Butt welding method for steel parts and related steel parts

Abstract

Butt welding method of two steel plates comprising the following steps: - A step of providing two steel plates having a composition such that the gamma factor of the unmodified target welding seam composition is strictly higher than 0.39, - A step of butt laser welding steel plates with additional material included in the weld so that the gamma factor of the modified target welding seam composition is 0.39 or less, Here, Gamma = C + Si / 30 + Mn / 20 + 4.8*P + 4*S - Al / 20.

Description

Technology Field

[0001] The present invention relates to a butt welding method for steel parts and related steel parts. Background Technology

[0002] Laser-welded blanks are a widely used solution in the sheet metal forming industry, particularly in the automotive industry. They allow for the combination of multiple sub-blanks of the same material, different grades, and thicknesses. This offers numerous advantages: optimal material properties and thicknesses are utilized in each area of ​​the blank, leading to optimized performance of the final part in terms of safety, weight, and environmental footprint. Additionally, designers can combine multiple parts into a single unit. Furthermore, maximizing material usage can reduce scrap, costs, and environmental footprint. Overall, laser-welded blanks simplify the entire production process, leading to increased productivity, cost savings, and reduced CO2 emissions.

[0003] The emergence of new grades for cold stamping possessing very high strength and also very high formability presents new challenges for manufacturing laser-welded blanks. In fact, grades with higher strength and formability come with more alloying elements, which introduces new properties, phenomena, and fracture risks during welding.

[0004] The present invention aims to solve the problem of manufacturing laser welded blanks using high-strength steels so that the resulting laser welded blanks have reliable resistance and formability, and the weld seam does not provide structural weaknesses to subsequent parts.

[0005] The object of the present invention is to provide a method for butt welding two steel plates according to claim 1, optionally comprising the features of claims 2 to 12. Brief explanation of the drawing

[0006] The present invention will be further described with reference to the following drawings: Figure 1 is a schematic diagram of a butt welding operation using a laser source. Figure 2a shows a modern method for testing welded assemblies. Figure 2b shows a newly developed method for testing welded assemblies. Specific details for implementing the invention

[0007] A steel plate refers to a flat plate of steel. A blank has an upper surface and a lower surface, which are also referred to as the upper side and the lower side or the upper surface and the lower surface. The distance between these surfaces is designated as the thickness of the plate. The thickness can be measured, for example, using a micrometer, with its spindle and anvil positioned on the upper surface and the lower surface. In a similar manner, the thickness can also be measured in the formed part. In the present invention, the thickness of the steel plate is, for example, 0.5 to 5.0 mm, preferably 0.5 to 4.0 mm, and more preferably 0.5 to 3.5 mm.

[0008] Tailor-welded blanks are manufactured by assembling several plates of steel or cutout blanks known as sub-blanks, for example by laser welding them together, in order to optimize part performance in different areas, reduce overall part weight, and reduce overall part cost.

[0009] Ultimate tensile strength, yield strength, and elongation are measured according to ISO standard ISO 6892-1 of October 2009. The tensile test specimen is cut from a flat area. If necessary, a small tensile test sample is taken to provide the total available flat area on the portion.

[0010] Hardness is a measure of resistance to local plastic deformation induced by mechanical indentation. It is a useful local measurement method that has excellent correlation with the mechanical properties of a material and does not require cutting a sample for tensile testing. In the present invention, hardness is measured using a Vickers indenter according to the standard ISO 6507-1. Vickers hardness is expressed using the unit Hv.

[0011] Referring to FIG. 1, butt-to-butt welding is a specific type of welding operation in which two steel plates (1, 2) to be welded are placed side by side along their respective edges, and a weld seam (3) is created by melting each edge to form a melt pool containing a mixture of both the steel plates and possible external materials used to support the welding process. The melt pool solidifies to form the weld seam (1). FIG. 1 illustrates the case of laser butt welding in which a laser beam (10) emitting a laser beam (11) is used as the energy source to create the melt pool. The weld edges may be straight or curved. The steel plates to be welded and the energy source used to melt the steel plates move relative to each other during the welding operation in the direction W at a speed known as the welding speed. In certain embodiments, a gap (4) may be left between the steel plates (1, 2). This allows for the incorporation of a significant amount of additional material, such as filler wire, into the molten pool without, for example, over-thickness of the weld seam. Such over-thickness is considered a geometric defect and will be detrimental to further processing of the weld blank, for example, to stamping operations.

[0012] Laser welding refers to a welding operation that provides the energy necessary to melt steel plates using at least a laser source. In certain embodiments, other energy sources, such as electric arcs or infrared heating, may be associated with the laser source to provide welding energy.

[0013] The quality of weld seams in terms of geometric defects is defined by the European standard EN 10359:2015 titled “Laser welded tailored blanks - technical delivery conditions”.

[0014] FIG. 2a illustrates the current state of technology regarding how the strength of a weld seam is evaluated. A tensile test sample is prepared by placing the weld seam (3) joining two steel plates (1 and 2) in the center of the sample in the transverse direction for comparison with the tensile strength F. This well-established method allows for a good evaluation of the relative strengths of the different elements constituting the samples, namely the two steel plates (1, 2) and the weld seam (3). According to this method, if the sample fractures at the outer side of the weld seam, the weld seam is considered to be sufficiently strong. That is, if the weld seam is at least harder than the weakest of the two steel plates, it is considered to be of good quality, which means that the weld is not a weak link in the assembly.

[0015] Although the aforementioned method provides a good assessment of the pure mechanical strength of the weld seam, it does not actually reflect the reality of the different deformation modes the weld seam will undergo under actual usage conditions. When a laser welded blank is stamped, the weld seam undergoes deformation in all directions as well as in the transverse direction.

[0016] The inventors have discovered that when laser welding steel plates, if at least one of the steel plates has a high strength, for example, a tensile strength exceeding 590 MPa, small cracks may be initiated perpendicular to the weld seam in the region where the weld is subjected to deformation having a longitudinal component. Surprisingly, this type of crack is observed only in high-strength steel and not in lower grades. The risk of this type of crack occurring cannot be assessed using the aforementioned method because the behavior of the weld when deformed longitudinally is not tested at all in traditional testing methods. Furthermore, there are statistical factors associated with this type of cracking. For identical part geometries containing the same steel grade and the same laser welding parameters, some parts may be crack-free while others may develop small cracks. This is attributed to naturally occurring variations in steel plate composition, welding processes, stamping processes, etc. Therefore, these small cracks are not entirely predictable and pose an additional problem in industrial settings because they are difficult to detect through quality control. While these cracks may appear as small cracks in the formed parts, they represent a critical weakness and can lead to part failure over the part's lifespan, causing serious safety issues.

[0017] The inventors have therefore developed a new methodology for assessing the risk of such small cracks occurring. The inventors have discovered that, as shown in FIG. 2b, when a weld seam is placed in the longitudinal direction of a tensile sample parallel to the tensile strength, the occurrence of small transverse cracks on a laser-welded assembly comprising at least one blank of high-strength steel can be observed. Furthermore, the inventors have found that a good criterion for ensuring that there is no possibility of small cracks occurring in continuous production is to perform a series of 10 tensile tests on longitudinally welded samples and compare the uniform elongation of the welded assembly with the weighted average of the uniform elongation of each of the steel plates. The inventors found that the risk of cracking is very low if the uniform elongation of the welded assembly Uweld is 50% or more of the weighted average (Usheet1*th1+Usheet2*th2) / (th1+th2) in all 10 tensile tests performed, where th1 and th2 are the thicknesses of the assembled steel plates (1, 2) and Usheet1 and Usheet2 are their respective uniform elongations.

[0018] In the remainder of the explanation, it will be stated that when Uweld is lower than (Usheet1*th1 + Usheet2*th2) / (th1 + th2), the test sample exhibits brittle failure; on the other hand, if Uweld is higher than the above weighted average, the test sample exhibits ductile failure.

[0019] Thanks to the statistical longitudinal testing of the newly developed weld seam described above, the inventors were able to investigate the problem of small cracks in a number of steel plates. When observing the cross-sections of the failure zones of the samples exhibiting small cracks, the inventors discovered that the fracture surface exhibits a mixture of ductile and brittle characteristics.

[0020] More specifically, the brittle feature part was associated with typical brittle fracture characteristics in some cases, and with dendritic shapes and some voids within the weld seam (3) in some cases. These observations suggest that several mechanisms are involved in the formation of small cracks, all of which are related to how the weld pool solidifies. While dendrites and voids indicate shrinkage problems, the brittle fracture surface indicates high-temperature cracking problems, which are likely related to separation problems during solidification. Indeed, without being bound by theory, it appears that the specific composition of the high-strength steel plate, which induces a specific composition of the weld pool during welding, can cause specific solidification problems of the weld pool, leading to brittleness of the weld seam.

[0021] Based on these observations, the inventors have discovered that the unmodified target chemistry of the welding seam (3) is likely to cause small cracks when tested under the following conditions (concentration expressed in weight%):

[0022] Gamma > 0.39%

[0023] Here, Gamma = C + Si / 30 + Mn / 20 + 4.8*P + 4*S - Al / 20

[0024] The unmodified target chemistry of the weld seam refers to the weighted average of all chemical elements entering the weld seam. Consider two steel plates (1 and 2) having thicknesses th1 and th2 and concentrations X1 and X2 of chemical element X, which are topped by metal coatings containing Xcoat1 and Xcoat2 of chemical element X, having total double-sided thicknesses thcoat1 and thcoat2. The unmodified target weld seam chemistry Xweld_unamended for element X is given as follows:

[0025]

[0026] Thanks to these observations, the inventors were able to further solve the problem of small cracks in critical assemblies by controlling the weld chemistry. This can be accomplished, for example, by adding an additional material addM to the weld pool through a filler wire, or by injecting metal powder into the weld pool, or by adding a material to at least one edge to be welded before the welding operation, for example, by applying it in the form of an additional local metal coating or paint, or by low-temperature spraying or any other available technical means.

[0027] Thanks to this additional material, the gamma factor of the weld seam can be lowered to 0.39% or less, which protects the welded assembly from the occurrence of small cracks.

[0028] More specifically, the modified target welding seam composition needs to be reduced to 0.39% or less using an additional material addM with an amount of %addM of additional material incorporated into the welding seam (3).

[0029] In the presence of an additional material addM of %addM with a concentration of chemical element XaddM, the modified target weld seam chemistry Xweld_amended is defined as follows:

[0030]

[0031] For example, the additional material addM has the effect of diluting elements with positive coefficients in the gamma factor formula (C, Si, Mn, P, or S), and thus has a composition that lowers the gamma factor. As can be seen from the array of gamma coefficients, the most important elements are P and S, which are associated with dendritic defects, separation, and high-temperature cracking problems. Therefore, an additional material with very low concentrations of P and S must be used. For example, the P content of the additional material should be lower than 0.008 wt%, more preferably lower than 0.006 wt%, and even more preferably lower than 0.004 wt%.

[0032] For example, it is also possible to adjust the gamma factor by using additional materials containing a significant amount of aluminum. In fact, aluminum has a negative coefficient in the gamma factor formula, and therefore, increasing the aluminum content of the weld seam will reduce the gamma factor. For example, the Al content of the additive material is greater than 1.0 wt%, more preferably greater than 2.0 wt%.

[0033] For example, it is also possible to combine the two embodiments by using a filler wire containing a significant amount of aluminum and very small amounts of P and S.

[0034] Another important factor to consider in all cases is the amount of additional material addM used, %addM.

[0035] In order to increase the amount of additional material and maintain a good weld shape, it is possible to increase the gap (4) between the two plates to be welded without harmful defects such as excessive thickness of the weld due to excess material. For example, the gap between the plates is increased to 0.1 mm or more. For example, when using a gap of 0.1 mm or more, the amount of filler wire is 15% or more.

[0036] When using filler wire, the amount of filler wire %addM can be adjusted by adjusting the ratio of the filler wire feed rate to the welding rate.

[0037] When using metal powder, the amount of %addM can be controlled by adjusting the injection speed.

[0038] When using metal coatings, paints, sprays, etc., the amount of %addM can be adjusted by adjusting the thickness and total volume of the additional material to be melted in the welding pool.

[0039] In certain embodiments, the characteristics of at least one of the steel plates to be welded in terms of chemical composition, microstructure, and mechanical properties correspond to one of the lines in the table below (chemical composition is expressed in weight%, the remainder being Fe and unavoidable impurities from the elaboration process; % residual austenite in the microstructure of the steel plate is expressed as surface % of the cross section; YP represents the yield point expressed in MPa; UTS represents the ultimate tensile strength expressed in MPa; and El% represents the elongation measured according to the aforementioned ISO 6892 standard):

[0040]

[0041] In a specific embodiment, the silicon content of at least one of the steel plates to be welded is in the range of 0.5% to 3.0%, preferably 0.9% to 3.0%, and more preferably 0.9% to 2.5% (lower and upper limits are included in the range). Advantageously, silicon acts to strengthen the steel and also stabilizes the austenite to retain a portion of the austenite within the microstructure of the steel plate at room temperature. This enables a combination of very high mechanical properties and high elongation to be achieved.

[0042] In a specific embodiment, the manganese content of at least one of the steel plates to be welded is in the range of 1% to 4%, preferably 1% to 3%, and more preferably 2% to 3% (lower and upper limits are included in the range). Advantageously, manganese acts to strengthen the steel and also stabilizes the austenite to retain a portion of the austenite within the microstructure of the steel plate at room temperature. This allows for a combination of very high mechanical properties and high elongation to be achieved.

[0043] Now, the present invention will be explained by the following examples, which are by no means limiting.

[0044] Table 1 shows the steel compositions used in the following examples along with the calculated gamma factors. The compositions are expressed in weight percent.

[0045]

[0046] In the examples, the additional material addM is in the form of a filler wire. The filler wire compositions used are listed in Table 2 along with their associated calculated gamma factors, and the compositions are expressed in weight percent.

[0047]

[0048] Welded assemblies were formed using the previously listed steel compositions and filler wire compositions according to different modalities detailed in Table 3. Samples manufactured using the method of the present invention have sample references starting with I, and assemblies not manufactured using the method of the present invention have sample references starting with R. Additionally, for samples manufactured not using the present invention, parameters not using the present invention are underlined.

[0049] For 10 welded assemblies in which the welds were positioned longitudinally, the types of weld defects were evaluated using the test method described above. The results of this test are also listed in Table 3.

[0050]

[0051] Not only are the uncorrected target gamma factors of the weld seam reported in Table 3, but the corrected target gamma factors of the weld seam are also reported. It should be noted that for assemblies where filler wire is not used, the corrected gamma factor is the same as the uncorrected gamma factor, which is evident when considering the formulas for corrected and uncorrected gamma. Additionally, it should be noted that all samples used were either uncoated or coated with metal coatings that do not contain any of the elements included in the gamma factor, so the metal coating did not participate in the calculation of the gamma factor.

[0052] All welded assemblies were tested according to the novel longitudinal statistical test method described above. In Table 3, if at least one of the 10 tensile test samples shows a brittle defect, small crack occurrence is reported as "Yes," and if all test samples show a ductile defect, it is reported as "No."

[0053] Assemblies R1 to R5 outside the present invention have an unmodified gamma exceeding 0.39 and are not welded using additional materials, so their unmodified gamma also exceeds 0.39, causing a risk of small cracks.

[0054] Samples R6 to R9 outside the present invention have an unmodified gamma greater than 0.39 and were welded using additional material. However, the combination of the specific composition of the filler wire used and the amount of added filler wire is not sufficient to bring the modified gamma factor to less than 0.39, so the resulting assembly is still at risk of small cracks.

[0055] On the other hand, samples I1 to I8 manufactured according to the present invention have an unmodified gamma greater than 0.39, and due to the addition of filler wire, have a modified gamma less than 0.39, resulting in an assembly with no tendency for small cracks to occur. More specifically, for samples I1 to I5, this improvement is achieved by using very low C, Mn, S, and P filler wire in an amount of 15% or less and maintaining a tight gap of less than 0.1 mm between the two plates. For samples I6 and I7, the same wire composition as I5 is used, but a larger amount of wire is added in combination with a larger gap between the two steel plates. This allows for a much lower modified gamma factor and a safety margin toward the 0.39 threshold. Sample I8 is manufactured using filler wire with a high aluminum content, which allows the modified gamma factor to be lowered from 0.42 to 0.35. This is interesting because the filler wire composition (wire 7) used has a low P content, which can be costly, but is managed to efficiently lower the gamma factor and thus prevent the risk of small cracks.

[0056] The remaining samples R11 to R16, which are outside the scope of the present invention, all have an unmodified gamma factor of less than 0.39 due to the chemical composition of the steel plate. They do not require additional material to obtain a risk-free weld and already have no small cracks without special measures.

[0057] In conclusion, by applying the process according to the present invention, a welded assembly can be manufactured from a very high-strength steel having a chemical composition that poses a risk of small cracks unless special countermeasures are taken.

Claims

Claim 1 A butt welding method of two steel plates (1, 2) having thicknesses th1 and th2, wherein the method comprises: -A / providing two steel plates having a chemical composition and a metal coating such that the gamma factor of an unmodified target welding seam composition is strictly higher than 0.39; -B / positioning the steel plates side by side; -C / but welding the steel plates (1, 2) using at least a laser source and using an additional material addM included in the welding seam (3) in an amount of weight% of %addM during the welding operation such that the gamma factor of the modified target welding seam composition is 0.39 or less, wherein for a given composition, the gamma factor is given by the following formula (all elements expressed in weight%) Gamma = C + Si / 30 + Mn / 20 + 4.8*P + 4*S - Al / 20, and for a given chemical element X, the two steel plates (1 and 2) The above chemical element X is topped by metal coatings having weight percentages X1 and X2 of the above chemical element X, both having full double-sided thicknesses thcoat1 and thcoat2, and containing Xcoat1 and Xcoat2 as weight% of element X, and the unmodified target weld seam composition Xweld_unamended for element X is the following formula Given by, and for a given chemical element X, the modified target welding seam composition Xweld_amended in the presence of an additional material addM in an amount of %addM having a weight % XaddM of element X is given by the following formula A butt welding method of steel plates provided by Claim 2 A butt welding method of steel plates according to claim 1, wherein at least one of the two provided steel plates to be welded has a silicon content of 0.5 wt% to 3.0%. Claim 3 A butt welding method of steel plates according to claim 2, wherein the two provided steel plates to be welded have a silicon content of 0.5 wt% to 3.0 wt%. Claim 4 A butt welding method of steel plates according to any one of claims 1 to 3, wherein the additional material addM is supplied in the form of filler wire. Claim 5 A butt welding method of steel plates according to any one of claims 1 to 3, wherein the additional material addM is supplied in powder form. Claim 6 A butt welding method of steel plates according to any one of claims 1 to 3, wherein the additional material addM is supplied in the form of a coating on at least one of the edges to be welded. Claim 7 A butt welding method of steel plates according to any one of claims 1 to 3, wherein the additional material addM contains less than 0.008 wt% P. Claim 8 In claim 7, the butt welding method of steel plates, wherein the additional material addM contains less than 0.006 wt% P. Claim 9 In claim 8, the butt welding method of steel plates, wherein the additional material addM contains less than 0.004 wt% P. Claim 10 A butt welding method of steel plates according to any one of claims 1 to 3, wherein the additional material addM contains more than 1.0 wt% of Al. Claim 11 In claim 10, the butt welding method of steel plates, wherein the additional material addM contains more than 2.0 wt% Al. Claim 12 A butt welding method of steel plates according to any one of claims 1 to 3, wherein a gap (4) of at least 0.1 mm is left between the steel plates (1, 2) to be welded, and the amount of additional material addM in the welding seam (3) %addM is 15% or more.

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