ASSEMBLY OF AN ALUMINUM COMPONENT AND A HOT-FORMED STEEL PART HAVING AN ALLOY COATING COMPRISING SILICON, IRON, ZINC AND MAGNESIUM, THE REMAINDER BEING ALUMINUM

Abstract

The present invention relates to an assembly of an aluminum base piece and a hot-formed steel piece provided with an alloy coating comprising, by weight percentage, 0.1% to 15.0% silicon, 15.0% to 70% iron, 0.1% to 20.0% zinc, 0.1% to 4.0% magnesium, the remainder being aluminum, on at least one of its surfaces positioned so as to be in contact with the aluminum base piece.

Description

ASSEMBLY OF AN ALUMINUM COMPONENT AND A HOT-FORMED STEEL PART HAVING AN ALLOY COATING COMPRISING SILICON, IRON, ZINC AND MAGNESIUM, THE REMAINDER BEING ALUMINUM The present invention relates to an assembly of an aluminum base piece and a hot-formed steel piece provided with an alloy coating composed of silicon, iron, zinc, optionally magnesium, the remainder being aluminum, on at least one of its surfaces positioned so as to be in contact with the aluminum base piece. These assemblies are intended, for example, to be used in the manufacture of automotive body parts, such as door openings and the like, but are not limited to that. For complex vehicle components, such as front / rear bumper crossmembers, door reinforcements, crossmember reinforcements, center foot reinforcements, floor reinforcements, tunnel reinforcements, deck rails, and roof crossbars, hot-formed steel parts coated with an aluminum alloy base coating are commonly used. These coated hot-formed steel parts ensure safety and good corrosion resistance thanks to the coating's barrier effect. The constant need to reduce weight to lower CO2 emissions leads to the search for hybrid solutions that combine, in particular, aluminum and a hot-formed steel part coated with an aluminum-based alloy. However, such a combination generates complex and evolving corrosion phenomena in both materials. Patent application EP1669153 discloses a welded steel / aluminum structure comprising: - a hot-dip coated steel sheet with Al having a coating layer, consisting, by mass, of 3% to 12% Si, 0.5% to 5% Fe and the remainder being Al except for unavoidable impurities, and a ternary alloy layer of Al-Fe-Si formed at an interface between a steel substrate and the coating layer; and - an aluminum or aluminum alloy sheet spot welded to the Al-coated steel sheet; - wherein an area ratio of a binary Al-Fe alloy layer to a total Al / Fe bond boundary is controlled to 90% or less, and there is an Al-Fe alloy-free region between the binary Al-Fe alloy layer and the ternary Al-Fe-Si alloy layer. However, the hot-dip coated aluminum steel sheet is a cold-rolled steel sheet, not a hot-formed steel piece, and therefore cannot be used to produce complex parts. Furthermore, the corrosion resistance of this assembly remains too low over time and thus needs improvement. Furthermore, it is known that an assembly of a hot-formed steel part with an aluminum panel has insufficient corrosion resistance, as the steel accelerates the kinetic corrosion of the aluminum panel. The objective of the present invention is, therefore, to provide an assembly of a hot-formed steel part with an aluminum base part that has excellent corrosion resistance compared to the prior art assembly. To this end, the invention relates to an assembly according to any of claims 1 to 4. The invention relates to the method for manufacturing this assembly according to any of claims 5 to 8. The invention also relates to a part according to claim 9 or 10 and to a vehicle according to claim 11. Finally, the invention relates to the use of the assembly according to claim 12. The invention will now be illustrated by indicative examples provided for informational purposes only, and without limitation, with reference to the accompanying figures in which: Figure 1 schematically represents an assembly according to the invention and Figure 2 represents the measurement curves that represent the evolution of the maximum degree of corrosion of the aluminum panels within the assemblies according to the invention compared to the assemblies according to the state of the art. The hot-formed steel portion of the designation means a hot-formed or hot-sealed steel sheet having a tensile strength of up to 2500 MPa and more preferably up to 2000 MPa. For example, the tensile strength is greater than or equal to 500 MPa, advantageously greater than or equal to 1200 MPa, and preferably greater than or equal to 1500 MPa. For example, the steel is selected from: Usibor®1000, Usibor®1500, Ductibor®1000, and Usibor®2000. The designation of a base element aluminum means pure aluminum and all its alloys comprise at least 85% by weight of aluminum, including the 1000 to 7000 series comprising aluminum without any additive alloying elements and alloys of various compositions: aluminum without an alloying element: 1000; aluminum + copper: 2000; aluminum + manganese: 3000; aluminum + silicon: 4000; aluminum + magnesium: 5000; aluminum + magnesium + silicon: 6000 and aluminum + zinc + magnesium: 7000. The invention relates to an assembly 1 of at least one aluminum base element 2 and a hot-formed steel piece 3 provided on at least one of its surfaces, with an alloy coating comprising silicon, iron, zinc, optionally magnesium, the remainder being aluminum, unavoidable impurities and possibly one or more additional elements selected from Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Sr, Ni or B1, the hot-formed steel piece 3 being joined to the aluminum base element 2. However, the electrochemical potential of the hot-formed steel part coated by itself LQ77 Ln / LZnZ / E / Yli is only found between -0.68 V and -0.75 V, that is, very close to that of steel, which is -0.6 V and -0.78 V with respect to a saturated calomel electrode (SCE), resulting in poor anodic protection. The assembly according to the present invention has high corrosion resistance compared to the prior art assembly. The inventors have discovered that there is a synergy between the aluminum base element and the hot-formed steel part coated with an alloy coating comprising silicon, iron, zinc, optionally magnesium, the remainder being aluminum. In fact, an oxide layer is believed to be present on the surface of the alloy coating of the hot-formed steel part.This oxide layer acts as a barrier, slowing corrosion rates and reducing electrogalvanic coupling between the aluminum component and the hot-formed steel part. Therefore, contrary to expectations, corrosion resistance is significantly improved in the assembly area. According to the invention, the first aluminum base element can take the form of a panel 2. This panel 2 has suitable dimensions adapted to the subsequent use of the assembly according to the invention. In a preferred embodiment, the assembly in question is a B-pillar. Advantageously, the aluminum base panel is positioned on the exterior side of the vehicle. The aluminum base panel 2 may also include, on at least part of its surface, one or more protective coatings, such as phosphate and / or cataphoretic coatings, like those commonly applied to vehicle body blanks. Assembly with the second coated hot-formed steel part 3 is generally performed before the body blank passes through baths containing phosphate and / or cataphoretic coatings. Parts located outside the assembly area are coated. The assembly area can only be partially coated due to the low penetration of the coating within it. The second element of the assemblies according to the invention is thus a hot-formed steel piece 3 provided on at least one of the surfaces thereof, with an alloy coating comprising in weight percentage, 0.1% to 15.0% silicon, 15.0% to 70% iron, 0.1% to 20.0% zinc, 0.1% to 4.0% magnesium, the remainder being aluminum, unavoidable impurities and possibly one or more additional elements selected from Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Sr or B1. The weight content of each additive is generally less than 0.3%. These additives can, among other things, improve the ductility or adhesion of the coating to the steel element. A skilled professional familiar with their effects on coating characteristics will know how to use them according to the desired additional or complementary objective. The coating may ultimately contain residual elements originating, for example, from the coating bath when a hot-dip coating process is used. It may be contaminated with impurities from the feed ingots or resulting from the steel element's passage through the coating bath. Preferably, the alloy coating of the hot-formed steel part comprises LQ77 LO / i7Π7 / Β / ΥI between 5.0% and 14% by weight of zinc and, for example, between 7.0% and 12.0% by weight of zinc. Preferably, the alloy coating of the hot-formed steel part comprises from 0.1% to 6.0% by weight of silicon and, for example, from 2.0% to 6.0% by weight of silicon. Preferably, the alloy coating of the hot-formed steel part comprises from 1.0% to 4.0% by weight of magnesium. Advantageously, the alloy coating of the hot-formed steel part comprises between 40% and 60% iron by weight. The iron is derived from the feed ingots or results from the steel element passing through the coating bath and from the steel during the austenitizing treatment. Preferably, the alloy coating comprises an intermetallic FeaAl layer and an interdiffusion Fe-Si-Al layer. Advantageously, the microstructure of the metallic coating comprises the Zn2Mg phase or the Mg2Si phase or both. The metallic coating generally has a thickness that is less than or equal to 30 pm or even 25 pm and greater than or equal to 3 pm or even 5 pm. As mentioned above, the two elements 2, 3 are mounted in such a way that the surface coated with the hot-formed steel part 3 is in at least partial contact with the aluminum base element 2. In a preferred embodiment, the assembly can be carried out in particular by adhesive bonding, welding, sealing, crimping, clinching or riveting the two elements in an assembly zone 4 located, in the example shown in Figure 1. For example, welding can be done by spot welding, laser ablation welding, or arc welding. Such a crimp could consist of a simple folding of one of the elements around the other, as shown in Figure 1 where element 2 is arranged so as to wrap around element 3. Of course, it is possible to carry out any other type of mechanical assembly known to the expert in the field. It is also possible to assemble the panels according to the invention by structural joining using a structural adhesive or sealant that allows assembly in such a way that the mechanical stresses to which one or the other of the elements 2, 3 are subjected are transferred to the other element 3, 2. Since the layers of adhesives and / or sealants are very thin (generally less than 5 mm, or even less than 1 mm and even less than 200 pm), such a joined assembly would be considered equivalent to putting the two elements 2, 3 in contact in the same way as a simple mechanical assembly. Obviously, it is possible to combine structural joining and mechanical assembly to improve the robustness of the assembly. The invention also relates to a method for manufacturing an assembly according to the present invention comprising: A. The provision of a steel sheet pre-coated on at least one of the surfaces with a pre-coating comprising, by weight percentage, 0.1% to 20.0% silicon, 0% to 10% iron, 0.1% to 25.0% zinc, 0.1% to 6.0% magnesium, the remainder being LQ77Ln / L7nZ / E / Yli aluminum, unavoidable impurities and possibly one or more additional elements selected from Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi, B. cutting the pre-coated steel sheet to obtain a target, C. the austenitizing treatment of the target to obtain a fully austenitic microstructure in the steel, D. the transfer of the target to a press tool, E. the hot forming of the target, F. Cooling the blank to obtain the hot-formed steel part (3) provided on at least one of the surfaces with an alloy coating comprising silicon, iron, zinc, optionally magnesium, the remainder being aluminum, unavoidable impurities and possibly one or more additional elements selected from Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi and G. the assembly of the hot-formed steel part provided on at least one of the surfaces with the alloy coating with an aluminum base element. For example, in step A), the steel sheet can be pre-coated by hot-dip coating, physical vapor deposition, or electrodeposition. Preferably, the steel sheet is pre-coated by hot-dip coating. Next, the steel sheet pre-coated with the pre-coating according to the present invention is cut to obtain a blank in step B). Preferably, in stage C), the austenitizing treatment is applied to the blank in a furnace under an unprotected atmosphere at an austenitizing temperature of between 800 °C and 1100 °C, more preferably between 800 °C and 1000 °C, and advantageously between 880 °C and 930 °C. Advantageously, the blank is held for a dwell time tm of between 1 minute and 12 minutes, preferably between 3 minutes and 9 minutes. During the heat treatment prior to hot forming, the coating forms an alloy layer comprising silicon, iron, zinc, optionally magnesium, the remainder being aluminum, which has high resistance to corrosion, abrasion, wear, and fatigue. The steel diffuses iron into the pre-coating. In step E), the blank is then transferred to a hot forming tool and hot formed, for example, at a temperature between 600 °C and 900 °C. The hot forming can be hot stamping or rolling. Preferably, the blank is hot stamped. In step F), the part is then cooled in the hot forming tool or subsequently transferred to a specific cooling tool. Preferably, in step G), the hot-formed steel part (3) is joined to the aluminum base element (2) by adhesive bonding, welding, sealing, crimping, clinching, or riveting. For automotive applications, after the phosphating step, the part is immersed in an electrophoretic coating bath. Typically, the phosphate layer thickness is between 1 and 2 µm, and the The electrophoretic coating thickness of LQZZLn / LZnZ / E / Yli is between 15 µm and 25 µm, preferably less than or equal to 20 µm. The cataphoretic layer guarantees additional protection against corrosion. After the electrophoretic coating step, other layers of paint can be deposited, for example, a primer paint coating, a base coat, and a top coat. Before applying the electrophoretic coating to the part, the part is degreased and subjected to a phosphating process to ensure the adhesion of the cataphoresis. The invention relates to a hot-formed steel part for a vehicle comprising at least one assembly 1 according to the present invention. The invention relates to a hot-formed steel part according to the present invention, which is a front crossmember, a rear bumper crossmember, a door reinforcement, a vertical windshield reinforcement, a B-pillar reinforcement, a floor and roof reinforcement, a roof crossmember, or a dashboard panel. The invention relates to a vehicle that includes at least the hot-formed steel part. Finally, the invention relates to the use of an assembly according to the present invention for the manufacture of hot-formed steel parts of a vehicle. In order to highlight the improved performance obtained by using the assemblies according to the invention, some specific examples of modalities will be detailed in comparison with assemblies based on the prior art. Examples The steel sheets made of Usibor® were coated with zinc or aluminum alloys of varying composition, as listed in Table 1, with a uniform thickness of 20 µm on both sides. The steel sheets were hot-formed at 900 °C for 5 minutes to obtain parts with the alloy composition described in Table 2. The assembly samples were created using panels made of AA6061 aluminum, and these hot-formed parts were coated. The AA6061 aluminum panels and the coated hot-formed steel parts were joined with adhesive tape, leaving a 120 mm gap in the assembly area, in accordance with SEP1160. The edges of the panels were bonded together with electrical wire to create an electrogalvanic connection. There was no coating present in the assembly area. The assembly samples of aluminum panels and hot-formed steel parts were subjected to the VDA 233-102 test, which involves subjecting the samples to a series of treatment cycles, each cycle lasting one week, with a succession of subcycles A, B and C of 24 hours each: - Cycle A: cycle that includes treatment for a period of three hours by placing the subject in the presence of a saline spray mist at 35 °C; - Cycle B: 24-hour cycle without treatment with a salt spray mist at a temperature ranging from 25°C to 50°C and a relative humidity ranging from 70% to 95%; - Cycle C: cycle without treatment with a salt spray mist, at a temperature ranging from -15 °C to 50 °C and a relative humidity ranging from 70% to 95%. The saline solution used is an aqueous solution containing 1% by weight of sodium chloride. At the end of each cycle, a first sample was subjected to corrosion etching degree measurements of the aluminum element by laser triangulation mapping across the entire surface of a sample, followed by extraction of the maximum value observed in micrometers. The results for 6 and 12 cycles are summarized and presented in Table 3, and the detailed results, cycle by cycle, are presented in Figure 2. Table 1 - Coating compositions of steel sheets The steel sheets before hot forming had the following coating compositions: LQ77 LO / i7P7 / E / YI Sample Zn (% by weight) Si (7% by weight) Fe (7% by weight) Mg (7% by weight) 7% Al by weight 1 Remainder - - - 0.2 2 - 9 - - Remainder 3 12 3 - 2 Remainder After hot forming at 900 °C for 5 minutes, the steel parts were coated with an alloy coating with the compositions listed in Table 2 below. Table 2 - Steel parts with alloy coating composition Sample Zn (7º by weight) Si (7º by weight) Fe (7º by weight) Mg (7º by weight) 7º of Al by weight 1 Remainder - 50 - 0.08 2 - 4 46 - Remainder 3* 4 1.8 50 0.8 Remainder *: according to the present invention Table 3 - Maximum depth of corrosion Maximum depth (pm) after 6 cycles Maximum depth (pm) after 12 cycles 1 450 420 2 300 380 3* 0 250 *: according to the present invention Test 3 according to the present invention shows a greater improvement in corrosion resistance compared to tests 1 and 2.

Claims

1. An assembly (1) of at least one aluminum base element (2) and a hot-formed steel piece (3), the steel piece being hardened on at least one of its surfaces with an alloy coating comprising, by weight, 0.1% to 15.0% silicon, 15.0% to 70% iron, 0.1% to 20.0% zinc, 0.1% to 4.0% magnesium, the remainder being aluminum, unavoidable impurities, and possibly one or more additional elements selected from Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Sr, Cr, Ni, or Bi, the hardened steel piece (3) being bonded to the aluminum base element (2).

2. An assembly according to claim 1, wherein the alloy coating comprises a FesAl intermetallic layer and a Fe-Si-Al interdiffusion layer.

3. An assembly according to claim 1 or 2, wherein the microstructure of the metallic coating comprises the Zn2Mg phase or the MgzSi phase or both.

4. An assembly according to any of claims 1 to 3, wherein the hot-formed steel part (3) is joined to the aluminum base element (2) by at least one means selected from: adhesive bonding, welding, sealing, crimping, clinching, or riveting.

5. A method for manufacturing an assembly according to any one of claims 1 to 4 comprising: A. Providing a steel sheet pre-coated on at least one surface with a pre-coating comprising, by weight percentage, 0.1% to 20.0% silicon, 0% to 10% iron, 0.1% to 25.0% zinc, 0.1% to 6.0% magnesium, the remainder being aluminum, unavoidable impurities, and possibly one or more additional elements selected from Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, or Bi; B. Cutting the pre-coated steel sheet to obtain a blank; C. Austenitizing the blank to obtain a fully austenitic microstructure in the steel; D. Transferring the blank to a press tool; E. Hot forming the blank; F.the cooling of the blank to obtain the hot-formed steel part (3) provided on at least one of the surfaces with an alloy coating comprising, by weight percentage, 0.1% to 15.0% silicon, 15.0% to 70% iron, 0.1% to 20.0% zinc, 0.1% to 4.0% magnesium, the remainder being aluminum, unavoidable impurities and possibly one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi and G. the assembly of the hot-formed steel part provided on at least one of the surfaces with the alloy coating with an aluminum base element.

6. A method according to claim 5, wherein in step C), the austenitizing treatment is carried out between 800 °C and 1100 °C.

7. A method according to claim 5 or 6, wherein in step E), the hot forming is performed by hot stamping or rolling.

8. A method according to any of claims 4 to 7, wherein in step G), the hot-formed steel part (3) is joined to the aluminum base element (2) by adhesive bonding, welding, sealing, crimping, clinching or riveting.

9. A vehicle part comprising at least one assembly (1) according to any of claims 1 to 4 or obtainable by the method according to any of claims 5 to 8.

10. A part according to claim 9, wherein is a front crossmember, a rear bumper crossmember, a door reinforcement, a vertical windshield reinforcement, a B-pillar reinforcement, a floor and roof reinforcement, a roof crossmember, or a dashboard panel.

11. A vehicle comprising at least one part according to claim 9 or 10.

12. Use of an assembly according to any of claims 1 to 4 for the manufacture of parts according to any of claims 9 or 10 or of a vehicle according to claim 11.