Spot welding method

Abstract

A method for manufacturing at least two steel substrates (3, 4) spot-welded together, the invention relates to a method for welding an assembly comprising a first steel substrate (3, 3 ') comprising:-a first steel substrate (3) being a press-hardened steel part obtained by press-hardening a steel sheet coated with an aluminum-based coating comprising, in weight percent, 7.0% to 9.0% of zinc, 1.0% to 10% of silicon, 1.0% to 10% of magnesium, up to 3.0% of iron, optional elements selected from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, the content of each element is less than 0.3% by weight, and at most 0.02% of unavoidable impurities, the balance being aluminum; -applying a spot welding cycle comprising: o at least three pulsations (22, 32, 42) each having the same maximum pulsation current (Cp), each pulsation duration p being the same and set between 20 ms and 60 ms; o is the same cooling time set to 30 ms to 50 ms so as to separate each pulsation.

Description

Technical Field

[0001] The present invention relates to a welding method for manufacturing components of a steel substrate spot-welded together by at least one spot weld joint. The present invention is particularly suitable for the manufacture of motor vehicles. Background Art

[0002] In view of saving the weight of vehicles, it is known to use high-strength steel sheets to achieve a lighter body and improve crash safety. Press-hardened steel components are also particularly used to reduce the weight of vehicles. In fact, the tensile strength of these steels is at least 1200 MPa and can be up to 2500 MPa. The press-hardened components can be coated with an aluminum-based coating having good corrosion resistance and thermal properties.

[0003] Generally, the method for manufacturing a coated press-hardened component comprises the following steps:

[0004] A) Providing a steel sheet pre-coated with a metal coating, which is a conventional aluminum-based coating,

[0005] B) Cutting the coated steel sheet to obtain a blank,

[0006] C) Heat-treating the blank at a high temperature to obtain a fully austenitic microstructure in the steel,

[0007] D) Transferring the blank into a pressing tool,

[0008] E) Thermoforming the blank to obtain a component,

[0009] F) Cooling the component obtained in step E) to obtain a press-hardened steel component.

[0010] The pre-coated steel sheet of step A) is usually obtained by hot-dip coating the steel sheet in a liquid metal bath.

[0011] After the component is manufactured in step F), it is assembled with other components of the vehicle by spot welding. However, welding of aluminum-based coated press-hardened components is difficult to achieve. In particular, such materials generally do not allow a wide welding range. The suitable welding current range is from the current when the minimum nugget diameter is formed to the current when spatter occurs. A wide welding current range is desired because even in the case of welding current fluctuations, the nugget diameter can be controlled within a specified range. A wide welding current range is also helpful because it means that the material is more resistant to electrode wear, misalignment, and power line voltage fluctuations. Automobile manufacturers usually require a welding range equal to or greater than 1 kA to be able to obtain good welding quality when their welding production lines are running and not have to frequently replace welding electrodes.

[0012] Furthermore, it has been observed that the welding range of press-hardened components depends on the press-hardening parameters used in their production. The higher the temperature and the longer the time used for press-hardening, the smaller the welding range will be. This is due to the presence of alloy phases through the diffusion of iron from the substrate into the coating. This is also due to the surface oxides generated during the press-hardening process. Especially when the coating contains additional elements besides aluminum, such as silicon, magnesium, or zinc, complex surface oxides will form according to heat treatment parameters such as time and temperature. These oxides must be removed before welding. Summary of the Invention

[0013] Accordingly, an object of the present invention is to provide a welding method for manufacturing press-hardened steel components, which allows increasing the welding range up to at least 1 kA without having to remove the oxide layer generated by the press-hardening process and minimizing welding spatter.

[0014] This object is achieved by providing the welding method according to claim 1. The method may also include any or all of the features according to claims 2 to 10. Another object of the present invention is to provide a vehicle including such a component according to claim 11.

[0015] Other features and advantages of the present invention will become apparent from the following detailed description of the present invention. Brief Description of the Drawings

[0016] To illustrate the present invention, various embodiments and tests of non-limiting examples will be described with particular reference to the following drawings:

[0017] - Figure 1 Illustrates the equipment for implementing the present invention.

[0018] - Figure 2 Illustrates an embodiment of the spot welding cycle according to the present invention. Detailed Description of the Invention

[0019] The present invention relates to a welding method for manufacturing a component of at least two steel substrates spot-welded together by at least one spot welding head.

[0020] As Figure 1 illustrated, a spot welder (not shown) including welding electrodes 1, 1' and a spot welding power source 2 is used. In this example, the electrodes allow joining two press-hardened steel components 3, 3' manufactured by press-hardening steel sheets coated with aluminum-based coatings 4, 4'. The current can be alternating current (AC) or direct current (DC). In a preferred embodiment, the current is medium-frequency direct current (MFDC) obtained through the conversion of an AC current supply.

[0021] The method according to the invention further includes applying a spot welding cycle 21, which includes:

[0022] - At least three pulses 22, 32, 42, each of the at least three pulses 22, 32, 42 having the same pulse current (Cp), the pulse current (Cp) being applied through a welding electrode connected to a spot welding power source to join together metal substrates, each pulse duration being the same and set to 20 ms to 60 ms,

[0023] - The same cooling time, the cooling time being set to 30 ms to 50 ms so as to separate each pulse.

[0024] The pulses used in the method according to the invention must be present in a number of at least three, and preferably at least five. In a preferred embodiment, the maximum number of pulses can be set to nine. After using such pulses separated by such a cooling time, the substrates are completely welded, which means that no other type of welding cycle is carried out in addition to such pulses.

[0025] The duration from one pulse to another is the same and is set in the range of 20 ms to 60 ms, preferably 30 ms to 50 ms.

[0026] If the pulse duration is shorter than 20 ms, the minimum nugget diameter may not be achieved. If the pulse duration is longer than 60 ms, early splashing may occur. The inventors have found that an increase in the number of pulses increases the current welding range.

[0027] For all pulses, the maximum pulse current (Cp) is the same and is preferably set to 0.1 kA to 30 kA. While applying the current, the welding force applied by the electrode is preferably set to 50 daN to 650 daN, and more preferably set to 250 daN to 500 daN.

[0028] The welding frequency is preferably set to 500 Hz to 5000 Hz, and more preferably set to 800 Hz to 2000 Hz.

[0029] The spot welding cycle according to the invention can include pulses of various shapes. Such pulse shapes can be the same or different in a given welding cycle. Figure 2 A preferred embodiment is illustrated, in which the spot welding cycle 21 includes five pulses having a rectangular form, namely the same rectangular pulse peaks 22, 32, 42, 52 and 62. Other shapes for such pulses are selected as follows:

[0030] - A parabolic form,

[0031] - A triangular form

[0032] Or any other suitable form, as long as the pulsations of the given welding cycle have the same maximum pulsation current (Cp).

[0033] In the framework of the present invention, the term press-hardened steel part refers to a hot-formed or hot-stamped steel part having a tensile strength of up to 2500 MPa, and more preferably up to 2000 MPa, after the blank is austenitized and further formed and quenched in a die. For example, the tensile strength is higher than or equal to 500 MPa, advantageously higher than or equal to 1200 MPa, preferably higher than or equal to 1500 MPa.

[0034] In cases where steel with high mechanical strength is required, especially for parts of the structure of a motor vehicle, steel having a tensile strength better than 500 MPa, advantageously between 500 MPa and 2000 MPa, before or after heat treatment can be used. The weight composition of the steel sheet is preferably as follows: 0.03% ≤ C ≤ 0.50%; 0.3% ≤ Mn ≤ 3.0%; 0.05% ≤ Si ≤ 0.8%; 0.015% ≤ Ti ≤ 0.2%; 0.005% ≤ Al ≤ 0.1%; 0% ≤ Cr ≤ 2.50%; 0% ≤ S ≤ 0.05%; 0% ≤ P ≤ 0.1%; 0% ≤ B ≤ 0.010%; 0% ≤ Ni ≤ 2.5%; 0% ≤ Mo ≤ 0.7%; 0% ≤ Nb ≤ 0.15%; 0% ≤ N ≤ 0.015%; 0% ≤ Cu ≤ 0.15%; 0% ≤ Ca ≤ 0.01%; 0% ≤ W ≤ 0.35%, with the balance being iron and inevitable impurities from the manufacture of the steel.

[0035] For example, the steel sheet is 22MnB5, which has the following weight composition: 0.20% ≤ C ≤ 0.25%; 0.15% ≤ Si ≤ 0.35%; 1.10% ≤ Mn ≤ 1.40%; 0% ≤ Cr ≤ 0.30%; 0.020% ≤ Ti ≤ 0.060%; 0.020% ≤ Al ≤ 0.060%; 0.002% ≤ B ≤ 0.004%, and the remainder is iron and inevitable impurities from the manufacture of the steel.

[0036] In another embodiment, the steel sheet has the following weight composition: 0.24% ≤ C ≤ 0.38%; 0.40% ≤ Mn ≤ 3%; 0.10% ≤ Si ≤ 0.70%; 0.015% ≤ Al ≤ 0.070%; Cr ≤ 2%; 0.25% ≤ Ni ≤ 2%; 0.015% ≤ Ti ≤ 0.10%; Nb ≤ 0.060%; 0.0005% ≤ B ≤ 0.0040%; and the remainder is iron and inevitable impurities from the manufacture of the steel.

[0037] Alternatively, the steel sheet may have the following weight composition: 0.30% ≤ C ≤ 0.40%; 0.5% ≤ Mn ≤ 1.0%; 0.40% ≤ Si ≤ 0.80%; 0.1% ≤ Cr ≤ 0.4%; 0.1% ≤ Mo ≤ 0.5%; 0.01% ≤ Nb ≤ 0.1%; 0.01% ≤ Al ≤ 0.1%; 0.008% ≤ Ti ≤ 0.003%; 0.0005% ≤ B ≤ 0.003%; 0.0% ≤ P ≤ 0.02%; 0.0% ≤ Ca ≤ 0.001%; 0.0% ≤ S ≤ 0.004%; 0.0% ≤ N ≤ 0.005%, with the balance being iron and inevitable impurities resulting from the manufacture of the steel.

[0038] In another embodiment, the steel sheet has the following weight composition: 0.040% ≤ C ≤ 0.100%; 0.80% ≤ Mn ≤ 2.00%; 0% ≤ Si ≤ 0.30%; 0% ≤ S ≤ 0.005%; 0% ≤ P ≤ 0.030%; 0.010% ≤ Al ≤ 0.070%; 0.015% ≤ Nb ≤ 0.100%; 0.030% ≤ Ti ≤ 0.080%; 0% ≤ N ≤ 0.009%; 0% ≤ Cu ≤ 0.100%; 0% ≤ Ni ≤ 0.100%; 0% ≤ Cr ≤ 0.100%; 0% ≤ Mo ≤ 0.100%, with the balance being iron and inevitable impurities resulting from the manufacture of the steel.

[0039] In another embodiment, the steel sheet has the following weight composition: 0.06% ≤ C ≤ 0.1%, 1% ≤ Mn ≤ 2%, Si ≤ 0.5%, Al ≤ 0.1%, 0.02% ≤ Cr ≤ 0.1%, 0.02% ≤ Nb ≤ 0.1%, 0.0003% ≤ B ≤ 0.01%, N ≤ 0.01%, S ≤ 0.003%, P ≤ 0.020%, less than 0.1% of Cu, Ni and Mo, with the balance being iron and inevitable impurities resulting from the manufacture of the steel.

[0040] In another embodiment, the steel sheet has the following weight composition: 0.015% ≤ C ≤ 0.25%; 0.5% ≤ Mn ≤ 1.8%; 0.1% ≤ Si ≤ 1.25%; 0.01% ≤ Al ≤ 0.1%; 0.1% ≤ Cr ≤ 1.0%; 0.01% ≤ Ti ≤ 0.1%; 0% ≤ S ≤ 0.01%; 0.001% ≤ B ≤ 0.004%; 0% ≤ P ≤ 0.020%; 0% ≤ N ≤ 0.01%; with the balance being iron and inevitable impurities resulting from the manufacture of the steel.

[0041] Alternatively, the steel sheet has the following weight composition: 0.2% ≤ C ≤ 0.34%; 0.5% ≤ Mn ≤ 1.24%; 0.5% ≤ Si ≤ 2.0%; 0% ≤ S ≤ 0.01%; 0% ≤ P ≤ 0.020%; 0% ≤ N ≤ 0.01%, with the balance being iron and inevitable impurities from the manufacture of the steel.

[0042] The method according to the invention is applicable to press-hardened steel components obtained by press-hardening a steel sheet coated with an aluminum-based coating and containing zinc, silicon, and magnesium.

[0043] The steel sheet for manufacturing press-hardened components can be manufactured by hot-dip galvanizing in a bath, the temperature of which is set to 600 °C to 700 °C, preferably 620 °C to 650 °C.

[0044] During the wiping process by an air knife, for the sum of both sides of the steel sheet, the coating weight is set in the range of 50 g / m 2 to 500 g / m 2 optionally 80 g / m 2 to 150 g / m 2 and preferably 90 g / m 2 to 120 g / m 2 range.

[0045] Before being coated, the steel sheet according to the invention can be obtained by hot rolling and optionally cold rolling according to the desired thickness, which can be for example 0.5 mm to 3.0 mm, preferably 1.0 mm to 2.0 mm.

[0046] The coating contains 7.0 wt% to 9.0 wt% of zinc by weight percentage, and advantageously contains 7.5% to 8.5% of zinc.

[0047] Optionally, the coating contains 1.0% to 10.0% of silicon and 1.0% to 10.0% of magnesium.

[0048] Preferably, the coating contains 1.0% to 4.0% of silicon and 1.0% to 4.0% of magnesium by weight percentage, and advantageously contains 2.5% to 3.5% of silicon and 1.5% to 3.0% of magnesium.

[0049] Optionally, the coating contains up to 3 wt% of iron. The iron comes from the dissolution of the steel sheet in the hot-dip coating bath and may vary during production. Optionally, the coating contains additional elements selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, and the content of each additional element is less than 0.3 wt% by weight.

[0050] In a preferred embodiment, calcium is added in an amount of up to 100 ppm by weight.

[0051] Finally, the coating may contain up to 0.02% by weight, preferably up to 0.01% by weight, of unavoidable impurities.

[0052] The press hardening process of such steel sheets is known to those skilled in the art and includes austenitizing a blank cut from such steel at a temperature of, for example, 840 °C to 950 °C, preferably 900 °C to 950 °C, for a duration of 3 minutes to 10 minutes, followed by quenching in a forming die. After press hardening, since the blank is heated, the above-mentioned coating will become alloyed by the diffusion of iron. An oxide layer will form on top of the alloy coating, and the oxide layer contains aluminum, zinc, and magnesium.

[0053] The welding method according to the present invention can be used to weld such press-hardened parts to similar press-hardened components (homogeneous welding) without removing the top oxide, or to any steel component. The welding method according to the present invention can also be used for composite welding between a press-hardened steel component and an aluminum substrate.

[0054] The present invention will now be explained in tests conducted for reference only. They are not restrictive.

[0055] Example

[0056] Steel sheets coated with an aluminum-based alloy having different average thicknesses are prepared and press-hardened under the conditions summarized in Table 1.

[0057] Table 1

[0058]

[0059]

[0060] The composition of U1500 is 0.22% by weight of carbon, 1.2% by weight of manganese, 0.25% by weight of silicon, 0.2% by weight of chromium, 0.04% by weight of aluminum, 0.04% by weight of titanium, and 0.003% by weight of boron.

[0061] The coating composition is also shown in Table 1.

[0062] Then, for each test, two identical press-hardened components are welded together. The welding range is determined as described below.

[0063] These methods will now be explained. For all methods, the welding range is the difference between the maximum current at which no spatter occurs and the minimum current that ensures the required minimum nugget size.

[0064] According to ISO 18278-2:2016, the welding test starts with a current of at least 3 kA, and the current is increased in steps of 0.2 kA. Three spot welds are made for each current level. When two of the three welds meet the minimum size requirement of 4√t - where t is the sheet thickness - at the same current, this current Imin is reached. This standard defines the minimum acceptable diameter value of the fusion core that ensures welding quality and strength. Then the current intensity is further increased in steps of 0.2 kA until spatter occurs in two of three consecutive welds at the same current level. This current level is defined as the upper welding limit of the current range: Imax.

[0065] According to SEP 1220-2:2011, the welding test starts with 3 kA, and the current is increased in steps of 0.2 kA. Two spot welds are made for each current level. When spatter is shown at the joint interface for both welds, the current is decreased in steps of 0.1 kA. When there is no spatter, a second and a third spot weld are made without changing the current. Imax is obtained when no spatter occurs in three consecutive welds at the same current level. To find Imin, start with the spot welds performed during the first current increase sequence. Imin is obtained when 5 spot welds at the same intensity all meet the minimum size requirement of 4√t.

[0066] Since the standardized method takes time and consumes a large amount of material, a simplified variant has been used for effectiveness purposes.

[0067] The simplified SEP 1220 starts with 4 kA, and the current is increased in steps of 0.4 kA. After spatter, the current is decreased in steps of 0.2 kA to define Imax when no spatter is shown in two spot welds. Then Imin is searched for and obtained when 2 spot welds at the same intensity both meet the minimum size requirement (4√t).

[0068] In addition, a rough method with a step size of 0.5 kA is also used to detect the approximate current range between the intensity (≥Imin) that ensures the minimum size requirement of 4√t and the higher intensity (≤Imax) without spatter. The current welding range obtained by this rough method is at least of the same order of magnitude as the current welding range obtained by the standardized method. The current welding range obtained by the standardized method may be larger.

[0069] For all methods, the current welding range calculated as (Imax - Imin) must be 1 kA or greater. The pulsation is in rectangular form.

[0070] The frequency is set to 1000 Hz, and the welding force is set to 350 daN to 500 daN according to ISO 18278-2:2016 for different thicknesses.

[0071] The results of the tests are summarized in Table 2.

[0072] Table 2

[0073]

[0074]

[0075] *: According to the present invention; underlined values: not according to the present invention.

[0076] Tests 3, 4, 5, and 16 are non-weldable, i.e., they do not reach the minimum welding range of 1 kA.

[0077] The tests according to the present invention all have a welding range equal to or greater than 1 kA, even for components produced at high press hardening temperatures and for long times, as demonstrated by Test 17.

Claims

1. A welding method for a component for manufacturing at least two steel substrates (3, 3') spot-welded together by at least one spot welding joint, said welding method comprises the following steps: - Providing said at least two metal substrates (3, 3'), wherein a first steel substrate (3) is a press-hardened steel component obtained by press-hardening a steel sheet coated with an aluminum-based coating, said coating containing by weight percentage: 7.0% to 9.0% zinc, 1.0% to 10% silicon, 1.0% to 10% magnesium, at most 3.0% iron, optional elements selected from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, each element having a content by weight of less than 0.3%; and at most 0.02% unavoidable impurities, the balance being aluminum, - Applying a spot welding cycle using a spot welding machine, said spot welding machine comprising welding electrodes (1, 1') and a spot welding power source (2), said spot welding power source (2) applying current through said at least two metal substrates, said spot welding cycle (21) comprising: o At least three pulses (22, 32, 42), said at least three pulses (22, 32, 42) each having the same maximum pulse current (Cp), said maximum pulse current (Cp) being applied through the at least two metal substrates joined together using welding electrodes connected to said spot welding power source, each pulse duration p being the same and set to 20 ms to 60 ms, o The same cooling time, said cooling time being set to 30 ms to 50 ms so as to separate each pulse.

2. The welding method according to claim 1, wherein, in step A), said coating contains by weight percentage: 7.5% to 8.5% zinc, 1.0% to 4.0% silicon, 1.0% to 4.0% magnesium, at most 3.0% iron, optional elements selected from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, each element having a content by weight of less than 0.3%; and at most 0.02% unavoidable impurities, the balance being aluminum.

3. The welding method according to claim 1 or 2, wherein, said maximum pulse current (Cp) is set to 0.1 kA to 30 kA.

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

5. The welding method according to any one of claims 1 to 4, wherein, said welding force is set to 50 daN to 650 daN.

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

7. The welding method according to any one of claims 1 to 6, wherein, said spot welding cycle comprises pulses having a pulse shape selected from the following: Rectangular form, Parabolic form, Triangular form.

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 press-hardened steel component.

10. The welding method according to any one of claims 1 to 9, wherein, the first substrate (3, 3') is obtained by press-hardening a steel sheet that has been heat-treated at a temperature of 840 °C to 950 °C for a duration of 3 minutes to 10 minutes.

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

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