A set of at least 2 metal substrates

Abstract

Summary of the Invention\nThe present invention is a method for manufacturing an assembly of at least two metal substrates that are spot welded to each other through at least one spot welded joint, this method comprising two steps, the assembly obtainable by this method and the use of this assembly for manufacturing an automotive vehicle.

Description

A set of at least 2 metal substrates The present invention relates to a method for manufacturing an assembly of at least two metal substrates and the assembly. The invention is particularly suitable for manufacturing automotive vehicles. In terms of reducing the weight of vehicles, the use of high-strength steel sheets is known to achieve lighter vehicle bodies and improve crash safety. Hardened parts are also used significantly in reducing the weight of vehicles. In fact, the tensile strength of these aluminum sheets is 1200MPa and can be up to 2500MPa. Hardened parts can be coated with an aluminum-based or zinc-based coating that has good corrosion resistance and thermal properties. Typically, the method of manufacturing a coated hardened part includes the following steps: A) Pre-coated steel sheet with a metallic coating, which is conventionally based on aluminum or zinc, B) Cutting the coated steel sheet to obtain a plate, C) Heat treatment of the plate at high temperature to achieve a complete austenitic microstructure in the plate, D) Transferring the plate into a press tool, E) Hot rolling of the plate to obtain a piece, F) Cooling the piece obtained in step E) in order to obtain a microstructure in the steel that is martensitic or martensito-bainite or is made of at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in an amount less than or equal to 10%. It usually continues by welding two coated hardened parts or a coated hardened part to another metal substrate. Because the coating becomes hard and thick, welding coated hardened parts based on aluminum or zinc is very difficult. Patent application EP3020499 discloses a resistance spot welding method comprising the following steps: - a pulsed process of closing a sheet assembly of two or more overlapping steel sheets including at least one high-strength steel sheet using a pair of welding electrodes connected to a spot welding source using an inverter direct current method, and performing a series of repeated current passing and current stopping, while pressing the steel sheets with the welding electrodes; and - A continuous current passing process in which (after the pulsing process) current is passed continuously for a longer period of time than the maximum current passing time of the pulsing process, while the steel sheets are pressed with welding electrodes. However, this method is only applicable to hot-pressed steel sheets coated with zinc-based coating and conventional aluminum-based coating. In fact, this method was tested on 1500MPa hot-pressed steel sheets coated with aluminum, 1500MPa grade hot-pressed steel sheets coated with galvanized, and 1500MPa grade hot-pressed steel sheets coated with Al with ZnO skin treatment. Specific aluminum- or zinc-based coatings containing other elements are not covered by this patent application. Patent application EP3085485 discloses a method of resistance welding a set of steel sheets including a high tensile strength steel sheet superimposed, wherein the resistance spot welding method, the conduction system, is pulse conduction using a DC inverter welding power source, and, in the set of current pulses constituting the pulse conduction, in the respective current pulses, the conduction time, the intervals of the current pulses, defined as the conduction break time, and the welding currents applied by the current pulses, are variably controlled. However, this method is specific to hot-pressed steel sheets which contain a solid solution of intermetallic compounds and iron on their surface by an alloying reaction between a conventional zinc-based coating (Zn, Zn-Fe, Zn-Ni, Zn-Al, Zn-Mg, pure Zn-Mg-Al, etc.) or a conventional aluminum-based coating (Al-Si, etc.) and the sheet of base material. These surfaces are formed with an oxide layer consisting essentially of zinc or aluminum. In addition, sometimes, the surface of the coating consisting essentially of intermetallic compounds of iron and aluminum is formed with a film consisting essentially of zinc oxide. In the examples, this method was tested on hot-pressed steel sheets coated with an alloy of aluminum coating containing 9 wt% Si and Fe and a very small amount of ZnO, and on hot-pressed galvanized steel sheets. Typically, the native oxide layer of these coatings has a thickness between 10 and 100 nm. When a thin ZnO layer is deposited on the hardened aluminum-based coated part before austenitization, the ZnO and the aluminum-based coating become alloyed.Because a very thin layer is deposited on the aluminum-based coating, however, after austenitization, the native oxide consisting essentially of aluminum is very thin, i.e., 10-100nm, which results in easy welding. Specific aluminum-based coatings or zinc coatings of other elements are not included in this patent application. Patent application GB2468011 discloses a method for applying a current for resistance welding a set of plates, wherein a material of at least one plate is a high tensile strength material, the method comprising the following steps: - an initial step of applying an initial amperage of a magnitude that continuously softens the surface of the high-tensile material joint for a predetermined period of time; - a secondary stage (when the initial predetermined time period has elapsed) of switching an energizing amount from the primary amperage to a secondary amperage that causes a nugget at the large junction; and - A third stage applies secondary amperage continuously for a predetermined secondary time period. This method is dedicated to a high tensile strength material or a hot-pressed material. The hot-pressed material can be coated with a plating layer. However, the nature of the plating layer is not specified. In addition, in the first step, a low amperage is applied to soften the surface of the joint, and in the second step, a high amperage is applied to enlarge the nugget at the joint of the high tensile material. However, the low amperage in the first step does not allow welding of certain hot-pressed coated parts where the coatings contain elements other than zinc or aluminum. Recently, new coatings have been developed for hot-formed steel sheets. Patent application WO2017017521 discloses a phosphateable hardened part coated with an alloyed coating comprising 0.4 to 20.0 wt% zinc, 1.0 to 3.5 wt% silicon, optionally 1.0 to 4.0 wt% magnesium, wherein the Zn / Si ratio is between 3.2 and 8.0. Patent application WO2017 / 017514 discloses a hardened part coated with an alloyed coating comprising 2.0 to 24.0 wt% zinc, 1.1 to 7.0 wt% silicon and optionally 1.1 to 8.0 wt% magnesium, the remainder being aluminum, wherein the Al / Zn ratio is above 2.9 to improve the liquid metal embrittlement (LME) strength. Patent application WO2017 / 017513 discloses a sacrificial steel sheet coated with a coating comprising 2.0 and 24 wt% zinc, 7.1 to 12.0% silicon, optionally 1.1 to 8.0 wt% magnesium, the remainder aluminum, the Al / Zn ratio being above 2.9, and the sacrificially hardened coated part being obtained after a press hardening process. The particular coatings have a native oxide layer of micrometric thickness.Due to the thickness and hardness of the native oxide layer, welding these coatings is very difficult. However, no method has been developed for welding these special coated hardened parts. Therefore, the object of the present invention is to provide a convenient welding method for the manufacture of hardened parts coated with special coatings based on aluminum or zinc that have recently been developed. In particular for production lines, the object is to achieve a welding range for such special hardened parts, which is equal to or greater than 1kA. This object is achieved by providing a welding method for manufacturing this assembly according to claim 1. The welding method may also include the features of claims 2 to 11. Another object is achieved by providing a kit according to claim 12. The kit may also comprise any of the features of claims 13 to 21. Finally, another object is achieved by providing the use of a set according to claim 22. Other features and advantages of the invention will become apparent from the following detailed description of the invention. To illustrate the invention, various non-limiting examples will be described, particularly with reference to the following figure: Figure 1 shows an example according to the present invention. Figures 2 to 5 show examples of a spot welding cycle according to the present invention. Other features and advantages of the invention will be apparent from the following detailed description of the invention. The term hardened steel part means a hot-pressed or hot-formed steel sheet having a tensile strength of up to 2500 MPa, and very 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, preferably greater than or equal to 1500 MPa. This invention relates to a welding method for manufacturing an assembly comprising the following steps: A- Providing at least two metal substrates, wherein the first metal substrate is a hardened steel piece coated with: An alloyed coating consisting of zinc, silicon, optionally magnesium, and the remainder of aluminum, on which the following is directly deposited A native acid layer consisting of ZnO and optionally MgO. B- Applying a spot welding cycle with a spot welding machine, which includes welding electrodes and a spot welding power source that applies an inverter direct current, through at least two metal substrates. Step A), said spot welding cycle includes the following sub-steps: i- a pulsing having a pulsing current (Cp) applied through at least two of said metal substrates joined together using welding electrodes connected to a spot welding power source and directly thereafter, ii-A welding step having a welding current (Cw) applied through at least two metal substrates and Where the Cp current is higher than the Cw current and where the pulsing duration is shorter than the welding duration. Without wishing to be bound by any theory, it appears that the welding method according to the present invention, which is carried out on two metal substrates comprising at least one hardened steel piece coated with a specific coating comprising zinc, silicon, optionally magnesium, the remainder being aluminum, allows the welding range to be equal to or greater than 1 kA and the spatter of the coating on the surface of the assembly is reduced. In fact, it appears that due to the oxidation of the hardened sheet by air, ZnO and optionally MgO are naturally present on the surface of the hardened steel piece. It is believed that the pulsing with a pulsing current (Cp) of high intensity relative to the welding current (Cw) breaks at least a part of the oxide layer of ZnO and optionally MgO or the alloy coating present on the coated hardened steel piece and opens a path for the welding current. However, if Cp is less than Cw, i.e. outside the scope of the present invention, it is believed that the ZnO and optionally MgO barrier layer will not be broken by pulsing due to the high amount of ZnO and optionally MgO.Furthermore, it appears that the method according to the present invention, including a pulsing, facilitates implementation on an industrial scale. As shown in Figure 1, a spot welding machine (not shown) is used, comprising welding electrodes 1, 1' and a spot welding source 2. In this example, the electrodes enable the joining of two hardened steel pieces 3, 3' coated with a coating according to the invention 4, 4', 4''. During welding, a nugget 5 is formed between the two hardened steel pieces by diffusion. The nugget is an alloy of the remaining coatings and the steel pieces. Thanks to the spot welding cycle according to the present invention, it is believed that at least a part of the coating is removed in the nugget. Furthermore, above the spot welded joint 6, 6', it is believed that at least a part of the native oxide layer and / or alloyed coating is absent. In fact, it appears that at least one pulse breaks the native oxide layer and initiates welding between two pieces of coated hardened steel, by melting and removing the coatings above the spot welded joint and into the nugget.Therefore, the current can flow through the two hardened steel pieces, which allows for improved welding. Finally, it is believed that no coating is needed between at least one pulse and the welding step. In fact, if cooling is performed between these steps, there is no risk of the nugget formation between the two hardened steel pieces stopping because the steel pieces start to solidify. On the contrary, when cooling is not performed, the steel pieces seem to remain in a liquid state and can be easily joined together. Preferably, in phase Bi, the pulse current (Cp) is between 0.1 and 30 kA, preferably between 0.1 and 20 kA, very preferably between 8.0 and 20 kA and advantageously between 8.0 and 15 kA. Advantageously, in phase Bi, the pulsing duration is from 5 to 60 ms, preferably from 4 to 30 ms. Preferably, in step B.ii, the welding current (Cw) is between 0.1 and 15kA, advantageously between 0.1 and 7.5 and very preferably between 2.0 and 7.5kA. Advantageously, in step B.ii, the welding time duration is from 150 to 500 ms and very preferably from 250 to 400 ms. Preferably, the welding force is between 50 and 550 daN. In a preferred embodiment, the welding force during the spot welding cycle is between 350 daN and 550 daN. In another preferred embodiment, the welding force during the spot welding cycle is between 50 daN and 350 daN. In this case, it appears that there is a better localization of the current at the centers of the electrodes, which allows for better weldability. Preferably, the welding frequency is between 500 and 5000Hz, very preferably between 500 and 3000Hz, and for example between 800 and 1200Hz. Preferably, the welding step B.ii comprises a series of pulses, at least one pulse Bi directly following the first pulse of the welding step. In this case, no cooling takes place between the pulsing and the first pulse. The first pulse is followed by one or more pulses, there being a time interval between each successive pulse. Preferably, the duration of the interval is from 20 to 80 ms and more preferably from 30 to 60 ms. The spot welding cycle according to the present invention can have different shapes. Figure 2 shows a preferred embodiment in which the spot welding cycle 21 has a rectangular shape including a vertical pulsing peak 22 and a rectangular welding peak 23. Figure 3 shows another preferred embodiment in which the spot welding cycle 31 has a parabolic shape including a parabolic pulsing peak 32 and a parabolic welding peak 33. Figure 4 shows another preferred embodiment in which the spot welding cycle 41 has a triangular shape including a triangular pulsing peak 42 and a triangular welding peak 43. According to other examples, the spot welding cycle has a parabolic and rectangular shape including a parabolic pulsing peak and a rectangular welding peak, or, a triangular and rectangular shape including a triangular pulsing peak and a rectangular welding peak. Figure 5 shows a preferred example in which the spot welding cycle includes a Bi pulse directly followed by a first welding step pulse. In this example, the spot welding cycle 51 has a rectangular shape including a rectangular pulsing peak 52 and three rectangular welding peaks 53, 53', 53''. The invention also relates to a set of at least two metal substrates which are spot welded by at least one spot welded joint obtainable by the method according to the present invention, said set comprising: - A primary metal substrate which is a hardened steel piece coated with: An alloyed coating consisting of zinc, silicon, optionally magnesium, the remainder aluminum, which is directly deposited on the underlying material. A native oxide layer consisting of ZnO and optionally MgO. - Said spot welded joint comprises a nugget, and said spot welded joint is such that at least a portion of the native oxide layer and / or alloy coating is absent thereon. Without wishing to be bound by any theory, it appears that when the assembly includes a special top coating on the hardened part welded using the welding method according to the present invention, the welding range is equal to or above 1 kA. In fact, it appears that the thickness of the native oxide layer is greater than that of the prior art coatings, the welding method according to the present invention breaks the native oxide layer and removes at least a portion of the native oxide layer and / or the alloyed coating, which allows for a good weldability of the assembly. Preferably, the alloy coating of the hardened steel part comprises 0.1 to 40.0 wt% zinc, most preferably between 0.1 and 20.0 wt% zinc, advantageously between 5.0 and 14 wt% zinc, for example between 7.0 and 12.0 wt%. Preferably, the alloy coating of the hardened steel part comprises 0.1 to 20.0 wt% silicon, very preferably from 0.1 to 15.0 wt% silicon and advantageously from 0.1 to 6.0 wt% silicon, for example between 2.0 and 6.0 wt% silicon. Preferably, the alloy coating of the hardened steel part comprises from 0.1 to 20.0% by weight magnesium, from 0.1 to 10.0%, preferably from 0.1 to 4.0% by weight magnesium. Optionally, the coating comprises additional elements selected from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr, or Bi, the weight content of each additional element being less than 0.3% by weight, and the remaining elements optionally comprising iron from the feed ingots or from the passage of the steel substrate in the molten path. For example, the amount of iron is up to 5% by weight. In a preferred embodiment, the secondary metal substrate is a steel substrate or an aluminum substrate. Preferably, the secondary steel substrate is a hardened steel part according to the present invention. In another preferred embodiment, the assembly includes a third metal substrate, which is a steel substrate or an aluminum substrate. In this case, there are two or more spot welded joints. Finally, this invention relates to the use of the assembly according to the present invention for the manufacture of an automotive vehicle. The invention will be described in the form of implemented examples for information purposes only. They are not limiting. Examples All samples, which were Usibor® 1500 steel sheets, were hot-dip coated with a coating consisting of 3 wt% silicon, 2 wt% magnesium, 10-12 wt% zinc, the balance aluminum. The steel sheets were then press-cooled at an austenitizing temperature of 900°C for 5 minutes. Then, for each sample, two identical press-hardened pieces were welded together. The welding range was determined according to the SEP1220-2 standard. The welding test started at 3 kA and increased to 0.2 kA for both spot welds. The spatter range was found when two consecutive spatters occurred at the same current level. When the spatter range was achieved, the welding current was reduced in 0.1 kA steps to have three consecutive welded samples at the same current level without removal. This current level is defined as the upper welding range of the current range: Imax. After that, the lower limit of Imin was found. The search for Imin was performed using the criterion 4√t, where t is the sheet thickness. This criterion defines the minimum acceptable diameter value that guarantees weld quality and durability. For verification, five consecutive welded samples with a weld diameter higher than the minimum weld diameter were obtained. For examples 1 to 12, 17 and 18, the welding cycle includes, optionally, a pulse with a pulse current Cp, and a welding step with a welding current Cw defined by Imin and Imax according to the SEP1220-2 standard. For examples 13 to 16, the welding cycle includes a pulse with a pulse current Cp and three or four welding steps with a welding current Cw defined by Imin and Imax according to the SEP1220-2 standard, a temporary step between each welding step. The frequency was 1000Hz. The obtained Imin, Imax and welding current range are presented in Table 1 below. Samples Zinc percentage in coating (wt.%) Welding force (daN) Welding step pulsation Number of welding steps (s) Duration (ms) Stop (ms) Welding current (Cw) Cp > Cw Welding current range (kA) Number of duration (ms) Current (Cp) (kA) Imin (kA) Imax (kA) 1 12 200 0 - - 1 - - - - - 0 2 12 450 0 - - 1 - - - - - 0 3 12 200 1 20 4.5 1 - - - - - 0 4* 12 200 1 20 10 1 340 - 4.2 5.2 yes 1 5* 12 200 1 20 11 1 340 - 4.46 5.65 yes 1.19 6* 12 200 1 20 12 1 340 - 4.46 5.88 yes 1.42 7* 12 200 1 20 13 1 340 - 4.6 6.2 yes 1.6 8* 12 200 1 20 14 1 340 - 4.87 6.66 yes 1.79 9 12 450 1 20 6 1 - - - - - 0 10 12 450 1 20 7 1 - - - - - 0 11* 12 450 1 20 16 1 340 - 5.67 7.07 yes 1.4 12* 12 450 1 40 10 1 340 - 5.43 6.64 yes 1.21 13 12 450 1 20 10 3 160 40 5.6 6.1 yes 0.5 14 12 450 1 20 10 3 160 40 4.9 5.4 yes 0.5 15 12 450 1 20 10 3 160 40 4.5 4.6 yes 0.1 16 12 450 1 20 10 4 160 40 - - - 0 17* 10 200 1 20 10 1 340 - 4 5.2 yes 1.2 18* 10 400 1 20 10 1 340 - 4.2 5.2 yes 1 *: according to the present invention Samples 1, 2, 3, 9, 10 and 16 are not weldable, i.e. the Imin and Imax criteria defined in the SEP1220-2 standard were not achieved. The samples according to the present invention have a welding range equal to or greater than 1kA. Example 2: Heterogeneous welding test Usibor® 1500 steel sheets were hot dip coated with a coating consisting of 3 wt.% silicon, 2 wt.% magnesium, 12 wt.% zinc, the balance aluminum. The steel sheets were then press hardened at an austenitizing temperature of 900°C for 5 min. They were welded to grade DP600 steel (C: 0.14wt.%, Mn: 2.1wt.% and Si: 0.4wt.%) coated with a zinc coating. The welding range was defined as in Example 1. The frequency was 1000Hz. The defined Imin, Imax and welding current ranges are given in Table 2 below. Examples Welding force (daN) Welding step pulsation Welding time (ms) Welding current Cp > Cw Welding current range (kA) Number of times duration (ms) Stop (ms) Current (kA) Imin (kA) Imax (kA) 19* 200 1 20 - 10 340 4.8 6.6 Yes 1.8 20* 300 1 20 - 10 340 5.2 6.4 Yes 1.2 *: according to the present invention Examples according to the present invention have a welding range equal to or greater than 1kA. Example 3: Electrode life test Electrode life is defined as the last weld number of a test strip, before more than two welds outside of the eight defined minimum weld diameters are obtained. The minimum weld diameter was 4.7mm. Two coated hardened steel pieces prepared as Sample 4 were welded together by a welding method according to the present invention which includes a pulse and welding step. The pulse current was 10 kA for 10 ms. The welding current was Imax determined for Sample 4 in Example 1. A series of spot weldings were performed with electrodes on the coated hardened pieces and the weld diameter was measured for each spot weld. The results are given in Table 3 below. Sample Number of spot welds Weld diameter (mm) 4* 10 5.2 100 5.2 200 5.2 300 5.2 400 5.2 500 5.3 600 5.4 *: according to the present invention In Example 4 according to the present invention, the weld diameter was always higher than the minimum weld diameter.

Claims

Claims 1- A welding method for manufacturing an assembly comprising the following steps: A- Providing at least two metal substrates (3, 3') wherein a first metal substrate (3) is a hardened steel piece and coated with:  An alloy coating (4) comprising zinc, silicon, optionally magnesium, with a remainder of aluminum, on which is directly deposited  A native oxide layer comprising ZnO and optionally MgO, B- Applying a spot welding cycle with a spot welding machine comprising welding electrodes (1, 1') and a spot welding power source (2) applying an inverter direct current through the at least two metal substrates of step A, said spot welding cycle (21, 31, 41, 51) comprising the following sub-steps: i- A pulsing (22, 32, 42, 52) comprising a pulsed current (Cp) applied through at least two of said metal substrates connected to each other using welding electrodes connected to a spot welding power source and directly thereafter, ii-a welding step (23, 33, 43, 53) comprising a welding current (Cw each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each other using welding electrodes connected to each otherThe metal substrate is uncooled between the pulsing step and the welding step, and wherein the current Cp is higher than the current Cw and wherein the pulsing duration is shorter than the welding duration.

2. A welding method according to claim 1, wherein in the Bi step, the pulsing current (Cp) is between 8.0 and 30.0kA.

3. A welding method according to claim 1 or 2, wherein in step Bi, the pulse duration is from 5 to 60 ms.

4. A welding method according to any one of claims 1 to 3, wherein in step B.ii, the welding current (Cw) is between 0.1 and 15kA.

5. A welding method according to any one of claims 1 to 4, wherein in step B.ii, the welding time duration is from 150 to 500ms.

6. A welding method according to any one of claims 1 to 5, wherein the welding force during the spot welding cycle is between 50 and 550 daN.

7. A welding method according to claim 6, wherein the welding force during the spot welding cycle is between 350 daN and 550 daN.

8. A welding method according to claim 6, wherein the welding force during the spot welding cycle is between 50 daN and 350 daN.

9. The welding method according to any one of claims 1 to 8, wherein the welding frequency is between 500 and 5000 Hz.

10. A welding method according to any one of claims 1 to 9, wherein the welding step B.ii comprises a series of pulses, and at least one pulse Bi is directly preceding the first pulse of the welding step.

11. A welding step according to any one of claims 1 to 10, wherein the spot welding cycle shape (21, 31, 41, 51) is selected from the following: A rectangular shape including a rectangular pulsing peak (22) and a rectangular welding peak (23), A parabolic shape including a parabolic pulsing peak (32) and a parabolic welding peak (33), A triangular shape including a triangular pulsing peak (42) and a triangular welding peak (43), A parabolic and rectangular shape including a parabolic pulsing peak and a rectangular welding peak and A triangular and rectangular shape including a triangular pulsing peak and a rectangular welding peak.

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