A spot welding method

Abstract

A welding method for producing a set of at least two steel substrates (3, 3') spot-welded to each other, comprising: a first steel substrate (3), which is a hot-formed steel piece produced by hot-forming a steel sheet with an aluminum-base coating, said coating comprising the following by weight percentage: 7.0 to 9.0% zinc, 1.0 to 10% silicon, 1.0 to 10% magnesium, maximum 3.0% iron, optional elements selected from lead, nickel, zirconium, hafnium, strontium, antimony, titanium, calcium, manganese, tin, lanthanum, cerium, chromium or bismuth with a weight content of each less than 0.3, and unavoidable impurities up to 0.02% and the balance of the composition consisting of aluminum, \n- Application of a spot welding cycle comprising:\n- At least three pulses (22, 32, 42), each having the same maximum pulse current (Cp), the duration of each pulse (p) being the same and between 20 and 60 milliseconds It is set,\n- Uniform cooling time, between 30 and 50 milliseconds, separating each pulse.

Description

A spot welding method The present invention relates to a method for welding to produce a set of steel substrates welded together by at least one spot joint. This method is particularly suitable for automobile manufacturing. In order to reduce the weight of automobiles, it is well known to use high-strength steel sheets to make lighter bodies and improve crash safety. Hot-rolled steel parts are also widely used to reduce the weight of automobiles. In fact, the tensile strength of these steels is at least 1200 MPa and can reach up to 2500 MPa. Hot-rolled steel parts may be coated with an aluminum-based coating that has good corrosion resistance and good thermal properties. Typically, the process of making a hot-dip coated molding involves the following steps: a) Providing steel sheet that is already coated with a metal coating (usually aluminum coating); b) Cutting the coated steel sheet to obtain a raw sheet; c) Performing heat treatment on the raw sheet at high temperature to create a fully austenitic microstructure in the steel; d) Transferring said sheet to the press tool; e) Hot forming of raw sheet to produce parts; f) Cooling the part produced in step (e) to obtain a hot-formed steel part. The pre-coated steel sheet in step (a) is usually prepared by hot dip galvanizing in a molten metal bath. After the part is produced in step (f), it is assembled to other vehicle components by spot welding. However, it is difficult to weld hot-formed parts with aluminum cladding. In particular, this type of material usually does not allow for a wide welding range. The appropriate welding current range starts from the current at which the minimum diameter of the weld core is formed and continues to the current at which molten spatter occurs. In addition, a wide welding current range is desirable because it allows the weld core diameter to be controlled within a certain range even if the welding current fluctuates. A wide welding range is also an advantage due to its greater resistance to electrode wear, component mismatch, and power grid voltage fluctuations. Automobile manufacturers usually require a range of 1 kA or higher to ensure weld quality and reduce the need for frequent electrode replacements in their welding lines. Furthermore, it has been shown that the welding range of hot-formed parts depends on the components of the hot-formed process leading to their production. The higher the temperature and time used in this process, the smaller the welding range will be. This is due to the presence of alloy phases resulting from the diffusion of iron from the substrate into the coating, as well as surface oxides formed during the hot-formed process. Especially when the coating contains elements other than aluminum, such as silicon, magnesium or zinc, complex surface oxides are formed in relation to thermal components such as time and temperature. These oxides must be removed before welding can be carried out. Therefore, the aim of the present invention is to provide a welding method for manufacturing hot-formed steel parts that allows for an increase in the welding current range to at least 1 kiloampere and minimizes the phenomenon of molten spatter, without the need to remove the oxide layer formed in the hot-forming process. This object is achieved by providing a method according to claim 1. Said method may also include any or all of the features set forth in claims 2 to 10. Another object of the invention is to manufacture a vehicle comprising such an assembly according to claim 11. Other features and advantages of the invention will become apparent from the detailed description below. To illustrate the invention, various constructions and various tests are presented as non-limiting examples and are explained with reference to the following figures: Figure 1: Shows an example of the equipment used to implement the present invention. Figure 2: illustrates a construction of a spot welding cycle according to the present invention. The present invention relates to a welding method for manufacturing an assembly comprising at least two steel substrates welded together by at least one spot weld joint. As shown in Figure 1, a spot welding apparatus (not shown in the figure) is used, comprising welding electrodes 1 and 1' and a spot welding power source 2. In this example, the electrodes connect two hot-formed steel pieces 3 and 3' obtained by hot-forming a steel sheet coated with an aluminum base coating 4 and 4'. The current can be alternating current or direct current and in a preferred embodiment is medium frequency direct current (MFDC) supplied from an alternating current converter. The method according to the invention further comprises the use of a spot welding cycle 21, which comprises the following: - At least three pulses 22, 32, 42, all having the same pulse current (Cp) are applied through the bonded metal substrates using welding electrodes connected to a spot welding power source. The length of each pulse is the same and is set in the range of 20 to 60 milliseconds; - An identical cooling time, between 30 and 50 milliseconds, between both pulses. The pulses used in the method according to the invention should be at least three and preferably at least five. In a preferred embodiment, the maximum number of pulses can be nine. After these pulses and cooling times have been used, the substrates are completely welded, meaning that no further welding cycles of any kind are performed on top of them. The duration of each pulse is the same from start to finish and is set in the range of 20 to 60 milliseconds, and preferably 30 to 50 milliseconds. If the pulse duration is less than 20 milliseconds, the minimum weld core diameter will not be achieved. If the pulse duration is greater than 60 milliseconds, the risk of premature melt ejection increases. The inventors have found that increasing the number of pulses increases the welding current range. The maximum pulse current (Cp) is the same for all pulses and is preferably in the range of 0.1 to 30 kA. The welding force applied by the electrodes simultaneously with the current is preferably in the range of 50 to 650 decanewtons and more preferably 250 to 500 decanewtons. The welding frequency is preferably set in the range of 500 to 5000 Hz, and more preferably 800 to 2000 Hz. The spot welding cycle according to the present invention can include pulses with various shapes. These shapes can be the same or different within a welding cycle. Figure 2 shows a preferred embodiment in which the spot welding cycle 21 includes five rectangular shaped pulses, i.e. identical rectangular pulse peaks 22, 32, 42, 52 and 62. Other options for the pulse shape are: - Parabolic shape - Triangular shape Or any other suitable form, provided that all pulses in a cycle have the same maximum pulse current (Cp). In the context of this invention, the term hot-formed steel part refers to a hot-formed or hot-punched steel part which, after austenitizing a raw sheet and forming and tempering it in a die, has a tensile strength of up to 2500 MPa, and preferably up to 2000 MPa. For example, a tensile strength of 500 MPa or more, desirably 1200 MPa or more, and preferably 1500 MPa or more. If steel with high mechanical strength is required, especially for structural parts of automobiles, steel with a tensile strength higher than 500 MPa, preferably between 500 and 2000 MPa, before or after heat treatment, can be used. The weight composition of the steel sheet is preferably as follows: 0.03% ≤ carbon ≤ 0.50% ; 0.3% ≤ manganese ≤ 3.0% ; 0.05% ≤ silicon ≤ 0.8% ; 0.015% ≤ titanium ≤ 0.2% ; 0.005% ≤ aluminum ≤ 0.1% ; 0% ≤ chromium ≤ 2.50% ; 0% ≤ sulfur ≤ 0.05% ; 0% ≤ phosphorus ≤ 0.1% ; 0% ≤ boron ≤ 0.010% ; 0% ≤ nickel ≤ 2.5% ; 0% ≤ molybdenum ≤ 0.7% ; 0% ≤ niobium ≤ 0.15% ; 0% ≤ nitrogen ≤ 0.015% ; 0% ≤ copper ≤ 0.15% ; 0% ≤ calcium ≤ 0.01% ; 0% ≤ Tungsten ≤ 0.35% The rest consists of iron and unavoidable impurities from steel production. For example, the steel sheet is 22MnB5 with the following weight composition: 0.20% ≤ Carbon ≤ 0.25%; 0.15% ≤ Silicon ≤ 0.35%; 1.10% ≤ Manganese ≤ 1.40%; 0% ≤ Chromium ≤ 0.30%; 0.020% ≤ Titanium ≤ 0.060%; 0.020% ≤ Aluminum ≤ 0.060%; 0.002% ≤ Boron ≤ 0.004% The rest consists of iron and unavoidable impurities from steel production. In another construction, the steel sheet has the following weight composition: 0.24% ≤ Carbon ≤ 0.38%; 0.40% ≤ Manganese ≤ 3%; 0.10% ≤ Silicon ≤ 0.70%; 0.015% ≤ Aluminum ≤ 0.070%; Chromium ≤ 2%; 0.25% ≤ Nickel ≤ 2%; 0.015% ≤ Titanium ≤ 0.10%; Niobium ≤ 0.060%; 0.0005% ≤ Boron ≤ 0.0040% The rest consists of iron and unavoidable impurities from steel production. Alternatively, the steel sheet can have the following weight composition: 0.30% ≤ carbon ≤ 0.40%; 0.5% ≤ manganese ≤ 1.0%; 0.40% ≤ silicon ≤ 0.80%; 0.1% ≤ chromium ≤ 0.4%; 0.1% ≤ molybdenum ≤ 0.5%; 0.01% ≤ niobium ≤ 0.1%; 0.01% ≤ aluminum ≤ 0.1%; 0.008% ≤ titanium ≤ 0.003%; 0.0005% ≤ boron ≤ 0.003%; 0.0% ≤ phosphorus ≤ 0.02%; 0.0% ≤ calcium ≤ 0.001%; ​​0.0% ≤ sulfur ≤ 0.004%; 0.0% ≤ nitrogen ≤ 0.005% The rest consists of iron and unavoidable impurities from steel production. In another construction, the steel sheet has the following weight composition: 0.040% ≤ Carbon ≤ 0.100%; 0.80% ≤ Manganese ≤ 2.00%; 0% ≤ Silicon ≤ 0.30%; 0% ≤ Sulfur ≤ 0.005%; 0% ≤ Phosphorus ≤ 0.030%; 0.010% ≤ Aluminum ≤ 0.070%; 0.015% ≤ Niobium ≤ 0.100%; 0.030% ≤ Titanium ≤ 0.080%; 0% ≤ Nitrogen ≤ 0.009%; 0% ≤ Copper ≤ 0.100%; 0% ≤ Nickel ≤ 0.100%; 0% ≤ Chromium ≤ 0.100%; 0% ≤ Molybdenum ≤ 0.100% The rest consists of iron and unavoidable impurities from steel production. In another construction, the steel sheet has the following weight composition: 0.06% ≤ Carbon ≤ 0.1%, 1% ≤ Manganese ≤ 2%, Silicon ≤ 0.5%, Aluminum ≤0.1%, 0.02% ≤ Chromium ≤ 0.1%, 0.02% ≤ Niobium ≤ 0.1%, 0.0003% ≤ Boron ≤ 0.01%, Nitrogen ≤ 0.01%, Sulfur ≤ 0.003%, Phosphorus ≤ 0.020% Less than 0.1% copper, nickel and molybdenum, The rest consists of iron and unavoidable impurities from steel production. In another construction, the steel sheet has the following weight composition: 0.015% ≤ Carbon ≤ 0.25%; 0.5% ≤ Manganese ≤ 1.8%; 0.1% ≤ Silicon ≤ 1.25%; 0.01% ≤ Aluminum ≤ 0.1%; 0.1% ≤ Chromium ≤ 1.0%; 0.01% ≤ Titanium ≤ 0.1%; 0% ≤ Sulfur ≤ 0.01%; 0.001% ≤ Boron ≤ 0.004%; 0% ≤ Phosphorus ≤ 0.020%; 0% ≤ Nitrogen ≤ 0.01% The rest consists of iron and unavoidable impurities from steel production. Alternatively, the steel sheet has the following weight composition: 0.2% ≤ Carbon ≤ 0.34%; 0.5% ≤ Manganese ≤ 1.24%; 0.5% ≤ Silicon ≤ 2.0%; 0% ≤ Sulfur ≤ 0.01%; 0% ≤ Phosphorus ≤ 0.020%; 0% ≤ Nitrogen ≤ 0.01% The rest consists of iron and unavoidable impurities from steel production. The method according to the invention is applied to hot-formed steel parts produced by a hot-forming process of a steel sheet with an aluminum-based coating containing zinc, silicon and magnesium. The steel sheet used to make the hot-dip galvanized part can be produced by hot-dip galvanizing in a bath at a temperature between 600 and 700 degrees Celsius, and preferably between 620 and 650 degrees Celsius. The coating weight is adjusted during the cleaning process using a gas knife in a range of 50 to 500 g / m2 and may be between 80 and 150 g / m2 and preferably between 90 and 120 g / m2 for the total of both sides of the steel sheet. Before coating, the steel sheet can be produced by hot rolling and optionally cold rolling, depending on the desired thickness, which can be, for example, between 0.5 and 3.0 mm and preferably between 1.0 and 2.0 mm. The coating contains 7.0 to 9.0 wt.% zinc, and preferably 7.5 to 8.5 wt.% zinc, by weight. Optionally, the coating can include 1.0 to 10.0% silicon and 1.0 to 10.0% magnesium. Preferably, the coating contains 1.0 to 4.0% silicon and 1.0 to 4.0% magnesium by weight, and desirably contains 2.5 to 3.5% silicon and 1.5 to 3.0% magnesium. Optionally, the coating may contain up to 3% by weight of iron. The iron is the result of dissolution of the steel sheet in the hot dip coating bath and may vary during production. Optionally, the coating may contain additional elements selected from nickel, zirconium, hafnium, strontium, antimony, lead, titanium, calcium, manganese, tin, lanthanum, cerium, chromium or bismuth, each of which is less than 0.3% by weight. In a preferred formulation, up to 100 ppm by weight of calcium is added. Finally, the coating can contain unavoidable impurities up to 0.02% by weight and preferably up to 0.01% by weight. The hot forming process of these steel sheets is known to the skilled person and involves austenitizing a cut blank of this steel at a temperature between 840 and 950 degrees Celsius, and preferably between 900 and 950 degrees Celsius, for 3 to 10 minutes, and then cooling it in a forming die. After hot forming, the coatings mentioned in the previous lines are alloyed by the penetration of iron due to the heating of the blank. An oxide layer is formed on the surface of the alloy layer, which includes aluminum, zinc and magnesium. The welding method according to the invention can be used to weld a hot-formed part to a similar hot-formed part (homogeneous welding) or to any other steel part without removing surface oxides. The method can also be used in composite welding between a hot-formed steel part and an aluminum substrate. In the following, the present invention will be explained by performing experiments that are merely illustrative and not limiting. Examples Steel sheets with various average thicknesses and coated with aluminum-base alloys were prepared and hot-formed under the conditions listed in Table 1. Table 1 Sample Steel Grade Steel Sheet Thickness (mm) Coating Composition (wt %) Total Coating Weight (g / m²) Heat Treatment Time (min) Heat Treatment Temperature (°C) Zirconium Silicon Magnesium Iron Aluminum 1 U1500 1.0 8.2 3.2 2.0 1.6 Remaining 120 5 900 2 U1500 1.0 8.2 3.2 2.0 1.6 Remaining 120 5 900 3 U1500 1.0 7.8 3.3 2.0 1.6 Remaining 120 5 900 4 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 5 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 6 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 7 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 8 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 9 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 10 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 11 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 12 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 13 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 14 U1500 1.2 8.1 3.1 1.9 1.6 Remaining 120 5 900 15 U1500 1.5 8.1 3.2 2.1 1.7 Remaining 120 5 900 16 U1500 1.5 8.1 3.2 2.1 1.7 Remaining 120 5 900 17 U1500 1.5 8.2 3.2 2.0 1.6 Remaining 120 6 930 The U1500 case has the following composition in weight percent: 0.22 carbon, 1.2 manganese, 0.25 silicon, 0.2 chromium, 0.04 aluminum, 0.04 titanium, and 0.003 boron. The composition of the coating can be seen in Table 1. Then, for each sample, two identical heated moldings were welded together. The welding interval was determined as follows. These methods are now explained. In all methods, the welding range is equal to the difference between the highest current at which spatter does not occur and the lowest current that ensures the minimum required size of the weld pool. According to ISO 18278-2:2016, the welding test starts with a current of at least 3 kA and increases in steps of 0.2 kA, and for each current level, three spot welds are made. When two of the three welds achieve the minimum required size in the form of 4√t, where t is the thickness of the sheet at the same current level, this current Imin has been achieved. This criterion defines the minimum acceptable weld pool diameter index that guarantees the quality and strength of the joint. The current intensity is then increased again in steps of 0.2 kA until two of the three consecutive welds, at the same current level, have spatter. This current level is defined as the upper limit of the welding range of the current range Imax. According to SEP 1220-2:2011, the welding test starts at 3 kA and the current is increased in steps of 0.2 kA, and two spot welds are made at each current level. When both welds show spatter at the contact surface, the current is reduced in steps of 0.1 kA. When no spatter occurs, the second and third welds are made without changing the current. Imax is achieved when three consecutive welds at the same current level are spatter-free. To find Imin, the welds made during the initial current increase phase are used. Imin is achieved when 5 spot welds at the same current intensity meet the minimum size of 4√t. Because standard methods are time-consuming and use a lot of materials, simplified versions have been used to increase efficiency. In the simplified SEP 1220 method, the test starts at 4 kA and the current is increased in 0.4 kA steps. After spraying, the current is reduced in 0.2 kA steps until Imax is determined with two welds without spraying. Imin is then sought and is obtained when two spot welds at the same current intensity achieve a minimum size of 4√t. In addition, an approximate method with 0.5 kA steps has been used to determine the approximate current range between the current that guarantees a minimum of 4√t (≥ Imin) and a higher current that is free of spatter (≤ Imax). This welding current range obtained with the approximate method is at least as large as the welding current range obtained with the standard method. The standard method may be larger. In all methods, the welding current range, calculated as (Imax – Imin), must be 1 kA or more. The pulse shape was rectangular. The frequency is set at 1000 Hz and the welding force is set according to ISO 18278-2:2016 standard, suitable for different thicknesses from 350 to 500 dekanewtons. The results of the experiments can be seen in Table 2. Table 2 Samples Electrode tip diameter (mm) Number of pulses Pulse duration (ms) Cooling time (ms) Welding interval determination method Welding interval (kA) 1* 5.5 5 30 30 SEP 1220-2 Simplified 2.0 2* 5.5 9 30 30 SEP 1220-2 Simplified 2.2 3 5.5 1 30 0 SEP 1220-2 0.0 4 5.5 1 320 0 SEP 1220-2 Simplified 0.0 5 5.5 1 50 30 Approximate method 0.0 6* 5.5 5 50 30 Approximate method 1.5 7* 5.5 7 50 30 Approximate method 1.7 8* 5.5 9 50 30 Approximate method 2.0 9* 5.5 3 30 30 Approximate method 1.0 10* 5.5 5 30 30 Approximate method 1.5 11* 5.5 7 30 30 Approximate method 2.0 12* 5.5 9 30 30 Approximate method 2.0 13* 5.5 7 30 30 SEP 1220-2 Simplified 1.7 14* 5.5 9 30 30 SEP 1220-2 Simplified 1.1 15* 6.0 5 50 33 ISO 18278-2 2.4 16 6.0 1 380 0 ISO 18278-2 0.6 17* 8.0 7 40 30 ISO 18278-2 1.7 *: In accordance with the present invention; underlined indicators: Not in accordance with the invention. Samples 3, 4, 5, and 16 were not weldable, meaning the minimum welding range of 1 kiloampere was not achieved. The tests performed according to this invention all have a welding current range equal to or greater than 1 kiloampere, even for parts produced with high hot forging temperatures and long times, as is particularly evident in test number 17.

Claims

Claims 1. A welding method for manufacturing an assembly comprising at least two steel substrates (3, 3') joined together by at least one spot weld joint, comprising the following steps: - providing at least two said metal substrates (3, 3'), wherein the first steel substrate (3) is a hot-formed steel part produced by hot-forming a steel sheet with an aluminum-based coating, the coating comprising the following in percentage by weight: 7.0 to 9.0% zinc, 1.0 to 10% silicon, 1.0 to 10% magnesium, maximum 3.0% iron, optional elements selected from lead, nickel, zirconium, hafnium, strontium, antimony, titanium, calcium, manganese, tin, lanthanum, cerium, chromium or bismuth with a weight content of each less than 0.3, and unavoidable impurities up to 0.02% And the rest is a compound consisting of aluminum.- Applying a spot welding cycle using a spot welding device comprising welding electrodes (1, 1') and a spot welding power source (2) that passes current through at least two said metal substrates, said spot welding cycle (21) comprising: - At least three pulses (22, 32, 42), each having the same maximum pulse current (Cp) and applied through at least two metal substrates connected to each other, using electrodes connected to the spot welding power source, such that the duration of each pulse (p) is the same and set between 20 and 60 milliseconds, - A uniform cooling time, between 30 and 50 milliseconds, separating each pulse.

2. The welding method according to claim 1, wherein in step (a), the coating comprises the following by weight percentage: between 7.5 and 8.5% zinc, between 1.0 and 4.0% silicon, between 1.0 and 4.0% magnesium, up to 3.0% iron, optional elements selected from lead, nickel, zirconium, hafnium, strontium, antimony, titanium, calcium, manganese, tin, lanthanum, cerium, chromium or bismuth in an amount by weight of each element of less than 0.3% and unavoidable impurities of up to 0.02%, the balance consisting of aluminum.

3. The welding method according to claim 1 or 2, wherein the maximum pulse current (Cp) is set in the range of 0.1 to 30 kiloamperes.

4. A welding method according to any one of claims 1 to 3, wherein the number of pulses is set between three and nine.

5. A welding method according to any one of claims 1 to 4, wherein the welding force is set between 50 and 650 decanewtons.

6. The welding method according to any one of claims 1 to 5, wherein the welding frequency is set between 500 and 5000 Hz.

7. The welding method according to any one of claims 1 to 6, wherein the spot welding cycle comprises a pulse with a pulse shape selected from the following: rectangular shape, parabolic shape, triangular shape.

8. The welding method according to any one of claims 1 to 7, wherein the second metal substrate (3') is a steel substrate or an aluminum substrate.

9. The welding method according to claim 8, wherein the second steel substrate is a hot-rolled steel casting.

10. The welding method according to any one of claims 1 to 9, wherein said first substrate (3, 3') is obtained by hot forming a steel sheet which has previously been heat treated at a temperature of 840 to 950°C for 3 to 10 minutes.

11. A vehicle comprising at least one assembly obtained by the method according to any one of claims 1 to 10.

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