Welding method for press hardening steels having an aluminum-based coating, associated welded blank and hot stamped part

Abstract

Method for butt-welding two sub-blanks of steel (10) comprising the steps of: -providing two sub-blanks (10) made of press-hardening steel grades, having a thickness of respectively t1 and t2 expressed in mm, and coated on both sides with an aluminum based metallic coating (2) comprising an intermetallic layer (21) in contact with a substrate (1) and a metallic layer (22) above it. -preparing said two sub-blanks (10) by removing at least said metallic layer (22) of said aluminum-based coating (2) on the top side of said weld edges (11) of each of said sub-blanks (10) to form removal zones (23) and by removing at least said metallic layer (22) of said aluminum-based coating (2) on the bottom side of one weld edge (11) to form another removal zone (23), while leaving the full amount of said aluminum-based coating (2) on the bottom side of the other weld edge (11), -positioning said two sub-blanks (10) side by side along their respective weld edges (11). -laser welding said two sub-blanks (10) using a laser beam (31) emitted by a laser head (30) traveling at a relative speed S_welding, expressed in m / min, and simultaneously adding a filler wire (41) having a diameter D_filler_wire, which is fed out of a filler wire feeder (40) at a speed S_filler_wire, expressed in m / min, to form a melt pool generated by said laser beam (31) and thus manufacture a laser welded blank (6) having a weld seam (5).

Description

[0001] Welding method for press hardening steels having an aluminum-based coating, associated welded blank and hot stamped part

[0002]

[0001] The present invention relates to a laser welding method for press hardening steels having an aluminum-based coating and to the related laser welded press hardening steel blank and press hardened steel part.

[0003]

[0002] Press hardening steel parts can be used as structural elements in automotive vehicles for anti-intrusion or energy absorption functions.

[0004]

[0003] Laser welded steel blanks associate in the same blank at least two or more different sub-blanks butt-welded together. This allows to combine within the same blank steel sheets having different chemical compositions and thicknesses. The benefits of using laser welded blanks in manufacturing automotive parts are multiple: optimization of parts properties, reduction of material scrap, simplification of the overall manufacturing process and vehicle weight lightning.

[0005]

[0004] Press hardening steels with an aluminum based metallic coating allow to produce high strength parts by hot stamping. However, the presence of the aluminum based coating presents a challenge when butt welding steel blanks together because the molten aluminum during welding interferes negatively with the final properties of the weld seam. One known method to handle this problem is to remove the metallic alloy portion of the aluminum based coating before welding on all four sides of the blanks to be welded (top and bottom sides of the two weld edges). This method to prepare the weld edges is known as ablation. However, in an industrial setting, ablating the top and bottom sides of both blanks to be welded can be challenging and can lower productivity and increase costs.

[0006]

[0005] The object of the present invention is to provide a welding process and resulting laser welded blank and hot stamped part, which can be applied without ablating all four sides to be welded.

[0007]

[0006] The object of the present invention is achieved by applying a laser welding process according to claim 1 , optionally having the features of claims 2 to 8 taken alone or according to any possible combination. Another object of the present invention is achieved by providing a hot stamped part using a laser welded blank according to claim 9, optionally comprising the features of claim 10 and 11 taken alone or according to any possible combination.

[0008]

[0007] The invention will now be described in detail and illustrated by examples without introducing limitations, and referring to the attached figures:

[0009] -Figure 1 is a schematic of the overall testing protocol that was developed by the inventors to evaluate the effect of a laser welding process on toughness,

[0010] -Figure 2 is a schematic of a sample used in the toughness measurement test that was developed by the inventors,

[0011] -Figure 3 is a schematic cross section of a sub-blank before laser welding it according to the invention,

[0012] -Figure 4 is a schematic cross section of the welding process according to the invention,

[0013] -Figure 5 is a schematic cross section of a laser welded blank according to the invention.

[0014]

[0008] A blank of steel refers to a flat sheet of steel, which has been cut to any shape suitable for its use. A blank has a top and bottom face, which are also referred to as a top and bottom side or as a top and bottom surface. The distance between said faces is designated as the thickness of the blank. The thickness can be measured for example using a micrometer, the spindle and anvil of which are placed on the top and bottom faces. In a similar way, the thickness can also be measured on a formed part.

[0015]

[0009] Laser welded blanks are made by assembling together several blanks of steel, known as sub-blanks, in order to optimize the performance of the part in its different areas, to reduce overall part weight and to reduce overall part cost. The sub-blanks forming the laser welded blanks are assembled by butt-welding them together, i.e. without overlap between the sub blanks. The sub-blanks are placed side by side and then laser welded together. In case of large laser welded blanks comprising many sub-blanks, the pre-positioning of said sub-blanks is done using for example complex jigs which give access only to the top face of said sub-blanks for the subsequent ablation and laser welding steps.

[0016]

[0010] By opposition to a laser welded blank, a monolithic blank is a blank which consists only of one sub-blank and which does not comprise any welds within it.

[0017]

[0011] A steel part refers to a part that was formed from a steel blank - said steel blank can be for example a laser welded blank, as described above.

[0018]

[0012] By average thickness of a part, or of a portion of a part, it is meant the overall average thickness of the material making up the part after it has been formed into a 3-dimensional part from an initially flat sheet. When referring to the thickness of a steel part, one refers to the local thickness measured using for example the above-described spindle and angle or else using for example a micrograph of a cross section.

[0019]

[0013] Hot stamping is a forming technology which involves heating a blank up to a temperature at which the microstructure of the steel has at least partially transformed to austenite, forming the blank at high temperature by stamping it and quenching the formed part to obtain a microstructure having a very high strength. Hot stamping allows to obtain very high strength parts with complex shapes and presents many technical advantages. It should be understood that the thermal treatment to which a part is submitted includes not only the above-described thermal cycle of the hot stamping process itself, but also possibly other subsequent heat treatment cycles such as for example the paint baking step, performed after the part has been painted in order to cure the paint. The mechanical properties of hot stamped parts below are those measured after the full thermal cycle, including optionally for example a paint baking step.

[0020]

[0014] The yield strength and ultimate tensile strength are measured according to ISO standard ISO 6892-1 , published in October 2009. The tensile test specimens are cut-out from flat areas of the hot stamped part. If necessary, small size tensile test samples are taken to accommodate for the total available flat area on the part.

[0021]

[0015] The bending angle is measured according to the VDA-238 bending standard. For the same material, the bending angle depends on the thickness. For the sake of simplicity, the bending angle values of the current invention refer to a thickness of 1.5mm. If the thickness is different than 1.5mm, the bending angle value needs to be normalized to 1.5mm by the following calculation where a1.5 is the bending angle normalized at 1.5mm, t is the thickness, and at is the bending angle for thickness t: a1 .5 = (at x t) I i .5

[0022]

[0016] Toughness reflects the ability of a material to absorb energy before breaking. It is a fundamental material property for parts used in the automotive industry. The presence of a laser weld within a blank can significantly deteriorate the toughness if the welding process is not correctly managed.

[0023]

[0017] The normalized toughness test according to ISO standard ISO 148-1 is not adapted to testing laser welded blanks for the automotive industry. Indeed, the standard provides for a minimum sample thickness of 2.5mm, which is higher than most typical blank thicknesses used in the automotive industry. Furthermore, the standard only provides for measurements on monolithic samples, without welds.

[0024]

[0018] The inventors have therefore developed a testing protocol to compare the toughness of the two sub-blanks making up a laser welded blank with the corresponding laser welded blank. This method allows to determine the effect that the laser welding process itself has on toughness. The method will hereafter be referred to as the “micro-Charpy” test and its result will be termed the “micro-Charpy ratio”.

[0025]

[0019] Referring to figure 1 , the production of samples for the micro-Charpy testing protocol which was developed by the inventors will now be described:

[0026] -A laser welded blank 100 is manufactured according to the laser welding process to be tested using two sub blanks 101 and 102 each having a length L1 of 300mm and a width L2 of 150mm - the thus produced laser welded blank 100 has a laser weld seam 103 in the middle and measures in total 300mm by 300mm,

[0027] -Said laser welded blank 100 is then hot stamped in a flat hot stamping press according to pre-determined hot stamping parameters, -micro-Charpy samples are cut out using water jet cutting out of the thus produced hot stamped flat part according to the following protocol:

[0028] -three micro-Charpy samples 111 are cut out from the fist subblank 101 ,

[0029] -three micro-Charpy samples 112 are cut out from the fist subblank 102,

[0030] -five micro-Charpy samples 113 are cut-out from the laser welded part itself - the long side of said micro-Charpy samples 113 being transverse to the weld seam 103 and said weld seam 103 being positioned in the middle of said long side.

[0031]

[0020] Referring to figure 2, the geometry of a micro-Charpy sample 110 will now be described - it should be noted that these geometrical characteristics, apart from the thickness of the sample and the imposed geometric tolerances, are the same as the sample used for standard Charpy testing according to ISO standard ISO 148-1 :

[0032] -Said micro-Charpy sample 110 has a length L3 of 55 mm and a width L4 of 10 mm.

[0033] -A notch 120 is positioned in the middle of one of the long sides of said sample 110. Said notch has a depth of 2mm, leaving an unnotched width L5 of 8mm. Furthermore, said notch 120 has an angle a of 45° and the tip 130 of said notch 120 has a radius of 2.5mm.

[0034]

[0021] The impact toughness at 23°C is measured on all of the above described micro-Charpy samples 111 , 112 and 113, using a standard Charpy testing equipment (the inventors have used a Zwick HIT50P machine) - the samples are maintained at 23°C by dipping them before testing for at least 5 minutes in a cryothermostatic bath set at the desired temperature. The result of said impact toughness test will be referred to as a micro-Charpy value. In the case of the monolithic samples of sub-blanks 101 and 102, the micro- Charpy value is the energy measured in the test divided by the thickness of the sample and expressed in J / mm. In the case of the weld containing test (samples 113), the micro-Charpy value is the energy measured in the test divided by the thickness of the thinnest of the two sub-blanks 101 and 102, also expressed in J / mm.

[0035]

[0022] The final result is expressed as the Micro-Charpy ratio, expressed in %:

[0036] -Micro-Charpy ratio = (Corrected average of micro-Charpy values on weld samples) I (minimum of the average of micro-Charpy value of the two sub-blanks)

[0037] -Wherein the Corrected average of micro-Charpy values on weld samples is the average of all micro-Charpy values performed on the five weld samples 113 from which is subtracted the standard deviation of said values on the five weld samples 113,

[0038] -Wherein the minimum of the average of micro-Charpy values of the two sub-blanks is the lowest value between the average of the micro- Charpy values of the three samples 111 of sub-blank 101 and the average of the micro-Charpy values of the three samples 112 of subblank 102.

[0039]

[0023] In other words:

[0040] Micro — Charpy ratio = average ( nicro Charpy Of Weld samples minimum (micro — charpy values subblank 101,

[0041]

[0024] The average of micro-Charpy values on weld samples is corrected using the standard deviation of said values in order to reflect in the final ratio possible low points in the values, which would denote an undesirable variation in product characteristics.

[0042]

[0025] The inventors have observed many rupture surfaces using the abovedescribed toughness tests and have come to the conclusion that when the micro-Charpy ratio is above 70%, the fracture surface of welded samples is fully ductile. This 70% minimum value was therefore chosen as the criterion to distinguish a good laser welding process from a bad laser welding process.

[0043]

[0026] In the following figures, the orientations and spatial references are all made using an X, Y, Z coordinates referential. X is the welding direction, the X axis is oriented such that the X coordinates increase while travelling along the welding direction. Y is the direction of the width of the steel blanks, i.e. the direction transverse to the welding direction and parallel to the top and bottom faces of the steel blanks. Z is the elevation direction, corresponding to the direction of the thickness of the steel blanks. The Z axis is oriented such that the Z coordinates increase when travelling from the bottom face of the steel blanks towards the top face of the steel blanks.

[0044]

[0027] Referring to figure 3, a sub-blank 10 to be welded comprises a steel substrate 1 having, on both top and bottom faces, an aluminum-based coating 2. Said coating 2 is superimposed on the substrate 1 and in contact therewith.

[0045]

[0028] The substrate 1 is made of a press hardenable steel, i.e. a steel capable of hardening after austenitizing and rapid cooling by quenching.

[0046]

[0029] The composition of the steel depends on the desired mechanical properties for the part. But preferably, in the steel substrate or in each area of the steel substrate, the steel has a composition comprising, by weight %:

[0047] 0.062% < C < 0.4%

[0048] 0.4% < Mn < 3.9%

[0049] 0.10% < Si < 1.5%

[0050] 0.005% < Al < 1.0%

[0051] 0.001 % < Cr < 2.0%

[0052] 0.001 % < Ti < 0.2%

[0053] 0.0005% < B < 0.010%

[0054] Ni < 2%

[0055] Nb < 0.1 %

[0056] Mo < 0.65% W < 0.30% N < 0.010%

[0057] 0.0001 % < S < 0.05%

[0058] 0.0001 % < P < 0.1 %

[0059] Ca < 0.005%

[0060]

[0030] the balance of the composition consisting of iron and unavoidable impurities resulting from elaboration.

[0031] The level of impurities resulting from the elaboration process will depend on the production route used. For example, when using a Blast Furnace route with a low level of steel scrap (recycled steel), the level of impurities will remain very low. On the other hand, when elaborating the steel using an electric furnace, with a very high ratio of recycled scrap steel, the level of impurities will be significantly increased. In this latter case, for example, the level of Cu can go up to 0.25%, Ni can go up to 0.25%, Sn can go up to 0.05%, As can go up to 0.03%, Sb can go up to 0.03% and Pb can go up to 0.03%.

[0061]

[0032] Hence, in an embodiment, the steel comprises up to 0.25% Cu, up to 0.05% Sn, up to 0.03% As, up to 0.03% Sb and / or up to 0.03% Pb as unavoidable impurities.

[0062]

[0033] The above composition is favorable to the achievement of high mechanical properties, in particular a tensile strength TS in the range of 950 MPa to 2100 MPa.

[0063]

[0034] In what follows, the contents in the elements are expressed by weight percent, unless otherwise explicated.

[0064]

[0035] The carbon content depends on the desired tensile strength TS of the hot-stamped coated steel part.

[0065]

[0036] Below a content of 0.062% of C, it is difficult to obtain a tensile strength of at least 950 MPa after hot-stamping under any cooling conditions. Above 0.4%, in combination with the other elements of the composition, the adhesion of the coating after hot stamping may not be satisfactory, and the resistance to delayed cracking and the toughness of the steel decrease. In an embodiment, the C content is of at most 0.38%.

[0066]

[0037] The C content depends on the desired tensile strength TS of the hot- stamped part, produced by hot-stamping the steel sheet. In an embodiment, the C content is comprised between 0.062% and 0.095%. If a higher tensile strength is desired, in the order of 1500 MPa, the C content can be increased to the range of 0.15% to 0.30%. If a further increase of the tensile strength, to at least 1800 MPa is needed, the C content can be added in a content of up to 0.4%.

[0038] Apart from its deoxidizing role, manganese has an important effect on quenchability, in particular when its content is of at least 0.4%. Above 3.9%, the stabilization of austenite by Mn may be too important, which may leads to the formation of a too pronounced banded structure. Preferably, the Mn content is of at most 3.0%.

[0067]

[0039] Silicon is added in a content of at least 0.10% to help deoxidizing the liquid steel and to contribute to the hardening of the steel by precipitating in solid solution. Its content is however generally limited in order to avoid excess formation of silicon oxides impairing the coatability of the steel. The silicon content is therefore generally lower than or equal to 1 .5%, for example lower than or equal to 0.80%.

[0068]

[0040] Aluminum may be added as a deoxidizer, in a content of at least 0.005%. Additionally, Al can protect boron by binding with N is the titanium content is insufficient. The Al content is preferably of at least 0.01 %. The Al content is generally lower than or equal to 1 .0% to avoid oxidation issues and avoid the formation of ferrite during hot stamping. Preferably, the Al content is of at most 0.1 %.

[0069]

[0041] Cr may be added to increase the quenchability of the steel and to contribute to achieving the desired tensile strength after hot stamping. When Cr is added, its content is higher than or equal to 0.01 %, preferably higher than or equal to 0.1 %, up to 2.0%. If no voluntary addition of Cr is performed, the Cr content may be present as an impurity in a content as low as 0.001 %.

[0070]

[0042] When titanium is added, its content is preferably of at least 0.008%, up to 0.2%. When the Ti content is comprised between 0.008% and 0.2%, precipitation at very high temperature occurs in the form of TiN and then, at lower temperature, in the austenite in the form of fine TiC, resulting in hardening. Furthermore, when titanium is added in addition to boron, titanium prevents combination of boron with nitrogen, the nitrogen being combined with titanium. Hence, the titanium content is preferably higher than 3.42*N, N being the N content by weight percent in the composition. However, the Ti content should preferably remain lower than or equal to 0.2%, preferably lower than or equal to 0.1 %, still preferably of at most 0.05%, to avoid precipitation of coarse TiN precipitates. If no voluntary addition of Ti is performed, Ti is present as an impurity in a content of at least 0.001 %.

[0071]

[0043] Boron is added in a content of at least 0.0005%, up to 0.010%, to increase the quenchability of the steel. Preferably, the B content is of at most 0.004%.

[0072]

[0044] In an embodiment, Ni may be added in a content of at most 2% and generally at least 0.25% and preferably up to 0.5% to reduce susceptibility to delayed fracture by concentrating on the surface of the part. If not added, Ni may be present as an impurity in a content which may be as low as 0.001 %. Depending on the production route used, the Ni content as an impurity can be as high as 0.25% (e.g. when producing the steel with a high ratio of recycled scrap steel) or as high as 0.1 % (e.g. when using a lower level of steel scrap).

[0073]

[0045] Up to 0.1 % of niobium is optionally added to provide precipitation hardening and microstructure refinement such as the prior austenitic grain size. Nb further improves the ductility of the steel. When Nb is added, its content is preferably of at least 0.01 %. The Nb content is preferably of at most 0.06% to avoid the formation of coarse (Ti, Nb) (C, N) precipitates.

[0074]

[0046] Molybdenum may be added in a content of at most 0.65%. When Mo is added, its content is preferably of at least 0.05%. Mo is preferably added together with Nb and Ti, to form co-precipitates which are very stable at high temperatures. Mo may also be added to increase the toughness of the steel playing a role grain boundary strengthener in solid solution state. An optimal effect is obtained when the Mo content is comprised between 0.15% and 0.25%.

[0075]

[0047] W may be added to increase the quenchability and the hardenability of the steel by forming tungsten carbides. When W is added, its content is higher than or equal to 0.001 %, and lower than or equal to 0.30%.

[0076]

[0048] Sulfur, phosphorus and nitrogen and generally present in the steel composition as impurities.

[0077]

[0049] The nitrogen content is generally of at least 0.0005%. The N content is generally of at most 0.010%, preferably of at most 0.005%, to prevent precipitation of coarse TiN precipitates.

[0050] When in excessive amounts, sulfur and phosphorus reduce the ductility. Therefore, their contents are limited to 0.05% and 0.1 % respectively.

[0078]

[0051] In particular, the presence of S in the liquid steel can lead to the formation of MnS precipitates which are detrimental to the properties. Preferably, the S content is of at most 0.01 %, better of at most 0.005%. Achieving a very low S content, i.e. lower than 0.0001 %, is very costly, and without any benefit. Therefore, the S content is generally higher than or equal to 0.0001 %.

[0079]

[0052] Preferably, the phosphorus content is of at most 0.05%, still preferably of at most 0.02%. Achieving a very low P content, i.e. lower than 0.0001 %, is very costly. Therefore, the P content is generally higher than or equal to 0.0001 %.

[0080]

[0053] The steel may undergo a treatment for globularization of sulfides performed with calcium, which has the effect of improving the bending angle, due to MnS globularization. Hence, the steel composition may comprise at least 0.0001 % of Ca, up to 0.005%.

[0081]

[0054] The remainder of the composition of the steel is iron and impurities resulting from the elaboration process. As detailed above, the impurities resulting from the elaboration process may include 0.25% or less of Cu, 0.05% or less of Sn, 0.03% or less of As, 0.03% or less of Sb and / or 0.03% or less of Pb.

[0082]

[0055] The composition of the steel may be selected depending on the desired mechanical properties, in particular in terms of strength and ductility.

[0083]

[0056] In particular, when a tensile strength in the range of 950 to 1200 MPa is desired, together with a bending angle higher than 75° the steel of the steel substrate has a composition in accordance with a first preferred composition, comprising, by weight%:

[0084] 0.062% < C < 0.095%

[0085] 1.4% < Mn < 1.9%

[0086] 0.2% < Si < 0.5%

[0087] 0.020% < Al < 0.070%

[0088] 0.02% < Cr < 0.1 %

[0089] With 1 .5% < (C + Mn +Si + Cr) < 2.7% 0.0035% < Ti < 0.072%

[0090] 0.0002% < B < 0.004%

[0091] 0.04% < Nb < 0.06% with 0.044% < (Nb+Ti) < 0.09%

[0092] 0.001% < N <0.009%

[0093] 0.0005% < S < 0.003%

[0094] 0.0001% < P <0.020%

[0095] Ca < 0.005%,

[0096]

[0057] the balance of the composition consisting of iron and unavoidable impurities resulting from elaboration.

[0097]

[0058] On the other hand, when a tensile strength of at least 1400 MPa, is required, the steel of the steel substrate or in at least one area of the steel substrate preferably has a composition in accordance with a second preferred composition comprising, by weight%:

[0098] 0.15% <C <0.30%

[0099] 0.5% < Mn < 3.0%

[0100] 0.10% < Si <0.50%

[0101] 0.005% < Al <0.1%

[0102] 0.01% < Cr< 1.0%

[0103] 0.001% < Ti <0.2%

[0104] 0.0002% < B <0.010%

[0105] 0.0005% < N < 0.010%

[0106] 0.0001% <S <0.05%

[0107] 0.0001% < P <0.1%

[0108] Ca < 0.005%

[0109]

[0059] the remainder being Fe and unavoidable impurities resulting from elaboration.

[0110]

[0060] If an even higher tensile strength is required, of 1800 MPa or higher, the composition of the steel substrate or in at least one area of the steel substrate is preferably according to a third preferred composition, comprising, by weight%:

[0111] 0.3% < C < 0.4%

[0112] 0.5%<Mn<1.0% 0.40% < Si < 0.80%

[0113] 0.01 % < Al < 0.1 %

[0114] 0.1 % < Cr < 1.0%

[0115] 0.008% < Ti < 0.03% 0.0005% < B < 0.003% Ni < 0.5% 0.01 % < Nb < 0.1 % 0.1 % < Mo < 0.5 % N < 0.005%

[0116] 0.0001 % < S < 0.004%

[0117] 0.0001 % < P < 0.02%

[0118] Ca < 0.0010%

[0119]

[0061] the balance of the composition consisting of iron and unavoidable impurities resulting from elaboration.

[0120]

[0062] The steel substrate of the hot-stamped coated steel part generally has a microstructure consisting of, in volume fraction, at least 60% martensite, at most 20% bainite and at most 5% ferrite and at most 15% austenite.

[0121]

[0063] The martensite fraction can be as high as 100%, and the bainite, ferrite and austenite fractions each as low as 0%.

[0122]

[0064] This microstructure description applies to the majority of the steel substrate, which means that this microstructure is present in at least 95% of the volume of the steel substrate.

[0123]

[0065] The microstructure is determined through the following method: a specimen is cut from the hot-stamped coated steel part, polished as detailed below, and etched with Nital 2% (10s), to reveal the microstructure. The section is afterwards examined through optical microscope with a 500x magnification and, if it is required to distinguish martensite from bainite, with a Scanning Electron Microscope (SEM) (Back Scattered Electron mode, Magnification 500x, EHT (Electron High Tension Voltage) = 15.00 kV, Scale 10 micrometers). The determination of the volume fraction of each constituent (martensite, bainite, ferrite, austenite) is performed with image analysis through a method known per se.

[0066] In an embodiment, the austenite fraction is of at most 5% by volume and / or the bainite fraction is of at most 10% by volume.

[0124]

[0067] In an embodiment, the microstructure consists of, by volume, at least 80% martensite, up to 10% of bainite, up to 5% austenite and up to 5% ferrite.

[0125]

[0068] In a preferred embodiment, the microstructure is essentially martensitic, i.e. consists of, by volume, at least 95% martensite and up to 5% of bainite and / or ferrite.

[0126]

[0069] Still preferably, the microstructure is fully martensitic.

[0127]

[0070] The substrate 1 may be obtained, depending on its desired thickness, by hot rolling and / or by cold rolling followed by annealing, or by any other appropriate method.

[0128]

[0071] The substrate 1 typically has a thickness comprised between 0.5 mm and 5 mm.

[0129]

[0072] The aluminum-based coating 2 is obtained by hot-dip coating, i.e. by immersion of the substrate 1 into a bath of molten metal. It comprises an intermetallic alloy layer 21 in contact with the substrate 1 and a metallic layer 22 extending atop the intermetallic alloy layer 21 .

[0130]

[0073] The intermetallic alloy layer 21 is formed by reaction between the substrate 1 and the molten metal of the bath. It comprises an intermetallic compound comprising at least one element from the metallic layer 22 and at least one element from the substrate 1 .

[0131]

[0074] The thickness of the intermetallic alloy layer 21 is generally of the order of a few micrometers. In particular, its mean thickness is typically comprised between 2 and 7 micrometers.

[0132]

[0075] The metallic layer 22 has a composition which is close to that of the molten metal in the bath. It is formed by the molten metal carried away by the strip as it travels through the molten metal bath during hot-dip coating.

[0133]

[0076] The metallic layer 22 has, for example, a thickness comprised between 19 pm and 33 pm or between 10 pm and 20 pm.

[0134]

[0077] The metallic layer 22 is a layer of aluminum, or a layer of aluminum alloy or a layer of aluminum-based alloy.

[0078] In this context, an aluminum alloy refers to an alloy comprising more than 50% by weight of aluminum. An aluminum-based alloy is an alloy in which aluminum is the main element, by weight.

[0135]

[0079] The intermetallic alloy layer 21 comprises intermetallic compounds of the Fex-Aly type, and more particularly Fe2AI5.

[0136]

[0080] The particular structure of the coating 2 obtained by hot-dip coating is in particular disclosed in patent EP 2 007 545.

[0137]

[0081] According to one embodiment, the metallic layer 22 is a layer of aluminum alloy further comprising silicon.

[0138]

[0082] According to one example, the metallic layer 11 comprises, by weight:

[0139] - 8% < Si < 11 %,

[0140] - 2% < Fe < 4%, the rest being aluminum and possible impurities.

[0141]

[0083] The laser welding method according to the invention comprises the following steps:

[0142] -Providing two sub-blanks made of press-hardening steel grades and coated on both sides with an aluminum based metallic coating, -Preparing said two sub-blanks by removing at least all of the metallic layer of said aluminum coating on the top side of weld edges of each of said sub-blanks to form two top side removal zones and by removing at least said metallic layer of said aluminum-based coating on the bottom side of one weld edge only to form one removal zone while leaving the full amount of said aluminum-based coating on the bottom side of the other weld edge,

[0143] -Positioning said two sub-blanks side by side along their respective weld edges,

[0144] -Laser welding said two sub-blanks using a laser head and a filler wire in order to form a laser welded blank.

[0145]

[0084] Each of these steps will now be described in detail.

[0146]

[0085] The two steel blanks 10 to be welded comprise a weld edge 11 , along which the laser welding operation will take place. Said weld edge is further characterized by the presence of removal zones 13 on the top side of said weld edge 11 and on the bottom side of the weld edge 11 of only one of the two sub-blanks. In said removal zones 13, the metallic layer 22 of the coating has been removed over a width which is for example comprised from 0.5 mm to 3.0 mm. In a particular embodiment, at least part of the intermetallic alloy layer 21 remains present in the removal area. Advantageously, this allows to provide temporary corrosion protection in the removal zones 13, in the case when the steel blank is stored in between the ablation operation and the welding operation. Advantageously, leaving at least part of the intermetallic layer 21 in the removal zones 13 also provides for increased corrosion protection during the hot stamping process and on the hot stamped part. In a particular embodiment, the metallic layer 22 and the intermetallic alloy layer 21 are both fully removed in the removal zone 13. Advantageously, this allows to minimize the amount of Al which will be melted and mixed into the weld seam during the welding operation. Alternatively, the amount of ablation varies in the different removal zones. For example, the removal zones on the top side of the weld edges each retain at least part of the intermetallic alloy layer 21 (this is known as partial ablation), while the removal zone located on the bottom side of only one sub-blank does not comprise any intermetallic layer 21 (this is known as full ablation). Any other combination of partial ablation and full ablation is possible in the three different removal zones 13. Advantageously, by thus mixing partial ablation and full ablation, it is possible to accrue the benefits of each configuration in the different portions of the weld area.

[0147]

[0086] Additionally to the aforementioned technical advantages of the removal zones 13 on the concentration of Al in the weld seam, the presence of removal zones 13 on the top side also allows to increase the efficiency of the laser beam used to weld the two sub-blanks together. Indeed, the metallic layer 22 has a very low absorptivity, On the other hand, the intermetallic layer 21 and the substrate 1 both have much higher absorptivity of the laser beam energy. The presence of the removal zones 13 on the top side therefore allows to absorb more of the laser beam energy when welding and therefore increases efficiency and productivity of the laser welding process.

[0087] The process to remove the metallic layer 22 in the removal zones 13 can be laser ablation, obtained by applying a pulsed laser beam over the area to remove the metallic layer 22. It can also be mechanical ablation, obtained by brushing I generally mechanically removing the metallic layer 22. It can also be any other type of removal method available. In the case of laser ablation, the pulsed laser beam can be advantageously applied simultaneously on the two weld edges 11 of the two sub-blanks 10 such as for example described in patent application PCT / IB2014 / 000612.

[0148]

[0088] It should be noted that the different ablation steps to create the three different removal zones (two on the top side and one on the bottom side) can be performed in any given order. For example, the ablation step to create one bottom side removal zone 13 is done prior to the ablation step to create both top side removal zones. For instance, said first ablation step to create one bottom side removal zone 13 is full ablation performed by mechanical ablation and said second ablation step to create simultaneously the top side removal zones is partial ablation using a pulsed laser beam according for examples to the teachings of patent application PCT / IB2014 / 000612. Industrially speaking, this way of proceeding can be performed by first performing “off-line” ablation on the bottom side of one sub-blank - off-line ablation being characterized as an ablation process performed on a stand alone equipment, not coupled to a subsequent laser welding step. The thus bottom side ablated sub-blank is then positioned side by side along their respective weld edges with another non-ablated sub-blank and the top side of each of said sub-blanks is then ablated simultaneously as described in patent application PCT / IB2014 / 000612. This type of process order is interesting for example when using equipments able to simultaneously ablate the top side and subsequently weld the sub-blanks, but which only allow access to the top side of the sub-blanks.

[0149]

[0089] In the case of laser ablation, typical process parameters for the ablation laser are listed below: o Laser Wave length: 1 micron o Laser Power : 0.5 to 4kW, preferably 1 to 2kW o Pulsed laser having a pulse length of 10 to 100 ns. o Spot size in Y direction : from 1 ,4mm to 4mm o Spot size in X direction: from 0.1 to 1 .0mm, preferably 0.2 to 0.7mm

[0150]

[0090] The two sub-blanks 10 to be welded are positioned side by side along their respective weld edges 11. In a specific embodiment, a gap is left between the two sub-blanks 10 to be welded. For example, a gap between 0.02mm and 0.6mm is left between said sub-blanks. It should be noted that in the case of the simultaneous inline ablation process described in patent application PCT / IB2014 / 000612, the sub-blanks are positioned side by side before performing simultaneous ablation - the blanks are not repositioned after the ablation process and are welded directly together in the same position, often times with a very short delay between ablation and welding, typically less than two minutes, preferably less than one minute, even more preferably less than thirty seconds. In this configuration, the bottom side ablation of one of the two sub-blanks will have been performed previously, before the step of positioning the two sub-blanks side by side along their respective weld edges.

[0151]

[0091] Referring to figure 4, Said sub-blanks 10 are subsequently welded together using a laser beam 31 emitted by a laser head 30 and simultaneously adding a filler wire 41 which is fed from a filler wire feeder 40 to the melt pool generated by the laser beam 31 . The weld pool itself is not represented on figure 4 for clarity’s sake.

[0152]

[0092] The welding operation is carried out by applying a relative speed in the welding direction X between the laser head 30 and the sub-blanks to be welded 10. Said relative speed will be referred to as the welding speed S_welding, expressed in m / min. In a particular embodiment it is the laser head 30 which moves along the welding direction. In a particular embodiment it is the sub-blanks 10 which move in the opposite direction to the welding direction, generating the desired relative movement.

[0153]

[0093] The filler wire 41 has a given diameter, known as the filler wire diameter D_filler_wire, expressed in mm, and is fed out of the filler wire feeder at a speed known as the filler wire speed S_filler_wire, expressed in m / min. The higher the filler wire speed is relative to a given welding speed, the more filler wire will be melted into the weld pool.

[0154]

[0094] Referring to figure 5, the welding operation generates a weld seam 5, which joins the sub-blanks 10 to form a laser welded blank 6. The weld seam 5 results from the solidification of the melt pool generated during the welding operation. Its chemical composition is a mixture of the chemical composition of the substrate 1 of each sub-blank 10, of the chemical composition of the metallic coatings 2 on the top and bottom surfaces of each sub-blank 10 and of the filler wire 41 that was fed into the melt pool. The amount of filler wire 41 melted into the weld seam 5 is referred to as the filler wire ratio, %fi ller_wire. It is dependent on the relative values of the welding speed and the filler wire speed. The higher the filler wire speed is compared to the welding speed, the more filler wire is melted into the weld seam 5.

[0155]

[0095] The laser welded blank 6 comprises intermediate zones 14 on either sides of the top side of said weld seam 5 as well as an intermediate zone 14 on one side only of the bottom side of the weld seam 5. Said intermediate zones 14 correspond to the area of the removal zones 13 on the sub-blanks before welding which extends beyond the area of the weld edges 11 which are incorporated within the melt pool during the welding operation and thus within the subsequently formed weld seam 5. Said intermediate zones have a width Wi, which is for example comprised from 5 microns to 2000 microns and more particularly from 5 microns to 1500 microns. The existence of said intermediate zones on either side of the weld seam are a guarantee that no aluminum from the metallic layer 22 on the top of the sub-blanks is integrated within the weld seam 5. When setting up the process to remove the metallic layer 22 in the removal zones 13 and the welding process to create the laser weld seam, it is important to ensure that the width of the removal zones 13 is greater than the width that will be occupied by the weld seam 5.

[0156]

[0096] Since the metallic layer of the aluminum-based coating is not removed on the bottom side of one of the weld edges 11 , a significant amount of aluminum is mixed into the weld seam generated by the welding operation.

[0097] The chemical composition of the weld seam 5 can be amended by adjusting the chemical composition and amount of filler wire 41 , which is fed into said weld seam.

[0157]

[0098] The inventors have found that by controlling the amount of chemical elements in the weld seam 5 and by controlling the welding process itself to obtain a good mixing of the different components that are mixed into the weld seam 5, it is possible to obtain a weld having a micro-Charpy ratio above 70%, even though the bottom the aluminum-based coating is not removed on the bottom side of the weld edges 11 .

[0158]

[0099] In particular, the inventors have found that a good weld seam is obtained when the following chemical composition ranges are obtained in the weld seam, all elements are expressed in weight% - the upper and lower values of the disclosed ranges are included within the acceptable chemical composition range:

[0159]

[0100] The Carbon content of the weld seam needs to be comprised from 0.15% to 0.38%. Carbon acts both as a hardening element and as a gammagene element, promoting the formation of austenite during the heating step of the hot stamping process. Below 0.15% of carbon, the hardness and toughness of the weld is too low. Above 0.38% of carbon, the weld can become fragile, with premature formation of cracks during deformation, which is detrimental to toughness, ductility and energy absorption in the case of a crash.

[0160]

[0101] The Nickel content of the weld seam needs to be comprised from 1 .0% to 12.0%. Nickel acts both as a hardening element and as a gammagene element, promoting the formation of austenite during the heating step of the hot stamping process. Below 1 .0% of Nickel, the diluted aluminum content of the weld seam, coming in particular from the aluminum-based metallic coating of the bottom side of the weld edges, is not compensated by the Nickel content of the weld seam. This results in an incomplete austenitization of the weld in the heating step of the hot stamping process. The ferrite which is not transformed to austenite is retained in the final hot stamped part, leading to welds with insufficient mechanical strength and toughness. Above 12.0% of Nickel, the austenite in the weld is extremely stable and part of it is not transformed to martensite during the quenching step of the hot stamping process. This retained austenite in the weld seam on the hot stamped part is detrimental to both mechanical strength and toughness.

[0161]

[0102] The inventors have also found that it is important to limit the amount of Chromium in the weld seam. Chromium can come from the base metals that are welded together and can also be used in the filler wire to compensate the effect of aluminum, as it is a gammagene element. However, it precipitates with carbon to form chromium carbides. These carbides consume part of the available carbon and weaken the weld seam. The inventors have found that Chromium in the weld seam should not exceed 5.0%, preferably 2.5%.

[0162]

[0103] The rest of the composition of the weld seam is at least 70% Fe, aluminum and optionally Mn, Co, V, Nb, Ti, Si, W, N, Ca, Mg, Sn, Cu, Mo, B the rest being inevitable impurities coming from the elaboration process of the welding wire and of the sub-blanks 10.

[0163]

[0104] The inventors have found that it is possible to control the chemical composition of the weld seam 5 within the above-described ranges of carbon, nickel and chromium content by controlling the process parameters of the welding operation, in combination with the known composition of the subblanks 10 and of the filler wire 41. Advantageously, this allows to have a process control methodology associated with a quality control feedback, without having to analyze the chemical composition of the weld seam itself in production, which is a destructive and time-consuming test.

[0164]

[0105] First of all, the inventors have established, through trial and error, a formula to estimate the filler wire ratio which will be noted %filler_wire_estimate:

[0165] %filler_wire_estimate = (S_f il ler_wire x IT x D_fil ler_wire2 / 4)

[0166] I (S_welding x (0.7 x (t1 +t2) / 2 + 0.4) x (t1 +t2) / 2 x 0.75)

[0167]

[0106] This formula was established and fine-tuned by comparing the process parameters to measured filler wire ratios. The filler wire ratio can for example be measured by singling out a chemical element which is only present in the filler wire and not in the sub-blanks, determining the concentration of said element within the weld seam and then comparing it with the concentration of said element in the filler wire.

[0168]

[0107] Once the filler wire ratio is estimated, the amount of carbon, nickel and chromium within the weld seam can be estimated through the following formulas, in which all the elemental concentrations are expressed in weight%, sub-blank1 and sub-blank2 designates the two sub-blanks to be welded, Ni_weld_estimate designates the Ni content of the weld estimated by the current method, Ni_filler_wire the known Ni content of the filler wire, Ni_sub-blank1 and Ni_sub-blank2 the known Ni contents respectively of sub- blankl and sub-blank2, the same notations being used for Carbon and Chromium:

[0169] Ni_weld_estimate = Ni_filler_wire x %filler_wire_estimate

[0170] + (100% - %filler_wire_estimate) x (Ni_sub-blank1 x t1 + Ni_sub-blank2 x t2) I (t1 + t2)

[0171] C_weld_estimate =

[0172] C_filler_wire x %filler_wire_estimate + (100% - %filler_wire_estimate) x (C_sub-blank1 x t1 + C_sub-blank2 x t2) I (t1 + t2)

[0173] Cr_weld_estimate =

[0174] Cr_filler_wire x %filler_wire_estimate + (100% - %filler_wire_estimate) x (Cr_sub-blank1 x t1 + Cr_sub-blank2 x t2) I (t1 + 12)

[0175]

[0108] For simplicity’s sake, the chemical compositions of the sub-blanks has been considered as a single uniform value in the above formulas. However, in the event in which for example the remaining coating 2 in the weld edges 11 contain some Ni, C or Cr, the contribution of said coatings is to be integrated within the corresponding sub-blank composition. For example, if the coating of sub-blank1 contains 20% of Ni, the amount of coating in the the weld edge of sub-blank1 represents 5% of the total thickness of sub- blankl and the substrate 1 of sub-blank 1 contains 0.1 % of Ni, then Ni_sub- blankl = 95% x 0.1 + 5% x 20 = 1 .095%.

[0176]

[0109] The inventors have found that when the welding process is controlled so that the following formulas are verified, the composition of the weld seam does indeed reach the desired targeted ranges:

[0177] 1.0% < Ni_weld_estimate < 12.0%

[0178] 0.15% < C_weld_estimate < 0.38%

[0179] Cr_weld_estimate < 5.0%, preferably Cr_weld_estimate < 2.5%

[0180]

[0110] The detrimental effect of aluminum coming from the bottom side coating of one of the weld edges is compensated by the addition of elements coming from the filler wire. This solution to the technical problems caused by aluminum is only efficient if the welding process allows for sufficient mixing of the elements coming from the different constituents of the weld pool and the subsequent weld seam 5. In particular, it is important to avoid localized peak concentrations of aluminum and of the compensating gammagene elements, in particular nickel. Indeed, these islands of high aluminum or gammagene element concentrations will be weak spots of the weld that can lead to premature failure and deteriorated toughness.

[0181]

[0111] In order to promote good mixing, the inventors have found that it is necessary to closely control the following process parameters:

[0182] -the size of the laser beam spot

[0183] -the linear energy of the laser beam

[0184]

[0112] The size of the laser beam spot will affect the size and shape of the melt pool and of the subsequent weld seam. The inventors have found that by ensuring a minimum spot size in the welding direction X, it is possible to obtain a sufficiently long melt pool in the welding direction which leaves time for the constituents of the melt pool to mix together and be sufficiently well distributed in the weld seam. The inventors have found that a minimum spot size of 0.8mm in the welding direction ensures good results. The inventors have found that it is also necessary to control the spot size in the transverse direction to welding Y, in order to have a good mixing of the weld edges 11 of the sub-blanks 10 within the melt pool. A minimum spot size of 0.8mm in the Y direction ensures good results.

[0185]

[0113] Finally, it is necessary to input a sufficient amount of energy into the welding process in order for the melt pool to be sufficiently large and remain liquid sufficiently long for good mixing. The linear energy of the welding process, expressed in J / mm, is defined as (the laser power P is expressed in Watts and S_welding in m / min):

[0186] Linear energy = P / S_welding x 60 / 1000

[0187]

[0114] The inventors have found that it is necessary to ensure that the linear energy verifies the following formula to obtain good results (t1 and t2 are the thicknesses of the sub-blanks, expressed in mm):

[0188] Linear energy > 51 + 5 x (t1 + t2) / 2

[0189]

[0115] The inventors have further found that by combining the abovedescribed minimum spot sizes and the above described minimum linear energy, it is possible to obtain a weld seam 5 having a large width W, expressed in mm, on its top side. In particular, the inventors have found that the above-described combination of welding process parameters leads to a weld seam verifying the following equation:

[0190] W > 0.75 x minimum (t1 , t2) + 0.34

[0191]

[0116] This minimum width of the top of the weld seam allows for a soft weld shape which is advantageous in terms of mechanical resistance.

[0192]

[0117] As a result of the above described edge preparation, weld chemical composition inputs and welding linear energy, the microstructure of the weld seam after hot stamping comprises more than 95% of martensite. Furthermore, the micro-charpy ratio of the resulting hot stamped part is equal to or greater than 70%.

[0193]

[0118] Advantageously, as a result of the above described process, the aluminum content of the weld is kept below a threshold level, expressed in weight % - for example the aluminum content of the weld is equal to or lessa than 1 .3%.

[0194]

[0119] The current invention also concerns a laser welded blank 6 comprising the following characteristics: -said laser welded blank 6 comprises a weld seam 5 connecting two sub_blanks 10 having a thickness t1 and t2, expressed in mm,

[0195] -said sub-blanks 10 each comprise a steel substrate 1 having, on both top and bottom faces, an aluminum-based coating 2 superimposed on said substrate 1 and in contact therewith,

[0196] -said substrates 1 are made of press-hardening steel grades, as described in more detail above,

[0197] -said aluminum-based coating 2 is obtained by hot-dip coating and comprises an intermetallic alloy layer 21 in contact with the substrate 1 and a metallic layer 22 extending atop the intermetallic alloy layer 21 , as described in more detail above, said laser welded blank 6 comprising the following characteristics:

[0198] -a top side having intermediate zones 14 on both sides of the weld seam 5, in which at least part of the metallic layer 22 of the sub-blanks 10 has been removed,

[0199] -a bottom side having an intermediate zone 14 on one side only of the weld seam 5 in which at least part of the metallic layer 22 of one of the subblanks 10 has been removed,

[0200] -the weld seam 5 has a top width W, expressed in mm, such that:

[0201] W > 0.75 x minimum (t1 , t2) + 0.34

[0202] -the weld seam 5 has the following chemical composition, expressed in weight %

[0203] 1.0% < %Ni < 12.0%

[0204] 0.15% < %C < 0.38%

[0205] 0% < %Cr < 5.0%, preferably 0% < %Cr < 2.5%, the rest of the composition comprising at least 70% Fe, the rest being Al, and optionally Mn, Co, V, Nb, Ti, Si, W, N, Ca, Mg, Sn, Cu, Mo, B the rest being inevitable impurities coming from the elaboration process.

[0206]

[0120] The current invention also concerns a hot stamped part. As a result of the hot stamping process, the steel on each side of the weld seam is shaped into a three dimensional part. However, the weld seam itself and the steel in the immediate vicinity of the weld seam can still be considered as flat. By immediate vicinity, it is meant a zone spanning a width of 3mm on each side of the weld seam. This is in accordance for example with the width range of micro-hardness measurements after heat treatment in and around the weld according to norm EN 10359:2023.

[0207]

[0121] Said hot stamped part comprises the following characteristics:

[0208] -Said hot stamped part is a welded hot press formed and cooled steel part comprising a first coated steel part portion and a second coated steel part portion connected by a weld seam

[0209] -Said steel part portions each have a substrate made of presshardening steel grades, as described in more detail above, and coated on each side with an aluminum based coating,

[0210] -said steel part portions have in the vicinity of the weld a thickness t1 and t2, expressed in mm,

[0211] -said hot stamped part comprises a top side having intermediate zones 14 on both sides of the weld seam 5, in which the thickness of the aluminum based coating is strictly smaller than the thickness of the aluminum based coating in areas located at a greater distance from the weld seam,

[0212] -said hot stamped part further comprises a bottom side having an intermediate zone 14 on one side only of the weld seam 5 in which the thickness of the aluminum based coating is strictly smaller than the thickness of the aluminum based coating in areas located at a greater distance from the weld seam on the same sub-blank,

[0213] -the weld seam 5 has a top width W, expressed in mm, such that:

[0214] W > 0.75 x minimum (t1 , t2) + 0.34

[0215] -the weld seam 5 has the following chemical composition, expressed in weight %

[0216] 1.0% < %Ni < 12.0%

[0217] 0.15% < %C < 0.38%

[0218] 0% < %Cr < 5.0%, preferably 0% < %Cr < 2.5%,

[0219] -the rest of the composition comprising at least 70% Fe, the rest being Al, and optionally Mn, Co, V, Nb, Ti, Si, W, N, Ca, Mg, Sn, Cu, Mo, B the rest being inevitable impurities coming from the elaboration process.

[0122] Optionally, said hot stamped part further comprises the following characteristic:

[0220] -the micro-charpy ratio of said hot stamped part is equal to or greater than 70%. -the chromium content of the weld seam is equal to or less than 2.5%.

[0221] -the %AI of the weld seam, expressed in weight %, is equal to or lower than 1.3%.

[0222] -The weld seam has a microstructure comprising a ratio of martensite greater than or equal to 95%.

[0223]

[0123] The invention will now be illustrated by means of examples, which are in no way intended to be limitative.

[0224]

[0124] Table 1 summarizes the chemical composition and thickness, of the sub-blanks. The element concentrations are all expressed in weight%. All the sub-blanks used in the examples are coated on both sides with a metallic coating containing 9% Si, 3% Fe. the rest being Al and unavoidable impurities - the coating thickness is 20 microns on each side.

[0225] Table 1

[0226]

[0125] Table 2 is a summary of the filler wire chemical compositions and diameters, the compositions are expressed in weight%, the rest being Fe and other inevitable impurities coming from the wire elaboration process:

[0227] Table 2

[0126] Table 3 summarizes the process parameters employed to weld the sub-blanks, the first four examples E1 - E4 are according to the invention while the last three examples E5 - E7 are not according to the invention. The values that are outside the range of the invention have been underlined.

[0127] The following ablation parameters are common to all the examples for which ablation was performed:

[0228] • Laser wavelength: 1030 nm

[0229] • Laser power: 1 ,5kW

[0230] • Pulse frequency: 20kHz • Spot size in X direction: 0.62mm

[0231] • Spot size in Y direction: 1 ,9mm

[0232]

[0128] Furthermore, in all the examples below, the welding laser wavelength is 1 micron. When ablation is referred to as 3Q, short for three quarters, it is according to the invention: i.e. the metallic alloy layer has been removed on the two weld edges of the top side and on the bottom side of the edge of subblank 1 only - the bottom side of sub-blank 2 is left unablated.

[0233]

[0234] Table 3

[0235] Table 3 (continued)

[0129] Table 4 is a summary of the hot stamping process parameters that were used to produce hot stamped parts from the above described laser welded blanks - these hot stamping process parameters allow to reach a fully martensitic microstructure on the steel sheets:

[0236]

[0237] Table 4

[0238]

[0130] Table 5 summarizes the characteristics of the weld seam of the hot stamped parts that were produced. The underlined properties are not according to the invention:

[0239] Table 5

[0240]

[0131] Table 6 summarizes the results of the micro-Charpy tests that were performed on the produced samples according to the above-described protocol:

[0241] Table 6

[0242]

[0132] Examples E1 to E4, in which the sub-blank preparation and welding process are carried out according to the invention, all have a micro-Charpy ratio above the desired minimum target of 70%.

[0243]

[0133] On the other hand, in the case of example E5, the linear energy of the welding operation is below the minimum required value of the invention, which results in a low weld top width value, and insufficient mixing within the weld. As a result, the associated micro-Charpy ratio is 56%, below the desired minimum target of 70%. Furthermore, the microstructure of the weld joint is not fully martensitic.

[0244]

[0134] Examples E6 and E7 have not undergone the prescribed sub-blank weld edge preparation, no ablation of the metallic coating prior to welding has been performed. This results in higher amounts of aluminum within the weld seam 5. Furthermore, the prescribed minimum weld spot size is also not respected, and in the case of E6 the linear energy of the welding operation is too low. This results in a low weld top width value and a low associated micro-Charpy ratio. Furthermore, the microstructure of the weld joint is not fully martensitic.

Claims

CLAIMS1 ) Method for butt-welding two sub-blanks of steel (10) comprising the steps of: -providing two sub-blanks (10) made of press-hardening steel grades, having a thickness of respectively t1 and t2 expressed in mm, and coated on both sides with an aluminum based metallic coating (2) comprising an intermetallic layer (21 ) in contact with a substrate (1 ) and a metallic layer (22) above it, -preparing said two sub-blanks (10) by removing at least said metallic layer (22) of said aluminum-based coating (2) on the top side of said weld edges (11 ) of each of said sub-blanks (10) to form removal zones (23) and by removing at least said metallic layer (22) of said aluminum-based coating (2) on the bottom side of one weld edge (11 ) to form another removal zone (23), while leaving the full amount of said aluminum-based coating (2) on the bottom side of the other weld edge (11 ),-positioning said two sub-blanks (10) side by side along their respective weld edges (11 ),-laser welding said two sub-blanks (10) using a laser beam (31 ) emitted by a laser head (30) and a welding speed S_welding, expressed in m / min, and simultaneously adding a filler wire (41 ) having a diameter D_f i I ler_wire, which is fed out of a filler wire feeder (40) at a speed S_filler_wire, expressed in m / min, to form a melt pool generated by said laser beam (31 ) and thus manufacture a laser welded blank (6) having a weld seam (5), wherein, the laser beam (31 ) has a power P, expressed in W, such that the linear energy, defined as P / (S_welding x 1000 / 60), and expressed in J / mm, satisfies the following condition :Linear energy > 51 + 5 x (t1 + t2) / 2 wherein, the laser beam (31 ) has a spot size equal to or greater than 0.8mm in the welding direction and equal to or greater than 0.8mm in the direction transverse to said welding direction and parallel to said sub-blanks top face,wherein the filler wire ratio %fi ller_wire, expressed in weight %, is estimated using the following formula:%filler_wire_estimate =(S_fi ller_wire x IT x D_fil ler_wire2 / 4) / (S_welding x (0.7 x (t1 + t2 ) / 2 + 0.4) x (t1 + t2) / 2 x 0.75) wherein the amount of nickel, carbon and chromium within the weld seam (5) are estimated through the following formulas, in which all the elemental concentrations are expressed in weight%, sub-blank1 and sub-blank2 designate the two sub-blanks to be welded, Ni_weld_estimate designates the estimated Ni content of the weld, Ni_filler_wire the known Ni content of the filler wire, Ni_sub-blank1 and Ni_sub-blank2 the known Ni contents respectively of sub-blank1 and sub-blank2, the same notations being used for Carbon and Chromium:Ni_weld_estimate = Ni_filler_wire x %filler_wire_estimate + (100% - %filler_wire_estimate) x (Ni_sub-blank1 x t1 + Ni_sub-blank2 x t2) I (t1 + t2)C_weld_estimate =C_filler_wire x %filler_wire_estimate+ (100% - %filler_wire_estimate) x (C_sub-blank1 x t1 + C_sub-blank2 x t2) I (t1 + t2)Cr_weld_estimate =Cr_filler_wire x %filler_wire_estimate + (100% - %filler_wire_estimate) x (Cr_sub-blank1 x t1 + Cr_sub-blank2 x t2) I (t1 + 12) and wherein,1.0% < Ni_weld_estimate < 12.0%0.15% < C_weld_estimate < 0.38%Cr_weld_estimate < 5.0%2) Method for butt-welding two sub-blanks of steel (10) according to claim 1 wherein the sub-blanks (10) preparation comprises the following steps: S1 / a first sub-blank (10) is first prepared by removing the metallic layer (22) of the aluminum-based coating (2) on the bottom side of the weld edge (11 ) of said first sub-blank (10) to form a first removal zone (23),S2 / said sub-blanks (10) are then positioned side by side along their respective weld edges (11 ), S3 / said sub-blanks (10) are subsequently prepared by removing simultaneously on both sub-blanks at least said metallic layer (22) of said aluminum-based coating (2) on the top side of said weld edges (11) of each of said sub-blanks (10) to form further removal zones (23), and wherein said sub-blanks (10) are not repositioned between step S3 and the welding step.3) Method according to claim 2, wherein the time between step S3 and welding the sub-blanks (10) is lower than or equal to two minutes.4) Method according to claim 3, wherein the time between step S3 and welding the sub-blanks is lower than or equal to one minute.5) Method according to any one of claims 1 to 4, wherein at least part of the intermetallic alloy layer (21 ) remains present in the two removal zones (13) located on the top side of said sub-blanks (10) in the step of preparing said sub-blanks (10) before welding.6) Method according to any one of claims 1 to 5, wherein the step of removing the metallic coating (22) to form the two removal zones (23) on the top side is carried out using a pulsed laser beam.7) Method according to any one of claims 1 to 6, wherein the welding speed S_welding is equal to or greater than 5.0 m / min.8) Method according to any one of claims 1 to 7, whereinCr_weld_estimate < 2.5%.9) Laser welded blank (6) comprising the following characteristics:-said laser welded blank (6) comprises a weld seam (5) connecting two sub_blanks (10) having a thickness t1 and t2, expressed in mm,-said sub-blanks (10) each comprise a steel substrate (1 ) having, on both top and bottom faces, an aluminum-based coating (2) superimposed on said substrate (1 ) and in contact therewith,-said substrates (1 ) are made of press-hardening steel grades,-said aluminum-based coating (2) is obtained by hot-dip coating and comprises an intermetallic alloy layer (21 ) in contact with the substrate (1 ) and a metallic layer (22) extending atop the intermetallic alloy layer (21 ),-said laser welded blank (6) comprising a top side having intermediate zones (14) on both sides of the weld seam (5), in which at least part of the metallic layer (22) of the sub-blanks (10) has been removed,-said laser welded blank (6) further comprising a bottom side having an intermediate zone (14) on one side only of the weld seam (5) in which at least part of the metallic layer (22) of one of the sub-blanks (10) has been removed,-the weld seam (5) has a top width W, expressed in mm, such that:W > 0.75 x minimum (t1 , t2) + 0.34-the weld seam (5) has the following chemical composition, expressed in weight %:1.0% < %Ni < 12.0%0.15% < %C < 0.38%0% < %Cr < 5.0%, the rest of the composition comprising at least 70% Fe, the rest being Al, and optionally Mn, Co, V, Nb, Ti, Si, W, N, Ca, Mg, Sn, Cu, Mo, B the rest being inevitable impurities coming from the elaboration process.10) Laser welded blank according to claim 9, wherein the chromium content of the weld seam is equal to or less than 2.5%.)Laser welded blank according to claim 9 or 10, wherein the aluminum content of the weld seam is equal to or less than 1 .3%. )Hot stamped part comprising the following characteristics:-Said hot stamped part is a welded hot press formed and cooled steel part comprising a first coated steel part portion and a second coated steel part portion connected by a weld seam,-Said steel part portions each have a substrate made of presshardening steel grades, and coated on each side with an aluminum based coating,-said steel part portions have in the vicinity of the weld seam, over a zone spanning width of 3mm on either side of the weld seam, a thickness t1 and t2, expressed in mm,-said hot stamped part comprises a top side having intermediate zones 14 on both sides of the weld seam 5, in which the thickness of the aluminum based coating is strictly smaller than the thickness of the aluminum based coating in areas located at a greater distance from the weld seam,-said hot stamped part further comprises a bottom side having an intermediate zone 14 on one side only of the weld seam 5 in which the thickness of the aluminum based coating is strictly smaller than the thickness of the aluminum based coating in areas located at a greater distance from the weld seam on the same sub-blank,-the weld seam 5 has a top width W, expressed in mm, such that:W > 0.75 x minimum (t1 , t2) + 0.34-the weld seam 5 has the following chemical composition, expressed in weight %1.0% < %Ni < 12.0%0.15% < %C < 0.38%0% < %Cr < 5.0%-the rest of the composition comprising at least 70% Fe, the rest being Al, and optionally Mn, Co, V, Nb, Ti, Si, W, N, Ca, Mg, Sn, Cu, Mo, B the rest being inevitable impurities coming from the elaboration process.13) Hot stamped part according to claim 12 having a micro-charpy ratio of equal to or greater than 70%.14) Hot-stamped part according to claim 12 or 13, wherein the %AI of the weld seam, expressed in weight %, is equal to or lower than 1 .3%.15) Hot-stamped part according to any one of claims 12 to 14, wherein the chromium content of the weld seam %Cr expressed in weight is equal to or less than 2.5%.16)Hot stamped part according to any one of claims 12 to 15, wherein said weld seam has a microstructure comprising a ratio of martensite greater than or equal to 95%.

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