Method for manufacturing welded part with high mechanical properties from rolled sheet with coating

TECHNICAL FIELD OF THE INVENTION

The invention relates to the manufacture of steel sheets or blanks coated with welding and heat treatment to produce parts with good mechanical properties and good corrosion resistance.

BACKGROUND

For some applications, attempts are made to make steel parts that combine high mechanical strength, high impact resistance and good corrosion resistance. This combination is particularly necessary in the automotive industry, which seeks ways substantial investment vehicles and provide excellent ability to absorb energy during collisions. This can be achieved, in particular, the use of steels with very high mechanical properties, having a martensitic or bainitic-martensitic microstructure: the details that prevent penetration inside, design details and parts to ensure the safety of vehicles, such as bumper beams, parts rigidity door, B-pillar or roof, must have, for example, given properties.

EP 0971044 discloses a manufacturing method in which use hot- or cold-rolled steel sheet coated with aluminum or an alloy thereof. After treatment to impart shape to obtain details and before the heat treatment at a temperature above Ac1 coating is exposed to an elevated temperature to form compounds on the surface of alloyed by interdiffusion between the steel and the aluminum coating. Such a fused joint allows to exclude any decarburization and any oxidation of the metal during the heat treatment in the furnace. Consequently, there is no need to apply special furnaces with atmosphere. Also, the presence of such fused compounds avoids some operations on the surface of the machined parts, such as blasting operations required for uncoated blanks. Next, details cooled under conditions to provide mechanical strength that may exceed 1500 MPa.

However, to facilitate vehicles were created by parts made from sheet steel blanks of different composition or different thickness are welded together end to end in a continuous manner. These welded parts are called "docking blanks." Laser beam welding is the preferred method of connecting such blanks, while useful elastic properties are used, the quality and productivity of the process. After cold forming of welded blanks receive the items, characterized by mechanical strength, formability, impact energy absorption, which are variable within these parts. Thus it is possible to set the required properties at a desired location, without subjecting the complex or costly parts ineffective treatment. The method of manufacture described in EP 0971044 and is intended for butt-connected blanks may be applied as follows. Steel sheets, if necessary of different composition or different thickness, and sheets coated with these preliminary metal coating is produced by welding butt joint blanks. Then, the welded blanks are heat treated to form the surface of the fused connection and followed by hot stamping and hardening. Thus obtained hardened components, thickness and mechanical properties of which are variable and are ideally suited to the requirements local to the load. However, such a method of manufacture and substantial inherent difficulties as welding steel blanks coated with pre initial part of the surface coating is in a transitional state within the molten zone formed by welding. Such exogenous metal elements are concentrated, particularly because of the strong convection currents in the liquid metal. These elements undergo segregation, particularly in interdendritic spaces in which the concentrated liquid fraction enriched most dissolved elements. If you then carry out the heat treatment at austenizing welded blanks for the purpose of hardening, the fortified area as a result of self-diffusion alloyed with iron or other elements of the matrix and form intermetallic areas. On subsequent mechanical loading, these intermetallic areas may be advantageous in some places of origin of destruction in static or dynamic conditions. Thus, the total strain weld after the heat treatment is significantly reduced due to the presence of these intermetallic zones formed by welding and subsequent surface austenitization and fusing.

Therefore, you must remove the source of the formation of these intermetallic areas, namely the initial surface metal coating that can be melted during butt welding. However, such removal itself poses a serious problem: indeed, can be removed, for example, mechanical pre-coating zone on both sides of the future weld. The width of the zone in which the precoat is removed, must be equal to at least the width of the future weld molten zone so that it does not contribute subsequent formation of intermetallic zones. In practice, it should be much more to take into account possible fluctuations in the width of the molten zone during the assembly operation. Therefore, after the welding operation there are zones on either side of the weld, which no longer has a metal surface pre-coating. During subsequent heat treatment to fusion and austenitization occurs scaling and decarburization in these zones lying on either side of the weld. These areas represent zones of preferential corrosion during use of the parts as they are not protected by any coating. Consequently, the search is conducted manufacturing method prevents the formation of intermetallic zone in welded assemblies, which are the source of the causes of destruction.

Also it is sought the manufacturing process, providing good corrosion resistance welded and heat-treated parts.

Moreover, the search is conducted economical manufacturing method capable of easily fit into the welding line and is compatible with the subsequent forming operation or heat treatment.

Also being sought product, which conducted operations: butt welding, subsequent heat treatment, forging and tempering, allow to make the item, have acceptable ductility and good corrosion resistance. Being particularly pathfinding total elongation in a direction transverse to the weld, greater than or equal to 4%.

Disclosure of invention

The aim of the present invention is to solve the above problems.

Therefore, the invention is a sheet comprising a steel substrate and the pre-coat, wherein the precoat layer is formed of an intermetallic alloy in contact with a substrate on which a layer of metal alloy. The at least one comprising precoating the leaf area located on the periphery of the sheet does not contain a layer of metal alloy.

Preferably, the pre-coat consisted of an aluminum alloy or aluminum base alloy.

Also preferably, the layer of metal alloy, referred to as pre coating comprising in wt.%: 8-11% silica, 2-4% iron, the rest - aluminum and inevitable impurities.

The width of the zone free of the metal alloy layer is preferably 0.2-2.2 mm.

According to a preferred embodiment, the width of the free zones of the metal alloy layer is variable.

The thickness of the intermetallic alloy layer is preferably from 3 to 10 microns.

It is also preferred that the free area of ​​the metal layer was obtained by removing a layer of metal alloy partial sweep brush, at least in the pre-coating containing sheet.

According to a preferred embodiment is free from a metallic alloy zone by partial removal of the metal alloy layer on at least one side comprising precoating sheet by laser beam.

Also an object of the invention is a welded sheet blank obtained butt welding at least two sheets according to one of the above embodiments, the connection is performed by welding along the edge adjacent the free metal alloy layer area.

Also disclosed herein is a piece obtained by heat treatment and deformation of a welded blank sheet according to the above embodiment, wherein the precoat is converted throughout its thickness by heat treatment in an intermetallic alloyed compound providing protection against corrosion and decarburization of the steel substrate.

Also an object of the invention is a sheet or sheet blank part according to one of the above embodiments, the steel has the following composition in wt.%: 0.10% ≤S≤0,5%, 0.5% ≤Mn≤3% 0,1≤Si≤1%, 0,01% ≤Cr≤1%, Ti≤0,2%, Al≤0,1%, S≤0,05%, S≤0,05%, P≤0 1%, 0.0005% ≤B≤0,010%, the rest - iron and unavoidable impurities from the smelting.

Preferably, the composition of the steel included in wt.%: 0.15% ≤S≤0,25%, 0.8% ≤Mn≤1,8%, 0.1% ≤Si≤0,35%, 0 01≤Cr≤0,5%, Ti≤0,1, Al≤0,1%, S≤0,05%, P≤0,1%, 0,002% ≤B≤0,005%, the rest - iron and unavoidable when melting impurities.

Also disclosed herein is a piece according to one of the above described embodiments, the microstructure of the steel which is martensitic, bainitic or bainitic-martensitic.

The object of the invention is a method in which:

- Prepared steel sheet;

- On the coated sheet for precoat consisting of an intermetallic alloy layer disposed thereon a layer of metal alloy;

- At least on one side of the metal layer is removed layer disposed in the periphery of the sheet.

Preferably the band width is 0.2 to 2.2 mm.

Also an object of the invention is a method for manufacturing the steel sheet pre-coated, wherein:

- Prepared steel sheet;

- On the coated sheet for precoat consisting of an intermetallic alloy layer disposed thereon a layer of a metal alloy,

- At least on one side of the metal alloy layer is removed in an area not totally contiguous to the periphery of the sheet, and

- The sheet is cut in a plane that is free from a metallic alloy zone located at the periphery of the cut sheet.

The width of the zone free of metal alloy and not completely adjacent to the periphery of the sheet is preferably from 0.4 to 30 mm.

Pre-coating is carried out preferably by dipping aluminizing. Removing the layer is made preferably sweep brush. According to a preferred embodiment of the layer removed by laser beam on the precoating.

Also an object of the invention is a method according to one of the above described embodiments, wherein the measured value emissivity or reflectivity in the zone in which the sacrificial layer of a metal alloy, the measured value is compared with a characteristic reference value emissivity or reflectivity of the layer of metal alloy and stop operation stripping at a time when the difference between the measured and the reference value exceeds a critical value.

Also disclosed herein is a method of removing a layer of metal alloy with a laser beam, characterized by measuring the intensity or wavelength of the radiation emitted at the point of impact of the laser beam, which is then compared to the measured value of a characteristic reference value emissivity layer of a metal alloy and that terminate operation of stripping at a time when the difference between the measured and the reference value exceeds a critical value.

Also an object of the invention is a method in which a butt-welded, at least two sheets, made according to one of the above described embodiments, wherein the weld is performed on an edge adjacent to a zone located at the periphery of the free layer of a metal alloy.

Before welding, the width of the zone free of metal alloy layer and situated on the periphery of the sheet, preferably greater than 20-40% at the half-width of the weld bead.

Before welding, the width of the zone free of metal alloy and not completely adjacent to the periphery of the sheet, preferably greater than 20-40% of the width of the weld bead.

Also an object of the invention is a method for manufacturing parts, in which is prepared a welded sheet blank made according to the above embodiment, then:

- Sheet blank is heated so that the resulting weld between the steel substrate and formed intermetallic compound is fused and that the steel has gained partially or totally austenitic structure,

- Hot-deformation of the blank sheet for details

- Item cooled at a rate necessary to impart the required mechanical properties.

The cooling rate is preferably higher than the critical quenching rate on martensite.

According to a preferred embodiment of the laser beam welding is carried out.

It is also preferred to apply an electric arc.

Also an object of the invention is the use of sheet or sheet blank part according to one of the above described embodiments for the manufacture of structural parts or components for safety of vehicles equipped with the engine.

BRIEF DESCRIPTION OF DRAWINGS

Other features and advantages of the invention are given below in the description by way of example with reference to the accompanying drawings:

Figure 1 - a schematic example of a sheet according to the invention before the welding operation;

Figure 2 - a schematic example of a second sheet according to the invention;

Figure 3 - a schematic example of a seam butt-welding according to the invention;

4 - macrograph weld according to the invention after heat treatment and alloying austenitization;

5 - macrograph reference weld on which visible adverse intermetallic zone in the molten metal;

6 - macrograph sheet according to the invention before the welding operation, which has been locally removed metal alloy with a laser beam.

EMBODIMENTS

Above it was shown that the complete removal of the metal coating on both sides of the joint before welding creates problems with respect to localized corrosion. The inventors have surprisingly found that the current portion of the coating removal can solve these problems.

For a better understanding of the invention it should be recalled first of all, on some properties of the webs or sheets of coated usually by immersion in a bath of molten zinc or aluminum or zinc or aluminum alloys.

These so-called continuous processes "dipping" form the following general morphology of the coatings:

- On the surface of the steel sheet substrate is precipitated intermetallic alloy layer a few microns thick, formed by reaction proceeds very quickly when immersed in the molten bath. Since these intermetallic alloys are relatively fragile, then try to limit the growth of the layer of the inhibitor into the molten bath. In the case of a coating of zinc alloy or aluminum alloy, forming said layer, often represent FexAly type alloy, namely Fe2Al5. In the case of coatings of zinc alloys presence of the intermetallic layer with high aluminum content due to the fact that zinc baths often contain a small amount of aluminum as an inhibitor.

Sometimes the layer of intermetallic alloys can have complex origins and divided, e.g., into two intermetallic sublayer, wherein the sublayer in contact with the substrate contains more iron.

This layer is a layer of intermetallic alloys, metal alloy whose composition is very similar to the composition of the bath. In fact, a more or less substantial metal layer entrained on the output sheet from the molten bath, wherein the thickness may be controlled by jets of air or nitrogen.

The inventors have shown that particularly effectively remove locally the last layer solutions for the issues raised above.

It is necessary to refer in particular to Figure 1, which shows a sheet according to the invention. The term "sheet" should be understood in a broad sense, it is designated in particular, any tape or any article obtained by cutting the strip of tape or a thin sheet. In this particular case, the sheet comprises two sides and four edges. The invention is of course not limited to a rectangular shape. 1 shows:

- Steel substrate 1. This substrate can be, in particular hot-rolled or cold-rolled sheet, depending on the desired thickness, or any other suitable type of sheet.

Applying to the substrate and contacting it with two pre-coating is present on both sides of the part. Needless precoating consists of:

- The intermetallic alloy layer 3 in contact with the substrate 1. As shown, it is a layer formed by reaction between the substrate and the molten metal bath.

Preferably, the pre-coat consisted of an aluminum alloy or aluminum base alloy. Indeed, this type of pre-coating is particularly suitable for the subsequent heat treatment, whereby the intermetallic compound formed by interdiffusion with the substrate 1 and, as will be shown below, for local removal of the surface layer. In particular, the metal alloy pre-coating may comprise 8-11 wt.% Silica, 2-4% iron, the rest - aluminum and inevitable impurities. Addition of silicon makes it possible in particular to reduce the thickness of the intermetallic layer 3.

5 also shows the periphery of the sheet. According to the invention the peripheral portion 6 contains a metal alloy layer 4 but retains the intermetallic alloy layer 3. This portion 6 serves to apply to the other sheet and butt welding in the plane defined by the edge 11 to form a slab.

According to the first embodiment of the removal of the layer 4 is preferably carried out by stripping brush 5 on the periphery; Indeed, the removal of material is carried out mainly a brush on the surface layer of lower hardness, i.e. layer 4 on the metal alloy. The harder layer 3 is saved after a brush. Particularly effective is the application of the precoating of aluminum or alloy thereof, as the difference between the hardness of the intermetallic alloy layer 3 and layer 4 metal alloy is substantial.

The specialist is able to align the brush cleaning various parameters, such as the choice of the type of brush, the rotational speed and relative translational movement of the pressure in the direction perpendicular to the surface, provide the most complete and rapid removal by matching these parameters with a particular type of precoat. As an example is the use of the brush plate mounted on the axis of rotation, driven in translational movement parallel to the edge portion 6.

According to a second embodiment of the removal of the layer 4 is produced by a laser beam aimed at the periphery of the sheet: the interaction between the beam of high energy density and precoated leads to vaporization and removal of the coating from the surface. Given the difference between the thermal and physical properties of the metal alloy layer 4 and the intermetallic layer 3, the inventors have shown that the sequence of short laser pulses together with those in the corresponding parameters leads to a selective ablation of the metal layer 4 while maintaining the layer 3. The laser interaction, pulsating, directed to the periphery of the coated sheet and performs translational motion relative to the sheet, provides, therefore, removal of the metal layer 4 at the periphery. Specialist can reconcile the different parameters, such as the selection of the laser beam, its power downs, pulse duration, the rate of relative translational movement between the laser and the sheet, focusing the beam on the surface, to achieve the most complete and rapid ablation by matching with a particular type of pre- coating. As an example is the use of a laser beam type laser speed with a rated power of a few hundred watts forming pulses of the order of fifty nanoseconds.

Of course, you can change the width of the area 6 removal through successive ablations.

Width of 6 free from metal layer should be chosen so as to allow:

- Welding without introducing an element pre-coating the molten zone

- Sufficient corrosion resistance of the welded assembly after the subsequent heat treatment for alloying and austenitization.

The inventors have found that these conditions are met in the case where the width of the zone 6 exceeds by 20-40% the half-width of the molten zone formed by butt welding blanks.

The minimum value of 20% ensures the position at which the pre-coating does not enter into the molten metal during welding, the value of 40% ensures satisfactory corrosion resistance.

Taking into account the conditions of welding sheets with a thickness of 1 to 3 mm width of the zone 6 will be 0.2-2.2 mm. This situation is represented in Figure 3, which schematically shows a section after the welding precoated sheet 2, which in turn is formed of an intermetallic alloy layer 3 and metallic layer 4. The molten zone is marked by 10, its axial plane in the direction of welding - numeral 9 . The dashed line shows the initial size of the zone 6, molten during welding.

3 shows the position in which the weld bead is formed completely symmetrically on two opposite sides. Under these conditions, the width of the zone 6 is the same on both sides. However, depending on the process parameters and welding thereof weld bead may have an asymmetrical appearance. According to the invention the width of the zone 6 may be aligned with the asymmetry so that this width is slightly more than a half width of the molten zone 10 on each of the two parties concerned. Under these conditions, the width of the zone 6 is different from the width of the zone 6 '3.

When welding in conditions vary along the assembly, for example due to local changes in the shape or thickness, width of the area 6 can also be matched with a corresponding change in the width of the molten zone along the periphery of the sheet welding. The width of the zone 6 is naturally enhanced if local conditions induce the formation of a wider weld bead.

In welding two sheets of different thickness the coated width of the zone 6 may also be different for welding the peripheral portion of each of the two sheets.

According to the embodiment in Figure 2 the layer 4 is removed in the area of ​​the coated sheet 7, wherein the deleted region is not completely adjacent to the periphery of the sheet 5. Then, the sheet is cut in an axial perpendicular plane of the sheet 8, such as a longitudinal cutting. Thus, a sheet shown in Figure 1. The width of the deleted area is 20-40% greater than the width of the molten zone, which could be produced by welding in the axial plane 8.

According to an embodiment of the invention, the width of the removed area is from 0.4 to 30 mm. The minimum value corresponds to the width, allowing to obtain after cutting in two an axial plane of the sheet 8 with leaving a very narrow area of ​​0.2 mm on each side of the sheets. The maximum value of 30 mm corresponds to the width of the removed area is well suited to industrial tools for such removal. Subsequent cutting can be performed in the axial plane 8 passing through the middle is not removed area, and the portion formed so as to be able to receive the sheet width removable zone which slightly exceeds half of the molten zone formed in the welding conditions and determined according to the invention.

As explained above, the width of removed areas can simultaneously prevent the introduction of the metal coating in the molten metal during the subsequent welding of the sheet and to provide corrosion resistance welded sheet blank after heat treatment.

Control over the removal of the metal layer 4 can be carried out by means of micrographic studies. However, it was also found that very quickly can monitor the effectiveness of removal operation using an optical control because in reality there is a difference between the external views of the metal layer 4 and an underlying intermetallic layer 3, since the latter has a darker color. Therefore, an operation to remove should continue and cease only at the moment when in the zone 6 major changes color compared to the color of the surface coating. Thus it is possible to control the removal process by measuring the emissivity or reflectance spectrometry for zone 6 of the light from the light source and one or more optical sensors directed toward the zone. The measured value corresponds to the reflected energy. This value is compared with a reference value emissivity or reflectivity of the metal layer 4 or with a value measured by another sensor directed at the metal layer. It is also possible to measure a change of reflected energy as a function of time. In the case where the surface layer 6 is exposed, the perceived energy becomes smaller than the energy corresponding to layer 4 of the metal alloy. By pre-calibration can thus accurately determine the time of approximations to the layer 3 during removal.

In the case of coating removal by laser ablation can also analyze the intensity or wavelength of the radiation emitted at the point of impact of the laser beam on the coated sheet. Indeed, a change of intensity and wavelength in the case where the layer 4 is removed and a laser beam fell on the layer 3. Therefore, control of the thickness of the sacrificial layer can be carried out as follows: measuring the intensity or wavelength of the radiation emitted at the point of impact of the laser beam, compared measured value with a reference value characteristic of the emissivity of the metal alloy layer 4 and the removal operation is stopped at the moment when the difference between the measured and the reference value exceeds a predetermined critical value.

Depending on the specific voltages, said step of removing the layer of metal alloy can be used at different stages of production, in particular one of the following:

- After unwinding rolls obtained on continuous rolling mills, and prior to being cut into sheets of smaller sizes;

- Cut sheets prior to welding.

In the method according to the invention is prepared by hot- or cold-rolled steel sheet of the following composition, wt.%: Carbon content 0.10 to 0.5%, preferably from 0.15 to 0.25%. This element has a great influence on the mechanical strength and hardenability after cooling conducted after processing to austenitization and fusion welded sheet blanks. When the content of below 0.10 wt.% Of the quenching becomes very low and the strength properties are not sufficient. Conversely, if over 0.5 wt.% Increases the risk of defect formation during quenching, particularly in the thickest parts. Carbon content in the range of from 0.15 to 0.25% produces a strength of about 1250 to 1650 MPa.

In addition to its role as a deoxidizer manganese exerts a significant influence on the hardenability, in particular when its content is not less than 0.5 wt.%, Preferably 0.8 wt.%. However, too high contents (3 wt.% Or preferably 1.8 wt.%) Poses a risk of excessive segregation.

The silicon content of the steel should be 0.1 to 1 wt.%, Preferably 0.1 to 0.35 wt.%. In addition to its influence on the deoxidation of liquid steel, this element contributes to the hardness. However, its content must be limited in order to avoid excessive formation of oxides and to improve their ability to cover.

When the content of more than 0.01% Cr improves the quenching characteristic and contributes to a considerable strength after hot forging at different sites items after cooling conducted after heat treatment and alloying austenitization. When the content of more than 1% (preferably 0.5%) not more chromium contributes to the formation of homogeneous mechanical properties.

Aluminum is an element that promotes deoxidation and precipitation of nitrogen. When the content above 0.1 wt.% In the smelting aluminates formed large, which makes it necessary to limit the content of said value.

When excessive amounts sulfur and phosphorus cause increased brittleness. Therefore it is preferable to limit the content by the values ​​0.05 and 0.1 wt.%.

- Boron, the content of which must be between 0.0005 to 0.010 wt.%, Preferably from 0.002 to 0.005 wt.%, An element has a big impact on quenchability. When the content of less than 0.0005% is no sufficient effect on hardenability. The full impact is provided by the content of 0.002%. The maximum boron content should be less than 0.010%, preferably 0.005% to not reduced viscosity.

Titanium has a high affinity to nitrogen and, therefore, helps to protect the boron so that this element is in the form necessary for the full effect on hardenability. However, if over 0.2%, in particular more than 0.1%, there is a risk of formation of large titanium nitrate molten steel adversely affect the viscosity.

After preparation of the sheets with one of the above methods assembling them by welding to obtain the welded sheet blanks. Certainly it is possible to collect more than two sheets in case of manufacturing complex parts target. The sheets may have different thicknesses or different compositions to meet the required local properties.

Welding is carried out after their joining edges of the sheets, with the availability of the metal alloy layer zone connected. Welding is performed along the edge adjacent to the zones 6, free of the metal alloy layer.

In the context of the invention may be used any means for the continuous welding is suitable for the specified thicknesses, production conditions and the quality of welds, namely:

- Laser beam welding,

- Electric arc welding, particularly welding a tungsten electrode in inert gas welding metal electrode inert gas, plasma welding, metal active gas electrode.

The welding conditions according to the invention does not lead to a re-melting of the metal coating 4, elements of which may then be in the molten zone. When this molten zone within the welding melts only the minimum amount of the intermetallic alloy layer 3. As follows from the above example, a very limited amount does not affect the quality of the metallurgical and mechanical properties of the weld after the heat treatment for alloying and austenitization.

Thereafter welded sheet blank is heated to a joint event:

- Processing surface fusion in the place in which the steel substrate diffusion elements, in particular iron, manganese, silicon, into the precoat. Thus formed on the surface alloyed intermetallic compound having a melting point substantially above the melting point of the metal alloy layer 4. The presence of this compound in the heat treatment allows to eliminate any oxidation and decarburization of any underlying steel.

Austenitizing the steel base can occur either partially or completely. Preferably heat the furnace so that the temperature of the parts ranged from Ac1 to Ac3 + 100 ° C. AC1 and AC3 represent respectively the initial and final temperatures of the austenitic transformation upon heating. According to the invention the duration of exposure at a given temperature is greater than or equal to 20 seconds required for ordering the microstructure and temperature at different points of the part.

In the context of the invention this heating phase is not formed brittle intermetallic zone in the molten metal are unfavorable for the mechanical properties of the part.

Consequently, the sheet blank is subjected to hot deformation to give it the final shape of the part, wherein this step contributes to lower yield strength and ductility of the steel increase with increasing temperature. Then, after reaching partially or fully austenitic structure at high temperature part was cooled under conditions necessary to impart the desired mechanical properties, in particular can be kept inside the tool part during the cooling, the instrument itself may be cooled for increased heat dissipation. To achieve high mechanical properties, preferably to obtain a martensitic, bainitic or bainitic-martensitic microstructure.

In the zone 6, on both sides of the weld, the intermetallic layer 3 of thickness of from 3 to 10 mm before the heat treatment is fused with the steel substrate and provide good corrosion resistance.

Example

As an example, other advantages of the invention are shown by the following embodiments. This used cold-rolled steel strip thickness of 1.5 mm with the following composition, wt.%:

Table 1Sostav steel (wt.%) SMnSiSPAlSrTiV0,2241,1600,2260,0050,0130,0440,1890,0410,0031

On the steel strip caused pre-coating by dipping in a bath of molten aluminum alloy containing 9.3% silicon, 2.8% iron, the rest - aluminum and inevitable impurities. The tape is then cut into sheets of format 300 × 500 mm 2. These sheets have on each of its sides in the form of a pre-coating layer of the intermetallic alloy containing predominantly Fe2Al3, Fe2Al5 and FexAlySiz. This layer thickness of 5 microns, in contact with the steel substrate is a layer of Al-Si alloy 20 um thick.

Before welding, the laser beam used four different ways to prepare:

- Method 1 (Inventive): a layer of an alloy of Al-Si was removed by stripping brush longitudinal width of 1.1 mm from the side edge of the sheet length of 500 mm. Stripping was performed the same way on both sides of the plate with a brush-type «Spirabande» 80 mm in diameter, mounted on a rotational system with angular transmission, while the entire complex made forward movement of the frame with counterweights. Stripping force is about 35 N at the contact portion of the brush with the sheet workpiece, moving speed of the brush - 10 m / min. Thus, when cleaning brush removes a layer of a metal alloy, and a stripping zone maintained only intermetallic alloy layer thickness of 5 microns.

Method II (according to invention). A layer of metal alloy Al-Si was removed by ablation by a laser beam width of 0.9 mm from the edge of the sheets. Ablation was performed in a similar manner on both sides of the laser beam such as a laser shutter with a rated power of 450 W and with the formation of 70 ns pulses. The energy of a single pulse was 42 mJ. The constant translational velocity of the laser beam relative to the sheet was 20 m / min. Figure 6 shows how ablation with a laser beam removes a layer 4 of a metal alloy, while maintaining in the treatment zone 3 merely intermetallic layer thickness of 5 microns.

R1 method (not part of the invention). Mechanically pre-coating completely removed, that is, a layer of metal alloy and intermetallic alloy. Such removal was performed on 1.1 mm width, i.e. in a similar way to the width of I, using a tool such as high-speed carbide plate in longitudinal translational motion. Thus, the subsequent welding was conducted in an area completely free of pre-coating on both sides of the seam.

Method R2 (does not belong to the invention). Laser beam welding was performed on precoated sheets without any special preparation of their periphery.

These sheets were welded with a laser beam under the following conditions: Nominal power: 6 kW, welding speed: 4 m / min. Although the width of the weld bead was established in the presence of process I zone free of metal alloy portion width of about 0.3 mm after production welds.

Welded blanks subjected to the heat treatment and alloying austenitization heating to a temperature of 920 ° C and holding at this temperature for 7 minutes. Under these conditions, there was a complete austenitic transformation of the steel substrate. It was found that during this phase of heating and holding the pre-coating aluminum-based silicic formed throughout its thickness intermetallic compound fused with the steel substrate. This fused coating with high melting point and high hardness has high corrosion resistance and prevents oxidation and decarburization of the underlying steel base during both heating and thereafter. After the heating phase to 920 ° C were subjected to hot deformation part and cooling.

Subsequent cooling between finding the tools provided martensitic structure. Mechanical strength Rm of the steel substrate after this treatment exceeds 1450 MPa.

Then, the following methods were evaluated welds made in such a way details:

- Micrographic sections, allowing to determine the possible presence of intermetallic areas in the connecting seams;

- Mechanical tests stretching transversely welds in samples 50 × 12.5 mm2 Rm for determining strength and total elongation;

- Accelerated corrosion tests conducted in accordance with DIN 50021, 50017, 50014. These tests comprise, after processing pairs salts cycles in which phase dried at 23 ° C alternated with wet phase at 40 ° C.

The results of these evaluations are presented in Table 2.

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The necessary conditions of quenching after the heat treatment the microstructure of the metal base and the melted zone formed by welding was fully martensitic for all four of the above methods.

Using the process I according to the invention, the molten zone did not contain intermetallic zone as shown in Figure 4.

But using the method mentioned R2 presence intermetallic zones (5) arranged, in particular, toward the periphery of the molten zone, in which the elements are pre-coating concentrate by the action of spontaneous movements of convection of the molten bath caused by the Marangoni effect. Such intermetallic zone of large size, the orientation of which may be substantially perpendicular to the mechanical load cause stress concentration and destruction of nucleation. Elongation in the lateral direction is considerably reduced by the presence of these intermetallic zones without these areas elongation exceeds 4%. It becomes less than 1% with them.

There was no significant difference between the mechanical properties (strength and elongation) by using method I and method according to the invention R1. This means that a thin layer of an intermetallic alloy after stripping brush preserved and melted during welding, does not cause formation of brittle zones in the molten metal, as shown in Figure 4.

Using the process decreases corrosion resistance R1; really become fully exposed on both sides of the weld resulting in the complete removal of the precoat. Because of the lack of protection from corrosion is noted, therefore, occurrence of red rust on the plot areas subjected to temperature extremes, both sides of the weld bead.

Thus, the method according to the invention allows simultaneous good weld ductility and good corrosion resistance.

Depending upon the steel composition, particularly carbon content, manganese, chromium and boron, maximum strength parts can be brought in line with the intended purpose. These components are effectively used for producing security items, in particular against the penetration of parts or components into the base, the reinforcing beams, center pillars in the construction of cars.