

   caractérisé en ce que, avant l'étape de mise en forme à proprement parler de ladite tôle, on soumet ladite tôle à un traitement thermique adapté pour porter au moins ladite couche externe à une température supérieure à 570°C et inférieure à 660°C, dans des conditions notamment de durée, vitesses de chauffage et de refroidissement, adaptées :

• pour que l'épaisseur de ladite couche externe reste supérieure ou égale à celle de la dite couche interne d'alliage,
• et pour que la projection de la longueur de toutes lesdites aiguilles ou lamelles sur une direction perpendiculaire au plan de ladite couche externe soit strictement inférieure à l'épaisseur de cette couche.

The invention also relates to a process for shaping a steel sheet coated with a metallic coating based on aluminum, which is mainly subdivided into two layers:

• an internal layer essentially composed of one or more alloys based on iron, aluminum and / or silicon,
• an outer layer which is essentially composed of an aluminum-based phase and incidentally other phases in the form of needles or elongated lamellae distributed in said aluminum-based phase, and whose thickness is greater than or equal to that of said internal alloy layer,

characterized in that, before the actual shaping step of said sheet, said sheet is subjected to a heat treatment suitable for bringing at least said outer layer to a temperature above 570 ° C and below 660 ° C , under conditions including duration, heating and cooling rates, suitable:

• so that the thickness of said external layer remains greater than or equal to that of said internal alloy layer,
• and so that the projection of the length of all said needles or strips on a direction perpendicular to the plane of said outer layer is strictly less than the thickness of this layer.

According to an additional characteristic of the invention, the duration of the heat treatment, in the phase where said temperature is higher than 570 ° C, is less than or equal to 15 seconds.

• la figure 1 est une représentation schématique de la structure des couches de revêtement d'une tôle aluminiée selon l'art antérieur.
• la figure 2 est une représentation schématique de la structure des couches de revêtement d'une tôle aluminiée selon l'invention.
• la figure 3 est une illustration du mode de pliage des tôles dans la méthode d'évaluation de la résistance à la fissuration.
• la figure 4 représente schématiquement le dispositif utilisé pour mettre en oeuvre l'invention telle que décrite dans l'exemple 1.
• les figures 5, 5 bis d'une part, et 6 d'autre part, sont des microphotographies en coupe illustrant les représentations schématiques respectivement des figures 1 et 2.

The invention will be better understood on reading the description which follows, given by way of nonlimiting example, and with reference to the appended figures in which:

• Figure 1 is a schematic representation of the structure of the coating layers of an aluminized sheet according to the prior art.
• Figure 2 is a schematic representation of the structure of the coating layers of an aluminized sheet according to the invention.
• Figure 3 is an illustration of the bending mode of the sheets in the method of evaluation of the resistance to cracking.
• FIG. 4 schematically represents the device used to implement the invention as described in example 1.
• FIGS. 5, 5 bis on the one hand, and 6 on the other hand, are microphotographs in section illustrating the schematic representations of FIGS. 1 and 2 respectively.

For the application of the metallic coating on a steel sheet 2, we carries out the quenching in a manner known per se and adapted to the steel grade used.

• dégraissage et nettoyage de la surface de la tôle,
• recuit de l'acier, généralement sous atmosphère inerte ou réductrice,
• directement en sortie de recuit, trempé dans un bain liquide à base d'aluminium,
• en sortie de trempé, essorage pour réguler l'épaisseur du revêtement et refroidissement pour solidifier le revêtement.

The conventional dip aluminizing process generally includes the following steps:

• degreasing and cleaning of the sheet surface,
• annealing of steel, generally in an inert or reducing atmosphere,
• directly after annealing, soaked in a liquid aluminum-based bath,
• at the end of the quench, spin to regulate the thickness of the coating and cooling to solidify the coating.

• une couche interne 1 composée essentiellement d'un ou plusieurs alliages à base de fer, aluminium et/ou silicium,
• une couche externe 3 composée essentiellement d'une phase à base d'aluminium.

Referring to FIGS. 1 and 5 and 5a, there is then obtained an aluminized sheet as previously described, the coating of which is mainly subdivided into two layers:

• an internal layer 1 essentially composed of one or more alloys based on iron, aluminum and / or silicon,
• an outer layer 3 essentially composed of an aluminum-based phase.

(The separation between steel substrate 2 and layer 1 has been marked in dotted on Figures 5 and 5a).

In a manner known per se, the steel grade, the conditions for applying the coating and the composition of the bath, in particular the content of alloying inhibitor, so that the thickness of the inner layer 1 alloy does not exceed that of the outer layer 3.

To limit the thickness of this layer 1, silicon is introduced as alloying inhibitor in the bath, with a content greater than or equal to 6% in weight; preferably, the silicon content is greater than or equal to 8%.

To limit the thickness of this layer 1, the annealing step can be performed under an atmosphere containing ammonia.

As seen in Figures 5 and 5a and as shown in Figure 1, the outer layer 3 comprises, in addition to the dendrites based aluminum, other phases 4 in the form of needles or elongated lamellae distributed in the thickness of this layer between the dendrites.

It is found that a significant proportion of needles and / or strips lead to the internal or external surface of the layer; the length of these “through” needles or strips is greater than or equal to the thickness layer; more precisely, the projection of the length of these needles or lamellae in a direction perpendicular to the plane of the layer is at less equal to the thickness of this layer.

This projection p is shown in FIG. 1 in the particular case any strip, the strip referenced 5.

We note for example that, for the lamellae referenced 6, the value of this projection corresponds to that of the thickness of layer 3.

• on soumet la tôle aluminiée à un traitement thermique adapté pour porter au moins la couche externe 3 du revêtement à une température supérieure à 570°C et inférieure à 660°C ;
• les conditions du traitement thermique, notamment la durée, les vitesses de chauffage et de refroidissement, sont adaptées :
• pour que l'épaisseur de ladite couche interne 1 d'alliage reste inférieure à celle de la dite couche externe,
• et pour que, dans ladite couche externe, la projection de la longueur toutes lesdites aiguilles ou lamelles sur une direction perpendiculaire au plan de ladite couche externe soit strictement inférieure à l'épaisseur de cette couche.

According to the invention, the following step is then carried out:

• the aluminized sheet is subjected to a heat treatment suitable for bringing at least the outer layer 3 of the coating to a temperature above 570 ° C and below 660 ° C;
• the conditions of the heat treatment, in particular the duration, the heating and cooling rates, are adapted:
• so that the thickness of said internal layer 1 of alloy remains less than that of said external layer,
• and so that, in said outer layer, the projection of the length of all said needles or strips on a direction perpendicular to the plane of said outer layer is strictly less than the thickness of this layer.

It can also be seen that the heat treatment according to the invention has has the effect of significantly reducing the proportion of needles and strips in this outer layer.

Preferably, the aluminum coating is applied so as to the thickness of said internal alloy layer is less than or equal to 5 μm and the heat treatment according to the invention is carried out so that the thickness of said internal alloy layer remains less than or equal to 5 μm.

The minimum temperature of the treatment according to the invention corresponds to the melting temperature of the phase of the outer layer corresponding to the Al-Si-Fe eutectic composition.

The maximum temperature of the treatment according to the invention corresponds to the melting temperature of the aluminum dendrites of the outer layer.

Preferably, in the heat treatment phase where the temperature is greater than 570 ° C, the treatment time is less than 15 seconds from so as to limit and / or prevent the increase in layer thickness internal alloy.

This heat treatment can be carried out in air, even if the coating oxidizes slightly on the surface.

Thus, based on these criteria for defining heat treatment, we see that we manage to significantly improve the resistance to coating cracking.

• on plie des échantillons de tôle 11 en angle fermé (voir figure 3) en intercalant dans le pli de la tôle une ou plusieurs cales 12, chaque cale ayant l'épaisseur de l'échantillon de tôle ; ainsi, des plis « 0T », « 1T » et « 2T », ... correspondent respectivement au pliage sans cale, avec une seule cale et avec deux cales ; la figure 3 représente donc un pliage « 2T ».
• sur une coupe métallographique pratiquée dans le pli, on observe alors, à l'extérieur du pli, le nombre de fissures débouchant à la surface du revêtement par millimètre de pli.

These findings can be made as follows:

• sheet metal samples 11 are folded at a closed angle (see FIG. 3) by inserting one or more shims 12 in the fold of the sheet, each shim having the thickness of the sheet sample; thus, folds "0T", "1T" and "2T", ... correspond respectively to folding without shims, with a single shim and with two shims; FIG. 3 therefore represents a “2T” folding.
• on a metallographic section made in the fold, we then observe, outside the fold, the number of cracks opening onto the surface of the coating per millimeter of fold.

More details on this method of evaluation can be found in the text of the standard known as "ECCA T7" entitled, in English, "Resistance to cracking on bending ", published by the" European Coil Coating Association ", standard T7, in the version of April 2, 1996.

Contrary to the official definition of this standard, the folding so that the direction of the fold corresponds to that of the rolling sheet metal.

By comparing observations made on aluminized sheets before heat treatment according to the invention and on the same sheets treated according to the invention, there is therefore, for identical folds, a significant reduction the number of cracks per millimeter of fold.

Due to the reduction in cracks, the corrosion resistance of the steel of these sheets, after deformation.

The aluminized sheet according to the invention therefore resists corrosion better after shaping, in the sense that the coating better protects the steel.

The structure of the coating of the aluminized sheet according to the invention is shown schematically in Figure 2 and shown in Figure 6; the general structure remains the same: on steel 2, an internal layer 7 of alloy and a layer external 8 mainly composed of aluminum.

• que les aiguilles et/ou lamelles restantes 9 sont beaucoup plus courtes qu'avant le traitement thermique, et qu'on est parvenu, grâce au traitement thermique selon l'invention, à obtenir que la projection de leur longueur dans une direction perpendiculaire au plan de cette couche est strictement inférieure à l'épaisseur de cette couche.
• que la couche externe peut maintenant contenir des inclusions en forme de « pavés », qui semblent contenir essentiellement du silicium.
• que la teneur moyenne en aluminium de la couche externe 8 est supérieure à la teneur moyenne en aluminium de la couche externe 3 des figure 1 , 5 ou 5 bis.
• que la proportion d'aiguilles et/ou de lamelles 9 a pu diminuer.

By comparison with the aluminized sheet before treatment (FIGS. 1 and 5, 5a), the following differences are mainly observed:

• that the remaining needles and / or strips 9 are much shorter than before the heat treatment, and that we have managed, thanks to the heat treatment according to the invention, to obtain that the projection of their length in a direction perpendicular to the plane of this layer is strictly less than the thickness of this layer.
• that the outer layer can now contain inclusions in the form of "blocks", which seem to contain essentially silicon.
• that the average aluminum content of the outer layer 8 is greater than the average aluminum content of the outer layer 3 of Figures 1, 5 or 5a.
• that the proportion of needles and / or strips 9 may have decreased.

By way of example, in FIG. 2, the most significant value has been represented at p ' high of this projection corresponding to the lamella or needle referenced 10 ; it can thus be seen that it is much less than the average thickness of the layer 8.

Without prejudging a definitive explanation, it is believed that the treatment according to the invention generates a structural rearrangement of the layer external leading to the disappearance and / or partition of the lamellae or needles of this layer.

Thus, in the event of deformation of this sheet, the cracks which appear for example in the fragile inner layer of alloy 7 can no longer be spread as easily in the outer layer 8.

The heat treatment according to the invention would therefore have the technical effect first to rearrange the structure of the outer layer so as to obtain a structure opposing the propagation of cracks.

The heat treatment according to the invention must also be adapted for prevent or limit the increase in thickness of the inner layer 7 of alloy, because this layer is particularly fragile.

The conditions of the heat treatment according to the invention can therefore be optimized in a manner known per se between these two compromises : sufficient rearrangement of the outer layer and slight increase in the thickness of the internal layer of alloy.

The heat treatment according to the invention is of short duration, which is an important advantage compared to long-term annealing treatments and at a lower temperature.

This heat treatment can therefore advantageously be carried out in line on conventional dip coating installations.

Preferably, this heat treatment is applied so as to heat the outer layer more than the inner alloy layer.

• des moyens de chauffage à la flamme,
• des moyens de chauffage par rayonnement infrarouge,
• des moyens de chauffage par induction, de préférence à haute fréquence pour obtenir une épaisseur de peau aussi faible que possible, voire comparable à l'épaisseur de la couche externe.

To do the heat treatment in this way, conventional heating means can be used, such as:

• flame heating means,
• means of heating by infrared radiation,
• induction heating means, preferably at high frequency to obtain a skin thickness as small as possible, or even comparable to the thickness of the outer layer.

The heat treatment according to the invention can also improve appreciably the surface reflectivity of the sheet, in particular in the field wavelengths between 1.5 and 5 µm; we get this advantage additional especially when performing heat treatment under non-oxidizing atmosphere.

But, in this case, it should be noted that the treatment according to the invention is not limited to a surface gloss treatment; indeed some effective brightening treatments cause significant increase of the thickness of the internal layer of alloy, which is contrary to the invention described here.

To limit the increase in the thickness of the internal alloy layer during the heat treatment according to the invention, preferably a steel grade containing a content greater than or equal to 10 ^-2 % by weight of nitrogen is used. free.

For example, steels calmed with aluminum and wound with low temperature after hot rolling; by winding at a temperature less than or equal to 610 ° C, the formation of aluminum nitrides (AIN) is limited and the content of free nitrogen is then maintained at a sufficiently high level.

When applying the metallic coating to the dip, this nitrogen free form of aluminum nitride-based phases at the interface between steel and the inner layer.

To limit the increase in the thickness of the internal alloy layer during the heat treatment according to the invention, it is possible, before application of the coating, nitriding the surface of the steel to be coated or simply performing the annealing before soaking in an atmosphere containing ammonia.

The following examples illustrate the invention.

Example 1 :

The purpose of this example is to illustrate the invention in the case of aluminizing of a steel grade called "aluminum calm".

The aluminized steel sheet according to the invention has the following analysis (element contents expressed in thousandths of% by weight): Composition of the steel of Example 1 Element VS Mn P S Yes Al Or Cr Cu NOT 10 ^-3 % 53 300 10 15 6 22 20 20 7 11

The other elements are in the state of traces; the titanium content is less than 10 ^-3 %.

A significant part of the nitrogen contained in this steel is said to be "free", the other part being essentially combined with aluminum in the form of aluminum nitride (AIN); the content of AIN having been evaluated at approximately 1.4 10 ^-3 % by weight of “nitrogen” equivalent, it is deduced therefrom that the content of free nitrogen is of the order of 10 ^-2 % by weight in this steel.

A coating based on both sides of this sheet is applied aluminum with a total thickness of approximately 15 µm; this coating is applied as previously described by soaking in an aluminum bath containing silicon.

The average silicon content in the coating is 7% about.

Then applied to this aluminized sheet heat treatment according to the invention; this treatment consists in heating the sheet at the speed of 4 ° C / s up to a temperature of 578 ° C, and, as soon as this temperature is reached, blowing with nitrogen to obtain a speed of cooling between 10 and 15 ° C / s.

To carry out this heat treatment, the schematic device is used in FIG. 4: it is a vertical oven 13 comprising two series of electrical resistors 14; the sample to be treated 15, made of aluminized sheet, is suspended from a support rod 16; to measure the temperature of heat treatment, a thermocouple 17, type K (chromel-alumel) is used, 0.2 mm in diameter and class 1 (± T ° C x 0.004, i.e. ± 2.4 ° C at 600 ° C); this thermocouple 17 is welded to the face coated with the aluminized sheet.

After heat treatment, an aluminized sheet is then obtained according to the invention.

Metallographic observations made on samples show that the thickness of the inner alloy layer of the coating has little varied following heat treatment: 2.7 µm before treatment, 4 µm after treatment ; this thickness therefore remains less than 5 μm.

The improvement in the crack resistance of the coating as previously described, counting the number of cracks outlets per millimeter of fold on a metallographic section.

The results obtained are reported in Table III below.

It is therefore observed that the coating according to the invention is very resistant better cracking than the coating according to the prior art which has not undergone heat treatment.

It is also found that the internal layer of alloy is less peeled off by deformation after the heat treatment according to the invention. Folding results of Example 1 Aluminized sheet Folding type Average number of cracks / mm Average crack width Observations Before heat treatment 0T 10 40 µm Inner layer separation and large cracks. 1T 8 62 µm 2T 5 7 µm 3T 2 7 µm After heat treatment (invention) 0T 5 41 µm Little or no internal layer detachments 1T 3 55 µm 2T 0 - 3T 0 -

Example 2 :

The purpose of this example is to illustrate the invention in the case of aluminizing a steel grade called "ultra low carbon" or "ULC" ("Ultra Low Carbon "in English).

The aluminized steel sheet according to the invention has the following analysis (element contents expressed in thousandths of% by weight): Composition of the steel of Example 2 Element VS Mn P S Yes Al Or Cr Cu NOT 10 ^-3 % 3 230 10 13 8 46 16 23 20 12

The other elements are in the state of traces.

A special feature of this steel also lies in its temperature of winding at the end of hot rolling: <620 ° C.

Due to its very low carbon content, the main hardening agent of this steel is the free nitrogen it contains; this steel therefore presents a much higher formability than the steel described in Example 1.

This steel is aluminized and then heat treated according to the invention in the same conditions as in Example 1.

An aluminized sheet according to the invention is then obtained.

As before, metallographic observations show that the thickness of the internal alloy layer of the coating varied little due to heat treatment.

The improvement in the crack resistance of the coating as in Example 1.

The results obtained are reported in Table V.

As previously, it is observed that the coating according to the invention is much more resistant to cracking than the coating according to the prior art which has not undergone heat treatment. Folding results of Example 2 Aluminized sheet Folding type Average number of cracks / mm Average crack width Before heat treatment 0T 11 31 µm 1T 8 28 µm 2T 6 7 µm 3T 2 3 µm After heat treatment (invention) 0T 10 17 µm 1T 3 10 µm 2T 1 3 µm 3T <1 3 µm

Claims (10)

Method for manufacturing a steel sheet (2) coated with a metallic coating based on aluminum, mainly subdivided into two layers: an internal layer (7) essentially composed of one or more alloys based on iron, aluminum and / or silicon, an outer layer (8) which is essentially composed of an aluminum-based phase and incidentally other phases in the form of needles or elongated lamellae (9) distributed in said aluminum-based phase, and of which l thickness is greater than or equal to that of said internal alloy layer, in which said metallic aluminum-based coating is applied by dipping in a liquid aluminum-based bath,
characterized in that, after solidification of said applied layer, said sheet is subjected to a heat treatment suitable for bringing at least said external layer (8) to a temperature above 570 ° C and below 660 ° C, under conditions in particular duration, heating and cooling rates, adapted: so that the thickness of said external layer (8) remains greater than or equal to that of said internal alloy layer (7), and so that the projection of the length of all said needles or lamellae (9) on a direction perpendicular to the plane of said outer layer (8) is strictly less than the thickness of this layer (8). Method according to claim 1 characterized in that said bath aluminum base contains at least 6% by weight of silicon. Method according to any one of claims 1 to 2 characterized in that said aluminum-based bath contains at least 8% by weight of silicon. Method according to any one of claims 1 to 3 characterized in that the duration of the heat treatment, in the phase where said temperature is greater than 570 ° C, is less than or equal to 15 seconds. Sheet steel coated with an aluminum-based metal coating, mainly divided into two layers: an internal layer (7) essentially composed of one or more alloys based on iron, aluminum and / or silicon, an outer layer (8) which is essentially composed of an aluminum-based phase and incidentally other phases in the form of needles or elongated lamellae (9) distributed in said aluminum-based phase, and of which l thickness is greater than or equal to that of said internal alloy layer (7), capable of being obtained by a process according to any one of the preceding claims,
characterized in that the projection of the length of all said needles or strips (9) on a direction perpendicular to the plane of said outer layer (8) is strictly less than the thickness of this layer (8). Sheet according to claim 5, characterized in that the thickness of said internal alloy layer (7) is less than or equal to 5 μm. Sheet according to any one of claims 5 to 6, characterized in that said coating includes nitride-based compounds aluminum interposed between the steel (2) of said sheet and said inner layer (7). Sheet according to any one of claims 5 to 7, characterized in that the free nitrogen content of said steel (2) is greater than or equal to about 10 ^-2 % by weight. Process for forming a steel sheet coated with a metallic coating based on aluminum, mainly subdivided into two layers: an internal layer (7) essentially composed of one or more alloys based on iron, aluminum and / or silicon, an outer layer (8) which is essentially composed of an aluminum-based phase and incidentally other phases in the form of needles or elongated lamellae (9) distributed in said aluminum-based phase, and of which l thickness is greater than or equal to that of said internal alloy layer (7), characterized in that, before the actual shaping step of said sheet, said sheet is subjected to a heat treatment suitable for bringing at least said outer layer (8) to a temperature above 570 ° C and below 660 ° C, under conditions including duration, heating and cooling rates, suitable: so that the thickness of said external layer (8) remains greater than or equal to that of said internal alloy layer (7), and so that the projection of the length of all said needles or lamellae (9) on a direction perpendicular to the plane of said outer layer (8) is strictly less than the thickness of this layer (8). Process according to Claim 9, characterized in that the duration of the heat treatment, in the phase where said temperature is higher than 570 ° C, is less than or equal to 15 seconds.

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