METHOD FOR WELDING COATED STEEL SHEETS

Description

[0001] The present invention relates to a method for welding coated steel sheets according to the preamble of claim 1.

[0002] In the prior art, it is known to produce welded blanks from steel sheets of different thicknesses and / or steel sheets with different compositions, which are then accessible for further processing, such as forming or heat treatment.

[0003] The idea behind this is that the different thickness or composition allows the properties of a finished formed component to be designed differently in different zones.

[0004] In addition, it is also known to weld sheets that have a corrosion protection coating, and in particular a metallic corrosion protection coating such as a zinc or aluminum coating.

[0005] It is particularly known to weld together highly hardenable manganese-boron steels, from which structural components of car bodies are subsequently manufactured.

[0006] Such tailor-made blanks made of steel sheets are also called "tailored blanks".

[0007] Well-known welding processes are arc welding and laser welding as well as laser arc hybrid welding.

[0008] Particularly with aluminum-silicon-coated sheets, it has been found that the aluminum-silicon layer causes problems when involved in the welding of the sheets using conventional welding methods. The coating elements evidently have a negative impact on the composition of the weld.

[0009] There are therefore approaches to partially remove aluminum-silicon layers before welding in order to reduce the aluminum-silicon concentration in the weld seam.

[0010] In addition, it is known from the prior art to use a filler wire or to add powder when welding such coated sheets.

[0011] Such welded blanks are also used in the production of hardened or partially hardened components and are heated and quenched for this purpose.

[0012] It is well known that so-called press-hardened sheet steel components are used, particularly in automobiles. These press-hardened sheet steel components are high-strength components that are used primarily as safety components in the bodywork. The use of these high-strength steel components makes it possible to reduce the material thickness compared to standard-strength steel, thus achieving low body weights.

[0013] When it comes to press hardening, there are basically two different ways to produce such components. A distinction is made between the so-called direct and indirect processes.

[0014] In the direct process, a steel sheet blank is heated above the so-called austenitizing temperature and, if necessary, held at this temperature until the desired degree of austenitization is reached. This heated blank is then transferred to a forming tool and formed into the finished component in a single-stage forming step. During this process, the cooled forming tool simultaneously cools the blank at a rate that exceeds the critical hardening rate. This produces the hardened component.

[0015] In the indirect process, the component is first almost completely formed, possibly in a multi-stage forming process. This formed component is then also heated to a temperature above the austenitizing temperature and, if necessary, held at this temperature for the required time.

[0016] This heated component is then transferred and inserted into a mold, which already has the dimensions of the component or the final dimensions of the component, possibly taking into account the thermal expansion of the preformed component. After the particularly cooled mold is closed, the preformed component is cooled and thus hardened only in this mold at a rate above the critical hardening rate.

[0017] The direct process is somewhat easier to implement, but only allows shapes that can actually be realized with a single forming step, i.e. relatively simple profile shapes.

[0018] The indirect method is somewhat more complex, but is capable of producing more complex shapes.

[0019] DE 10 2012 111 118 B3 discloses a method for laser welding one or more workpieces made of press-hardenable steel, in particular manganese-boron steel, in which butt welding is carried out and in which the workpiece or workpieces have a thickness of at least 1.8 mm and / or a thickness jump of at least 0.4 mm occurs at the butt joint, wherein filler wire is fed into the molten pool created with a laser beam during laser welding. In order to ensure that the weld seam can be reliably hardened into a martensitic structure during hot forming, this document provides for the addition of at least one alloying element from the group comprising manganese, chromium, molybdenum, silicon and / or nickel to the filler wire, which promotes the formation of austenite in the molten pool created with the laser beam, wherein this alloying element contains at least one alloying element with a thickness of at least0.1 percent by weight greater mass fraction is present in the filler wire than in the press-hardenable steel of the workpiece or workpieces.

[0020] DE 10 2014 001 979 A1 discloses a method for laser welding one or more workpieces made of hardenable steel in a butt joint, wherein the steel is in particular a manganese-boron steel and the workpieces have a thickness between 0.5 and 1.8 mm and / or a thickness jump of between 0.2 and 0.4 mm occurs at the butt joint. During laser welding, a filler wire is introduced into the molten pool, the molten pool being generated exclusively by the single laser beam. To ensure that the weld seam can be reliably hardened into a martenstic structure during hot forming, the document stipulates that the filler wire contains at least one alloying element from the group comprising manganese, chromium, molybdenum, silicon, and / or nickel, thus promoting the formation of austenite.

[0021] From EP 2 737 971 A1 a tailor welded blank is known and a method for its production, wherein the sheet is produced by welding sheets of different thickness or

[0022] Composition, whereby quality problems in the weld zone are to be reduced. A filler wire is also used here, which is designed so that no ferrite is formed in the temperature range of 800 to 950°C. This process is said to be particularly suitable for AlSi-coated sheets, whereby this wire should also have a higher content of austenite-stabilizing elements, which consist primarily of carbon or manganese.

[0023] EP 1 878 531 B1 discloses a method for hybrid laser-arc welding of surface-coated metallic workpieces, wherein the surface coating is intended to contain aluminum. The laser beam is intended to be combined with at least one arc to melt the metal and weld the part(s), and at least one of the parts has deposits of the aluminum-silicon coating on the surface of one of its lateral cut surfaces to be welded prior to welding.

[0024] From EP 2 942 143 B1 a method for joining two blanks is known, wherein the blanks are steel sheets with a coating comprising a layer of aluminum or an aluminum alloy, wherein the two parts are welded together using a laser beam and an arc, wherein the arc torch comprises a filler wire electrode and the filler wire electrode is made of a steel alloy comprising stabilizing elements, wherein the laser and arc are moved in a welding direction, wherein the arc welding torch and the laser beam are arranged subsequently in the welding direction.

[0025] From EP 2 883 646 B1 a method for joining two blanks is known, wherein at least one of the blanks comprises a layer of aluminum or an aluminum alloy, wherein during the welding process a metal powder is fed into the welding zone and the metal powder is an iron-based powder comprising gamma-stabilizing elements and the laser beam welding is a two-point laser beam welding.

[0026] EP 2 007 545 B1 discloses a method for producing a welded part with very good mechanical properties, wherein a steel sheet has a coating consisting of an intermetallic layer and a metal alloy layer located on the intermetallic layer. For the purpose of welding the sheets, the metal alloy layer on the intermetallic layer is to be removed from the periphery of the sheet, i.e. the areas to be welded, wherein this layer is an aluminum alloy layer. This coating is to be removed using a laser beam, so that this layer, which is in the form of an aluminum-silicon layer, is evaporated off before welding in order to prevent harmful effects of the aluminum in the weld seam. At the same time, the intermetallic layer is to remain in place in order to potentially exert corrosion-inhibiting effects.

[0027] US 960 43 11 B2 discloses a full ablation process in which a metallic and an intermetallic layer are completely vaporized by laser. WO 2017 / 103149 A1 discloses the welding of an AlSi-coated sheet. A disadvantage of the prior art is that in processes in which a powder is introduced into the weld seam, dosing the powder is difficult. A disadvantage of laser hybrid welding processes is that they are inherently very complex and difficult to perform. Scaling of the weld seam is disadvantageous in that the load-bearing cross-section is reduced, and decarburization of the weld seam also reduces the load-bearing cross-section, while also calling into question the mechanical load-bearing capacity of the weld seam.The disadvantage of laser ablation of aluminum-silicon layers is that, on the one hand, it is difficult to carry out the laser ablation safely and achieve reliable ablation; on the other hand, such a step represents an additional process step that makes production more complex and expensive.

[0028] The basic problem is that when aluminum-silicon layers are applied to sheets, the weld seam becomes less strong during welding, which is obviously due to the aluminum that is incorporated into the weld seam.

[0029] The object of the invention is to produce stable weld seams with minimal effort. This object is achieved by a method having the features of claim 1.

[0030] Advantageous further training is indicated in subclaims.

[0031] According to the invention, aluminum-silicon-coated sheets are welded together without completely or partially removing the aluminum-silicon layer, but the negative influence of the aluminum on the mechanical properties of the welded joint is neutralized. Furthermore, the invention prevents decarburization and scaling of the weld seam, increases the heat resistance of the weld seam, and also improves the weld seam for subsequent hot forming processes in such a way that the poorer cooling conditions prevailing at the weld seam due to the tool are compensated. To neutralize the aluminum and its negative effects according to the invention, welding is carried out with a special filler wire whose chemistry or alloy layer is tailored to counteract the effects of the aluminum.

[0032] In particular, the welding wire has a defined chromium content, which strongly inhibits scale formation and edge decarburization.

[0033] Accordingly, in contrast to the prior art, no gamma stabilization is carried out, but welding is carried out with a small amount of nickel and manganese. Surprisingly, it has been found that a high-strength weld seam is nevertheless created. This makes it possible to weld without ablation and to suppress the negative influence of aluminum on the mechanical properties of the welded joint. In addition, decarburization and scaling of the weld seam are almost completely avoided and the weld seam's heat resistance is increased. This is achieved, among other things, by welding with a higher chromium content, which improves hardenability. This is important because the invention has recognized that, due to the tool used, poor cooling conditions often exist at the weld seam during hot forming, which is reflected in a drop in the hardness of the weld seam after the press hardening process.

[0034] Overall, the use of a filler wire specifically tailored to the material largely neutralizes the aluminum, and the chromium content in the weld greatly inhibits scale formation and edge decarburization. A suitable filler wire has a carbon content that corresponds to 0.80 to 2.28 times the carbon content of the base material, preferably 0.88 to 1.51 times the carbon content of the base material, particularly preferably 0.90 to 1.26 times the carbon content of the base material, even more preferably 0.90 to 1.17 times the carbon content of the base material, with a chromium content of 8 to 20%, a nickel content of less than 5%, preferably less than 1%, a silicon content of 0.2 to 3%, a manganese content of 0.2 to 1%, and optionally a molybdenum content of up to 2%, preferably 0.5 to 2%.

[0035] When welding with such a filler wire, it is possible to significantly inhibit scale formation and edge decarburization during the subsequent hardening process and to "neutralize" the effect of the aluminum described in the description.

[0036] The invention is explained by way of example with reference to the figures. They show: Figure 1: A cross-section through a weld seam between two sheets of different thicknesses, in which a welding process according to the prior art was used and a scaled and decarburized weld seam can be seen; Figure 2: The decarburized zone in a weld seam according to the prior art and a weld seam according to the invention, ground in cross-section; Figure 3: The hardness profile within a weld seam, in which the weld seam is shown in a cross-section with the hardness test points; Figure 4: An overview of the strength level of weld seams with different gap widths and different wire materials according to the invention and not according to the invention and different welding progress speeds; Figure 5: An overview of the compositions of the additional wires of the Figure 4 shown weld seams of the wire materials according to the invention and not according to the invention.

[0037] According to the invention, the welding of two sheets of different thicknesses, preferably CMn steels, in particular a hardenable CMnB steel, in particular 22MnB5 steel materials, is accomplished by using a welding filler wire. In particular, according to the invention, aluminum-silicon-coated steel sheets with a tensile strength of > 900 MPa after hardening are joined by ablation-free welding.

[0038] The preferred chemical alloy of the filler wire or flux cored wire consists of the following elements: C= 0.80 - 2.28 × C base material Cr= 8 - 20 mass% Ni≤ 5, preferably ≤ 1 mass% Si= 0.2 - 3 mass% Mn= 0.2 - 1 mass% Optional Mo=< 2, preferably 0.5 - 2.5 mass% Optional V and / or W in total < 1% mass-%

[0039] Rest iron and unavoidable impurities due to melting

[0040] Preferably, the carbon of the filler wire or flux-cored wire is adjusted as follows, or the filler wire has the following composition: C= (0.88 to 1.51) × C base material Cr= 10 - 18 mass% Ni= ≤ 1 mass% Si= 0.3 - 1 mass% Mn= 0.4 - 1 mass% Mon= 0.5 - 1.3 mass% V = 0.1 - 0.5 mass% W= 0.1 - 0.5 mass%

[0041] Rest iron and unavoidable impurities due to melting. Particularly preferred: C = 0 , 90 bis 1,26 × C Grundmaterial

[0042] Even more preferred: C = 0 , 90 bis 1,17 × C Grundmaterial

[0043] As already mentioned, the materials used were aluminum-silicon-coated sheets made of 22MnB5 with a coating of 60 g / m² per side. For tensile testing, sheets with a thickness of 1.5 mm were used. These sheets were provided with weld edges and welded using a Trumpf 4006 welding laser (4.4 kW) with a focal diameter of 0.6 mm.

[0044] The base material is a steel of the general alloy composition (in mass%) Carbon (C) 0,03-0,6 Manganese (Mn) 0,3-3,0 Aluminum (Al) 0,01-0,07 Silicon (Si) 0,01-0,8 Chromium (Cr) 0,02-0,6 Nickel (Ni) <0,5 Titanium (Ti) 0,01-0,08 Niobium (Nb) <0,1 Nitrogen (N) < 0,02 Boron (B) 0,002-0,02 Phosphorus (P) < 0,01 Sulfur (S) < 0,01 Molybdenum (Mo) < 1

[0045] Rest iron and smelting-related impurities.

[0046] This means that the carbon content of the filler wire can range from 0.024 to 1.086 mass%.

[0047] During production, the carbon content of the filler wire is of course specifically selected based on the carbon content of the base material.

[0048] The base material can preferably have the following alloy composition: Carbon (C) 0,03-0,36 Manganese (Mn) 0,3-2,00 Aluminum (Al) 0,03-0,06 Silicon (Si) 0,01-0,20 Chromium (Cr) 0,02-0,4 Nickel (Ni) <0,5 Titanium (Ti) 0,03-0,04 Niobium (Nb) <0,1 Nitrogen (N) < 0,007 Boron (B) 0,002-0,006 Phosphorus (P) < 0,01 Sulfur (S) < 0,01 Molybdenum (Mo) < 1

[0049] Rest iron and smelting-related impurities.

[0050] Specifically, 22MnB5, for example, can have the following composition: C=0.22 Si=0.19 Mn=1.22 P=0.0066 S=0.001 Al=0.053 Cr=0.26 Ti=0.031 B=0.0025 N=0.0042,

[0051] The remainder is iron and impurities resulting from the melting process, all values ​​being in mass%.

[0052] With this specific composition of the base material, the carbon content of the filler wire can be in the range between 0.216 and 0.257 mass%.

[0053] During the tests, the process parameters were varied as follows: » Welding laser Trumpf 4006 4.4 kW (Focus size Ø=0.6 mm) » Variation of process parameters: » vw = 4 - 7.5 m / min » V d = 2.3 - 6.4 m / min » Gap = 0 / 0.1 mm » Hardening » Furnace temperature: 930°C » Furnace dwell time: 310 sec » Transfer time: approx. 6 sec » Water-cooled plate tool

[0054] Where vw defines the welding progress speed and V d the wire feed speed.

[0055] The samples were subsequently cured at a furnace temperature of 930°C and a furnace residence time of 310 seconds. The transfer time between furnace removal and insertion into a water-cooled plate mold was 6 seconds.

[0056] After welding with welding wires according to the invention, weld seams are produced according to Figure 2 below. One can see a homogeneous structure without a decarburized zone as in Figure 2 above, which shows a state-of-the-art weld. Such a state-of-the-art weld is also shown in Figure 1where significant scaling and an underlying decarburized zone are visible. Scaling of the weld reduces the load-bearing cross-section, and decarburization of the weld also reduces the load-bearing cross-section, so that in this case, the tensile specimens crack in the weld area. However, the goal must be to ensure that the tensile specimens crack in the base material rather than in the weld, thus ensuring that the base material determines the mechanical properties.

[0057] In Figure 3 The hardness profile of a weld seam welded with a filler wire according to the invention can be seen, with the hardness absorption points in Figure 3 can be seen on the right and the corresponding hardness curve in Figure 3 Left. It can be seen that although there are slight fluctuations in the hardness curve, these remain at a high level and do not decrease in any way compared to the edge zones or the base material.

[0058] Figure 4 shows the mean values ​​of tensile specimens using different wire materials, different progress speeds, and different gap widths.

[0059] Wire materials 1 and 7 were judged to be unsuitable, while wire materials 3, 6 and 8 have the composition according to the invention and have the smallest fluctuation range across the entire process range and the entire process possibilities.

[0060] It is noteworthy that the minimum strengths specified by most users are far exceeded by the samples welded with the wire materials according to the invention. The wire compositions are in Figure 5 summarized.

[0061] The following compositions were tested as wire material (see Figure 5 ) Wire No. Diameter [mm] C Si Mn Cr Mon Ni W V 1 1,2 0,12 0,8 1,9 0,45 0,55 2,35 not according to the invention 3 1,2 0,2 0,65 0,55 17 1,1 0,4 according to the invention 6 1,2 0,5 3 0,5 9,5 according to the invention 7 0,7 0,25 0,3 0,5 1,45 0,4 3,6 0,2 not according to the invention 8 0,8 0,2 0,5 0,5 12 1 0,1 0,5 0,35 according to the invention

[0062] All values ​​in mass%, remainder iron and unavoidable impurities due to melting.

[0063] Wires numbered 3, 6, and 8 exhibited particularly favorable properties, although the fracture pattern and any susceptibility to brittle fracture were evident in wire number 6 due to the slightly higher carbon and silicon content. Overall, however, the results for all of these wires were considered satisfactory.

[0064] These strength results are as previously mentioned in Figure 4 shown.

[0065] The invention is advantageous in that aluminum-silicon-coated hardenable steel sheets, in particular made of a hardenable boron-manganese steel, can be welded together without a complex and unreliable ablation step and without the weld seam representing a weakening.

Claims

1. Method of welding steel sheets made of steel materials coated with an aluminium silicon corrosion protection layer, in particular CMnB and CMn steel materials hardenable by quench hardening, wherein a welding filler wire is used when welding the sheets, wherein the welding filler wire consists of the following composition: C=(0.80 - 2.28) x %C base materialCr=8 - 20%Ni<5%Si=0.2 - 3%Mn=0.2 - 1%Mo=0.5 - 2% optionally V and / or W in total < 1% remainder iron and unavoidable smelting-related impurities, wherein all values are in wt%; where a steel is used as base material of the general alloy composition in wt%: carbon (C)0.03-0.6manganese (Mn)0.3-3.0aluminum (Al)0.01-0.07silicon (Si)0.01-0.8chromium (Cr)0.02-0.6nickel (Ni)<0.5titanium (Ti)0.01-0.08niobium (Nb)<0.1nitrogen (N)< 0.02boron (B)< 0.02phosphorus (P)< 0.01sulphur (S)< 0.01molybdenum (Mo)< 1 remainder iron and smelting-related impurities.

2. Method according to claim 1, characterized in that a welding wire is used whose nickel content is less than 1% by weight.

3. Method according to one of the preceding claims, characterized in that the sheets are laser welded together.

4. Method according to one of the preceding claims, characterized in that the welding progress is 4 to 15 m / min.

5. Method according to one of the preceding claims, characterized in that gap widths of 0 to 0.3 mm, in particular 0 to 0.1 mm, are set.

6. Method according to one of the preceding claims, characterized in that the carbon content of the filler wire is set at C= (0.88 to 1.51) x % C base material preferably C= (0.90 to 1.26) x % C base material particularly preferably C= (0.90 to 1.17) x % C base material.

7. Method according to one of the preceding claims, characterized in that the base material used is a steel which is a boron-manganese steel which can be hardened by an austenitizing and quenching process particularly preferably to a tensile strength of more than 900 MPa.

8. Method according to one of the preceding claims, characterized in that a steel is used as base material with the general alloy composition in wt%: carbon (C)0.03-0.36manganese (Mn)0.3-2.00aluminium (Al)0.03-0.06silicon (Si)0.01-0.20chromium (Cr)0.02-0.4nickel (Ni)<0.5titanium (Ti)0.03-0.04niobium (Nb)<0.1nitrogen (N)< 0.007boron (B)< 0.006phosphorus (P)< 0.01sulphur (S)< 0.01molybdenum (Mo)< 1 remainder iron and smelting-related impurities.

9. Method according to one of the preceding claims, characterized in that that the base material used is a steel with the alloy composition C=0.22, Si=0.19, Mn=1.22, P=0.0066, S=0.001, Al=0.053, Cr=0.26, Ti=0.031, B=0.0025, N=0.0042, the remainder being iron and smelting-related impurities, wherein all values are in wt%.

10. Method according to one of the preceding claims, characterized in that the filler wire has a carbon content in the range of 0.024 to 1.086 wt%, particularly preferably 0.186 to 0.5082 wt%, even more preferably between 0.20 to 0.257 wt%.