EP1805341A1

EP1805341A1 - Hot-dip coating method in a zinc bath for strips of iron/carbon/manganese steel

Info

Publication number: EP1805341A1
Application number: EP05809221A
Authority: EP
Inventors: Pascal Drillet; Daniel Bouleau
Original assignee: Arcelor France SA
Current assignee: ArcelorMittal France SA
Priority date: 2004-10-20
Filing date: 2005-10-10
Publication date: 2007-07-11
Anticipated expiration: 2025-10-10
Also published as: ZA200703345B; CN101072892A; CA2584449A1; RU2007118637A; CN100554487C; FR2876711B1; ATE394517T1; PL1805341T3; WO2006042930A1; FR2876711A1; DE602005006603D1; JP2008517157A; EP1805341B1; KR20070064373A; US20080083477A1; US7556865B2; ES2306247T3; MX2007004728A; RU2363756C2; JP4828544B2

Abstract

The invention relates to a hot-dip coating method in a liquid bath of zinc, comprising aluminium, for a strip of running austenitic iron/carbon/manganese steel, whereby said strip is subjected to a thermal treatment in an oven within which a reducing atmosphere with relation to iron is present, to give a strip covered with a thin layer of manganese oxide, said strip covered with the thin layer of manganese oxide is then run through said bath, the content of aluminium of which is adjusted to give a value at least equal to the content necessary such that the aluminium completely reduces the layer of manganese oxide such as to form a coating on the surface of the strip, comprising a layer of iron/manganese/zinc alloy and an outer layer of zinc.

Description

Hot-dipped coating process in a zinc bath of iron-carbon-manganese steel strip

The present invention relates to a method of hot dip coating in a zinc-based liquid bath comprising aluminum, a strip of austenitic iron-carbon-manganese steel in scrolling.

The steel belts conventionally used in the automotive field, such as for example the dual-phase steel belts, are coated with a zinc-based coating to protect them against corrosion before they are shaped or delivered. This zinc layer is generally applied continuously either by electrodeposition in an electrolytic bath containing zinc salts, or by vacuum deposition, or by hot quenching of the high speed band in a bath of molten zinc.

Before being coated with a zinc layer by hot dipping in a zinc bath, the steel strips undergo a recrystallization annealing in a reducing atmosphere in order to give the steel a homogeneous microstructure and to improve its properties. mechanical characteristics. Under industrial conditions, this recrystallization annealing is carried out in an oven in which there is a reducing atmosphere. For this purpose, the strips run in the oven consisting of an enclosure completely isolated from the outside atmosphere, comprising three zones, a first heating zone, a second temperature holding zone, and a third cooling zone, in which there is an atmosphere composed of a reducing gas with respect to iron. This gas may be chosen for example from hydrogen, and mixtures of nitrogen and hydrogen, and has a dew point of between -40 ^° C. and -15 ° C. Thus, in addition to the improvement of the mechanical properties of the steel, the recrystallization annealing of the steel strips under a reducing atmosphere makes it possible to bond the zinc layer to the steel, because the iron oxides present on the surface of the steel band are reduced by the reducing gas.

For certain automotive applications that require increased lightening and strength of metal structures in the event of impact, is beginning to replace conventional steel grades with austenitic iron-carbon-manganese steels which have superior mechanical properties, including a particularly advantageous combination of strength and elongation at break, excellent workability and high resistance to fracture in the presence of defects or concentration of stresses. The applications concern, for example, parts that contribute to the safety and durability of motor vehicles or even pieces of skin.

These steels must also, after recrystallization annealing, be protected against corrosion by a layer of zinc. However, the inventors have shown that it was impossible, under the usual conditions, to coat a steel-carbon-manganese steel strip traveling at a high speed (greater than 40 m / s) by a layer of zinc, a hot dipping coating process in a zinc bath. In fact, the oxides of the MnO and (Mn ₁ Fe) O type which are formed during the heat treatment which the strip undergoes before being coated render the surface of the strip non-wetting for liquid zinc.

The present invention therefore aims to provide a method for coating by hot dipping in a zinc-based liquid bath, an iron-carbon-manganese steel strip running through a coating based on zinc.

To this end, the subject of the invention is a method of hot-dip coating in a zinc-based liquid bath comprising aluminum, said bath having a temperature T2, of an austenitic steel-carbon steel strip. manganese comprising: 0.30% <C <1.05%, 16% <Mn <26%, Si <1%, and Al <0.050%, the contents being by weight, said process comprising the steps of:

- Subjecting said strip heat treatment in an oven inside which prevails a reducing atmosphere vis-à-vis the iron, said heat treatment comprising a heating phase at a heating rate V1, a maintenance phase at a temperature T1 and during a holding time M, followed by a cooling phase at a cooling rate V2, to obtain a strip covered on both sides with a continuous undercoat of mixed iron oxide and manganese (Fe, Mn) O amorphous, and a continuous or discontinuous outer layer of crystalline MnO manganese oxide, then - scrolling said covered strip of oxide layers in said bath to coat it with a zinc-based coating, the aluminum content in said bath being adjusted to a value at least equal to the amount necessary for the aluminum to reduce completely the crystalline MnO manganese oxide layer and at least partially the amorphous (Fe ₁ Mn) O oxide layer, so as to form on the surface of the strip said coating comprising three layers of iron-manganese-zinc alloy and a superficial layer of zinc. The invention also relates to the austenitic iron-carbon-manganese steel strip coated with a zinc-based coating obtainable by this method.

The features and advantages of the present invention will appear better in the following description given by way of non-limiting example.

The inventors have thus demonstrated that by creating favorable conditions for the bi-layer of mixed oxide (Fe ₁ Mn) O and of manganese oxide forming on the surface of the iron-carbon steel strip. manganese, being reduced by the aluminum contained in the zinc-based liquid bath, the surface of the strip became wetting with respect to the zinc, which allowed to coat it with a coating based on zinc.

The thickness of this steel strip is typically between 0.2 and 6 mm, and may be issued from either the hot band or the cold band train. The austenitic iron-carbon-manganese steel used according to the invention comprises, in% by weight: 0.30% <C <1.05%, 16% <Mn <26%, Si <1%, Al < 0.050%, S <0.030%, P <0.080%, N <0.1%, and optionally, one or more elements such as: Cr <1%, Mo ≤ 0.40%, Ni <1%, Cu <5%, Ti <0.50%, Nb <0.50%, V ≤ 0.50%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation.

Carbon plays a very important role in the formation of the microstructure: it increases the stacking fault energy and promotes the stability of the austenitic phase. In combination with a manganese content ranging from 16 to 26% by weight, this stability is obtained for a carbon content greater than or equal to 0.30%. However, for a carbon content greater than 1.05%, it becomes difficult to avoid a precipitation of carbides which occurs during certain thermal cycles of industrial manufacture, in particular during winding cooling, and which degrades the ductility and tenacity.

Preferably, the carbon content is between 0.40 and 0.70% by weight. Indeed, when the carbon content is between 0.40% and 0.70%, the stability of the austenite is increased and the strength is increased. Manganese is also an essential element for increasing strength, increasing stacking fault energy and stabilizing the austenitic phase. If its content is less than 16%, there is a risk of formation of martensitic phases which significantly reduce the ability to deform. On the other hand, when the manganese content is greater than 26%, the ductility at room temperature is degraded. In addition, for cost reasons, it is not desirable for the manganese content to be high.

Preferably, the manganese content in the steel according to the invention is between 20 and 25% by weight. Silicon is an effective element for deoxidizing steel as well as for hardening in the solid phase. However, beyond a content of 1%, Mn ₂ SiO ₄ and SiO ₂ layers are formed on the surface of the steel, which show a reduction ability of the aluminum contained in the water-based bath. zinc significantly lower than the mixed oxide (Fe ₁ Mn) O and manganese oxide MnO layers.

Preferably, the silicon content in the steel is less than 0.5% by weight. Aluminum is also a particularly effective element for the deoxidation of steel. Like carbon, it increases the stacking fault energy. However, its excessive presence in steels with a high manganese content has a disadvantage: In fact, manganese increases the solubility of nitrogen in the liquid iron, and if too much aluminum is present in the steel, Nitrogen combined with aluminum precipitates in the form of aluminum nitrides hindering the migration of grain boundaries during hot processing and greatly increases the risk of crack appearances. An Al content less than or equal to 0.050% makes it possible to avoid a precipitation of AlN. Correlatively, the nitrogen content must be less than or equal to 0.1% in order to prevent this precipitation and the formation of volume defects (blowholes) during solidification.

Furthermore, in excess of 0.050% by weight of aluminum, oxides such as MnAl ₂ O ₄ , MnO ₁ Al ₂ O _3, which are more difficult to reduce, are formed during the recrystallization annealing of the steel. aluminum contained in the zinc-based coating bath, as oxides (Fe ₁ Mn) O and MnO. Indeed, these oxides comprising aluminum are much more stable than oxides (Fe ₁ Mn) O and MnO. Therefore, even if it is possible to form on the surface of the steel a zinc-based coating, it will in any case little adherent because of the presence of alumina. Thus, to achieve good adhesion of the zinc-based coating, it is essential that the aluminum content in the steel is less than 0.050% by weight.

Sulfur and phosphorus are impurities that weaken the grain boundaries. Their respective content must be less than or equal to 0.030 and 0.080% in order to maintain sufficient hot ductility.

Chromium and nickel can be used as an option to increase the strength of the steel by hardening in solid solution. However, since chromium decreases the stacking fault energy, its content must be less than or equal to 1%. Nickel contributes to a significant elongation rupture, and in particular increases the toughness. However, it is also desirable, for cost reasons, to limit the nickel content to a maximum content of less than or equal to 1%. For reasons similar, the molybdenum may be added in an amount less than or equal to 0.40%.

Similarly, optionally, addition of copper to a content of less than or equal to 5% is a means of hardening the steel by precipitation of metallic copper. However, beyond this content, copper is responsible for the appearance of surface defects hot sheet.

Titanium, niobium and vanadium are also elements that can optionally be used to obtain precipitation hardening of carbonitrides. However, when the Nb or V, or Ti content is greater than 0.50%, excessive precipitation of carbonitrides can cause a reduction in toughness, which should be avoided.

After being cold rolled, the austenitic iron-carbon-manganese steel strip is heat-treated to recrystallize the steel. The recrystallization annealing makes it possible to give the steel a homogeneous microstructure, to improve its mechanical characteristics, and in particular to give it ductility to allow its use in stamping.

This heat treatment is performed in an oven inside which there is an atmosphere composed of a reducing gas vis-à-vis the iron, to avoid excessive oxidation of the surface of the strip, and allow good adhesion of zinc. This gas is selected from hydrogen, and nitrogen-hydrogen mixtures. Preferably, the gaseous mixtures comprising between 20 and 97% by volume of nitrogen and between 3 and 80% by volume of hydrogen, and more preferably between 85 and 95% by volume of nitrogen and between 5 and 15%, are chosen. in volume of hydrogen. Indeed, although hydrogen is an excellent iron reducing agent, it is preferred to limit its concentration because of its high cost relative to nitrogen. By making prevail in the enclosure of the furnace a reducing atmosphere vis-à-vis the iron, thus prevents the formation of a thick layer of calamine, that is to say whose thickness is much greater than 100 nm. In the case of iron-carbon-manganese steels, calamine is a layer of iron oxide comprising a small proportion of manganese. Gold not only this calamine layer prevents any adhesion of zinc on steel, but also it is a layer that tends to crack easily which makes it all the more undesirable. Under industrial conditions, the atmosphere prevailing in the furnace is certainly reducing with respect to iron, but not for elements such as manganese. Indeed, the gas constituting the atmosphere in the furnace comprises traces of moisture and / or oxygen that can not be avoided, but it is possible to control by imposing the dew point of said gas.

Thus, the inventors have observed that, according to the invention, at the end of the recrystallization annealing, the lower the dew point in the oven, or in other words the lower the oxygen partial pressure, the lower the The manganese oxide formed on the surface of the iron-carbon-manganese steel strip is fine. This observation may seem at odds with Wagner's theory that the lower the dew point, the higher the density of oxides formed on the surface of a carbon steel strip. Indeed, when the amount of oxygen decreases on the surface of the carbon steel, the migration of the oxidizable elements contained in the steel to the surface accelerates, which promotes the oxidation of the surface. Without wishing to be bound by any theory, the inventors believe that in the case of the invention, the amorphous oxide layer (Fe ₁ Mn) O becomes rapidly continuous. It therefore constitutes a barrier for the oxygen of the atmosphere in the furnace, which is no longer in direct contact with the steel. An increase in the oxygen partial pressure in the furnace therefore leads to an increase in the thickness of the manganese oxide and does not cause internal oxidation, ie no layer is observed. of additional oxide between the surface of the austenitic iron-carbon-manganese steel and the amorphous oxide layer (Fe ₁ Mn) O. The recrystallization annealing carried out under the conditions of the invention thus makes it possible to form on both sides of the strip a continuous sub-layer of mixed oxide of iron and manganese (amorphous Fe ₁ Mn) O, the thickness of which is preferably between 5 and 10 nm, and a continuous or discontinuous outer layer of crystalline MnO manganese oxide whose thickness is preferably between 5 and 90 nm, preferably between 5 and 50 nm, and more preferably between 10 and 40 nm. The MnO outer layer has a granular appearance, and the size of the MnO crystals increases sharply as the dew point also increases. Indeed, their average diameter varies from approximately 50 nm for a dew point of -80 ⁰ C ₁ the layer of MnO being then discontinuous, up to 300 nm for a dew point of +10 ⁰ C, the MnO layer being in this case continues.

The inventors have demonstrated that, when the content by weight of aluminum in the zinc-based liquid bath is less than 0.18% and when the manganese oxide layer MnO is greater than 100 nm, the latter It is not reduced by the aluminum contained in the bath, and the zinc-based coating is not obtained due to the non-wetting effect of MnO with respect to the zinc.

For this purpose, the dew point according to the invention, at least in the zone for maintaining the temperature of the oven, and preferably in the entire enclosure of the oven, is preferably between -80 and 20 ° C, advantageously between - 80 and -40 ⁰ C and more preferably between -60 and -40 ° C. Indeed, under the usual industrial conditions it is possible under particular conditions to lower the dew point of a recrystallization annealing furnace to a value below -60 ^° C., but not below -80 ° C.

Above 20 ^° C., the thickness of the manganese oxide layer becomes too great to be reduced by the aluminum contained in the zinc-based liquid bath under industrial conditions, that is to say during a period of time. time less than 10 seconds.

The range -60 to -40 ⁰ C is advantageous because it allows to form a bi-oxide layer of relatively reduced thickness which will be easily reduced by the aluminum contained in the zinc-based bath.

The heat treatment comprises a heating phase at a heating rate V1, a holding phase at a temperature T1 and during a holding time M, followed by a cooling phase at a cooling rate V2. The heat treatment is preferably carried out at a heating rate V1 greater than or equal to 6 ° C / s, because below this value the holding time M of the strip in the oven is too long and does not correspond to the industrial requirements. of productivity. The temperature T1 is preferably between 600 and 900 ^° C. In fact, below 600 ^° C., the steel will not be completely recrystallized and its mechanical characteristics will be insufficient. Beyond 900 ⁰ C, not only the grain size of the steel increases which is harmful for obtaining good mechanical characteristics, but also the thickness of the manganese oxide layer MnO strongly increases and makes difficult, if not impossible, the subsequent deposition of a coating based on zinc, because the aluminum contained in the bath will not have completely reduced the MnO. The lower the temperature T1, the lower the amount of MnO formed, and the easier it will be to reduce it by aluminum, that is why T1 is preferably between 600 and 820 ^° C., advantageously less than or equal to 750 ° C, and more preferably between 650 and 750 ° C.

The holding time M is preferably between 20 s and 60 s, and advantageously between 20 and 40 s. The recrystallization annealing is generally carried out by a radiant tube heater.

Preferably, the strip is cooled to an immersion temperature of the T3 band between (T2 - 10 ^° C.) and (T2 + 30 ° C.), T2 being defined as being the temperature of the liquid bath based on zinc. By cooling the strip to a temperature T3 close to the temperature T2 of the bath, it is avoided to cool or heat the liquid zinc in the vicinity of the strip running in the bath, which allows to form on the strip a coating based on zinc having a homogeneous structure all along the strip.

The strip is preferably cooled at a cooling rate V2 of greater than or equal to 3 ° C./s, advantageously greater than 10 ° C./s, so as to avoid the enlargement of the grains and to obtain a steel strip having good mechanical characteristics. . Thus, the strip is generally cooled by injection of an air flow on both sides. When, after undergoing recrystallization annealing, the austenitic iron-carbon-manganese steel strip is covered on both sides by the two-layer oxides, it is passed through the zinc-based liquid bath containing water. 'aluminum. The aluminum contained in the zinc bath contributes not only to the at least partial reduction of the two-layer oxide, but also to obtaining a coating having a homogeneous surface appearance.

A homogeneous surface appearance is characterized by a uniform thickness, whereas a heterogeneous appearance is characterized by strong thickness heterogeneities. In contrast to carbon steels, Fe ₂ AI ₅ and / or FeAIs are not formed on the surface of iron-carbon-manganese steel, or if is formed, it is immediately destroyed by the formation of the phases (Fe ₁ Mn) Zn. However, Fe ₂ Al ₅ and / or FeAl _{3 type} matts are found in the bath.

The aluminum content in the bath is adjusted to a value at least equal to the content necessary for the aluminum to completely reduce the crystalline MnO manganese oxide layer and at least partially the oxide (Fe ₁ Mn) O layer. amorphous. For this purpose, the weight content of aluminum in the bath is between 0.15 and 5%. Below 0.15%, the aluminum content will be insufficient to completely reduce the manganese oxide layer MnO and at least partially the layer of (Fe ₁ Mn) O, and the surface of the steel strip will not exhibit sufficient wettability with respect to zinc. Above 5% of aluminum in the bath, a coating of a type different from that obtained by the invention will form on the surface of the steel strip. This coating will include an increasing proportion of aluminum as the aluminum content in the bath increases.

In addition to aluminum, the zinc-based bath may also contain iron, preferably at a content such that it is supersaturation with respect to Fe ₂ Al ₅ and / or FeAl ₃ .

To maintain the bath in the liquid state, it is brought to a temperature T2 preferably greater than or equal to 430 ⁰ C, but to avoid excessive evaporation of zinc, T2 is less than or equal to 480 ⁰ C. The band is in contact with the bath for a contact time C preferably between 2 and 10 seconds, and more preferably between 3 and 5 seconds. Below 2 seconds, the aluminum does not have enough time to completely reduce the MnO layer of manganese oxide and at least partially the mixed oxide layer (Fe ₁ Mn) O, and thus make the surface of wetting steel vis-à-vis zinc. Above 10 seconds, the two-layer oxides will certainly be completely reduced, however the line speed may be industrially too low, and the coating too alloyed and then difficult to adjust in thickness.

These conditions make it possible to coat the strip on both sides with a zinc-based coating having in the order from the steel / coating interface a layer of iron-manganese-zinc alloy composed of two cubic phase r and cubic with a n-face, a zinc-manganese-zinc alloy layer δ 1 of hexagonal structure, a zinc-manganese-zinc alloy layer of monoclinic structure, and a zinc surface layer. The inventors have thus verified that according to the invention, and contrary to what happens in the case of a coating of a carbon steel strip in a zinc bath containing aluminum, it is not formed. no Fe ₂ AI ₅ layer at the steel / coating interface. According to the invention, the aluminum of the bath reduces the bi-oxide layer. However, the MnO layer is more easily reducible by the aluminum of the bath than the oxide layers based on silicon. This results in a local depletion of aluminum which leads to the formation of a coating comprising FeZn phases instead of the expected Fe ₂ Al ₅ (Zn) coating, which is formed in the case of carbon steels. To improve the weldability of the coated strip by the zinc-based coating comprising three layers of iron-manganese-zinc alloy and a surface layer of zinc according to the invention, it is subjected to a heat treatment of alloying so as to completely combine said coating. Thus, a strip coated on both sides by a zinc-based coating comprising in order from the steel / coating interface a layer of iron-manganese-zinc alloy composed of two cubic phase r and cubic to face centered r 1, a layer of iron-manganese alloy-Σinc δ 1 of hexagonal structure, and possibly a layer of iron-manganese-zinc alloy ζ of monoclinic structure.

In addition, the inventors have demonstrated that these compounds (Fe ₁ Mn) Zn are favorable to the adhesion of the paint. The heat treatment of alloying is preferably carried out directly at the outlet of the zinc bath, at a temperature of between 490 and 540 ^° C., for a duration of between 2 and 10 seconds.

The invention will now be illustrated by examples given for information only, and not limiting, and with reference to the appended figures in which:

FIGS. 1, 2 and 3 are photographs of the surface of an annealed iron-carbon-manganese austenitic steel strip with respectively a dew point of -80 ^° C., of -45 ° C. and of + 10 ° C, under the conditions described below; FIG. 4 is a SEM micrograph showing in cross-section the oxide bilayer formed on an iron-carbon-manganese austenitic steel after recrystallized recrystallization with a dew point; +10 ⁰ C, under the conditions described below,

FIG. 5 is a SEM micrograph showing in cross-section the zinc-based coating formed after immersion in a zinc bath comprising 0.18% by weight of aluminum, on a ferric carbon-manganese austenitic steel annealed with a point. dew point -80 ^° C. under the conditions described below.

1) Influence of the dew point on the suitability for coating

The tests were carried out using samples cut from an iron-carbon-manganese austenitic steel strip, which after hot rolling and cold rolling has a thickness of 0.7 mm. The chemical composition of this steel is shown in Table 1, the content being expressed in% by weight. Table 1

The samples were recrystallized in an Infra Red oven, the dew point (PR) of which was varied from -80 ^° C. to + 100 ^° C., under the following conditions:

- gaseous atmosphere: nitrogen + 15% by volume of hydrogen

- heating rate V1: 6 ° C / s

- heating temperature T1: 810 ° C

- holding time M: 42 s

- cooling rate V2: 3 ° C / sec

immersion temperature T3: 480 ^° C.

Under these conditions, the steel is completely recrystallized, and Table 2 shows the characteristics of the oxide bi-layer comprising an amorphous continuous lower layer (Fe ₁ Mn) O, and an upper layer MnO, formed on the samples after annealing according to the dew point.

Table 2

After being recrystallized, the samples are cooled to a temperature T3 of 48O ⁰ C and are immersed in a zinc bath comprising, by weight, 0.18% aluminum and 0.02% iron, whose temperature T2 is 460 ^° C. The samples remain in contact with the bath for a period of contact C of 3 seconds. After immersion, the samples are examined to see if a zinc-based coating is present on the surface of the sample. Table 3 shows the result obtained as a function of the dew point. Table 3

The inventors have demonstrated that if the oxide bilayer formed on the iron-carbon-manganese austenitic steel strip after recrystallization annealing was greater than 110 nm, the presence in the bath of 0.18% by weight of Aluminum was insufficient to reduce the bi-oxide layer and give the strip sufficient wettability of the zinc to the steel to form a zinc-based coating.

2) Influence of the aluminum content in the steel The tests were carried out using samples cut from an iron-carbon-manganese austenitic steel strip, which after hot rolling and cold rolling has a thickness of 0, 7 mm. The chemical composition of the steels used is shown in Table 4, the content being expressed in% by weight. Table 4

* according to the invention

The samples have undergone recrystallization annealing in an Infra¬ red furnace whose dew point (PR) is -80 ^° C., under the following conditions: - gaseous atmosphere: nitrogen + 15% by volume of hydrogen

- heating rate V1: 6 ° C / s

- heating temperature T1: 810 ° C

- holding time M: 42 s

- cooling rate V2: 3 ° C / sec

immersion temperature T3: 480 ^° C.

Under these conditions, the steel is completely recrystallized, and Table 5 shows the structures of the various oxide films that formed on the surface of the steel after annealing in function.

Table 5

* according to the invention

After having been recrystallized, the samples are cooled to a temperature T3 of 480 ^° C. and are immersed in a zinc bath comprising 0.18% of aluminum and 0.02% of iron, the temperature of which is 460 ^° C. C. The samples remain in contact with the bath for a contact time C of 3 seconds. After immersion, the samples are coated with a zinc coating.

To characterize the adhesion of this zinc-based coating formed on the steel A and steel B samples, an adhesive tape was applied to the coated steel and then torn off. Table 6 shows the results after tearing off the adhesive tape of this adhesion test. The adhesion is qualified by grading the gray levels on the tape, starting from 0 for which the tape has remained clean after tearing up to level 3 with the highest level of gray. Table 6

Steel A Bad adhesion, gray level: 3

Good adhesion, gray level: 0, no trace of the coating

* Zinc-based steel B on the adhesive tape according to the invention

Claims

A hot dipping coating method in a zinc-based liquid bath comprising aluminum, said bath having a temperature T2, of an iron-carbon-manganese austenitic steel strip comprising: 0.30% <C <1.05%, 16% <Mn ≤ 26%, Si <1%, and Al <0.050%, the contents being expressed by weight, said process comprising the steps of: - subjecting said strip to a heat treatment in an oven inside which there is a reducing atmosphere with respect to the iron, said heat treatment comprising a heating phase at a heating rate V1, a holding phase at a temperature T1 and during a holding time M, followed by a cooling phase at a cooling rate V2, to obtain a band covered on both sides with a continuous sublayer of mixed iron oxide and manganese (Fe ₁ Mn) O amorphous, and a continuous or discontinuous outer layer of crystalline MnO manganese oxide, then flowing said covered strip of said oxide layers in said bath to coat the strip with a zinc-based coating, the aluminum content in said bath being adjusted to a value at least equal to the amount required for the aluminum to reduce. completely the crystalline MnO manganese oxide layer and at least partially the amorphous oxide layer (Fe ₁ Mn) O, so as to form on the surface of the strip said coating comprising three layers of iron-manganese-zinc alloy and a surface layer of zinc.

2. Method according to claim 1, characterized in that said reducing atmosphere vis-à-vis the iron is composed of a gas selected from hydrogen, and nitrogen-hydrogen mixtures.

3. Method according to claim 2, characterized in that said gas comprises between 20 and 97% by volume of nitrogen and between 3 and 80% by volume of hydrogen.

4. Method according to claim 3, characterized in that said gas comprises between 85 and 95% by volume of nitrogen and between 5 and 15% by volume of hydrogen.

5. Method according to any one of claims 1 to 4, characterized in that said gas has a dew point between -80 and 20 ⁰ C.

6. Method according to claim 5, characterized in that said gas has a dew point between -80 and -40 ⁰ C.

7. The method of claim 6, characterized in that said gas has a dew point between -60 and -40 ° C.

8. Method according to any one of claims 1 to 7, characterized in that the heat treatment of the strip is performed at a heating rate V1 greater than or equal to 6 ° C / s at a temperature T1 between 600 and 900 ° C, during a hold time M between 20 s and 60 s, and at a cooling rate V2 greater than or equal to 3 ° C / s to a immersion temperature of the T3 band between (T2 - 10 ⁰ C) and (T2 + 30 ^° C).

9. Process according to claim 8, characterized in that the temperature T1 is between 650 and 820 ° C.

10. Method according to claim 9, characterized in that the temperature T1 is less than or equal to 750 ° C.

11. Method according to one of claims 8 to 10, characterized in that the holding time M is between 20 and 40 s.

12. Method according to any one of claims 1 to 11, characterized in that implements the heat treatment in a reducing atmosphere so that one forms a mixed oxide layer (Fe ₁ Mn) ( O) having a thickness of between 5 and 10 nm, and a crystalline MnO manganese oxide layer having a thickness between 5 and 90 nm, before completely reducing the MnO layer by aluminum bath.

13. The method of claim 12, characterized in that the crystalline manganese oxide MnO layer has a thickness between 5 and 50 nm.

14. The method of claim 13, characterized in that the crystalline MnO manganese oxide layer has a thickness between 10 and 40 nm.

15. Process according to any one of claims 1 to 14, characterized in that the zinc-based liquid bath comprises between 0.15 and 5% by weight of aluminum.

16. Method according to any one of claims 1 to 15, characterized in that the temperature of the zinc-based liquid bath T2 is between 430 and 48O ⁰ C.

17. Method according to any one of claims 1 to 16, characterized in that the strip is in contact with the zinc-based liquid bath for a contact time C between 2 and 10 s.

18. The method of claim 17, characterized in that the contact time C is between 3 and 5 s.

19. Process according to any one of claims 1 to 18, characterized in that the carbon content in the steel is between 0.40 and 0.70% by weight.

20. Process according to any one of claims 1 to 19, characterized in that the manganese content in the steel is between 20 and 25% by weight.

21. A method according to any one of claims 1 to 20, characterized in that after having coated the austenitic steel strip by the coating comprising three layers of iron-manganese-zinc alloy and a surface layer of zinc, is subjected said heat-treated strip so as to completely combine said coating.

22. Iron-carbon-manganese austenitic steel strip obtainable according to any one of claims 1 to 20, the chemical composition of which comprises the contents being expressed by weight: 0.30% <C <1.05%

16% <Mn <26%

If <1% Al ≤ 0.050% S <0.030% P <0.080%

N ≤ 0.1%, and optionally, one or more elements such as

Cr ≤ 1% Mo <0.40% Ni <1%

Cu <5% Ti <0.50% Nb <0.50% V <0.50%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation, said strip being coated on both zinc-coated coatings comprising in the order to from the steel / coating interface a zinc-manganese alloy layer composed of two cubic T phase and cubic face-centered π, a layer of iron-manganese-zinc alloy δ 1 of hexagonal structure, a layer of iron-manganese-zinc alloy of monoclinic structure, and a superficial layer of zinc.

23. The austenitic iron-carbon-manganese steel strip obtainable according to claim 21, the chemical composition of which comprises the contents being expressed by weight: 0.30% <C <1.05%

16% ≤ Mn <26%

If <1% Al <0.050% S <0.030% P <0.080%

N <0.1%, and optionally, one or more elements such as

Cr <1% Mo <0.40% Ni <1%

Cu <5% Ti <0.50% Nb <0.50% V <0.50%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation, said strip being coated on at least one of its faces by zinc-based coating having in the order from the steel / coating interface a layer of iron-manganese-zinc alloy composed of two cubic phase r and cubic face-centered n, a layer of iron-manganese-zinc alloy δ 1 of hexagonal structure, and possibly a superficial layer of iron-manganese-zinc alloy ζ of monoclinic structure.

24. Steel strip according to one of claims 22 or 23, characterized in that the silicon content is less than 0.5% by weight.

25. Steel strip according to any one of claims 22 to 24, characterized in that the carbon content is between 0.40 and 0.70% by weight.

26. Steel strip according to any one of claims 22 to 25, characterized in that the manganese content is between 20 and 25% by weight.