Spot welding method

Abstract

1. A welding method for the manufacture of an assembly of at least two steel substrates (3, 3') spot-welded to one another, in which: a first steel substrate (3) is a press-hardened steel part obtained by press-hardening a steel sheet coated with an aluminum-based coating, said coating containing, in percentages by weight, 7.0-9.0% zinc, 1.0-10% silicon, 1.0-10% magnesium, and up to 3.0% iron and optional elements selected from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi; 1. A welding method comprising: a welding material containing less than 0.3% by weight of optional elements and a maximum of 0.02% of unavoidable impurities, the remainder being aluminum; applying a spot welding cycle, the cycle consisting of at least three pulsations (22, 32, 42), each having the same maximum pulsation current (Cp), each having an identical pulsation duration p set to 20 to 60 ms, and identical cooling times separating the pulsations, set to 30 to 50 ms.

Description

[Technical Field]

[0001] The present invention relates to a welding method for the production of an assembly of steel substrates spot-welded to one another via at least one spot weld joint. The invention is particularly well suited for the production of motor vehicles. [Background technology]

[0002] To reduce the weight of vehicles, it is known to use high-strength steel sheets to reduce the weight of the vehicle body and improve crashworthiness. Press-hardened steel parts are also used, particularly to reduce the weight of vehicles. In fact, the tensile strength of these steels can be as low as 1200 MPa and as high as 2500 MPa. Press-hardened parts can be coated with aluminum-based coatings that have good corrosion resistance and thermal properties.

[0003] Typically, the manufacturing method of coated press-hardened parts comprises the following steps: A) providing a steel sheet pre-coated with a metal coating, which is a conventional coating based on aluminum; B) cutting the coated steel sheet to obtain a blank; C) heat treating the blank at high temperature to obtain a fully austenitic microstructure in the steel; D) transferring the blank to a press tool; E) hot forming the blank to obtain a part; F) cooling the part obtained in step E) to obtain a press-hardened steel part; Includes:

[0004] The pre-coated steel sheet of step A) is usually obtained by hot-dip coating of the steel sheet in a liquid metal bath.

[0005] Once the part is manufactured in step F), it is assembled to other vehicle components by spot welding. However, welding aluminum-based coated, press-hardened parts is difficult to achieve. In particular, such materials typically do not have a wide welding range. An appropriate welding current range is from the current at which the smallest nugget diameter is formed to the current at which expulsion occurs. A wide welding current range is desirable because it allows the nugget diameter to be controlled within a specified range even when the welding current fluctuates. A wide welding current range is also useful because it means the material is more resistant to electrode wear, misfit, and power line voltage fluctuations. A typical requirement from automobile manufacturers is a welding current range of 1 kA or more, allowing the automobile manufacturer's welding line to operate with good weld quality and without the need to frequently change welding electrodes.

[0006] Furthermore, it has been observed that the weld area of ​​press-hardened parts depends on the press-hardening parameters used to produce the press-hardened parts. The higher the temperature and the longer the press-hardening time, the smaller the weld area. This is due to the presence of alloy phases due to the diffusion of iron from the substrate into the coating. It is also due to surface oxides produced by the press-hardening process. In particular, when the coating contains additional elements to aluminum, such as silicon, magnesium, or zinc, complex surface oxides develop depending on the heat treatment parameters, such as time and temperature. These oxides must be removed before welding. Summary of the Invention [Problem to be solved by the invention]

[0007] It is therefore an object of the present invention to provide a welding method for the production of press-hardened steel parts that allows the welding range to be increased to at least 1 kA without the need to remove the oxide layer resulting from the press-hardening process, thereby minimizing weld expulsion. [Means for solving the problem]

[0008] This object is achieved by providing a welding method according to claim 1. The method may also include any or all of the features of claims 2 to 10. Another object of the invention is a vehicle comprising such an assembly according to claim 11.

[0009] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0010] To illustrate the invention, various embodiments and implementations will now be described by way of non-limiting examples, with particular reference to the following figures: [Brief explanation of the drawings]

[0011] [Figure 1] 1 shows an apparatus for carrying out the present invention. [Figure 2] 1 illustrates one embodiment of a spot welding cycle according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention relates to a welding method for producing an assembly of at least two steel substrates spot welded to one another via at least one spot weld joint.

[0013] As shown in Figure 1, a spot welding machine (not shown) is used, comprising welding electrodes 1, 1' and a spot welding power supply 2. In this example, the electrodes make it possible to join two press-hardened steel parts 3, 3' produced by press-hardening steel sheets coated with an aluminum-based coating 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 by conversion of an AC current supply.

[0014] The method according to the present invention further comprises applying a spot welding cycle 21, the spot welding cycle comprising: - at least three pulsations 22, 32, 42, each having the same pulsation current (Cp) applied through the metal substrates joined together using a welding electrode connected to a spot welding power supply, wherein the duration of each pulsation is the same and is set to 20 to 60 ms; - Identical cooling times, set at 30-50 ms, separating each pulsation; It consists of:

[0015] The number of pulsations used in the method of the present invention must be at least three, preferably at least five. In a preferred embodiment, the maximum number of pulsations can be set to nine. After using such pulsations separated by such cooling times, the substrates are fully welded, which means that no other welding cycles of any kind are performed in addition to them.

[0016] The duration is the same from one pulsation to the other and is set within the range of 20 to 60 ms, preferably 30 to 50 ms.

[0017] If the pulsation duration is less than 20 ms, the minimum nugget diameter may not be achieved. If the pulsation duration is longer than 60 ms, premature expulsion may occur.

[0018] The inventors have found that increasing the number of pulsations increases the current welding range.

[0019] The maximum pulsation current (Cp) is the same for all pulsations and is preferably set to 0.1 to 30 kA. The welding force applied by the electrode simultaneously with the current is preferably set to 50 daN to 650 daN, more preferably 250 daN to 500 daN.

[0020] The welding frequency is preferably 500 to 5000 Hz, more preferably 800 to 2000 Hz.

[0021] Spot welding cycles according to the present invention can include pulsation peaks of various shapes. Such pulsation peaks may be the same or different in a given welding cycle. Figure 2 shows a preferred embodiment in which a spot welding cycle 21 comprises five pulsations with rectangular configurations, i.e., identical rectangular-shaped pulsation peaks 22, 32, 42, 52, and 62. Other shape options for such pulsations are as follows:

[0022] - Parabolic shape, - Triangular shape Or any other suitable shape, provided that the pulsations in a given welding cycle all have the same maximum pulsation current (Cp).

[0023] In the frame of the present invention, the term press-hardened steel part refers to a hot-formed or hot-stamped steel part which, after austenitization of the blank and further forming in a die and quenching, has a tensile strength of up to 2500 MPa, more preferably up to 2000 MPa, for example a tensile strength of 500 MPa or more, suitably 1200 MPa or more, preferably 1500 MPa or more.

[0024] If steels with high mechanical strength are required, in particular for parts of automotive structures, steels with a tensile strength of more than 500 MPa, preferably between 500 and 2000 MPa before or after heat treatment, can be used. The weight composition of the steel plate 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%, and the remainder being unavoidable impurities from the production of iron and steel.

[0025] For example, the steel sheet is 22MnB5 with the following composition by weight: 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%, the rest being unavoidable impurities from the production of iron and steel.

[0026] In another embodiment, the steel sheet has the following composition by weight: 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 being unavoidable impurities resulting from the production of iron and steel.

[0027] Alternatively, the steel plate may have the following composition by weight: 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%, and the remainder being unavoidable impurities resulting from the production of iron and steel.

[0028] In another embodiment, the steel sheet has the following composition by weight: 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 remainder being unavoidable impurities from the production of iron and steel.

[0029] In another embodiment, the steel sheet has the following composition by weight: 0.06%≦C≦0.1%, 1%≦Mn≦2%, Si≦0.5%, AI≦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% Cu, Ni and Mo, the remainder being unavoidable impurities resulting from the production of iron and steel.

[0030] In another embodiment, the steel sheet has the following composition by weight: 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%, and the rest being unavoidable impurities from the production of iron and steel.

[0031] Alternatively, the steel plate has the following composition by weight: 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 remainder being unavoidable impurities from the production of iron and steel.

[0032] The method according to the invention is applied to press-hardened steel parts obtained by press-hardening of steel sheets coated with an aluminium-based coating and containing zinc, silicon and magnesium.

[0033] The steel sheets used to manufacture press-hardened parts can be produced by hot-dip galvanizing in a bath, the temperature of which is set at 600-700°C, preferably 620-650°C.

[0034] The coating weight is 50 to 500 g / m2 in total on both sides of the steel sheet. 2 , preferably 80 to 150 g / m 2 , and more preferably 90 to 120 g / m 2 is set during the gas knife wiping process in the range of

[0035] Before being coated, the steel sheet according to the present invention can be obtained by hot rolling and optionally cold rolling according to the desired thickness, which can be, for example, 0.5 to 3.0 mm, preferably 1.0 to 2.0 mm.

[0036] The coating contains 7.0 to 9.0% by weight zinc, preferably 7.5 to 8.5% zinc.

[0037] Optionally, the coating comprises 1.0 to 10.0% silicon and 1.0 to 10.0% magnesium.

[0038] Preferably, the coating contains, in weight percentages, 1.0 to 4.0% silicon and 1.0 to 4.0% magnesium, preferably 2.5 to 3.5% silicon and 1.5 to 3.0% magnesium.

[0039] Optionally, the coating includes up to 3% by weight of iron, which results from dissolution of the steel sheet in the hot dip coating bath and may vary during manufacturing. Optionally, the coating includes an additional element selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the weight content of each additional element is less than 0.3% by weight.

[0040] In a preferred embodiment, up to 100 ppm by weight of calcium is added.

[0041] Finally, the coating may contain up to 0.02% by weight, preferably up to 0.01% by weight, of unavoidable impurities.

[0042] The press-hardening process for such steel sheets is well known to those skilled in the art and involves austenitizing a blank cut from such steel at a temperature that can be, for example, 840-950°C, preferably 900-950°C, for 3-10 minutes, followed by quenching in a forming die. After press-hardening, the coating is alloyed by the diffusion of iron resulting from heating the blank. An oxide layer is formed on the alloyed coating, said oxide layer containing aluminum, zinc, and magnesium.

[0043] The welding method according to the invention can be used to weld such press-hardened parts to similar press-hardened parts (homogeneous welds) or to any steel part without removing the top oxide, and the welding method according to the invention can also be used for hybrid welding of press-hardened steel parts to aluminum substrates.

[0044] The invention will now be described based on test articles conducted for informational purposes only and not by way of limitation.

[0045] Example Steel sheets of different average thicknesses coated with aluminum-based alloys were prepared and press-hardened under the conditions summarized in Table 1.

[0046] Table 1

[0047] [Table 1]

[0048] U1500 has a composition by weight of 0.22% carbon, 1.2% manganese, 0.25% silicon, 0.2% chromium, 0.04% aluminum, 0.04% titanium, and 0.003% boron.

[0049] The coating compositions are also shown in Table 1.

[0050] For each specimen, two identical press-hardened parts were then welded together. The weld area was determined as follows:

[0051] These methods are described below: In all methods, the welding range is the difference between the maximum current at which no splash occurs and the minimum current that ensures the smallest required nugget size.

[0052] According to ISO 18278-2:2016, the welding test begins with a current of at least 3 kA, and the current is increased in 0.2 kA steps, with three spot welds made at each current level. If two of the three welds meet the minimum size requirement of 4√t, where t is the plate thickness at the same current, this current is designated Imin. This standard defines the minimum allowable nugget diameter to ensure weld quality and strength. The current intensity is then increased by 0.2 kA steps until two of three consecutive welds have splashing occurring at the same current level. This current level is designated the upper limit of the current range, Imax.

[0053] According to SEP1220-2:2011, the welding test begins at 3 kA and the current is increased in 0.2 kA steps, with two spot welds performed for each current level. When both welds show expulsion at the joint interface, the current is reduced to 0.1 kA steps. If there is no splash, a second and third spot weld is performed without changing the current. Imax is achieved when three consecutive welds do not exhibit splash at the same current level. To determine Imin, start with the spot welds performed during the first current increase sequence. Imin is obtained when five spot welds with the same strength meet the minimum size requirement of 4√t.

[0054] The standardized method is time consuming and consumes a lot of material, so simplified variations are used for efficiency purposes.

[0055] A simplified SEP1220 starts at 4kA and the current is increased in 0.4kA steps. After expulsion occurs, the current is decreased in 0.2kA steps to define Imax for two spot welds that do not exhibit expulsion. Imin is then sought and achieved when two spot welds of equal strength meet the minimum size requirement (4√t).

[0056] Furthermore, a gross method with 0.5 kA steps is used to check the approximate current range between the strength that ensures the minimum size requirement of 4√t (≥ Imin) and the higher strength without expulsion (≤ Imax). The current welding range obtained with such a gross method is at least as large as the current welding range obtained with the standardized method, which may be larger.

[0057] For all methods, the current welding range, calculated as (Imax-Imin), must be greater than or equal to 1 kA. The pulsation was rectangular in shape.

[0058] The frequency was set to 1000 Hz, and the welding force was set according to ISO18278-2:2016 for various thicknesses from 350 daN to 500 daN.

[0059] The test article results are summarized in Table 2.

[0060] Table 2

[0061] [Table 2]

[0062] Specimens 3, 4, 5 and 16 were not weldable, i.e. the minimum weld range of 1 kA was not achieved.

[0063] All specimens according to the invention have a weld range of 1 kA or more, even for parts manufactured at high press hardening temperatures and long times, as demonstrated in particular by specimen 17.

Claims

1. 1. A welding method for the manufacture of an assembly of at least two steel substrates (3, 3') spot-welded to one another via at least one spot weld joint, comprising: - providing at least two metal substrates (3, 3'), wherein a first steel substrate (3) is a press-hardened steel part obtained by press-hardening a steel sheet coated with an aluminium-based coating, said coating comprising, in percentages by weight, 7.0-9.0% zinc, 1.0-10% silicon, 1.0-10% magnesium, maximum 3.0% iron, optional elements selected from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, each with a weight content of less than 0.3%, and maximum 0.02% unavoidable impurities, the remainder being aluminium; - applying a spot welding cycle using a spot welding machine, the spot welding machine comprising welding electrodes (1, 1') and a spot welding power supply (2) for applying a current through said at least two metal substrates, said spot welding cycle (21) comprising: At least three pulsations (22, 32, 42), each having the same maximum pulsation current (Cp) applied through the at least two metal substrates to be joined together using a welding electrode connected to a spot welding power supply, and each pulsation duration p being the same and set to 20 to 60 ms; Identical cooling times, set at 30-50 ms, separating each pulsation; and A welding method comprising:

2. 2. The welding method of claim 1, wherein in step A) the coating comprises, in percent by weight, 7.5-8.5% zinc, 1.0-4.0% silicon, 1.0-4.0% magnesium, maximum 3.0% iron, optional elements selected from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, each of which has a weight content of less than 0.3%, maximum 0.02% inevitable impurities, and the remainder being aluminum.

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

4. The welding method according to any one of claims 1 to 3, wherein the number of pulsations is set to 3 to 9.

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

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

7. The spot welding cycle is Rectangular shape, Parabolic shape, Triangular shape, 7. The welding method of claim 1, including pulsation with a pulsation shape selected from the group consisting of:

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

9. The welding method of claim 8 wherein the second steel substrate is a press-hardened steel component.

10. A welding method according to any one of claims 1 to 9, wherein the first substrate (3, 3') is obtained by press hardening of a steel plate previously heat treated at a temperature between 840 and 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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