Method for butt welding steel parts to related steel parts

Abstract

1. A method for butt welding two steel sheets, comprising the steps of providing two steel sheets having a composition such that the gamma factor of the unmodified target weld seam composition is strictly greater than 0.39, and butt laser welding them together with additional material incorporated in the weld such that the gamma factor of the modified target weld seam composition is 0.39 or less, where gamma=C+Si / 30+Mn / 20+4.8*P+4*S-Al / 20.

Description

Background Art

[0001] Laser welded blanks are a widely used solution in the steel sheet metal forming industry, particularly in the automotive industry. This enables the combination of several sub-blanks of the same blank, different grades and thicknesses. This has many advantages, with the optimal material properties and thickness being used in each area of the blank, resulting in optimized performance of the final part in terms of safety, weight, environmental footprint, etc. Furthermore, it enables designers to combine several parts into one. Also, since the material can be used to its maximum extent, it is possible to reduce scrap, costs and the environmental footprint. Overall, laser welded blanks simplify the entire production process, leading to improved productivity, cost limitation and reduction of CO2 emissions.

Summary of the Invention

Problems to be Solved by the Invention

[0002] The emergence of new grades for cold stamping that also have very high strength and very high formability presents new challenges for manufacturing laser welded blanks. In fact, higher strength and higher formability grades involve more alloying elements, which bring new properties, phenomena and risks of breakage to the weld seam.

[0003] The present invention aims to address the problem of manufacturing laser welded blanks using high-strength steel such that the resulting laser welded blanks have high reliability resistance and formability and the weld seam does not present a structural weakness for subsequent parts.

Means for Solving the Problems

[0004] The object of the present invention is achieved by providing a method for butt welding two steel sheets according to claim 1, optionally including the features of claims 2 to 12.

[0005] The present invention will be further described with reference to the following figures. [Brief explanation of the drawing]

[0006] [Figure 1] This is a schematic diagram of a butt welding operation using a laser source. [Figure 2A] This diagram illustrates a state-of-the-art method for testing welded assemblies. [Figure 2B] This diagram illustrates a newly developed method for testing welded assemblies. [Modes for carrying out the invention]

[0007] A steel sheet refers to a flat steel sheet. It has an upper surface and a lower surface, also called an upper surface and a lower surface or upper and lower surface. The distance between the surfaces is indicated as the thickness of the sheet. The thickness can be measured using, for example, a micrometer, with its spindle and anvil positioned on the upper and lower surfaces. Similarly, the thickness can also be measured on a molded part. The thickness of the steel sheet in this invention 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] Tailored blanks are fabricated by assembling several sheets of steel, or notched blanks known as subblanks, together, for example by laser welding, to optimize the performance of the part in its different areas, reduce the overall part weight, and lower the overall part cost.

[0009] Ultimate tensile strength, yield strength, and elongation are measured according to ISO standard ISO 6892-1, published in October 2009. Tensile test specimens are cut from a flat area. If necessary, small tensile test samples are taken to accommodate the entire available flat area on the part.

[0010] Hardness is a measure of a material's resistance to localized plastic deformation induced by mechanical indentation. It correlates well with the material's mechanical properties and is a useful local measurement method that eliminates the need to cut a sample for tensile testing. In this invention, hardness measurement is performed using a Vickers indenter in accordance with the ISO 6507-1 standard. Vickers hardness is expressed in units of Hv.

[0011] Referring to Figure 1, butt welding is a specific type of welding operation in which two steel plates to be welded 1, 2 are positioned side by side along their respective edges, and a weld seam 3 is created by melting each edge to form a molten pool containing a mixture of both steel plates and external material that may be used to support the welding process. The molten pool solidifies to form the weld seam 1. Figure 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 molten pool. The weld edge can be either straight or curved. The steel plates to be welded and the energy source used to melt the steel plates move relative to each other in a direction W during the welding operation at a speed known as the welding velocity. In specific embodiments, a gap 4 can be left between the steel plates 1, 2. This makes it possible to incorporate a considerable amount of additional material, such as filler wire, into the molten pool without, for example, exceeding the thickness of the weld seam. Such excess thickness is considered a geometric defect and is detrimental to further processing of the weld blank, for example, to stamping operations.

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

[0013] The quality of welded seams in terms of geometric defects is defined by the European standard EN10359:2015, entitled "Laser welded tailored blanks - Technical delivery conditions".

[0014] Figure 2A illustrates the latest technology for evaluating the strength of weld seams. A tensile test specimen is prepared with a weld seam 3 joining two steel plates 1 and 2, positioned in the lateral center of the specimen and compared to its tensile strength F. This established method provides a good evaluation of the relative strengths of the different elements constituting the specimen, namely the two steel plates 1 and 2 and the weld seam 3. According to this method, if the specimen breaks outside the weld seam, the weld seam is considered sufficiently strong. In other words, a weld seam is considered to be of good quality if it is at least harder than the weakest of the two steel plates, meaning that the weld is not a weak link within the assembly.

[0015] While the above 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 that the weld seam undergoes under actual life conditions. When a laser-welded blank is punched out, the weld seam deforms not only laterally but in all directions.

[0016] The inventors have found that when laser welding steel plates in which at least one has high strength, for example, a tensile strength exceeding 590 MPa, small cracks may be initiated perpendicular to the weld seam in the region of the weld that undergoes deformation with 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 evaluated using the above methods because the behavior of the weld when deformed longitudinally is not tested at all by conventional test methods. Furthermore, there are statistical factors associated with this type of crack. For the same part shape with the same steel grade and the same laser welding parameters, some parts may be crack-free while others may develop small cracks. This is due to spontaneous variations in steel plate composition, welding process, stamping process, etc. Therefore, these small cracks are not entirely predictable and are difficult to detect by quality control, making them a further problem in industrial environments. Although these cracks may be small on a formed part, they represent a critical weakness of the part, leading to failure of the part during its lifespan and, in some cases, causing serious safety problems.

[0017] Therefore, the inventors developed a new methodology for evaluating the risk of these small cracks occurring. The inventors found that, as shown in Figure 2B, aligning the weld seam in the longitudinal direction of a tensile specimen parallel to the tensile strength can lead to the occurrence of small lateral cracks in laser-welded assemblies with at least one blank of high-strength steel. Furthermore, the inventors found that a good criterion for ensuring that small cracks are less likely to occur in continuous production is to perform a series of 10 tensile tests on specimens welded in the longitudinal direction and compare the uniform elongation of the welded assembly to the weighted average of the uniform elongations of each steel sheet. The inventors found that the risk of cracking is very low when the uniform elongation Uweld of the welded assembly is at least 50% of the weighted average of all 10 tensile tests performed (Usheet1*th1+Usheet2*th2) / (th1+th2) (wherein th1 and th2 are the thicknesses of the assembled steel sheets 1 and 2, and Usheet1 and Usheet2 are their respective uniform elongations).

[0018] In the remainder of the explanation, if Uweld is lower than (Usheet1*th1+Usheet2*th2) / (th1+th2), the test specimen is said to exhibit brittle fracture, while if Uweld is higher than the aforementioned weighted average, the test specimen is said to exhibit ductile fracture.

[0019] The aforementioned newly developed statistical longitudinal test of weld seams allowed the inventors to investigate the problem of small cracks in numerous steel plates. By observing the cross-section of the fracture zone of samples exhibiting small cracks, the inventors found that the fracture surface showed a mixture of ductile and brittle characteristics.

[0020] More specifically, the brittle feature portions are associated with typical brittle fracture features in some cases, dendritic shapes and some voids in the weld seam 3 in some cases. These observations suggest that several mechanisms are involved in the formation of small cracks and that all are related to the way the weld pool solidifies. Dendrites and voids point to shrinkage problems, and brittle fracture surfaces point to hot cracking problems related to segregation problems during solidification in some cases. In fact, without wishing to be bound by theory, the specific composition of the high-strength steel plate that results in a specific composition of the weld pool during welding can lead to specific solidification problems in the weld pool and the vulnerability of the weld seam.

[0021] Based on these observations, the inventors have found that small cracks may occur when the unmodified target chemical action of the weld seam 3 is verified for the following conditions (concentrations are expressed in wt%): Gamma > 0.39% where Gamma = C + Si / 30 + Mn / 20 + 4.8*P + 4*S - Al / 20

[0022] The unmodified target chemical action of the weld seam means the weighted average of all chemical elements entering the weld seam. For two steel plates 1 and 2 having thicknesses th1 and th2, and concentrations X1, X2 of a chemical element X, and optionally both covered by metal coatings having total thicknesses thcoat1 and thcoat2 on both sides and containing Xcoat1 and Xcoat2 of the chemical element X, the unmodified target composition Xweld_unamended of the weld seam for element X is

[0023]

Equation

[0024] Based on these observations, the inventors were able to further solve the problem of small cracks in important assemblies by adjusting the welding chemistry. This can be done, for example, by adding additional material addM to the weld pool through a filler wire, or by injecting metal powder into the weld pool, or by adding material to at least one weld edge before the welding operation, for example in the form of an additional local metal coating, or in the form of a paint, or by applying it by cold spray or any other available technical means.

[0025] This additional material can reduce the gamma factor of the weld seam to 0.39% or less, which protects the welded assembly from the occurrence of small cracks.

[0026] More specifically, the modified target weld seam composition needs to be reduced to 0.39% or less using additional material addM, and the amount %addM of the additional material is incorporated into weld seam 3.

[0027] In the presence of %addM of additional material addM having a concentration XaddM of chemical element X, the modified target weld seam chemistry Xweld_amended is defined as follows:

[0028]

Number

[0029] For example, the additional material addM has a composition that dilutes elements (C, Si, Mn, P or S) having a positive coefficient in the gamma factor formula and thus has the effect of lowering the gamma factor. As can be seen from the arrangement of the gamma coefficients, the most important elements are P and S, which are related to the problems of dendritic defects, segregation and hot cracking. Therefore, it is necessary to use an additional material with very low concentrations of P and S. For example, the P content of the additional material is less than 0.008 wt%, more preferably less than 0.006 wt%, and even more preferably less than 0.004 wt%.

[0030] For example, the gamma factor can be adjusted by using additional material 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 decreases its gamma factor. For example, the Al content of the additional material is greater than 1.0% by weight, and more preferably greater than 2.0% by weight.

[0031] For example, the two embodiments can be combined by using a filler wire containing a significant amount of aluminum and very small amounts of P and S.

[0032] Another important factor to consider in all cases is the amount of additional material (addM) used (%addM).

[0033] To maintain a good weld shape without harmful defects such as excessive weld thickness due to excess material, it is possible to increase the gap 4 between the two sheets to be welded by increasing the amount of additional material. For example, the gap between sheets can be increased to more than 0.1 mm. For example, when using a gap of 0.1 mm or more, the amount of filler wire exceeds 15%.

[0034] When using filler wire, the amount of filler wire (%addM) can be adjusted by adjusting the ratio of the filler wire feeding rate to the welding rate.

[0035] When using metal powder, the amount of %addM can be adjusted by adjusting the spraying speed.

[0036] 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 melted in the weld pool.

[0037] In certain embodiments, at least one characteristic of the welded steel sheet relating to its chemical composition, microstructure, and mechanical properties corresponds to one of the rows in the following table (the chemical composition is expressed in weight percent, with the remainder being Fe and unavoidable impurities derived from the refining process; the % retained austenite in the microstructure of the steel sheet is expressed in surface percent of the cross-section; YP represents the yield point expressed in MPa; UTS represents the ultimate tensile strength expressed in MPa; and El% is the elongation measured according to the ISO 6892 standard mentioned above).

[0038] [Table 1]

[0039] In certain embodiments, the silicon content of at least one of the welded steel sheets is in the range of 0.5% to 3.0%, preferably 0.9% to 3.0%, and more preferably 0.9% to 2.5% (the lower and upper limits are included in the range). Advantageously, silicon acts to strengthen the steel and stabilize the austenite to retain a portion of the austenite in the microstructure of the steel sheet at room temperature. This makes it possible to achieve a combination of very high mechanical properties and high elongation.

[0040] In certain embodiments, the manganese content of at least one of the welded steel sheets is in the range of 1% to 4%, preferably 1% to 3%, and more preferably 2% to 3% (the lower and upper limits are included in the range). Advantageously, the manganese acts to strengthen the steel and also stabilizes the austenite to retain some of the austenite in the microstructure of the steel sheet at room temperature. This makes it possible to achieve a combination of very high mechanical properties and high elongation.

[0041] The present invention will be explained below with reference to the following examples, but these are by no means limiting.

[0042] Table 1 lists the steel compositions used in the following examples, along with the calculated gamma factors. Compositions are expressed in weight percent.

[0043] [Table 2]

[0044] In the examples, the additional material addM is in the form of 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.

[0045] [Table 3]

[0046] Welded assemblies were formed using the steel and filler wire compositions listed above, according to the different modalities detailed in Table 3. Samples produced using the method of the present invention have a sample reference beginning with I, while assemblies outside the scope of the method of the present invention have a sample reference beginning with R. Furthermore, for samples produced outside the scope of the present invention, the parameters outside the scope of the present invention are underlined.

[0047] The type of weld failure was evaluated using the test method described above on 10 welded assemblies in which the welds were positioned longitudinally. The results of these tests are also reported in Table 3.

[0048] [Table 4]

[0049] The uncorrected target gamma factor for weld seams, along with the corrected target gamma factor for weld seams, are reported in Table 3. Note that for assemblies without filler wire, the corrected gamma factor is the same as the uncorrected gamma factor, which is evident from the formulas for both corrected and uncorrected gamma. Also note that all samples used were either uncoated or coated with metal coatings that did not support any of the elements included in the gamma factor, and therefore, metal coatings did not interfere with the calculation of the gamma factor.

[0050] All welded assemblies were tested according to the novel longitudinal statistical testing method described above. In Table 3, the occurrence of small cracks is reported as "yes" if at least one of the 10 tensile test specimens exhibited brittle fracture, and as "no" if all test specimens exhibited ductile fracture.

[0051] Assembled parts R1 to R5, which are outside the scope of the present invention, have an unmodified gamma greater than 0.39 and are not welded with additional material so that their unmodified gamma also exceeds 0.39, which poses a risk of small crack formation.

[0052] Samples R6-R9, which fall outside the scope of the present invention, have an uncorrected gamma factor greater than 0.39 and were welded using additional materials. However, the specific composition of the filler wire used and the amount of filler wire added were not sufficient to reduce the corrected gamma factor to less than 0.39, and as a result, the resulting assemblies are still at risk of developing small cracks.

[0053] On the other hand, samples I1 to I8 produced according to the method of the present invention have an uncorrected gamma greater than 0.39, and with the addition of filler wire, they have a corrected gamma less than 0.39, which results in assemblies that are less prone to the occurrence of small cracks. More specifically, in samples I1 to I5, this improvement is achieved by using very low amounts of C, Mn, S, and P filler wire (less than 15%) and maintaining a narrow gap of less than 0.1 mm between the two sheets. In the case of 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 makes it possible to further reduce the corrected gamma factor and have a safety margin toward the threshold of 0.39. Sample I8 is produced using filler wire with a high aluminum content, which makes it possible to reduce the corrected gamma factor from 0.42 to 0.35. This is interesting because the filler wire composition used (wire 7) does not have as low a phosphorus content as the other filler wires used, which may be costly, but still manages to efficiently reduce the gamma factor and thus prevent the risk of small crack formation.

[0054] All remaining samples R11-R16, which are outside the scope of this invention, 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 risk-free welding and are already free of small cracks without any specific countermeasures.

[0055] In conclusion, by applying the method according to the present invention, it is possible to produce welded assemblies using very high-strength steel having a chemical composition that would otherwise pose a risk of small crack formation if specific countermeasures were not implemented.

Claims

1. A method for butt welding two steel plates (1, 2) having thicknesses th1 and th2, - A / The step of providing two steel sheets having a chemical composition and, optionally, a metallic coating, such that the gamma factor of the unmodified target weld seam composition is strictly greater than 0.

39. -B / Step of arranging and positioning the steel plates, -C / The process includes the step of butt welding the steel plates (1, 2) using at least a laser source and additional material addM incorporated into the weld seam (3) in weight % of % addM during the welding operation, such that the gamma factor of the modified target weld seam composition is 0.39 or less, For a given composition, the gamma factor is given by the following formula (all elements are expressed in weight percent): Gamma=C+Si / 30+Mn / 20+4.8*P+4*S-Al / 20, For a given chemical element X, the two steel plates (1) and (2) are covered with a metal coating having weight percentages X1 and X2 of the chemical element X, and in some cases, the sum of the thicknesses of both sides is thcoat1 and thcoat2, and both contain Xcoat1 and Xcoat2 of element X in weight percentage, and the unmodified target weld seam composition Xweld_unmodified for element X is, [Math 1] Given by, and in the presence of an amount of %addM of an additional material addM having a weight percentage XaddM of element X, the modified target weld seam composition Xweld_amended is, [Math 2] The method.

2. The method according to claim 1, wherein at least one of the two provided welded steel plates has a silicon content in the range of 0.5% by weight to 3.0%.

3. The method according to claim 2, wherein the two provided welded steel plates have a silicon content in the range of 0.5% by weight to 3.0% by weight.

4. The method according to any one of claims 1 to 3, wherein the additional material addM is supplied in the form of filler wires.

5. The method according to any one of claims 1 to 3, wherein the additional material addM is supplied in the form of a powder.

6. The method according to any one of claims 1 to 3, wherein the additional material addM is supplied in the form of a coating to at least one of the welded edges.

7. The method according to claim 1, wherein the additional material addM contains less than 0.008% by weight of P.

8. The method according to claim 7, wherein the additional material addM contains less than 0.006% by weight of P.

9. The method according to claim 8, wherein the additional material addM contains less than 0.004% by weight of P.

10. The method according to claim 1, wherein the additional material addM contains more than 1.0% by weight of Al.

11. The method according to claim 10, wherein the additional material addM contains more than 2.0% by weight of Al.

12. The method according to claim 1, wherein a gap (4) of at least 0.1 mm is left between the steel plates (1, 2), and the amount of additional material addM in the weld seam (3) is 15% or more.

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