Manufacturing method of press-hardened laser welded steel component, and press-hardened laser welded steel component

Abstract

To provide a manufacturing method of press-hardened laser welded steel components.SOLUTION: A manufacturing method of press-hardened laser welded steel components comprises: butt-welding first and second precoated steel plates (1) to obtain a blank; heating the blank to a heat treatment temperature that is at least 10°C lower than a full austenitizing temperature of a weld joint (22) and at least 15°C higher than a minimum temperature Tmin. In the formula (I), Ac3(WJ) is a full austenitizing temperature of the weld joint (22). In the formula (II), Ts1 and Ts2 are the maximum tensile strengths of the strongest substrate and the weakest substrate after press hardening, CFW is a carbon content rate of a filler material, β is a proportion of a filler material, and ρ is a ratio between the thicknesses of the weakest substrate and the strongest substrate, and holding the blank at the heat treatment temperature for a time between 2 and 10 minutes; and press-forming the blank into a component and cooling the component.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for producing a press-hardened laser-welded steel part and to the press-hardened laser-welded steel part thus obtained. [Background technology]

[0002] Steel parts of this kind are used in particular in the automotive industry, and more particularly for the manufacture of crash management parts, such as anti-intrusion or impact absorbing parts, structural parts or parts which contribute to the safety of the motor vehicle.

[0003] For these types of parts, automotive manufacturers dictate that the weld joint should not constitute the weakest zone of the welded steel part.

[0004] To prevent corrosion, the steel sheets used in the manufacture of such welded steel parts are precoated with an aluminum-based precoat via hot-dip galvanization in an aluminum-containing bath. If the steel sheets are welded without prior preparation, the aluminum-based precoat is diluted with the steel substrate in the molten metal during the welding operation. Aluminum tends to increase the full austenitization temperature of the molten metal, thus preventing the complete transformation to austenite during hot forming using conventional heat treatment temperatures. As a result, it may no longer be possible to obtain a fully martensite or bainite microstructure in the weld joint during the press cooling that occurs during the hot forming process.

[0005] Furthermore, it is not possible to use higher heat treatment temperatures that would allow complete austenitization of the welded joint, as this would result in overalloying of the coat with potential negative effects on paint adhesion and / or spot weldability of the press-hardened parts.

[0006] Faced with this situation and when manufacturing components from such pre-coated steel sheets, two types of solutions have been developed in the prior art to be able to obtain a fully martensitic structure in the welded joint after hot forming and quenching using conventional heat treatment temperatures.

[0007] In particular, EP 2007545 describes a first solution consisting of removing a surface layer of metal alloy at the weld end of the pre-coated steel sheet, so as to significantly reduce the total aluminum content in the weld joint and, as a result, obtain a full austenitization temperature close to the full austenitization temperature of the base material of the pre-coated steel sheet.

[0008] Furthermore, EP2737971, US2016 / 0144456 and WO2014075824 describe a second solution which consists in welding pre-coated steel sheets with a filler wire containing an austenite stabilizing element such as carbon, manganese or nickel to compensate for the presence of aluminum in the weld joint and reduce its full austenitization temperature, such that a fully martensitic structure is obtained in the weld joint after hot forming and quenching using conventional heat treatment temperatures.

[0009] However, these methods are not entirely satisfactory.

[0010] Indeed, the first solution is relatively time-consuming. Moreover, the second method may require the addition of relatively large amounts of austenite-forming elements in order to be able to obtain a fully martensitic structure in the welded joint after heat treatment. This addition increases the production costs and may also lead to unsatisfactory joint geometries or problems resulting from inhomogeneous mixing between the materials from the pre-coated steel sheet and the filler wire in the welded joint, with the risk of having locally retained austenite. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] European Patent Application Publication No. 2007545 [Patent Document 2] European Patent Application Publication No. 2737971 [Patent Document 3] US Patent Application Publication No. 2016 / 0144456 [Patent Document 4] International Publication No. 2014 / 075824 Summary of the Invention [Problem to be solved by the invention]

[0012] The object of the present invention is therefore to provide a method for producing a welded steel blank from two pre-coated plates, which makes it possible to obtain, at relatively low cost, a part after press hardening which has satisfactory crash performance properties even for a relatively high aluminum content in the weld joint. [Means for solving the problem]

[0013] For this purpose, the present invention comprises the following successive steps: - providing a first precoated steel sheet and a second precoated steel sheet, each of the first and second precoated steel sheets comprising a steel substrate, at least one of the first and second precoated steel sheets having on at least one of its major surfaces an aluminum-containing precoat comprising at least 50% by weight aluminum; The first pre-coated steel sheet has a first thickness, and the second pre-coated steel sheet has a second thickness; the substrate of the first pre-coated steel sheet has, after press hardening, a maximum tensile strength strictly greater than the maximum tensile strength of the substrate of the second pre-coated steel sheet after press hardening; providing a product of the first thickness and the maximum tensile strength after press hardening of the first pre-coated steel sheet strictly greater than the product of the second thickness and the maximum tensile strength of the second pre-coated steel sheet; - removing the aluminium-containing precoat on at least one main surface of the weld edge or the edge to be welded of at least one of the first and second precoated steel sheets over at least a part of its thickness, at least if the theoretical average aluminium content in the weld joint obtained by butt welding the first and second precoated steel sheets provided in the providing step, optionally with a filler metal containing at most 0.05% aluminium, is strictly greater than 1.25% by weight, so that the theoretical average aluminium content in the weld joint obtained by butt welding the first and second precoated steel sheets thus produced, optionally with a filler metal containing at most 0.05% aluminium, is comprised between 0.5% and 1.25% by weight, - butt welding the first pre-coated steel sheet and the second pre-coated steel sheet using laser welding to obtain a welded joint between the first and second pre-coated steel sheets, thereby obtaining a welded blank, the welding process optionally including the use of a filler metal; - heating the welded blank to a heat treatment temperature, which is at least 10°C below the full austenitization temperature of the welded joint and at a minimum temperature T min at least 15° C. above

number

number

[0014] According to a particular embodiment of the method, the step of removing the aluminum-containing precoat is carried out when - if the theoretical average aluminum content of the welded joint obtained by butt welding the first and second pre-coated steel sheets provided in the providing step, optionally using a filler metal containing a maximum of 0.05% by weight of aluminum, is strictly more than 1.25% by weight; And optionally, when the theoretical average aluminum content of the welded joint obtained by butt welding the first and second pre-coated steel sheets provided in the providing step, optionally using a filler metal containing aluminum at most 0.05% by weight, is between 0.5% by weight and 1.25% by weight, more specifically, more than 0.5% by weight; This step is carried out, optionally using a filler metal containing a maximum of 0.05% by weight of aluminum, so that the theoretical average aluminum content of the welded joint obtained by butt welding the first and second pre-coated steel sheets thus produced is between 0.5% by weight and 1.25% by weight;

[0015] at the end of the heating process, the microstructure of the substrate of the first and second pre-coated steel sheets is fully austenitic; - the ratio of the maximum tensile strength of the substrate of the first pre-coated steel plate after press hardening to the maximum tensile strength of the substrate of the second pre-coated steel plate after press hardening is 1.2 or more;

[0016] - the carbon content of the substrate of the first pre-coated steel sheet is at least 0.05% by weight higher than the carbon content of the substrate of the second pre-coated steel sheet;

[0017] - each of the first and second precoated steel sheets provided in the providing step comprises an aluminum-containing precoat on at least one of its major surfaces comprising at least 50% by weight aluminum;

[0018] - the first and second precoated steel sheets provided in the providing step comprise an aluminum-containing precoat on both of their main surfaces, the aluminum-containing precoat comprising at least 50% by weight of aluminum;

[0019] - during butt welding, the aluminium-containing precoat remains intact on both main surfaces of at least one of the first precoated steel sheet and the second precoated steel sheet, e.g. of each of the first and second precoated steel sheets;

[0020] the method further comprises a step of preparing a weld edge of at least one of the first and second pre-coated steel sheets, which is intended to be at least partially incorporated into the weld joint, by removing, prior to butt-welding, the aluminum-containing pre-coat on at least one main surface thereof over at least a portion of its thickness, possibly using a filler metal containing at most 0.05% by weight of aluminum, even if the theoretical average aluminum content of the weld joint obtained by butt-welding the first and second pre-coated steel sheets provided in the providing step is comprised between 0.5% by weight and 1.25% by weight,

[0021] - the method further comprises a step of preparing a weld edge of at least one of the first and second pre-coated steel sheets, which is intended to be at least partially incorporated into the weld joint, by removing, prior to butt welding, the aluminum-containing pre-coat on at least one of its main faces over at least a part of its thickness, optionally with a filler metal containing at most 0.05% aluminum, even if the theoretical average aluminum content of the weld joint obtained by butt welding the first and second pre-coated steel sheets provided in the providing step is comprised between 0.5% and 1.25% by weight, the removing step being carried out in such a way that the theoretical average aluminum content of the weld joint obtained by butt welding the first and second pre-coated steel sheets thus prepared, optionally with a filler metal containing at most 0.05% aluminum, is comprised between 0.5% and 1.25% by weight,

[0022] For at least one of the first and second precoated steel sheets, the base steel comprises, by weight: 0.10%≦C≦0.5% 0.5%≦Mn≦3% 0.1%≦Si≦1% 0.01%≦Cr≦1% Ti≦0.2% Al≦0.1% S≦0.05% P≦0.1% B≦0.010% the balance being iron and impurities resulting from manufacturing,

[0023] For at least one of the first and second precoated steel sheets, the base steel comprises, by weight: 0.15%≦C≦0.25% 0.8%≦Mn≦1.8% 0.1%≦Si≦0.35% 0.01%≦Cr≦0.5% Ti≦0.1% Al≦0.1% S≦0.05% P≦0.1% B≦0.005% the balance being iron and impurities resulting from manufacturing,

[0024] For at least one of the first and second precoated steel sheets, the base steel comprises, by weight: 0.040%≦C≦0.100% 0.70%≦Mn≦2.00% Si≦0.50% S≦0.009% P≦0.030% 0.010%≦Al≦0.070% 0.015%≦Nb≦0.100% Ti≦0.080% N≦0.009% Cu≦0.100% Ni≦0.100% Cr≦0.2% Mo≦0.100% Ca≦0.006% the balance being iron and impurities resulting from manufacturing,

[0025] For at least one of the first and second precoated steel sheets, the base steel comprises, by weight: 0.06%≦C≦0.100% 1.4%≦Mn≦1.9% 0.2%≦Si≦0.5% 0.010%≦AI≦0.070% 0.04%≦Nb≦0.06% 3.4×N≦Ti≦8×N 0.02%≦Cr≦0.1% 0.0005%≦B≦0.004% 0.001%≦S≦0.009% the balance being iron and impurities resulting from manufacturing,

[0026] For at least one of the first and second precoated steel sheets, the base steel comprises, by weight: 0.24%≦C≦0.38% 0.40%≦Mn≦3% 0.10%≦Si≦0.70% 0.015%≦AI≦0.070% 0%≦Cr≦2% 0.25%≦Ni≦2% 0.015%≦Ti≦0.10% 0%≦Nb≦0.060% 0.0005%≦B≦0.0040% 0.003%≦N≦0.010% 0.0001%≦S≦0.005% 0.0001%≦P≦0.025% The titanium and nitrogen contents satisfy the following relationship: Ti / N>3.42 The carbon, manganese, chromium and silicon contents satisfy the following relationship:

number

[0027] - the laser welding is carried out using protective gas, in particular helium and / or argon, and

[0028] The first and second pre-coated steel sheets have different thicknesses.

[0029] The present invention further relates to a press hardened laser welded steel part, the steel part comprising a first coated steel part portion and a second coated steel part portion, Each coated steel component portion includes a steel substrate, and at least one of the first coated steel component portion and the second coated steel plate has an aluminum-containing coating on at least one of its major surfaces, the aluminum-containing coating comprising at least 30% by weight aluminum; the first coated steel component portion has a first thickness, the second coated steel portion has a second thickness, the substrate of the first coated steel component portion has a maximum tensile strength strictly greater than the maximum tensile strength of the substrate of the second coated steel component portion, and the product of the first thickness and the maximum tensile strength of the first coated steel component portion is strictly greater than the product of the second thickness and the maximum tensile strength of the second coated steel component portion; The first and second coated steel component portions are joined by a weld joint, the weld joint having an aluminum content comprised between 0.5 wt.% and 1.25 wt.%, the microstructure of the weld joint comprising martensite and / or bainite and comprising a fraction of intercritical ferrite comprised between 15% intercritical ferrite fraction and 5% maximum intercritical ferrite fraction, the maximum intercritical ferrite fraction being determined using the formula:

number

[0030] According to a particular embodiment of the steel part, the ratio of the ultimate tensile strength of the substrate of the first coated steel part to the ultimate tensile strength of the substrate of the second coated steel part is equal to or greater than 1.2;

[0031] for at least one of the first and second coated steel component portions, the substrate steel comprises, by weight: 0.10%≦C≦0.5% 0.5%≦Mn≦3% 0.1%≦Si≦1% 0.01%≦Cr≦1% Ti≦0.2% Al≦0.1% S≦0.05% P≦0.1% B≦0.010% the balance being iron and impurities resulting from manufacturing,

[0032] for at least one of the first and second coated steel component portions, the substrate steel comprises, by weight: 0.15%≦C≦0.25% 0.8%≦Mn≦1.8% 0.1%≦Si≦0.35% 0.01%≦Cr≦0.5% Ti≦0.1% Al≦0.1% S≦0.05% P≦0.1% B≦0.005% the balance being iron and impurities resulting from manufacturing,

[0033] for at least one of the first and second coated steel component portions, the substrate steel comprises, by weight: 0.040%≦C≦0.100% 0.70%≦Mn≦2.00% Si≦0.50% S≦0.005% P≦0.030% 0.010%≦Al≦0.070% 0.015%≦Nb≦0.100% Ti≦0.080% N≦0.009% Cu≦0.100% Ni≦0.100% Cr≦0.2% Mo≦0.100% Ca≦0.006% the balance being iron and impurities resulting from manufacturing,

[0034] for at least one of the first and second coated steel component portions, the substrate steel comprises, by weight: 0.24%≦C≦0.38% 0.40%≦Mn≦3% 0.10%≦Si≦0.70% 0.015%≦AI≦0.070% 0%≦Cr≦2% 0.25%≦Ni≦2% 0.015%≦Ti≦0.10% 0%≦Nb≦0.060% 0.0005%≦B≦0.0040% 0.003%≦N≦0.010% 0.0001%≦S≦0.005% 0.0001%≦P≦0.025% The titanium and nitrogen contents satisfy the following relationship: Ti / N>3.42 The carbon, manganese, chromium and silicon contents satisfy the following relationship:

number

[0035] for at least one of the first and second coated steel component portions, the substrate steel comprises, by weight: 0.06%≦C≦0.100% 1.4%≦Mn≦1.9% 0.2%≦Si≦0.5% 0.010%≦AI≦0.070% 0.04%≦Nb≦0.06% 3.4×N≦Ti≦8×N 0.02%≦Cr≦0.1% 0.0005%≦B≦0.004% 0.001%≦S≦0.009% the balance being iron and impurities resulting from manufacturing.

[0036] The invention will be better understood from reading the following specification, given by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0037] [Figure 1] 2 is a schematic cross-sectional view of the start of the welding step of the method according to the invention; FIG. [Diagram 2] 3 is a schematic cross-sectional view of a welded blank obtained at the end of the welding process; FIG. [Diagram 3] FIG. 2 is a perspective view of the precoated steel sheet after the preparation process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Throughout the patent application, elemental contents are expressed as weight percent (wt%).

[0039] The present invention relates to a method for producing press hardened laser welded steel parts.

[0040] More specifically, the method comprises a first step of providing a first pre-coated steel sheet 1 and a second pre-coated steel sheet 2 .

[0041] Each of the precoated steel sheets 1, 2 has two opposing main surfaces 5, 6 and at least one side surface 13 extending between the two opposing main surfaces 5, 6 from one of the main surfaces 5, 6 to the other main surface. In the example shown in Fig. 3, the precoated steel sheets 1, 2 have four side surfaces 13. For example, the side surface 13 forms an angle between 60° and 90° with one of the main surfaces 5, 6.

[0042] As shown in Figure 1, each precoated steel sheet 1,2 includes a metal substrate 3,4 having on at least one of its major surfaces an aluminum-containing precoat 7,8. The precoat 7,8 is superimposed on and in contact with the substrate 3,4.

[0043] The metal substrates 3, 4 are more particularly steel substrates.

[0044] Moreover, the steel of the base materials 3, 4 is more particularly a steel having a ferrito-perlitic microstructure.

[0045] Preferably, the substrates 3, 4 consist of a steel intended for heat treatment, more particularly a press-hardenable steel, for example a manganese-boron steel such as type 22MnB5 steel.

[0046] According to one embodiment, the steel of the substrates 3, 4 is 0.10%≦C≦0.5% 0.5%≦Mn≦3% 0.1%≦Si≦1% 0.01%≦Cr≦1% Ti≦0.2% Al≦0.1% S≦0.05% P≦0.1% B≦0.010% the balance being iron and impurities resulting from manufacturing.

[0047] More specifically, the steel of the substrates 3, 4 is 0.15%≦C≦0.25% 0.8%≦Mn≦1.8% 0.1%≦Si≦0.35% 0.01%≦Cr≦0.5% Ti≦0.1% Al≦0.1% S≦0.05% P≦0.1% B≦0.005% the balance being iron and impurities resulting from manufacturing.

[0048] According to the alternative, the steel of the substrate 3, 4 is by weight 0.040%≦C≦0.100% 0.70%≦Mn≦2.00% Si≦0.50%, more specifically Si≦0.30% S≦0.009%, more specifically S≦0.005% P≦0.030% 0.010%≦Al≦0.070% 0.015%≦Nb≦0.100% Ti≦0.080% N≦0.009% Cu≦0.100% Ni≦0.100% Cr≦0.2% Mo≦0.100% Ca≦0.006% the balance being iron and impurities resulting from manufacturing.

[0049] According to the alternative, the steel of the substrate 3, 4 is by weight 0.24%≦C≦0.38% 0.40%≦Mn≦3% 0.10%≦Si≦0.70% 0.015%≦AI≦0.070% 0%≦Cr≦2% 0.25%≦Ni≦2% 0.015%≦Ti≦0.10% 0%≦Nb≦0.060% 0.0005%≦B≦0.0040% 0.003%≦N≦0.010% 0.0001%≦S≦0.005% 0.0001%≦P≦0.025% The titanium and nitrogen contents satisfy the following relationship: Ti / N>3.42 The carbon, manganese, chromium and silicon contents satisfy the following relationship:

number

[0050] According to the alternative, the steel of the substrate 3, 4 is by weight 0.06%≦C≦0.100% 1.4%≦Mn≦1.9% 0.2%≦Si≦0.5% 0.010%≦AI≦0.070% 0.04%≦Nb≦0.06% 3.4×N≦Ti≦8×N 0.02%≦Cr≦0.1% 0.0005%≦B≦0.004% 0.001%≦S≦0.009% the balance being iron and impurities resulting from manufacturing.

[0051] The substrate 3, 4 can be obtained, depending on its desired thickness, by hot rolling and / or cold rolling followed by annealing or by any other suitable method.

[0052] The substrates 3, 4 advantageously have a thickness comprised between 0.6 mm and 5 mm, more particularly comprised between 0.8 mm and 5 mm, and even more particularly comprised between 1.0 mm and 2.5 mm.

[0053] According to one example, the thickness of the substrate 3 of the first precoated steel sheet 1 is different from the thickness of the substrate 4 of the second precoated steel sheet 2 .

[0054] According to an alternative, the substrates 3, 4 of the first and second pre-coated steel sheets 1, 2 have the same thickness.

[0055] According to the invention, the substrate 3 of the first precoated steel sheet 1 has, after press hardening, a maximum tensile strength Ts1 which is strictly greater than the maximum tensile strength Ts2 of the substrate 4 of the second precoated steel sheet 2 after press hardening.

[0056] In this context, "after press hardening" means after heating to a temperature equal to or greater than the full austenitization temperature Ac3 of the steel substrate considered, hot press forming so as to obtain hardening compared to the initial state, and subsequent cooling.

[0057] For example, the maximum tensile strength Ts1 of the substrate 3 of the first precoated steel sheet 1 after press hardening is between 1400 MPa and 1600 MPa or between 1700 MPa and 2000 MPa.

[0058] For example, the maximum tensile strength Ts2 of the substrate 3 of the second precoated steel sheet 2 after press hardening is between 500 MPa and 700 MPa or between 1000 MPa and 1200 MPa.

[0059] For example, the ratio (Ts1 / Ts2) of the maximum tensile strength Ts1 of the substrate 3 of the first precoated steel sheet 1 after press hardening to the maximum tensile strength Ts2 of the substrate 4 of the second precoated steel sheet 2 after press hardening is 1.2 or more, more specifically, 1.4 or more.

[0060] Furthermore, the first precoated steel sheet 1 has a first thickness t1, and the second precoated steel sheet 1 has a second thickness t2.

[0061] The thicknesses t1, t2 are, for example, between 0.6 mm and 5 mm, more particularly between 0.8 mm and 5 mm, and even more particularly between 1.0 mm and 2.5 mm.

[0062] According to one embodiment, the thicknesses t1 and t2 are identical. According to an alternative, the thicknesses t1 and t2 are different.

[0063] The product of the first thickness t1 and the maximum tensile strength Ts1 of the first pre-coated steel sheet 1 is strictly greater than the product of the second thickness t2 and the maximum tensile strength Ts2 of the second pre-coated steel sheet 1.

[0064] In particular, the compositions of the substrates 3 and 4 of the first and second precoated steel sheets 1, 2 are selected from the compositions described above.

[0065] For example, the steel of the substrate 3 of the first precoated steel sheet 1 is, by weight, 0.15%≦C≦0.25% 0.8%≦Mn≦1.8% 0.1%≦Si≦0.35% 0.01%≦Cr≦0.5% Ti≦0.1% Al≦0.1% S≦0.05% P≦0.1% B≦0.005% the balance being iron and impurities resulting from manufacturing.

[0066] According to another example, the steel of the substrate 3 of the first pre-coated steel sheet 1 is, by weight: 0.24%≦C≦0.38% 0.40%≦Mn≦3% 0.10%≦Si≦0.70% 0.015%≦AI≦0.070% 0%≦Cr≦2% 0.25%≦Ni≦2% 0.015%≦Ti≦0.10% 0%≦Nb≦0.060% 0.0005%≦B≦0.0040% 0.003%≦N≦0.010% 0.0001%≦S≦0.005% 0.0001%≦P≦0.025% The titanium and nitrogen contents satisfy the following relationship: Ti / N>3.42 The carbon, manganese, chromium and silicon contents satisfy the following relationship:

number

[0067] For example, the steel of the substrate 4 of the second precoated steel sheet 2 is 0.040%≦C≦0.100% 0.70%≦Mn≦2.00% Si≦0.50%, more specifically Si≦0.30% S≦0.009%, more specifically S≦0.005% P≦0.030% 0.010%≦Al≦0.070% 0.015%≦Nb≦0.100% Ti≦0.080% N≦0.009% Cu≦0.100% Ni≦0.100% Cr≦0.2% Mo≦0.100% Ca≦0.006% the balance being iron and impurities resulting from manufacturing.

[0068] According to another example, the steel of the substrate 4 of the second pre-coated steel sheet 2 is, by weight 0.06%≦C≦0.100% 1.4%≦Mn≦1.9% 0.2%≦Si≦0.5% 0.010%≦AI≦0.070% 0.04%≦Nb≦0.06% 3.4×N≦Ti≦8×N 0.02%≦Cr≦0.1% 0.0005%≦B≦0.004% 0.001%≦S≦0.009% the balance being iron and impurities resulting from manufacturing.

[0069] Preferably, the carbon content of the substrate 3 of the first precoated steel sheet 1 is at least 0.05% by weight higher than the carbon content of the substrate 4 of the second precoated steel sheet 2 .

[0070] According to the invention, for at least one of the first precoated steel sheet 1 and the second precoated steel sheet 2 the aluminium-containing precoat 7, 8 comprises at least 50% by weight aluminium.

[0071] Preferably, the precoats 7, 8 are obtained by hot-dip galvanization, i.e. by immersing the substrates 3, 4 in a bath of molten metal. In this case, the precoats 7, 8 comprise at least an intermetallic alloy layer 9, 10 in contact with the substrates 3, 4, as shown in FIG.

[0072] The intermetallic alloy layers 9, 10 contain an intermetallic compound containing at least iron and aluminum. The intermetallic alloy layers 9, 10 are formed in particular by a reaction between the substrates 3, 4 and the molten metal of the bath. More specifically, the intermetallic alloy layers 9, 10 contain Fe x -Al y type, more specifically Fe2Al5 intermetallic compounds.

[0073] In the example shown in Figure 1, the precoats 7, 8 further comprise metal alloy layers 11, 12 extending over the intermetallic alloy layers 9, 10. The metal alloy layers 11, 12 have a composition close to that of the molten metal in the bath. The metal alloy layers 11, 12 are formed by the molten metal carried away by the sheet as it passes through the molten metal bath during hot dip galvanizing.

[0074] The metal alloy layers 11, 12 are for example layers of aluminium, aluminium alloy or aluminium-based alloy.

[0075] In this context, an aluminium alloy refers to an alloy containing more than 50% aluminium by weight, and an aluminium-based alloy is an alloy in which aluminium is the main component by weight.

[0076] For example, the metal alloy layers 11 and 12 are layers of an aluminum alloy further containing silicon. More specifically, the metal alloy layers 11 and 12 are - 8%≦Si≦11%, - 2%≦Fe≦4% with the balance being aluminum and possible impurities.

[0077] The metal alloy layers 11, 12 have a thickness comprised for example between 19 μm and 33 μm, or between 10 μm and 20 μm.

[0078] 1, where the precoats 7, 8 include metal alloy layers 11, 12, the thickness of the intermetallic alloy layers 9, 10 is generally on the order of a few μm. In particular, their average thickness is typically comprised between 2 and 7 micrometers.

[0079] The particular structure of the precoats 7, 8 comprising the intermetallic alloy layers 9, 10 and the metal alloy layers 11, 12 obtained by hot dip coating is disclosed in particular in patent EP2007545.

[0080] According to another embodiment, the aluminum-containing precoat 7,8 only comprises the intermetallic alloy layer 9,10 described above. In this case, the thickness of the intermetallic alloy layer 9,10 is for example comprised between 10 μm and 40 μm. Such a precoat 7,8 consisting of an intermetallic alloy layer 9,10 can for example be obtained by subjecting the precoat 7,8 comprising the intermetallic alloy layer 9,10 and the metal alloy layer 11,12 disclosed above to a prealloying treatment. Such a prealloying treatment is carried out at a temperature and holding time selected to alloy the precoat 7,8 with the substrate 3,4 over at least a part of the thickness of the precoat 7,8.

[0081] More specifically, the pre-alloying treatment comprises the following steps: heating the sheet to a pre-alloying temperature comprised between 620 and 1000°C, keeping the pre-alloyed sheet at this temperature for a time that varies between a few minutes and a few hours depending on the treatment temperature used. In this case, the intermetallic alloy layers 9, 10 are themselves composed of Fe2Al5, FeAl3, FeAl, Fe6Al 12 It may be composed of different intermetallic sublayers, such as Si5 and FeAl3 sublayers.

[0082] Advantageously, as shown in FIG. 1, the substrates 3, 4 are provided on both of their main faces with an aluminium-containing precoat 7, 8 as described above.

[0083] The first and second precoated steel sheets 1, 2 may carry the same precoat 7, 8.

[0084] Alternatively, the precoats 7, 8 of the first and second precoated steel sheets 1, 2 may have different compositions.

[0085] Next, a theoretical average aluminum content in the weld joint 22 obtained by butt welding between the above-mentioned first and second precoated steel sheets 1, 2, optionally using a filler metal, is determined.

[0086] If a filler metal is intended to be used, it is preferred that the filler metal is a steel-based filler metal having an aluminium content of 0.05% by weight or less.

[0087] This determination may be made in any manner known to one of skill in the art.

[0088] For example, the theoretical average aluminum content in the weld joint 22 can be determined using the following formula:

number

[0089] The above formula can also be used when a filler metal is used, as long as the filler metal contains an aluminum content of 0.05% by weight or less.

[0090] The above formula can still be used if the substrates 3, 4 contain aluminum, as long as the aluminum content of the substrates 3, 4 is less than or equal to 0.05% by weight.

[0091] The proportion β of steel-based filler metal optionally added to the weld pool is for example comprised between 0 and 0.5, ie the proportion expressed as a percentage is between 0 and 50%.

[0092] Theoretical average aluminum content of welded joint 22 Al th weld would strictly exceed 1.25% by weight, the method according to the invention further comprises a step of preparing the weld edge 14 of at least one of the pre-coated steel sheets 1, 2 such that after preparation the theoretical average content of aluminum in the weld joint is comprised between 0.5% and 1.25% by weight.

[0093] More specifically, the weld edges 14 of the pre-painted steel sheets 1 , 2 considered are edges of the pre-painted steel sheets 1 , 2 which are intended to be welded to the other pre-painted steel sheet 1 , 2 .

[0094] 3, the weld edge 14 includes a peripheral portion of the pre-coated steel sheets 1, 2 that is intended to be at least partially incorporated into the weld joint 22 during butt welding. More specifically, the weld edge 14 comprises a side surface 13 of the pre-coated steel sheets 1, 2 and a portion of the pre-coated sheets 1, 2 extending from the side surface 13 and including a portion of the pre-coat 7, 8 and a portion of the substrate 3, 4.

[0095] More specifically, the step of producing the weld edge 14 comprises removing the aluminium-containing precoat 7, 8 on at least one of the main faces 5, 6 of the first and second precoated steel sheets 1, 2 over at least a part of its thickness. The precoat 7, 8 is removed from the side surface 13 of the precoated steel sheets 1, 2 over a removal zone 18 extending at the weld edge 14. The removal zone 18 can extend over a width comprised between 0.5 mm and 2 mm from the side surface 13 of the precoated steel sheets 1, 2. An example of a precoated steel sheet 1 produced in this way is shown in FIG. 3.

[0096] The removal is preferably carried out using a laser beam.

[0097] Advantageously, in the removal zone 18, the metal alloy layers 11, 12 are removed, while the intermetallic alloy layers 9, 10 remain over at least a part of their thickness.

[0098] More specifically, in the removal zone 18, the metal alloy layers 11, 12 are removed, while the intermetallic alloy layers 9, 10 remain intact.

[0099] The residual intermetallic alloy layers 9, 10 protect the areas of the welded blank immediately adjacent the weld joint 22 from oxidation and decarburization during subsequent hot forming steps, and from corrosion during service.

[0100] In the example shown in FIG. 3, the metal alloy layers 11, 12 are removed at the weld edge 14 over a removal zone 18, leaving the intermetallic alloy layers 9, 10 intact.

[0101] In particular, the proportion of the precoat 7, 8 to be removed, as well as the number of main faces of the precoated steel sheets 1, 2 on which the precoat 7, 8 is to be removed, are determined by the theoretical average aluminum content Al of the weld joint 22 after removal. th weld is between 0.5% and 1.25% by weight.

[0102] In particular, Precoat 7 and 8 are - only on one main surface 5, 6 of the first or second precoated steel sheet 1, 2, or on two main surfaces in total, for example on one main surface 5, 6 of each of the first and second pre-coated steel sheets 1, 2, or on two main surfaces 5, 6 of one of the first and second pre-coated steel sheets 1, 2, or on three main surfaces 5, 6 in total, i.e. on two main surfaces 5, 6 of one of the first and second pre-coated steel sheets 1, 2 and on only one main surface 5, 6 of the other pre-coated steel sheet 1, 2, or on a total of four main surfaces 5, 6, i.e. on the two main surfaces 5, 6 of the first and second precoated steel sheets 1, 2 It may be removed over at least a portion of its thickness.

[0103] The theoretical average aluminum content Al of the welded joint 22 obtained by butt welding the first and second precoated steel sheets 1 and 2 provided in the providing step, optionally using a filler metal having an aluminum content of 0.05% by weight or less. th weld is contained between 0.5% and 1.25% by weight, welding is carried out without prior removal of the precoats 7, 8, in particular for the first and second precoated steel sheets 1, 2. More specifically, in this case welding is carried out with the first and second precoated steel sheets 1, 2, whose precoats 7, 8 remain intact at least on the weld edges 14.

[0104] Optionally, a filler metal having an aluminum content of 0.05% by weight or less is used to obtain a weld joint 22 obtained by butt welding between the first and second precoated steel sheets 1, 2 provided in the providing step. The theoretical average aluminum content Al th weldThe precoats 7, 8 may be removed over at least a portion of their thickness on at least one main surface 5, 6 of at least one of the precoated steel sheets 1, 2, for example at the weld edge 14 only on at least one main surface 5, 6 of at least one of the two precoated steel sheets 1, 2, even if the aluminum content is between 0.5% and 1.25% by weight, more precisely greater than 0.5% by weight. For example, the precoats 7, 8 are removed over at least a portion of their thickness on the weld edge 14 only on one main surface 5, 6 of each of the two precoated steel sheets 1, 2. This optional removal step is carried out in accordance with the theoretical average aluminum content Al of the weld joint 22 obtained by welding between the first and second precoated steel sheets 1, 2 thus produced, optionally with a filler metal having an aluminum content of 0.05% by weight or less. th weld is carried out so that it remains between 0.5% and 1.25% by weight.

[0105] In particular, such removal is achieved by the thermal treatment temperature T t may be carried out to further reduce the heat treatment temperature T t is determined as described below. In fact, the austenitizing temperature Ac3(WJ) of the weld joint 22 decreases with decreasing aluminum content. In particular, this optional removal step reduces the heat treatment temperature T t would strictly exceed 950 °C. In fact, to maintain good paintability and weldability, a heat treatment temperature T below 950 °C is required. t It is preferable to use:

[0106] Theoretical average aluminum content of welded joint 22 Al th weld After the determination and, if necessary or desired, the preparation step, the method further comprises the step of butt-welding the first pre-coated steel sheet 1 to the second pre-coated steel sheet 2 using laser welding to obtain a weld joint 22 between the first and second pre-coated steel sheets 1, 2, resulting in a welded steel blank 15.

[0107] The weld joint 22 has an aluminum content between 0.5 and 1.25% by weight.

[0108] According to one embodiment, the welding process includes the use of a filler metal.

[0109] The filler metal is advantageously a steel-based filler metal having an aluminium content of 0.05% by weight or less, the filler metal having a low aluminium content dilutes the aluminium from the coating.

[0110] For example, the filler metal may further include austenite forming elements to partially balance the ferrite forming and / or intermetallic compound forming effects of the aluminum from the precoats 7,8.

[0111] The filler metal may be, for example, a filler metal wire or powder.

[0112] The proportion of filler metal added to the weld pool is, for example, comprised between 0 and 0.5.

[0113] According to one example, the filler metal has the following composition, by weight: 0.1%≦C≦1.2% 0.01%≦Mn≦10% 0.02%≦Ni≦7% 0.02%≦Cr≦5% 0.01%≦Si≦2% Optionally, Trace amount≦Mo≦1% Trace ≦Ti≦0.1% Trace amount≦V≦0.1% Trace amount≦B≦0.01% Trace amount≦Nb≦0.1% Trace amount≦Al≦0.05% The balance is iron and impurities inevitably resulting from production.

[0114] According to a particular example, the filler metal may have one of the compositions W1, W2 or W3 set forth in Table 1 below.

[0115] [Table 1]

[0116] In all these compositions, the contents are expressed as weight percent.

[0117] Additionally, for each composition, the balance of the composition is iron and unavoidable impurities.

[0118] In Table 1 above, "-" means that the composition contains at most trace amounts of that element.

[0119] According to a variant, the welding process is an autogenous welding process, which means that the welding is carried out without the use of filler metal. In this case, the composition of the welded joint 22 depends only on the composition of the substrates 3, 4 of the first and second precoated steel sheets 1, 2 and the amount of precoat 7, 8 incorporated in the welded joint 22.

[0120] The welding operation results in the formation of a weld joint 22 at the junction between the two plates 1,2.

[0121] The welding process is a laser welding process, where a laser beam 24 is directed towards the joint between the two pre-coated steel sheets 1,2.

[0122] The laser welding process is carried out, for example, using a CO2 laser or a solid-state laser or a semiconductor laser.

[0123] The laser source is preferably a high-power laser source, which can be selected from, for example, a CO2 laser with a wavelength of 10 micrometers, a solid-state laser source or a semiconductor laser source with a wavelength of 1 micrometer, for example a diode laser with a wavelength between 0.8 and 1 micrometer.

[0124] The power of the laser is selected depending on the thickness of the first and second precoated steel sheets 1, 2. In particular, the power is selected so as to allow fusion of the weld edges 14 of the precoated steel sheets 1, 2 as well as sufficient mixing in the weld joint 22. In the case of a CO2 laser, the laser power is for example comprised between 3 and 12 kW. In the case of a solid-state or semiconductor laser, the laser power is for example comprised between 2 and 8 kW.

[0125] The diameter of the laser beam 24 at its impact point 26 on the pre-coated steel sheets 1, 2 can be equal to about 600 μm for both types of laser source.

[0126] During the welding process, the welding is carried out, for example, under a protective atmosphere, which prevents, inter alia, oxidation and decarburization of the area where the welding is being performed, the formation of boron nitride in the weld joint 22, and possible cold cracking due to hydrogen absorption.

[0127] The protective environment is formed for example by an inert gas or a mixture of inert gases, which can be helium or argon or a mixture of these gases.

[0128] Welding may be performed using laser light as the sole heat source.

[0129] Optionally, the laser welding process includes an additional heat source in addition to the laser beam, such as an electric arc or induction heating, which contributes to melting the edges of the first and second pre-coated steel sheets 1, 2 to form a weld joint 22.

[0130] Optionally, the welding process includes the use of a filler wire 20, as shown in dashed lines in Figure 1. In this case, the laser beam 24 is additionally configured to melt the filler wire 20 at the impact point 26 of the laser beam 24.

[0131] During the welding process, the distance between the opposing weld edges 14 of the two pre-coated steel sheets 1, 2 is, for example, 0.3 mm or less, more specifically 0.1 mm or less. Providing such a gap between the opposing weld edges 14 of the two sheets 1, 2 promotes possible material deposition from the filler wire 20 during the welding operation and prevents the formation of excessive thickness at the weld joint 22.

[0132] At the end of the welding process, a welded steel blank 15 is obtained, as shown in FIG.

[0133] After the welding step, the method according to the invention comprises the step of heating the thus obtained welded steel blank 15 in a heat treatment oven.

[0134] More specifically, the heating step heats the welded steel blank 15 to a heat treatment temperature T t This involves heating to

[0135] According to the present invention, the heat treatment temperature T t is at least 10° C. lower than the fully austenitizing temperature Ac3(WJ) of the weld joint 22.

[0136] The fully austenitizing temperature Ac3(WJ) of the weld joint 22 is expressed in °C and is determined from the composition of the weld joint 22 using, for example, the following formula: Ac3(WJ)=102.2×Al+439×C+181.9×Mn+364.1×Si+148×Al 2 -425.2×C 2 -29.2×Mn 2 -497.8×Si 2 −400×Al×C+9.9×Al×Mn−50.5×Al×Si−208.9×C×Mn+570.3, where Al, C, Mn and Si refer to the aluminum, carbon, manganese and silicon contents of the weld joint 22, respectively, expressed in weight percent.

[0137] The above formula for Ac3(WJ) can be used for the content ranges shown in Table 2 below.

[0138] [Table 2]

[0139] In Table 2 above, - All contents are expressed as percentages by weight. - "-" means there is no lower limit.

[0140] According to the present invention, the heat treatment temperature T t is the minimum temperature T min In this connection, the minimum temperature T min is defined as follows:

number

number

[0141] In this regard, if a substrate has a lower ultimate tensile strength Ts after press hardening, it is weaker than another.

[0142] Therefore, the minimum temperature T min can be calculated based on the following: - Chemical composition of welded joint 22, - Characteristics of pre-coated steel sheets 1, 2 and substrates 3, 4 - Filler metal proportions and composition, if used

[0143] The step of heating the welded blank 15 further comprises heating the welded steel blank 15 to a heat treatment temperature T t The method includes a step of holding the mixture at 40° C. for a period of time comprised between 2 and 10 minutes.

[0144] At the end of the heating process, the welded steel blank 15 has been heated to a temperature that is at least 10° C. lower than the fully austenitizing temperature Ac3(WJ) of the welded joint 22, so that the microstructure of the welded joint 22 is not fully austenitic. The intercritical ferrite fraction in the welded joint 22 is approximately 10° C. below the heat treatment temperature T t and the fully austenitizing temperature Ac3(WJ) of the weld joint 22. In particular, at the end of the heating process, the intercritical ferrite fraction α IC is 15% or more, and the maximum transformation interval ferrite fraction α max IC At least 5% lower than (15%≦α IC ≦α max IC -5%).

[0145] The maximum transformation interval ferrite fraction is expressed as a percentage and can be determined using the following formula:

number

[0146] As known to those skilled in the art, the intercritical ferrite fraction varies with, for example, the heat treatment temperature T t This can be measured by directly quenching the welded blank 15 after heating to 300° C. After adapted Nital etching, the intercritical ferrite appears as a pale component on a greyish martensite matrix.

[0147] The intercritical ferrite fraction of the weld joint 22 can also be determined by analysis of a manganese elemental mapping image of the sample, which shows the distribution of manganese content within the sample. Such a mapping image can be obtained, for example, by analysis of the sample by electron probe microanalysis (EPMA). In this Mn mapping image, the areas showing a minimum of Mn content correspond to intercritical ferrite regions, while the areas with higher Mn content correspond to phases resulting from the transformation of austenite formed during intercritical annealing. The surface fraction of intercritical ferrite therefore corresponds to the surface fraction of the areas of minimum Mn content in this image. This method is described, for example, in Hanlon, D; Rijkenberg, A; Leunis, E et al.: Quantitative phase analysis of multi-phase steels, PHAST (2007), ISBN 92-79-02658-5, pages 77-79. Indeed, it is known that during intercritical annealing, a partitioning of manganese occurs between austenite and ferrite, with manganese migrating from ferrite to austenite, so that at the end of the intercritical annealing, the Mn content of the intercritical ferrite is strictly less than that of the austenite. Phases that form from the austenite upon subsequent cooling, such as martensite, transformed ferrite and / or bainite, inherit the Mn content of the austenite, while the intercritical ferrite retains its lower Mn content resulting from said partitioning. Thus, on a Mn elemental mapping image, the intercritical ferrite can be distinguished from other phases, especially from other types of ferrite, and corresponds to the region where the Mn content is minimal.

[0148] In the context of this patent application, all percentages relating to microstructure are expressed as surface percentages.

[0149] At the end of the heating process, the microstructure of the substrates 3, 4 of the first and second precoated steel sheets 1, 2 is fully austenitic. In particular, due to the presence of aluminum from the precoats 5, 6 at the weld edges 14 of the precoated steel sheets 1, 2 during welding, the fully austenitizing temperature Ac3 of the substrates 3, 4 is strictly lower than the fully austenitizing temperature Ac3(WJ) of the weld joint 22.

[0150] At the end of the heating process, the welded steel blank 15 is hot formed in a press with a press forming tool into a steel part, for example by hot stamping using an adapted hot stamping tool.

[0151] Preferably, the transfer time between the heat treatment oven and the press forming tool is less than 10 seconds. The transfer time is, for example, comprised between 5 and 10 seconds. The transfer time is selected to be as short as possible in order to avoid metallurgical transformations in the welded blank 15, in particular the formation of ferrite before hot forming.

[0152] The steel part thus formed is then cooled at a cooling rate equal to or greater than the critical martensite or bainite cooling rate of the most hardenable of the substrates 3, 4 of the first and second precoated steel sheets 1, 2.

[0153] Advantageously, the cooling step is performed in a press moulding tool, for example by using a moulding tool equipped with a cooling system, for example comprising cooling channels formed in the press moulding tool.

[0154] According to the invention, at the end of the cooling process, the weld joint 22 is a mixture of martensite and / or bainite and a ferrite fraction of at least 15% and up to the maximum transformation interval α maxIC Intercritical ferrite fraction α at least 5% lower than max IC (15%≦α IC ≦αmax IC -5%). max IC can be determined as described above.

[0155] At the end of the cooling step, at least one of the substrates 3, 4 has a predominantly martensite and / or bainite microstructure, which arises during the cooling step as a result of transformation of the austenite formed during the heating step.

[0156] According to one example, both substrates 3, 4 have a predominantly martensite and / or bainite structure.

[0157] In this context, "predominantly" means that the microstructure consists of martensite and / or bainite, with a maximum of 5% ferrite.

[0158] The invention also relates to press-hardened laser-welded steel parts obtained using the above method.

[0159] The part is in particular a crash management part, for example an anti-intrusion or impact absorbing part, a structural part or a part which contributes to the safety of the motor vehicle.

[0160] The press hardened laser welded steel part comprises a first coated steel part portion and a second coated steel part portion joined by a weld joint 22 as described above.

[0161] More specifically, the first coated steel part and the second coated steel part result from hot pressing and cooling in a press-forming tool of the first and second pre-coated steel sheets 1, 2, respectively.

[0162] More specifically, each coated steel component portion includes a steel substrate having on at least one of its major surfaces an aluminum-containing coating comprising iron and at least 30% by weight aluminum.

[0163] In particular, the aluminium-containing coat of the first and second steel component parts results from at least partial alloying of the precoats 7, 8 during hot press forming.

[0164] The substrates of the first and second steel component parts have the compositions described above for the pre-coated steel sheets 1, 2. They result from hot pressing and cooling of the substrates 3, 4 of the pre-coated steel sheets 1, 2.

[0165] The substrate of the first coated steel component part has a maximum tensile strength Ts1 strictly greater than the maximum tensile strength Ts2 of the substrate of the second coated steel component part.

[0166] For example, a first coated steel component portion has a first thickness and a second coated steel component portion has a second thickness, and the product of the first thickness and the maximum tensile strength of the first coated steel component portion is strictly greater than the product of the second thickness and the maximum tensile strength Ts2 of the second coated steel component portion.

[0167] The weld joint 22 has an aluminum content between 0.5% and 1.25% by weight.

[0168] The weld joint 22 is a mixture of martensite and / or bainite and a ferrite fraction of 15% or more and a maximum transformation interval α max IC Intercritical ferrite fraction α at least 5% lower than IC (15%≦α IC ≦α max IC -5%).

[0169] Maximum transformation interval ferrite fraction α max IC can be determined as described above.

[0170] On press-hardened laser-welded steel parts, the proportion of filler metal β added to the weld pool during the welding operation is determined by the aluminum content Al of the weld joint 22 by any of the applied methods. weld It can be determined by measuring the aluminum content of the coating of the welded steel plate.coating Knowing this, and considering that the amount of aluminum in the filler metal is negligible, Eq.

number

number

[0171] The ultimate tensile strength of the welded joint 22 is equal to or greater than the ultimate tensile strength of the weakest base material 4 after press hardening.

[0172] The steel on at least one side of the weld joint 22, which corresponds to the steel of at least the first substrate 3, has a predominantly martensite and / or bainite structure. For example, the steel on both sides of the weld joint 22, which corresponds to the steel of the first substrate 3 and the steel of the second substrate 4, has a predominantly martensite and / or bainite structure.

[0173] The inventors of the present invention have surprisingly found that when the welded blank 15 is heat treated under the above conditions, the maximum tensile strength of the welded joint 22 is strictly greater than the maximum tensile strength of the substrate 4 of the second pre-coated steel sheet 2, i.e. the substrate having the lowest maximum tensile strength. Thus, when a tensile test is carried out in a direction perpendicular to the welded joint 22, the part obtained after the above heat treatment does not break at the welded joint 22, even though the structure of the welded joint 22 after the heat treatment is not completely martensite or bainitic.

[0174] The method according to the invention is therefore particularly advantageous, since it allows to obtain satisfactory mechanical properties at low cost. Indeed, when precoated steel sheets with an aluminum-containing precoat are welded together, it is no longer necessary to adjust the composition of the weld joint, for example by removing the precoat on both sides of the precoated steel sheet or by adding large amounts of austenite-forming elements to the weld using a filler material such as a filler wire, so that the full austenitization temperature of the weld joint is below the full austenitization temperature of the base material. In particular, avoiding the removal of the precoat on both sides of the steel sheet reduces the total processing time. Furthermore, by reducing the amount of austenite-forming elements that must be added by the filler material, or even avoiding the use of filler material, the production costs are significantly reduced and problems resulting from the addition of a high proportion of filler material, in particular related to the geometry of the weld joint and to obtaining a homogeneous mixture between the material from the precoated steel sheet and the material from the filler material of the weld joint, are prevented. EXAMPLES

[0175] The inventors of the present invention conducted experiments E1 to E36 in which welded steel blanks 15 were produced using precoated steel sheets 1 and 2. Each of the precoated steel sheets 1 and 2 had a substrate 3 and 4 having the following composition (see Table 5), and a precoat 7 and 8 formed by hot-dip galvanization on both main surfaces, the precoat 7 and 8 comprising a metal alloy layer 11 and 12 containing 88% by weight of aluminum, 10% by weight of silicon and 2% by weight of iron.

[0176] The total weight per unit area of ​​the precoats 7, 8 on both main surfaces of each precoated steel sheet 1, 2 is 150 g / m 2 It was.

[0177] After removing the metal alloy layers 11, 12 from only one of the main surfaces 5, 6 of the precoated steel sheets 1, 2 and leaving the intermetallic compound alloy layers 9, 10 intact, the total weight per unit area of ​​the remaining precoats 7, 8 on each of the precoated steel sheets 1, 2 is 100 g / m 2 It was.

[0178] The composition of the substrate used in the experiments is disclosed below in Table 3. The composition of the filler wire used in the experiments is shown below in Table 4.

[0179] [Table 3]

[0180] [Table 4]

[0181] In Tables 3 and 4 above, the compositions are expressed in weight percent.

[0182] Furthermore, for each of the compositions in Tables 3 and 4, the balance of the composition is iron and unavoidable impurities.

[0183] A "-" means that the composition contains at most trace amounts of that element.

[0184] The fully austenitized temperature Ac3 and the maximum tensile strength Ts of the above-mentioned base materials S1, S2 and S3 are as follows. S1: 834℃, Ts=1500MPa S2: 858℃, Ts=1050MPa S3: 806℃, Ts=700MPa

[0185] The precoated steel sheets 1 and 2 were butt-welded by laser welding using a disk laser with an output of 5.6 kW or a YAG laser with an output of 4 kW.

[0186] In all cases, a protective atmosphere consisting of helium or argon was used to avoid oxidation and decarburization of the area where the weld was being made, as well as boron nitride formation in the weld joint and possible low-temperature cracking due to hydrogen absorption. The gas flow rate was ≥ 15 L / min.

[0187] Next, the welded blank 1 is subjected to a heat treatment at a temperature T tand held at this temperature for 6 minutes, the blank was transferred to a hot press forming tool with a transfer time selected to prevent the formation of ferrite between the heating oven and the hot forming tool, and then subjected to a heat treatment of cooling in the press forming tool for 1 minute at a cooling rate of 30°C / s or more to obtain a press hardened blank.

[0188] The experimental conditions used for Experiments E1 to E36 are summarized in Tables 5 and 6 below.

[0189] Tensile specimens were then cut from the thus obtained heated blank in a direction perpendicular to the weld joint.

[0190] Tensile tests were carried out at ambient temperature (approximately 20°C) on longitudinal tensile specimens of type EN 12.5x50 (240x30 mm) drawn parallel to the rolling direction using the methods disclosed in the following standards: NF EN ISO 4136 and NF ISO 6892-1. Five tensile tests were carried out for each heat-welded blank.

[0191] The results of the tensile tests are shown in Table 6 below under the column entitled "Failure Location" which indicates where failure occurred during the tensile test.

[0192] In this section, - "BM" refers to base metal failure, i.e., failure of one of the substrates of a precoated plate; - "Weld" refers to the failure of a welded joint. - "Mix" refers to the case where some of the tensile specimens fail at the weld joint and others fail at the base metal.

[0193] [Table 5] TIFF2024167263000020.tif102157TIFF2024167263000021.tif22157

[0194] In Table 5 above, 150 g / m 2The precoat weight of 100 g / m corresponds to the case where no preparation step was performed before welding, i.e. the precoat remained intact on both main faces of the precoated steel sheet at the time of welding, whereas 2 The precoat weight corresponds to the case where the precoated steel sheets are produced before welding by removing the metal alloy layers 11, 12 from only one main surface of each of the precoated steel sheets 1, 2, leaving the intermetallic compound alloy layers 9, 10 intact.

[0195] [Table 6] TIFF2024167263000023.tif101157TIFF2024167263000024.tif14157

[0196] In Tables 5 and 6 above, examples that are not in accordance with the present invention are underlined.

[0197] These results show that when the welded blank 15 is heated to a heat treatment temperature included in the above temperature range, where the holding time at the heat treatment temperature before press forming and cooling is between 2 and 10 minutes, failure occurs in the weakest base metal of the assembly (the "base material of the second precoated steel sheet" in Tables 5 and 6 above) but not in the welded joint 22 (Experiments E1, E2, E5, E10, E12, E13, E16, E18, E22 and E29 to E32).

[0198] Conversely, the minimum heat treatment temperature T min It is observed that for heat treatment temperatures strictly less than +15°C and holding times at said heat treatment temperature comprised between 2 and 10 minutes, failure always occurs at the welded joint 22 (experiments E3, E4, E6 to E8, E14, E15, E17, E19, E21, E23, E27 to E27 and E33 to E36) or at least in part of the tensile specimens for the experiments considered (experiments E9, E11, E20, E24 and E28, reference "mix" in the table).

[0199] The inventors further confirmed that in all experiments according to the present invention, the weld joint 22 had a strength of 15% to αmax IC -5% fraction of intercritical ferrite, α IC It was noted that the alloy had a microstructure containing

[0200] These results prove that when the welded blank 15 is heat treated using the heat treatment conditions according to the invention, the welded joint 22 has a maximum tensile strength strictly greater than that of the weakest substrate corresponding to the substrate 4 of the second pre-coated steel sheet 2. It is therefore this substrate 4 and not the welded joint 22 that forms the weakest zone of the part. For this reason, failure occurs in the substrate 4 of the second pre-coated steel sheet 2 and not in the welded joint 22 itself. These results are surprising, since the welded joint 22 is not fully austenitized and therefore does not have a predominantly martensite and / or bainite microstructure after heat treatment.

[0201] The method according to the invention is therefore particularly advantageous as it makes it possible to determine the optimum method parameters (including the minimum heat treatment temperature and the amount of filler metal added) in order to obtain a part with satisfactory properties while minimizing manufacturing costs and times.

Claims

1. A method for manufacturing a press-hardened laser-welded steel part, comprising the following consecutive steps, namely - A step of providing a first pre-coated steel sheet (1) and a second pre-coated steel sheet (2), wherein each of the first and second pre-coated steel sheets (1, 2) contains a steel substrate (3, 4), and at least one of the first and second pre-coated steel sheets (1, 2) has an aluminum-containing pre-coat (7, 8) containing at least 50% by weight of aluminum on at least one of its main surfaces, The first pre-coated steel sheet (1) has a first thickness (t １ ), and the second pre-coated steel sheet (2) has a second thickness (t ２ ). The base material (3) of the first pre-coated steel sheet (1) has a maximum tensile strength (Ts ２ ), after press hardening, that is strictly greater than the maximum tensile strength (Ts １ ) of the base material (4) of the second pre-coated steel sheet (2) after press hardening, and The first thickness (t １ ), and the maximum tensile strength (Ts １ ) of the first pre-coated steel sheet (1) after press hardening, the product of which is strictly greater than the product of the second thickness (t ２ ) and the maximum tensile strength (Ts ２ ) of the second pre-coated steel sheet (2) after press hardening, A providing step in which the maximum tensile strength (Ts1) of the substrate (3) of the first pre-coated steel sheet (1) after press hardening is between 1400 MPa and 1600 MPa or between 1700 MPa and 2000 MPa, and the maximum tensile strength (Ts2) of the substrate (4) of the second pre-coated steel sheet (2) after press hardening is between 500 MPa and 700 MPa or between 1000 MPa and 1200 MPa, and then, - A welding step of butting and welding the first pre-coated steel sheet (1) and the second pre-coated steel sheet (2) using laser welding to obtain a weld joint (22) between the first and second pre-coated steel sheets (1, 2), thereby obtaining a welded blank (15), the welding step optionally including the use of a filler material (20), and the weld joint (22) having an aluminum content between 0.5% and 1.25% by weight, the butting and welding step, - A step of heating the welding blank (15) to the heat treatment temperature (T ｔ ), wherein the heat treatment temperature (T ｔ ) is at least 10 °C lower than the complete austenitization temperature (Ac3 (WJ)) of the welded joint (22) and at least 15 °C higher than the minimum temperature T ｍｉｎ , where 【Number 1】 wherein, Ac3 (WJ) is the complete austenitization temperature of the weld joint (22), expressed in °C, and Al is the aluminum content of the weld joint (22), expressed in % by weight, α ｍａｘ ＩＣ is the maximum transformation range ferrite content of the welded joint (22) calculated by the following formula, 【Number 2】 wherein, Ts １ is the maximum tensile strength of the strongest base material (3) after press hardening, expressed in MPa, Ts ２ is the maximum tensile strength of the weakest base material (4) after press hardening, expressed in MPa, C ＦＷ is the carbon content of the filler metal, expressed in wt%, β is the ratio of the filler material added to the weld pool, included between 0 and 1, ρ is the ratio of the thickness of the pre-coated steel sheet (2) including the weakest base material to the thickness of the pre-coated steel sheet (1) including the strongest base material (ρ = t ２ / t １ ), and The step of holding the welded blank (15) at a heat treatment temperature (T ｔ ) for a time included between 2 and 10 minutes - A step of press-forming the welded blank (15) into a steel part, and - A step of cooling the steel part thus formed at a cooling rate equal to or higher than the critical martensite or bainite cooling rate of the most hardenable substrate among the substrates (3, 4) of the first and second pre-coated steel sheets (1, 2) to obtain a press-hardened welded steel part A method comprising the above steps.

2. After press hardening, the maximum tensile strength (Ts １ of the base material (3) of the first pre-coated steel sheet (1) and the maximum tensile strength (Ts ２ of the base material (4) of the second pre-coated steel sheet (2) is 1.2 or more. The method according to claim 1.

3. The method according to claim 1 or 2, wherein the carbon content of the substrate (3) of the first pre-coated steel sheet (1) is at least 0.05% by weight higher than the carbon content of the substrate (4) of the second pre-coated steel sheet (2).

4. The method according to any one of claims 1 to 3, wherein each of the first and second pre-coated steel sheets (1, 2) provided in the providing step includes an aluminum-containing pre-coat (7, 8) containing at least 50% by weight of aluminum on at least one of its main surfaces (5, 6).

5. The method according to any one of claims 1 to 4, wherein the first and second pre-coated steel sheets (1, 2) provided in the providing step include an aluminum-containing pre-coat (7, 8) containing at least 50% by weight of aluminum on both of their main surfaces (5, 6).

6. The method according to any one of claims 1 to 5, wherein during butt welding, the aluminum-containing pre-coat (7, 8) remains completely provided on both main surfaces (5, 6) of at least one of the first pre-coated steel sheet (1) and the second pre-coated steel sheet (2), for example, each of the first and second pre-coated steel sheets (1, 2).

7. Before the butt welding step, a step of removing the aluminum-containing pre-coat (7, 8) on at least one main surface (5, 6) of at least one of the welding edge portions (14) of the first and second pre-coated steel sheets (1, 2) over at least a part of its thickness, if the theoretically average aluminum content rate (Al th weld) in the welded joint (22) obtained by butt welding the first and second pre-coated steel sheets (1, 2) provided in the providing step by optionally using a filler metal containing at most 0.05% by weight of aluminum is strictly greater than 1.25% by weight, so that the theoretically average aluminum content rate (Al th weld) of the welded joint (22) obtained by butt welding the first and second pre-coated steel sheets (1, 2) thus produced by optionally using a filler metal containing at most 0.05% by weight of aluminum is included between 0.5% by weight and 1.25% by weight, the removing step The method according to any one of claims 1 to 6, further comprising:

8. Using a filler metal containing up to 0.05% by weight of aluminum, even when the theoretically average aluminum content of the welded joint (22) obtained by buttwelding the first and second pre-coated steel sheets (1, 2) provided in the providing step is included between 0.5% by weight and 1.25% by weight, prior to the buttwelding, by removing the aluminum-containing pre-coat (7, 8) on at least one main surface (5, 6) thereof over at least a part of its thickness, the method according to claim 7, further comprising the step of producing at least one welded edge portion (14) of at least one of the first and second pre-coated steel sheets (1, 2).

9. Before the buttwelding step, the step of producing at least one welded edge portion (14) of at least one of the first and second pre-coated steel sheets (1, 2) by removing the aluminum-containing pre-coat (7, 8) on at least one main surface (5, 6) of the first and second pre-coated steel sheets (1, 2) over at least a part of its thickness, The method according to any one of claims 1 to 6, further comprising:

10. For at least one of the first and second pre-coated steel sheets (1, 2), the steel of the base material (3, 4) is, by weight, 0.10% ≤ C ≤ 0.5% 0.5% ≤ Mn ≤ 3% 0.1% ≤ Si ≤ 1% 0.01% ≤ Cr ≤ 1% Ti ≤ 0.2% Al ≤ 0.1% S ≤ 0.05% P ≤ 0.1% B ≤ 0.010% The method according to any one of claims 1 to 9, comprising the balance being iron and impurities resulting from the manufacturing process.

11. For at least one of the first and second pre-coated steel sheets (1, 2), the steel of the base material (3, 4) is, by weight, 0.15% ≤ C ≤ 0.25% 0.8% ≤ Mn ≤ 1.8% 0.1% ≤ Si ≤ 0.35% 0.01% ≤ Cr ≤ 0.5% Ti ≤ 0.1% Al ≤ 0.1% S ≤ 0.05% P ≤ 0.1% B ≤ 0.005% The method according to claim 10, comprising the balance being iron and impurities resulting from the manufacturing process.

12. For at least one of the first and second pre-coated steel sheets (1, 2), the steel of the base material (3, 4) is, by weight, 0.040% ≤ C ≤ 0.100% 0.70% ≤ Mn ≤ 2.00% Si ≤ 0.50% S ≤ 0.009% P ≤ 0.030% 0.010% ≤ Al ≤ 0.070% 0.015% ≤ Nb ≤ 0.100% Ti ≤ 0.080% N ≤ 0.009% Cu ≤ 0.100% Ni ≤ 0.100% Cr ≤ 0.2% Mo ≤ 0.100% Ca ≤ 0.006% The method according to any one of claims 1 to 11, comprising the balance being iron and impurities resulting from production.

13. For at least one of the first and second pre-coated steel sheets (1, 2), the steel of the base material (3, 4) is, by weight, 0.06% ≤ C ≤ 0.100% 1.4% ≤ Mn ≤ 1.9% 0.2% ≤ Si ≤ 0.5% 0.010% ≤ Al ≤ 0.070% 0.04% ≤ Nb ≤ 0.06% 3.4 × N ≤ Ti ≤ 8 × N 0.02% ≤ Cr ≤ 0.1% 0.0005% ≤ B ≤ 0.004% 0.001% ≤ S ≤ 0.009% The method according to any one of claims 1 to 12, comprising the balance being iron and impurities resulting from production.

14. For at least one of the first and second pre-coated steel sheets (1, 2), the steel of the base material (3, 4) is, by weight, 0.24% ≤ C ≤ 0.38% 0.40% ≤ Mn ≤ 3% 0.10% ≤ Si ≤ 0.70% 0.015% ≤ Al ≤ 0.070% 0% ≤ Cr ≤ 2% 0.25% ≤ Ni ≤ 2% 0.015% ≤ Ti ≤ 0.10% 0% ≤ Nb ≤ 0.060% 0.0005% ≤ B ≤ 0.0040% 0.003% ≤ N ≤ 0.010% 0.0001% ≤ S ≤ 0.005% 0.0001% ≤ P ≤ 0.025% Comprising, the content ratio of titanium and nitrogen satisfies the following relationship, Ti / N > 3.42 The content ratio of carbon, manganese, chromium and silicon satisfies the following relationship, 【Number 3】 The steel optionally contains one or more of the following elements, 0.05% ≤ Mo ≤ 0.65% 0.001% ≤ W ≤ 0.30% 0.0005% ≤ Ca ≤ 0.005% The method according to any one of claims 1 to 13, comprising the balance being iron and impurities inevitably resulting from production.

15. The method according to any one of claims 1 to 14, wherein the laser welding is carried out using a shielding gas.

16. The method according to claim 15, wherein the shielding gas is helium or argon, or a mixture of these gases.

17. The method according to any one of claims 1 to 16, wherein the first and second pre-coated steel sheets (1, 2) have different thicknesses.

18. The method according to any one of claims 1 to 17, wherein the welding is carried out without using a filler metal.

19. The welding is carried out using a filler metal, and the filler metal preferably has, by weight, the following composition, namely 0.1% ≤ C ≤ 1.2% 0.01% ≤ Mn ≤ 10% 0.02% ≤ Ni ≤ 7% 0.02% ≤ Cr ≤ 5% 0.01% ≤ Si ≤ 2% Optionally, trace amount ≤ Mo ≤ 1% trace amount ≤ Ti ≤ 0.1% trace amount ≤ V ≤ 0.1% trace amount ≤ B ≤ 0.01% trace amount ≤ Nb ≤ 0.1% trace amount ≤ Al ≤ 0.05% The method according to any one of claims 1 to 17, having the above and the balance being iron and impurities inevitably resulting from the production.

Images