Method for manufacturing a welded component with very high mechanical characteristics from a coated lamination sheet

METHOD OF MANUFACTURING A WELDED PART WITH VERY HIGH MECHANICAL CHARACTERISTICS FROM A LAMINATED AND COATED SHEET The invention relates to the manufacture of coated steel sheets or blanks intended to be welded and then heat treated in order to obtain parts with high mechanical characteristics and good corrosion resistance.

For certain applications, attempts are made to produce steel parts combining high mechanical strength, high impact resistance and good corrosion resistance. This type of combination is particularly desirable in the automotive industry where significant weight reduction in vehicles and excellent energy absorption capacity in the event of collisions are sought. This can be obtained in particular thanks to the use of steels with very high mechanical characteristics, the microstructure of which is martensitic or bainito-martensitic: anti-intrusion parts, structural parts, or participating in the safety of motor vehicles such as the crossbars of windshields. shock, door, center pillar or roof reinforcements, for example, require the qualities mentioned above.

EP 0971044 discloses a manufacturing process in which a hot or cold rolled steel sheet is supplied from steel coated with aluminum or aluminum alloy. After shaping to obtain a part, and before heat treatment at a temperature above Ad, the coating is subjected to a temperature rise in order to form an alloyed compound on the surface by interdiffusio n between the steel and the coating of aluminum. This composite alloy makes it possible to avoid any decarburization of the metal as well as any oxidation during the heat treatment in an oven. It therefore eliminates the need for furnaces comprising a special atmosphere, the presence of this alloyed compound also makes it possible to avoid certain surface operations on the treated parts, such as shot blasting, operations necessary in the case of sheets having no coating. The parts are then cooled under conditions capable of imparting mechanical strength which may exceed 1500 MPa.

Furthermore, with the aim of making vehicles lighter, parts have been developed made up of steel blanks of different compositions or of different thicknesses welded together end to end continuously. These welded parts are called "raboutes blanks". LASER beam welding is a preferred method of assembling these blanks, which takes advantage of the flexibility, quality and productivity characteristics of this process. After cold stamping of these welded blanks, parts are thus obtained having properties of mechanical strength, drawability, shock absorption, which vary within these parts. It is thus possible to have the required properties in the right place without unnecessary or costly penalizing all the parts. The application of a manufacturing process as described in patent EP 0971044 to butted blanks can be as follows: From steel sheets optionally having different compositions or thicknesses, these sheets comprising a metallic precoating , welding is carried out to obtain butted blanks. These welded blanks then undergo a heat treatment to form an alloyed surface compound and then are hot stamped and quenched. In this way, hardened parts are obtained whose thicknesses and intrinsic mechanical characteristics are variable and ideally meet local stress requirements.

However, this method of manufacture comes up against significant difficulties: during the welding of the coated steel blanks, part of the initial surface precoating is in fact transferred within the molten zone created by the welding operation. These exogenous metallic elements are concentrated due in particular to strong convection currents in the liquid metal. These elements segregate in particular within interdendritic spaces where the liquid fraction richest in solute elements is concentrated. If one then carries out a heat treatment of austenitization of the welded blanks with a view to quenching, these enriched zones combine by interdiffusion with the iron or other elements of the matrix and form intermetallic zones. During a subsequent mechanical stress, these intermetallic zones can be privileged sites of initiation of fracture in static or dynamic conditions, the overall deformation of the welded joints after heat treatment is thus significantly reduced by the presence of these resulting intermetallic zones. welding and subsequent treatment of alloying and austenitization.

It is therefore desirable to eliminate the source of formation of these intermetallic zones, namely the initial surface metallic coating capable of being remelted during the butt welding operation. But this elimination itself poses an important problem: indeed, one can eliminate, for example by a mechanical process, the pre-coated area on either side of the future welded joint, the width of this area where the pre-coated coating is removed must be at least equal to that of the future remelted zone by welding so as not to favor the subsequent formation of intermetallic zones. In practice, it must be significantly larger to take account of possible fluctuations in the width of the molten zone during the assembly operation. There are therefore after the welding operation areas on either side of the welded joint no longer comprising a surface metal precoating, during the subsequent heat treatment of alloying and austenitization, there is the formation of scale. [- / ا and to decarburization within these areas located on either side of the weld joint. These are preferential corrosion zones when the parts 20 are put into service since they are not protected by any coating.

A manufacturing process is therefore sought to avoid the formation of intermetallic zones within welded assemblies, sources of initiation of rupture.

A manufacturing process is also sought for obtaining good corrosion resistance of welded and heat-treated parts.

We are also looking for an economical manufacturing process, capable of integrating without difficulty into welding lines, and compatible with the subsequent stamping or heat treatment phases.

We are also looking for a product on which the end-to-end welding operations, then heat treatment, stamping and quenching, lead to the manufacture of a part having satisfactory ductility and good resistance to corrosion. In particular, a total elongation greater than or equal to'4٥ / o across the welded joint is sought.

the object of the present invention is to solve the problems mentioned above.

For this purpose) the invention relates to a sheet consisting of a steel substrate and a pre-coating, the pre-coating being constituted by a layer of intermetallic alloy in contact with the substrate, topped with a layer of metal alloy. On at least one pe-coated face of the sheet, an area located at the periphery of the sheet is devoid of the layer of metal alloy.

Preferably, the precoating is an aluminum alloy or based on aluminum.

Preferably, the metal alloy layer of the precoating comprises, in composition by weight, from 8 to 11٥ / ο of silicon, from 2 to 4٥ / ο of iron, the rest of the composition being aluminum and impurities. inevitable.

the width of the zone devoid of the metal alloy layer is preferably between 0.2 and 2.2 mm.

According to a preferred embodiment, the width of the zone devoid of the metallic layer is variable.

the thickness of the intermetallic alloy layer is preferably between 3 and 10 micrometers.

Again preferably, the zone devoid of metal alloy is obtained by partially removing by brushing the layer of metal alloy on at least one pre-coated face of the sheet.

According to a preferred embodiment, the zone devoid of metal alloy is obtained by partially eliminating, by means of an IASER beam, the layer of metal alloy on at least one pre-coated face of said sheet.

The subject of the invention is also a welded blank obtained from the butt welding of at least two sheets according to one of the above methods, the welded connection being made on the edge adjacent to the zone devoid of layer of metal alloy.

the invention also relates to a part obtained from the heat treatment and the deformation of a welded blank according to the above method, the pre-coating being transformed over its entire thickness by the

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heat treatment in an intermetallic alloy compound providing protection against corrosion and decarburization of the steel substrate.

The invention also relates to a sheet, a blank or a part according to one of the modes described above, the. steel composition comprising, the contents being expressed by weight: 0.10٥ / ο &lt;c &lt;0.5٥ / ο, 0.5% ك Μη &lt;3٥ / ο, 0.1٥ / ٥ كا Si 0.01٥ ,% 1 كا / ο كا Or 1 كا%, ΤΟ, 2٥ / ο, Al 0.1 ك%. S 0٠05٥ كا / ο, 0.1 كام%. 0.0005٥ / ο كا Β 0.010٠ كا / ο, the rest of the composition being constituted by iron and inevitable impurities resulting from the production.

the composition of the steel preferably comprises, the contents being expressed by weight: 0.15٥ / ο كا c 0.25٥ كا / ο, 0.8 كا 0 اه Μη 1.8٥ كا / ο, 0.1 كا ٥/٥ Si كا 0.35%) 0.01% كا Cr 0.1 كاآ,% 0.5 كا%, AI 0.1 كا%, S% 0.002,% 0.1 كام,% 0.05 كا كا B 0.005 كا%, the rest of the composition being constituted by iron and inevitable impurities resulting from the production The invention also relates to a part according to one of the above modes, the steel microstructure of which is martensitic, bainitic, or bainitomartensitic.

The invention relates to a process according to which:

a steel sheet is supplied; the sheet is coated so as to obtain a pre-coating consisting of a layer of intermetallic alloy surmounted by a layer of metal alloy,

- Removing on at least one face of the sheet, the layer of metal alloy in a zone at the periphery of the sheet.

The width of the zone is preferably between 0.2 and 2.2mm! A subject of the invention is also a process for manufacturing a pre-coated steel sheet, according to which:

a steel sheet is supplied, the sheet is coated so as to obtain a pre-coating consisting of a layer of intermetallic alloy topped with a layer of metal alloy, then

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the metal alloy layer is removed on at least one face of the sheet in a zone which is not completely contiguous to the periphery of the sheet, then,

- The tô.le is cut according to a plan.de so that the zone devoid of metal alloy is located at the periphery of the cut sheet.

The width of the zone devoid of metal alloy and not completely contiguous with the periphery of the sheet, is preferably between 0.4 and 30 mm.

The pre-coating is preferably carried out by tempering aluminizing. The removal of the layer is preferably carried out by brushing.

According to a preferred embodiment, the removal of the layer is carried out by means of the impact of a LASER beam on the precoating.

The subject of the invention is also a method according to one of the above modes in which the value of emissivity or reflectivity of the zone on which the layer of metal alloy is removed is measured, the measured value is compared with a reference value characteristic of the emissivity or of the reflectivity of the metal alloy layer and the removal operation is stopped when the difference between the measured value and the reference value is greater than a critical value.

The subject of the invention is also a method according to which the layer is removed by means of a LASER beam, characterized in that the intensity or the wavelength of the radiation emitted at the point of impact is measured. of the LASER beam, that this measured value is compared with a reference value characteristic of the emissivity of the metal alloy layer and that the removal operation is stopped when the difference between the measured value and the reference value is greater than a critical value.

Another subject of the invention is a method according to which at least two sheets manufactured according to one of the above methods are butt welded, the welded connection being made on the edge adjacent to the zone of the periphery devoid of layer. of metal alloy.

Before welding, the width of the zone devoid of the metal layer located at the periphery of the sheet metal is preferably 20 to 40٥ / ο greater than the half-width of the weld bead.

Before welding, the. the width of the zone, devoid of metal alloy and ηο.η completely contiguous with the periphery of the sheet metal, is preferably 20 to 40% greater than the width of a weld bead.

The subject of the invention is also a method for manufacturing a part according to which a welded blank manufactured according to the above method is supplied, then:

- The blank is heated so as to form, by alloying between the steel substrate and the coating an intermetallic alloy compound and so as to impart a partially or totally austenitic structure to the steel, then

- the blank is hot deformed to obtain a part,

- The part is cooled with a speed suitable for conferring the intended mechanical characteristics.

The cooling rate is preferably greater than the critical martensitic quenching rate.

According to a preferred embodiment, the welding is carried out by LASER beam. Preferably again, the welding is carried out with an electric arc. A subject of the invention is also the use of sheet metal, blank, or part according to one of the above modes, for the manufacture of structural or safety parts for a land motor vehicle.

Other characteristics and advantages of the invention will become apparent from the description below given by way of example and made with reference to the following attached figures:

- Figure 1 shows a schematic example of sheet metal according to the invention, before the welding operation.

- Figure 2 shows a second schematic example of sheet metal according to the invention.

- Figure 3 shows a schematic example of a butt welded joint according to the invention.

- Figure 4 shows a macrography of a welded joint according to the invention!

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after heat treatment of austenitization and alloying.

- Figure 5 shows a macrography of a reference welded joint showing harmful intermetallic zones within the molten metal.

- Figure 6 shows a macrography of sheet metal according to the invention before the welding operation, the metal alloy of which has been locally removed by IASER beam.

It has been seen above that the total elimination of the metallic coating on either side of the joint before the welding operation led to problems of localized corrosion. The inventors have surprisingly demonstrated that the elimination of a precise part of this coating made it possible to solve the problems mentioned above.

In order to fully understand the invention, certain characteristics of the coated strips or sheets usually produced by immersion in baths of zinc or molten aluminum, or zinc or aluminum alloys, will first be recalled.

These continuous so-called "dip" processes lead to the following general morphology of the coatings:

- On the surface of the steel substrate of the sheet, there is a precipitation of a coating of intermetallic alloys with a thickness of a few micrometers, formed by a very rapid reaction upon immersion in the molten bath. These intermetallic alloys being relatively fragile, an attempt is made to limit the growth of this layer by adding inhibitors to the molten bath. In the case of coating of zinc or aluminum alloys, the alloys making up this layer are often of the FeAly type, in particular Fe2٨l5٠ In the case of zinc alloy coatings, the presence of this intermetallic layer rich in aluminum s' explains by the fact that the baths of 'Zinc often contain a small quantity of aluminum which plays a role of inhibition.

This layer of intermetallic alloys can sometimes be of a complex nature and may be subdivided, for example, into two intermetallic sublayers, the sublayer in contact with the substrate being more riclie in iron.

- This layer of intermetallic alloys is topped with a layer of metal alloy, the composition of which is very close to that of the bath. A more or less important metal layer is in fact entrained at the outlet of the molten bath by the sheet, the thickness of which can be controlled by means of air or nitrogen jets.

the inventors have demonstrated that, in a particularly advantageous manner, this last layer should be locally eliminated in order to solve the problems mentioned above.

Reference will be made more particularly to FIG. 1 illustrating a sheet according to the invention, the term sheet metal should be understood in a broad sense and designates in particular any strip or any object obtained by cutting from a strip, a coil. or a leaf. This sheet has two faces and four edges in this particular case. The invention is of course not limited to this rectangular geometry, FIG. 1 shows:

- A steel substrate 1. This substrate may in particular be in the form of a hot or cold rolled sheet depending on the desired thickness, or of any other suitable form.

- Superimposed on the substrate and in contact with it, a pre-coating 2 is present on both sides of the part. This pre-coating is itself composed:

- A layer of intermetallic alloy 3 located in contact with the substrate 1. As we have seen, this is a layer formed by reaction between the substrate and the molten metal of the bath.

Advantageously, the pre-coating is an aluminum alloy or based on aluminum. This type of pre-coating is in fact particularly well suited to subsequent heat treatment leading to the formation of an intermetallic compound by interdiffusion with the substrate 1 and, as will be seen, to local removal of the surface layer. In particular, the metal alloy of the pre-coating may contain from 8 to 11% by weight of silicon, from 2 to 4% of iron, the remainder of the composition being aluminum and inevitable impurities, the addition of silicon makes it possible in particular to reduce the thickness of the intermetallic layer 3.

- The periphery 5 of the sheet has also been shown. According to the invention, a portion 6 of the periphery is devoid of the metal alloy layer 4 but retains the intermetallic alloy layer 3. This part 6 is

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intended to be docked to another sheet, then to be butt welded in a plane defined by the edge 11 to form a blank.

- According to a first embodiment, the removal of the layer 4 is carried out advantageously thanks to a brushing operation carried out at the periphery

5: in fact, the removal of material carried out by the brush essentially concerns the surface layer whose hardness is the lowest, that is to say the metal alloy layer 4. The harder layer 3 will be left in place by the passage of the brush, the use of aluminum or aluminum-based precoating will be particularly advantageous since the difference in hardness between the intermetallic alloy layer 3 and the metal layer 4 is very large.

Those skilled in the art will know how to adapt the various parameters specific to the brushing operation such as the choice of the nature of the brush, of the speed of rotation and of relative translation, of the pressure perpendicular to the surface so as to achieve removal in the most complete and rapid manner, adapting them to the particular nature of the precoating. By way of example, one could use a blade brush mounted on a rotary axis driven by a translational movement parallel to the edge of part 6.

- According to a second embodiment, the removal of the layer 4 is carried out by a LASER beam directed towards the periphery of the sheet: The interaction between this beam at high energy density and the precoating causes vaporization and a expulsion from the surface of the latter. Taking into account the different thermal and physical properties between the metal alloy layer 4 and the intermetallic layer 3, the inventors have demonstrated that a succession of short LASER pulses with suitable parameters leads to a selective ablation of the layer. metal 4 leaving layer 3 in place. The interaction of a pulsed LASER beam directs towards the periphery of a coated sheet, and in relative translation with respect to this sheet, therefore leads to a removal of the metallic layer 4 at the periphery. Those skilled in the art will know how to adapt the various parameters such as the choice of the LASER beam, of the incident energy, of the duration of the pulses, of the relative speed of translation between the beam and the sheet, and of the focusing of the beam on the surface in order to achieve the ablation of the

39458 fastest and most complete way by adapting them to the particular nature of the pre-coating. By way of example, a Q-switch type LASER could be used, with a nominal power of a few hundred watts and delivering pulses of the order of about fifty nanoseconds. The width of the removal zone 6 could naturally be varied by means of successive contiguous ablations.

The width of zone 6 devoid of the metal layer must be adjusted so as to allow:

- welding without introduction of precoating element in the molten zone

- sufficient corrosion resistance of the welded assembly after subsequent heat treatment of alloying and austenitization

The inventors have demonstrated that these conditions were met when the width of zone 6 was greater, in a proportion of 2٥% to 40%, than the half width of the molten zone created during the butt welding of blanks.

The minimum value of 20% ensures that the precoating is not introduced into the molten metal during welding, the value of 40% ensures satisfactory resistance to corrosion.

Taking into account the welding conditions for sheets with a thickness ranging from 1 to 3mm, the width of zone 6 is between 0.2 and 2.2 mm.

This situation is represented in FIG. 3 which illustrates schematically a section after welding of a sheet comprising a precoating 2, itself formed of a layer of intermetallic alloy 3 and of a metal layer 4. the molten zone is designated by 10, its axial plane in the direction of welding by 9. ' The hatched lines illustrate the initial extent of a zone 6 remelted by the welding operation.

FIG. 3 illustrates the situation where the weld bead is generally symmetrical on the two opposite faces of the sheet. Under these conditions, the width of zone 6 is identical on both sides. However, depending on the welding process used and the parameters for implementing this process, the bead may have an asymmetrical appearance. According to the invention, the width of the zone 6 can then be coordinated with this asymmetry so that this width is slightly greater than the half-width of the molten zone 10 on each of the two respective faces. Under these conditions, the width of zone 6 will differ from that of zone 6 'illustrated in figure 3.

In the case of welding conditions changing along an assembly, for example to take account of a local modification of geometry or thickness, the width of zone 6 can also be coordinated with the corresponding evolution of the variation width of the molten zone along the welded periphery of the sheet metal. the width of zone 6 will naturally increase as local conditions lead to the formation of a wider bead.

In the case of welding two sheets of different thickness, comprising a coating, the width of zone 6 can also be different on the welded periphery part of each of the two sheets.

According to a variant of the invention illustrated in FIG. 2, the layer 4 is removed from a zone 7 of a coated sheet, the removal zone being not completely contiguous with the periphery 5 of the sheet. The sheet is then cut along an axial plane 8 perpendicular to the latter, for example by slitting. We then obtain a sheet as illustrated in figure 20 1. the width of the removal is greater, in a proportion of 20٥ / ο to

40%, to the width of the molten zone which would be obtained by a welding operation carried out along the axial plane 8.

According to a variant of the invention, the width of the removal is between 0.4 and 30mm. the minimum value corresponds to a width making it possible to produce, after cutting along the axial plane 8, two sheets having a very narrow removal zone of 0.2 mm on each of the two sheets. The maximum value of 30mm corresponds to a removal width well suited to industrial tools for such removal. A subsequent cut can be carried out, not on the axial plane 8 situated in the middle of the removal zone, but at a suitable location so as to obtain a sheet whose removal width is slightly greater than the half width of the molten zone obtained by a welding operation, defined by the conditions of the invention.

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As explained above, the removal widths make it possible to ensure both that the metal coating is not introduced into the molten metal during a subsequent welding of the sheet, and the resistance corrosion of the welded blank after heat treatment.

The control of the removal of the metal layer 4 can be carried out by means of micrographic examinations. But it has also been demonstrated that the efficiency of the removal operation can be very quickly controlled by optical control: there is indeed a difference in appearance between the metal layer 4 and the intermetallic layer. underlying 3, which is darker in color. The removal operation must therefore continue and be stopped when a significant change in color is observed in zone 6 with respect to the surface coating. It is thus possible to control the removal by means of emissivity or reflectivity measurement by spectrometry: zone 6 is illuminated by means of a light source, one or more optical sensors being directed towards this zone. The measured value corresponds to the reflected energy. This value is compared with a reference value corresponding to the emissivity or reflectivity of the metal layer 4 or with a value measured by another sensor directed towards this metal layer. It is also possible to measure the variation, as a function of time, of the reflected energy. In the case where the layer 6 is flush with the surface, the energy collected is lower than that corresponding to the metal alloy layer 4. By means of a prior calibration, it is therefore possible to determine the precise moment when the removal reaches layer 3.

In the case of coating removal by LASER ablation, it is also possible to analyze the intensity or the wavelength of the radiation emitted at the point of impact of the LASER beam on the pre-coated sheet. In fact, a modification of the intensity and of the wavelength is observed when layer 4 has been eliminated and the LASER beam impacts layer 3. A control of the thickness of the removed layer can therefore be carried out as follows: the intensity or the wavelength of the radiation emitted at the point of impact of the LASER beam is measured, this measured value is compared with a value reference characteristic of

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The emissivity of the metal alloy layer 4 and the removal operation is stopped when the difference between the measured value and the referenced value is greater than a predetermined critical value.

Depending on the specific constraints, this step of removing the metal alloy layer can be implemented at different stages of production, in particular at one of the following:

- After unwinding of coils manufactured on continuous rolling trains, before cutting in the form of smaller sheets

- Before the welding step, on cut sheets.

In the process according to the invention, a hot or cold rolled steel sheet is supplied with the following weight composition: a carbon content of between 0.10 and 0.5%, and preferably between 0.15 and 0, 25% by weight. This element plays a large role in the hardenability and in the mechanical resistance obtained after the cooling which follows the alloying and austenitization treatment of the welded blanks. Below 0.0% by weight, the hardenability is too low and the strength properties are insufficient. On the other hand, beyond a content of 0.5% by weight, the risk of formation of defects is increased during quenching, particularly for the thicker parts. A carbon content of between 0.15 and 0.25% makes it possible to obtain a resistance of between approximately 1250 and 1650 MPa.

Besides its role as a deoxidizer, manganese also has a significant effect on hardenability, in particular when its content by weight is at least 0.5% and preferably 0.8%. However, too large an amount (3% by weight, or preferably 1.8%) leads to risks of excessive segregation.

- The silicon content of the steel must be between 0.1 and 1% by weight, and preferably between 0.1 and 0.35%. Besides its role in the deoxidation of liquid steel, this element contributes to hardening. Its content must, however, be limited to avoid excessive formation of oxides and to promote the coatability.

- Above a content greater than 0.01 ٥/٥, chromium increases hardenability and contributes to obtaining significant strength after

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The hot forming operation, and this in the various parts of the part after cooling following the heat treatment of austenitization and alloying. Beyond a content equal to 1% (preferably 0.5%), the contribution of chromium to obtaining this homogeneity of mechanical properties is saturated.

- Aluminum is an element that promotes deoxidation and nitrogen precipitation. In an amount greater than 0.1% by weight, coarse aluminates are formed during production, which encourages them to limit their content to this value.

- In excessive quantities, sulfur and phosphorus lead to increased fragility, which is why it is preferable to limit their respective content to 0.05 and 0.1 ٥/٥ by weight.

- Boron, the content of which should be between 0.0005 and 0.010% by weight, and preferably between 0.002 and 0.005٥ / ٥ by weight, is an element which plays an important role on the hardenability. Below a content of 0.0005٥ / ٥, a sufficient effect on hardenability is not obtained. The full effect is obtained for a content of 0.002٥ / ٥. The maximum boron content must be less than 0.010٥ / ٥, and preferably 0.005%, so as not to degrade the toughness.

- Titanium has a strong affinity for nitrogen and therefore helps protect boron so that this element is in free form to play its full effect on hardenability. Beyond 0.2٥ / ο, and more particularly 0.1 ٥/٥, there is however a risk of forming coarse titanium nitrides in the liquid steel which play a detrimental role on the toughness.

After preparation of the sheets according to one of the processes described above, they are assembled by welding so as to obtain a welded blank. Of course, more than two sheets can be assembled for the production of complex end pieces. The sheets can be of different thickness or composition to locally meet the required properties.

the welding is carried out after docking the sheets edge to edge, the zones devoid of a layer of metal alloy being contiguous to each other. Welding is therefore carried out along the edge adjacent to the zones 6 devoid of a layer of metal alloy.

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In the context of the invention, any continuous welding means suitable for the thicknesses and for the productivity and quality conditions required for the welded joints, and in particular:

- LASER beam welding

- Electric arc welding, in particular by TIG ("Tungsten Inert Gas), plasma, MIG (" Metal Inert Gas) OR MAG ("Metal Active Gas") processes

Under the conditions of the invention, the welding operation does not lead to the reflow of a portion of the metallic coating 4, the elements of which would then be found in the molten zone. Only a minimal amount of the intermetallic alloy layer 3 is remelted by this operation within the molten zone. As will be shown by the example below, this very limited quantity has no influence on the metallurgical quality and the mechanical properties of the welded joint after heat treatment of alloying and austenitization.

The welded blank is then reheated to jointly produce:

- A surface alloying treatment where the οη there is a diffusion of elements of the steel substrate, in particular iron, manganese, silicon, within the precoating. In this way, an intermetallic alloy compound is formed at the surface, the melting point of which is notably higher than that of the metal alloy layer 4. The presence of this compound during the heat treatment makes it possible to avoid any oxidation and any decarburization of the metal. the underlying steel.

- Austenitization of the base steel, this austenitization being able to be partial or total. The heating is advantageously carried out in an oven such that the part reaches a temperature between Acl and Ac3 + 100٥C. Ad and Ac3 respectively denote the start and end temperatures of austenitic transformation on heating. According to the invention, the holding time at this temperature is greater than or equal to 20 s so as to standardize the temperature and the microstructure in the various points of the room.

Under the conditions according to the invention, one does not form, during this

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reheating phase, fragile intermetallic zones within the molten metal, harmful to the mechanical properties of the part.

A hot deformation of the blank is then carried out in order to give it its final shape in the form of a part, this step being favored by the reduction in the flow limit and the increase in the ductility of the steel with the temperature. . Starting from a partially or totally austenitic structure at high temperature, the part is then cooled under appropriate conditions so as to confer the intended mechanical characteristics: in particular, the part can be held within a tool during cooling. , the tool can itself be cooled to promote heat removal. In order to obtain high mechanical properties, the objective will preferably be to obtain martensitic, bainitic or bainitic-martensitic microstructures.

In zone 6 on either side of the welded joint, the intermetallic layer 3, with a thickness of between 3 to 10 micrometers before heat treatment, is combined with the steel substrate and makes it possible to obtain good resistance to corrosion. .

Example:

By way of example, the following embodiments will illustrate other advantages conferred by the invention. We considered a cold-rolled steel strip 1.5 mm thick, of the following weight composition:

<td>VS</td><td>Mn</td><td>Yes</td><td>S</td><td>P</td><td>Al</td><td>Cr</td><td>Ti</td><td>B</td>

<td> 0,224</td><td> 1,160</td><td> 0,226</td><td> 0,005</td><td> 0,013</td><td> 0,044</td><td> 0,189</td><td> 0,041</td><td> 0,0031</td>

Table 1: Composition of the steel (٥/٥ weight)

The steel strip was pre-dipped in a molten bath with an aluminum alloy comprising 9.3% silicon and 2.8% iron, the remainder being aluminum and inevitable impurities. . The strip was then cut in the form of sheets in the format of 300 × 500 mm2. These comprise on each of their faces, a pre-coating comprising a

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intermetallic alloy layer predominantly comprising leAa. leAls and Fe * AlySiz. This layer of 5 micrometers thick in contact with the steel substrate is surmounted by a layer of ΑΙ-Si metal alloy 20 micrometers thick.

Before LASER beam welding, four different preparation methods were implemented:

- Method I (according to the invention): The d'all-Si metal alloy layer was removed by longitudinal brushing over a width of 1.1 mm from the edge of the sheets, on the 500 mm long side. The brushing was carried out in an identical manner on both sides by means of a "spiraband" type brush with 80mm diameter blades mounted on a rotary system with angular transmission, the whole guide in translation on a bench with counterweight. The brushing force is approximately 35Ν at the brush / blank contact, the speed of movement of the brush of lOm / min. The metal alloy layer is thus removed by brushing, leaving only the 5 micron intermetallic alloy layer remaining on the brushed zone.

- Method II (according to the invention): the couche-Si metal alloy layer was removed by IASER ablation over a width of 0.9 mm from the edge of the sheets, the IASER ablation was performed with a identically on both sides by means of a Q-switch type laser with a nominal energy of 45OW delivering pulses of 70ns. the pulse energy is 42mJ. the constant speed of relative translation of the LASER beam with respect to the sheets is 20 m / min. FIG. 6 shows that the metal alloy layer 4 is thus removed by LASER ablation, leaving only the 5 micron intermetallic alloy layer 3 on the treated area.

- RI method (not in accordance with the invention): All of the precoating, ie the metal alloy layer and the intermetallic alloy, was removed by mechanical removal. This removal was carried out over a width of 1.1 mm, therefore identical to that of modality I, by means of a tool of the carbide insert type for rapid machining, in longitudinal translation. In this way, the subsequent welding

<img file="MA30458B1_D0011.tif" />

is carried out on an area completely devoid of pre-coating on either side of the joint.

- Method R2 (not in accordance with the invention): LASER welding was carried out on pre-coated sheets which had not undergone any particular preparation of their periphery.

These sheets were welded by LASER beam under the following conditions: Nominal power 6kW, welding speed: 4m / minute. Taking into account the width of the weld bead, the presence, in modality I, of a zone devoid of metal alloy over a width of approximately 0.3 mm after making the welded joints.

the welded blanks have undergone a heat treatment of alloying and austenitization comprising heating to a temperature of 92O٥C, a hold of 7 minutes being carried out at this temperature. These conditions lead to a complete austenitic transformation of the steel of the substrate. During this heating and holding phase, it is observed that the aluminum-silicon-based pre-coating forms, over its entire thickness, an intermetallic compound by alloying with the base steel. This high melting point, high hardness alloy coating has high corrosion resistance and prevents oxidation and decarburization of the underlying base steel during and after the heating phase.

After the heating phase to 92O٥C, the parts were hot deformed and cooled.

Subsequent cooling between tools leads to a martensitic structure. The mechanical strength Rm of the steel substrate obtained after such a treatment is greater than 1450 MPa.

The welded connections in the parts thus obtained were then characterized by the following techniques:

- Micrographic sections made it possible to assess the possible presence of intermetallic zones within the welded joints

- Mechanical tensile tests across welded joints on 12.5x50 mm2 specimens made it possible to evaluate the resistance Rm and the total elongation.

- Accelerated corrosion tests were carried out according to DIN 50021, 50017, 50 014. These tests include, after spraying with salt mist, cycles in which dry phases at 23٥c alternate with wet phases at 4O٠C.

The results of these characterizations are presented in Table 2:

<td>Modality</td><td>Fragile intermetallic areas within welded joints</td><td>Rm (MPa)</td><td>A (٥/٥)</td><td>Corrosion resistance</td>

<td>1 (according to the invention)</td><td>Absence</td><td>&gt; 1450MPa</td><td> &gt;4%</td><td>O</td>

<td>It (according to the invention)</td><td>Absence</td><td>&gt; 1450MPa</td><td> &gt;4%</td><td>O</td>

<td>RI (not in accordance with the invention)</td><td>Absence</td><td>&gt; 1450MPa</td><td> &gt;4%</td><td> !</td>

<td>R2 (not in accordance with the invention)</td><td>Presence</td><td> 1230</td><td>1% ج</td><td>O</td>

Table 2: Characteristics of welded joints after heat treatment

O: Satisfactory _ ·: Unsatisfactory

Under the quenching conditions required after heat treatment, the microstructure of the base metal and of the molten zone during welding is totally martensitic for the four above methods.

In the case of modality I according to the invention, the molten zone does not contain any intermetallic zone, as shown in FIG. 4.

On the other hand, in modality R2, we note the presence of intermetallic zones (figure 5) in particular towards the periphery of the molten zone where the elements of the precoating have been concentrated by the spontaneous convection movements of the liquid bath due to a Marangoni effect. These large intermetallic zones, the orientation of which can be substantially perpendicular to the mechanical stress, play a role of stress concentration and initiation with respect to failure, the elongation in the transverse direction is notably reduced by the presence of these intermetallic zones: in the absence of these zones, the elongation is greater than 4٥ / ο. It becomes less than 1% when these are present.

There are no significant differences in mechanical characteristics (strength and elongation) between the method according to the invention I, and the method RI. This means that the thin layer of intermetallic alloy left in place by brushing and remelted by welding does not result in the formation of fragile areas within the molten metal, as shown in Figure 4.

In the case of the RI modality, the corrosion resistance is reduced: in fact, the steel is completely exposed on either side of the welded joint by the total removal of the pre-coating. In the absence of corrosion protection, we then notice the appearance of red rust in the areas affected by the heat on either side of the bead.

Thus, the process according to the invention makes it possible to simultaneously obtain good ductility of the welded joint after treatment as well as good resistance to corrosion.

Depending on the composition of the steel, in particular its carbon content as well as manganese, chromium and boron, the maximum strength of the parts can be adapted to the intended use. These parts will be used with profit for the manufacture of security parts, and in particular of anti-intrusion or base parts, of reinforcement bars, of central feet, for the construction of motor vehicles.