Welded steel blank and method for producing associated welded steel blank

Abstract

To provide a method for producing welded steel blank (1).SOLUTION: This method comprises butting and joining sheets (2) using a filler wire (20) so as to produce a welded joint (22) having an aluminum content AlWJ included between 0.1 wt.% and 1.2 wt.%, each being provided with a steel base material (3) having a precoating (5) on each of two main surfaces (4) and each sheet (2) being provided with a removal zone (18) from which the precoating (5) is removed by a removal rate at a welded edge on each main surface (4). The composition of the wire (20) and the ratio of the added wire (20) are set such that the welded joint (22) has (a) a hardening coefficient FTWJ and (b) AlWJ has a nickel content NiWJ≤14-3.4xAlWJ and a chrome content CrWJ≤5-2xAlWJ which are aluminum contents of the welded joint (22).SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for producing a welded steel blank, the welded steel blank thus obtained, a method for producing a welded, hot-press-formed and cooled steel part from the welded steel blank, and the welded, hot-press-formed and cooled steel part thus obtained. [Background technology]

[0002] Methods for producing welded parts from steel sheets of different composition and / or thickness butt-welded to one another are known from the prior art. More specifically, the welded blank is usually heated to a temperature that allows the austenitization of the steel, then hot-formed in a hot press forming tool and cooled. The composition of the steel can be selected both to allow the subsequent heating and forming operations and to give the welded steel part high mechanical strength, high impact strength and good corrosion resistance.

[0003] Steel parts of this type are used in particular in the motor vehicle industry, and more particularly for the manufacture of anti-intrusion parts, structural parts or parts which contribute to the safety of motor vehicles.

[0004] To prevent corrosion, steel sheets are precoated with an aluminum-based precoating by hot-dipping in an aluminum-containing bath. If the steel sheets are welded without any prior preparation, the aluminum-based precoating is diluted with the steel substrate in the molten metal during the welding operation. In the range of aluminum content of the precoating, two phenomena can then occur:

[0005] When the aluminum content in the molten metal is locally high, intermetallic compounds are formed in the weld joint as a result of the dilution of some of the pre-coating in the molten metal and the alloying that occurs during the subsequent heating of the weld joint before the hot forming step. These intermetallic compounds are the most likely sites for incipient cracking.

[0006] Additionally, aluminum tends to increase the austenitization temperature (Ac3) of the weld joint, and this modification of the austenite domains becomes even more important due to the high levels of aluminum in the weld joint, which in some cases can prevent the complete austenitization of the weld joint that would occur during hot stamping and heating prior to forming, which is the first step required to obtain a martensitic structure in the weld joint after hot press forming and cooling.

[0007] Furthermore, aluminum also has a detrimental effect on the hardenability of the weld joint, since it increases the critical cooling rate required to obtain a martensite or bainite structure in the weld joint during cooling.

[0008] As a result, it is no longer possible to obtain martensite or bainite during cooling after hot forming, and the weld joint thus obtained contains ferrite. The weld joint then exhibits a hardness and mechanical strength less than those of the two adjacent sheets and therefore constitutes the weakest area of ​​the part.

[0009] EP 2007545 describes a solution consisting of removing a superficial layer of a metal alloy at the weld edge of a pre-coated steel sheet intended to be at least partially incorporated into the weld metal zone. The removal can be carried out by brushing or by using a laser beam. The intermetallic alloy layer is preserved to ensure corrosion resistance and to prevent the phenomena of decarburization and oxidation during the heat treatment prior to the forming operation. In this case, the effect of aluminum is reduced by the local elimination of the superficial layer of the coating.

[0010] However, the inventors of the present patent application have observed that even if the surface layer of the metal alloy is removed at the weld edge of the pre-coated steel sheet, the weld joint may still have insufficient mechanical properties. Indeed, the aluminum concentration in the weld joint may still be too high due to the presence of protrusions from the coating on the side of the steel sheet at the weld edge resulting from the removal operation and / or in the case of thin steel sheets having a thickness of, for example, 1.0 mm or less.

[0011] EP2737971, US2016 / 0144456 and WO2014075824 attempt to provide a method for welding precoated sheets without prior removal of the precoat, using a filler wire containing austenite stabilizing elements such as carbon, manganese or nickel, with the aim of obtaining a fully martensitic structure in the weld joint after hot press forming and cooling, despite the presence of aluminum in the weld resulting from melting of the precoat.

[0012] However, these methods are not entirely satisfactory since they address only one of the problems related to the presence of aluminum in the weld pool, namely the compensation of the austenitizing temperature (Ac3) and, in some cases, the use of high-carbon filler wires, which can induce segregation in the weld joint. In fact, the inventors of the present invention have found that with the methods disclosed in the above-mentioned documents, it is not possible to obtain satisfactory mechanical properties in the parts obtained after hot press forming and cooling, especially for aluminum contents of 0.7% by weight or more in the weld joint. In particular, in such parts there is a high risk of the weld joint breaking under a tensile test in the weld transverse direction.

[0013] The methods disclosed in WO2015 / 086781 and EP2942143 also address this problem and describe welding of pre-coated steel sheets using specific welding methods with specific filler materials.

[0014] More specifically, WO 2015 / 086781 proposes using twin-spot laser welding whilst supplying a filler material in the form of a metal powder having the following composition in weight percentages: C: 0-0.03 wt%, Mo: 2.0-3.0 wt%, Ni: 10-14 wt%, Mn: 1.0-2.0 wt%, Cr: 16-18 wt% and Si: 0.0-1.0 wt%, the remainder being iron.

[0015] EP 2942143 proposes using hybrid laser / arc welding using an arc welding torch positioned in front of the laser beam while supplying a filler material in the form of a filler wire having the following composition: C: 0-0.3% by weight, Mo: 0-0.4% by weight, Ni: 6-20% by weight, Mn: 0.5-7% by weight, Cr: 5-22% by weight and Si: 0-1.3% by weight, Nb: 0-0.7% by weight, the balance being iron.

[0016] These methods are also unsatisfactory: in fact, the inventors of the present invention have observed that the use of filler wires as described therein leads to a high risk of fracture of the part after hot pressing and cooling in the zone directly adjacent to the weld.

[0017] Furthermore, the use of hybrid laser-arc welding is undesirable because it is not possible to reach the same welding speeds as laser welding, thus reducing the overall productivity of the process.

[0018] Furthermore, powder additions are generally more difficult to implement in a large scale industrial setting than filler wires.

[0019] All the above-mentioned methods based on the addition of filler material only specify the chemical composition range of the filler material, and since the welding parameters and conditions affect the filler material velocity, one filler wire may induce very different chemical compositions in the weld joint. Therefore, the description of the composition of the filler wire alone does not seem to be sufficient to solve the above-mentioned problem. [Prior art documents] [Patent documents]

[0020] [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 [Patent Document 5] International Publication No. 2015 / 086781 [Patent Document 6] European Patent Application Publication No. 2942143 Summary of the Invention [Problem to be solved by the invention]

[0021] It is therefore an object of the present invention to provide a method for producing a welded steel blank from two such precoated sheets, which, after hot press forming and cooling, makes it possible to obtain a part with satisfactory crash performance properties, even with a relatively high aluminium content in the welded joint. [Means for solving the problem]

[0022] To this end, it is particularly desirable to avoid complete brittle fracture in the weld joint.

[0023] To this end, the present invention provides a continuous - providing two precoat sheets, each precoat sheet comprising a steel substrate (3) having a precoating on each of its two main faces, the precoating comprising an intermetallic alloy layer comprising at least iron and aluminium, and optionally a metal alloy layer extending on top of the intermetallic alloy layer, the metal alloy layer being a layer of aluminium, a layer of aluminium alloy or a layer of aluminium-based alloy; Each precoat sheet is provided on each of its main faces with a removal zone at the weld edge intended to be at least partially incorporated into the weld joint, in which the precoat has been removed over a removal rate comprised between 30% and 100% of the thickness of the precoat; - butt-welding the precoated sheets using a filler wire to produce a weld joint at the junction between the precoated sheets, the weld joint having an average aluminum content Al WJ is contained between 0.1% by weight and 1.2% by weight, A method for producing a welded steel blank, comprising: - The composition of the filler wire and the proportion of filler wire added to the weld pool determine whether the weld joint thus obtained is (a)

[0024]

number

[0025] According to particular embodiments, the method may comprise one or more of the following features, taken alone or according to any technically possible combination: the substrate steel of at least one of the precoated sheets, and for example the substrate steel of each precoated sheet, by weight: 0.10%≦C≦0.5% 0.5%≦Mn≦4.5% 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 due to manufacturing.

[0026] - the substrate of each of the precoated sheets is made of press-hardenable steel; - the average aluminum content of the welded joint is greater than or equal to 0.15% by weight; - the average aluminum content of the welded joint is not more than 0.8% by weight; the average nickel content of the welded joint is comprised between 0.1% and 13.6% by weight, more precisely between 0.2% and 12.0% by weight; - The welded steel blank, after hot press forming and cooling, has a Charpy energy of 25 J / cm at 20°C. 2 or greater, and the ultimate tensile strength of the hot press-formed and cooled steel welded steel blank is greater than or equal to the ultimate tensile strength of the weakest substrate among the substrates of the precoated sheet, the weakest substrate being such that the product of thickness and ultimate tensile strength after hot press-formed and cooling is the lowest; - the filler wire has a carbon content comprised between 0.01% and 0.45% by weight; - for at least one precoat sheet, and for example for both precoat sheets, the removal rate is strictly less than 100% of the precoating thickness; - for at least one of the precoat sheets, the precoating comprises a metal alloy layer extending on an upper surface of the intermetallic alloy layer, the metal alloy layer being a layer of aluminum, an aluminum alloy or a layer of an aluminum-based alloy, and for at least one of the precoat sheets, and e.g. for both precoat sheets, the metal alloy layer has been removed throughout its thickness, but the intermetallic alloy layer remains intact in removal zones on each major surface of the precoat sheet; - for at least one precoat sheet provided in the providing step, and for example for both precoat sheets, in a removal zone on each main surface of the precoat sheet, the removal percentage is equal to 100% such that the precoating is removed over its entire thickness; - the method further comprises, prior to the providing step, a step of producing two precoated sheets from each initial precoated sheet, which step comprises a substep of obtaining a removal zone on each main surface of each precoated sheet by removing the precoating over a fractional removal rate comprised between 30% and 100% of the thickness of the precoating by laser ablation at the welded edges of the precoated sheets.

[0027] - Producing two precoated sheets includes the steps of: - providing two initial precoated sheets; - arranging the two initial precoat sheets adjacent to each other while leaving a predetermined gap between them; and - simultaneously removing the pre-coating on two adjacent initial pre-coated sheets by laser ablation so as to simultaneously generate removal zones on the adjacent faces of these two initial pre-coated sheets, wherein during said removing step the laser beam overlaps the two adjacent initial pre-coated sheets, and optionally during a welding step the two adjacent pre-coated sheets thus prepared are welded together such that the laser beam spot overlaps the two adjacent pre-coated sheets, the time between the end of the laser ablation and the start of the welding being preferably less than or equal to 10 seconds; - the method further comprises preparing a weld edge of at least one of the precoated sheets prior to butt welding using at least one of the following processing steps: brushing, machining, chamfering and / or beveling; - the welding step is carried out using a laser beam; - The two precoated sheets have the same thickness; - The two pre-coated sheets have different thicknesses; for at least one of the precoated sheets, and for example for both precoated sheets, 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 due to manufacturing.

[0028] - For one of the precoated sheets, the base steel is, by weight: 0.040%≦C≦0.100% 0.80%≦Mn≦2.00% Si≦0.30% 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≦0.100% Mo≦0.100% Ca≦0.006%, the balance being iron and impurities due to manufacturing.

[0029] - For one of the precoated sheets, the base steel 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% wherein the titanium and nitrogen contents satisfy the following relationship: Ti / N>3.42 And the contents of carbon, manganese, chromium and silicon satisfy the following relationships:

[0030]

number

[0031] The welding is carried out using a protective gas, in particular helium and / or argon.

[0032] The present invention further provides a method for producing a welded, hot press formed and cooled steel part, comprising the steps of: - carrying out the method defined above to obtain a welded steel blank; - heating the welded steel blank in order to obtain a fully austenitic structure in the base material of the precoated sheet; - hot press forming the welded steel blank in a press tool to obtain a steel part; - cooling the steel part in the press tool; The present invention relates to a method comprising the steps of:

[0033] According to a particular embodiment of this method for producing a welded, hot press formed and cooled steel part, during the cooling step the cooling rate is equal to or greater than the cooling rate of the most hardenable bainite or martensite of the substrate of the precoated sheet.

[0034] The present invention also relates to a welded steel blank comprising two precoated sheets, each precoated sheet comprising a steel substrate having a precoating on each of its major surfaces, the precoating comprising an intermetallic alloy layer comprising at least iron and aluminium, and optionally a metal alloy layer extending on a top surface of the intermetallic alloy layer, the metal alloy layer being a layer of aluminium, a layer of an aluminium alloy or a layer of an aluminium-based alloy; The precoated sheets are joined by a weld joint, the weld joint having an average aluminum content comprised between 0.1% and 1.2% by weight, the weld joint being (a)

[0035]

number

[0036] According to particular embodiments of the welded steel blank, the welded steel blank comprises one or more of the following characteristics, taken alone or according to any technically possible combination: - the steel substrate of at least one of the precoated sheets, and for example both precoated sheets, is, by weight: 0.10%≦C≦0.5% 0.5%≦Mn≦4.5% 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 production. - the substrate of each of the precoated sheets is made of press-hardenable steel; - for each precoated sheet, the width of the intermediate zone is comprised between 5 μm and 2000 μm from the edge of the welded joint; - for at least one precoat sheet, and for example for both precoat sheets, the removal rate is equal to 100% of the precoating thickness; - for at least one precoat sheet, and for example for both precoat sheets, the removal rate is strictly less than 100% of the precoating thickness; - for at least one of the precoat sheets, and e.g. for both precoat sheets, the precoating comprises a metal alloy layer extending on an upper surface of the intermetallic alloy layer, the metal alloy layer being a layer of aluminum, an aluminum alloy or a layer of an aluminum-based alloy, and for at least one of the precoat sheets, and e.g. for both precoat sheets, the metal alloy layer has been removed throughout its thickness, but the intermetallic alloy layer remains integral in removal zones on each major surface of the precoat sheet; the nickel content of the welded joint is comprised between 0.1% and 13.6% by weight, more precisely between 0.2% and 12.0% by weight; - The welded steel blank, after hot press forming and cooling, has a Charpy energy of 25 J / cm at 20°C. 2and the ultimate tensile strength of the hot press-formed and cooled welded steel blank is equal to or greater than the ultimate tensile strength of the weakest substrate among the substrates of the precoated sheet, the weakest substrate being the substrate having the lowest product of thickness and ultimate tensile strength after hot press-formed and cooling; - After hot press forming and cooling, the maximum hardness change ΔHV(WJ) of the entire weld joint is 1 / 2 the average hardness HV of the weld joint. 平均 (WJ) is 20% or less; - each intermediate zone contains solidification streaks, and the solidification streaks on adjacent major surfaces of the two precoat sheets are symmetrical with respect to a vertical median plane between the two precoat sheets; - each intermediate zone has an inner edge located at the weld joint and an outer edge located away from the weld joint, and the distance between the outer edges of adjacent intermediate zones of the two precoated sheets is constant along the longitudinal direction of the weld joint; for at least one of the precoated sheets, and for example for both precoated sheets, 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; - For one of the precoated sheets, the base steel is, by weight: 0.040%≦C≦0.100% 0.80%≦Mn≦2.00% Si≦0.30% 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≦0.100% Mo≦0.100% Ca≦0.006%, the balance being iron and impurities resulting from manufacturing; - For one of the precoated sheets, the base steel 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% wherein the titanium and nitrogen contents satisfy the following relationship: Ti / N>3.42 And the contents of carbon, manganese, chromium and silicon satisfy the following relationships:

[0037]

number

[0038] The invention further relates to a welded, hot press formed and cooled steel component comprising a first coated steel component portion and a second coated steel component portion, each coated steel component portion comprising a steel substrate having on at least one of its major surfaces a coating comprising at least iron and aluminum; The first and second coated steel component portions are joined by a weld joint, the weld joint having an average aluminum content comprised between 0.1% and 1.2% by weight, the weld joint comprising: (a)

[0039]

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[0040] According to a particular embodiment of the welded, hot press formed and cooled steel part, the welded, hot press formed and cooled steel part may comprise one or several of the following features, taken alone or according to any possible combination: the steel of the substrate of at least one of the first and second steel part parts, and for example the steel of the first and second steel part parts, is, by weight, 0.10%≦C≦0.5% 0.5%≦Mn≦4.5% 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 production, - the substrate of each of the first and second steel component portions is made of press-hardenable steel; - each intermediate zone includes solidification striations, the solidification striations on adjacent major surfaces of the two coated steel part portions being symmetrical with respect to a vertical mid-plane between the two coated steel part portions; - each intermediate zone has an inner edge located at the weld joint and an outer edge located away from the weld joint, the distance between the outer edges of adjacent intermediate zones of the two coated steel part parts being constant along the longitudinal direction of the weld joint; - Average hardness of the welded joint HV 平均(WJ) is 700HV or less; the average nickel content in the welded joint is comprised between 0.1% and 13.6% by weight, more precisely between 0.2% and 12.0% by weight; - The Charpy energy of a welded joint at 20°C is 25 J / cm 2 and the ultimate tensile strength of the welded, hot pressed and cooled steel part is greater than or equal to the ultimate tensile strength of the weakest substrate among the substrates of the coated steel part section, the weakest substrate being the substrate having the lowest product of thickness and ultimate tensile strength; - the maximum hardness change ΔHV(WJ) across the welded joint is not more than 20% of the average hardness HVmean(WJ) of the welded joint; the substrate steel of at least one of the first and second coated steel part parts, and e.g. the substrate steel of the first and second coated steel part parts, 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; - the base steel of one of the first and second coated steel component portions is, by weight, 0.040%≦C≦0.100% 0.80%≦Mn≦2.00% Si≦0.30% 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≦0.100% Mo≦0.100% Ca≦0.006%, the balance being iron and impurities resulting from manufacturing; - the base steel of one of the first and second coated steel component portions 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% wherein the titanium and nitrogen contents satisfy the following relationship: Ti / N>3.42 And the contents of carbon, manganese, chromium and silicon satisfy the following relationships:

[0041]

number

[0042] The invention further relates to the use of the above-mentioned welded, hot-pressed and cooled steel parts for producing anti-intrusion or energy-absorbing parts for motor vehicles.

[0043] The invention will be better understood on reading the following specification, given by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0044] [Figure 1] FIG. 1 is a perspective view of a precoated sheet including a removal zone during precoating around the periphery of the sheet. [Diagram 2] FIG. 2 is a perspective view of an initial precoat sheet. [Diagram 3] 2 is a schematic cross-sectional view of the start of the welding step of the method according to the invention; [Figure 4] 3 is a schematic cross-sectional view of the end of the welding step of the method according to the invention; [Diagram 5] FIG. 1 is a schematic cross-sectional view of a welded steel blank according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] Throughout the patent application, elemental contents are expressed as weight percentages (wt%).

[0046] The present invention relates to a method for producing a welded steel blank 1 .

[0047] The method comprises a first step of providing two precoated sheets 2 .

[0048] As shown in Fig. 1, each precoat sheet 2 has two main surfaces 4 and at least one side surface 13 extending between them from one main surface 4 to the other main surface 4. In the example shown in Fig. 1, the precoat sheet 2 has four side surfaces 13. For example, the side surface 13 forms an angle with one of the main surfaces 4 that is included between 60° and 90°.

[0049] Each precoat sheet 2 comprises a metal substrate 3 having on each of its major surfaces a precoating 5. The precoatings 5 ​​overlie and are in contact with the substrate 3.

[0050] The metal substrate 3 is more particularly a steel substrate.

[0051] The steel of the substrate 3 is more particularly a steel having a ferritic-pearlitic microstructure.

[0052] Preferably, the substrate 3 is made of a steel for heat treatment, more particularly a press-hardenable steel, and for example a manganese-boron steel, such as type 22MnB5 steel.

[0053] According to one embodiment, the steel of the substrate 3 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 remainder being iron and impurities resulting from production.

[0054] More specifically, the steel of the substrate 3 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 remainder being iron and impurities resulting from production.

[0055] According to an alternative embodiment, the steel of the substrate 3 comprises, by weight: 0.040%≦C≦0.100% 0.80%≦Mn≦2.00% Si≦0.30% 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≦0.100% Mo≦0.100% Ca≦0.006% the remainder being iron and impurities resulting from production.

[0056] According to an alternative embodiment, the steel of the substrate 3 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% wherein the titanium and nitrogen contents satisfy the following relationship: Ti / N>3.42, And the contents of carbon, manganese, chromium and silicon satisfy the following relationships:

[0057]

number

[0058] According to one example, the substrates 3 of the two precoat sheets 2 have the same composition.

[0059] According to another example, the substrates 3 of the two precoat sheets 2 have different compositions. In particular, the two substrates 3 have different compositions, each selected from the four compositions described above. For example, the steel of the substrate 3 of one precoat sheet 2 has the first composition described above, and the steel of the substrate 3 of the other precoat sheet 2 has a composition selected from the second, third or fourth composition described above.

[0060] The substrate 3 may be obtained, depending on its desired thickness, by hot rolling and / or cold rolling followed by annealing or by any other suitable method.

[0061] The substrate 3 has a thickness advantageously comprised between 0.8 mm and 5 mm, and more particularly between 1.0 mm and 3.0 mm. The two precoat sheets 2 may have the same or different thicknesses.

[0062] The pre-coating 5 is obtained by hot-dip coating, i.e. by immersion of the substrate 3 in a bath of molten metal.

[0063] The pre-coating 5 comprises at least an intermetallic alloy layer 9 in contact with the substrate 3. The intermetallic alloy layer 9 comprises at least iron and aluminium. The intermetallic alloy layer 9 is formed in particular by reaction of the substrate 3 with the molten metal of the bath. More particularly, the intermetallic alloy layer 9 comprises Fe x -Al y type intermetallic compounds, more specifically Fe2Al5.

[0064] In the example shown in figure 1, the pre-coating 5 further comprises a metal alloy layer 11 extending on top of the intermetallic alloy layer 9. The metal alloy layer 11 has a composition close to the molten metal in the bath. It is formed by the molten metal carried away by the sheet as it travels through the molten metal bath during hot dip coating. The metal alloy layer 11 is a layer of aluminium, or a layer of an aluminium alloy, or a layer of an aluminium-based alloy.

[0065] In this context, an aluminium alloy refers to an alloy containing more than 50% aluminium by weight. An aluminium-based alloy is an alloy in which aluminium is the predominant component by weight.

[0066] For example, the metal alloy layer 11 is a layer of an aluminum alloy further containing silicon. More specifically, the metal alloy layer 11 is, by weight, - 8%≦Si≦11%, - 2%≦Fe≦4%, with the balance being aluminum and possible impurities.

[0067] The metal alloy layer 11 has a thickness comprised, for example, between 19 μm and 33 μm, or between 10 μm and 20 μm.

[0068] 1, in which the pre-coating 5 comprises a metal alloy layer 11, the thickness of the intermetallic alloy layer 9 is generally of the order of a few micrometers. In particular, its average thickness is typically comprised between 2 and 8 micrometers.

[0069] The particular structure of the pre-coating 5 comprising the intermetallic alloy layer 9 and the metal alloy layer 11 obtained by hot dip coating is disclosed in particular in EP2007545.

[0070] According to another embodiment, the pre-coating 5 comprises only the intermetallic alloy layer 9 described above. In this case, the thickness of the intermetallic alloy layer 9 is comprised, for example, between 10 μm and 40 μm. Such a pre-coating 5 consisting of an intermetallic alloy 9 can for example be obtained by subjecting the pre-coating 5 comprising the intermetallic alloy layer 9 and the metal alloy layer 11 disclosed above to a pre-alloying treatment. Such a pre-alloying treatment is carried out at a temperature and a holding time chosen so as to alloy the pre-coating 5 with the substrate 3 over at least a part of the thickness of the pre-coating 5. More specifically, the pre-alloying treatment can comprise the following steps: heating the sheet to a pre-alloying temperature comprised between 700 ° C and 900 ° C and keeping the pre-alloy sheet at this temperature for a time comprised between 2 minutes and 200 hours. In this case, the intermetallic alloy layer 9 can be composed of any of the following alloys: Fe2Al5, FeAl3, FeAl, Fe6Al. 12 It may be composed of different intermetallic sub-layers, such as Si5 and FeAl3 sub-layers.

[0071] Advantageously, as shown in FIG. 1, the substrate 3 has on both of its main faces 4 a pre-coating 5 as described above.

[0072] 1 , for each precoat sheet 2, the precoating 5 is removed at the weld edge 14 of the precoat sheet 2 on each major surface 4 of the precoat sheet 2 to produce a removal zone 18 at the weld edge 14. More specifically, the precoating 5 is removed over a removal percentage F comprised between 30% and 100% (inclusive) of the thickness of the precoating 5.

[0073] Weld edge 14 comprises a peripheral portion of precoated sheet 2 that is intended to be at least partially incorporated into weld joint 22 during butt welding. More specifically, weld edge 14 comprises a side 13 of precoated sheet 2 and a portion of precoated sheet 2 extending from side 13 and comprising a portion of precoat 5 and a portion of substrate 3.

[0074] The removal of the removal rate F of the precoating 5 at the weld edge 14 is preferably carried out using a laser beam, i.e. by laser ablation.

[0075] The removal zone 18 may extend from the side surface 13 of the sheet 2 over a width comprised between 0.5 mm and 3 mm.

[0076] Advantageously, the removal rate F is strictly less than 100%, which means that only a part of the pre-coating 5 is removed in the removal zone 18 and a part of it remains.

[0077] For example, in the embodiment shown in Figure 1, in the removal zone 18, the metal alloy layer 11 is removed, but the intermetallic alloy layer 9 remains over at least a portion of its thickness. In this case, the remaining intermetallic alloy layer 9 protects the area of ​​the welded blank 1 directly adjacent the weld joint 22 from oxidation and decarburization during subsequent hot press forming steps, and from corrosion during service life.

[0078] According to an embodiment, during the removal step, the intermetallic alloy layer 9 remains in its integrity or over a portion of its initial thickness that is strictly less than 100%, such as, for example, only more than 60%, 80% or 90% of its initial thickness.

[0079] According to an alternative embodiment (not shown), during the removal step, the pre-coating 5 is removed over its entire thickness in the removal zone 18. In this embodiment, the removal rate corresponds to 100% of the thickness of the pre-coating 5. In this embodiment, no pre-coating 5 is present in the removal zone 18.

[0080] More specifically, the method includes, prior to the providing step, the step of producing two precoated sheets 2 as shown in FIG. 1 from respective initial precoated sheets 2' as shown in FIG.

[0081] The initial precoat sheet 2' has substantially the same geometric shape and composition as the precoat sheet 2, the only difference being the absence of the removal zone 18. In other words, the precoating 5 of the initial precoat sheet 2' remains integral on both major surfaces 4 of the initial precoat sheet 2'. It completely covers the two major surfaces of the initial precoat sheet 2'.

[0082] This step includes the sub-step of obtaining a removal zone 18 on each main face 4 of each precoat sheet 2 by removing the precoating 5 over a removal rate F at the weld edge 14 by laser ablation.

[0083] Optionally, the method further comprises preparing a weld edge 14 of at least one of the precoat sheets 2, and for example both precoat sheets 2.

[0084] Preparation of the weld edge 14 may include at least one of the following processing steps: - brushing of the welded edges 14, - machining of the welded edges 14, chamfering of the weld edge 14, and / or - beveling of the weld edge 14;

[0085] The brushing step makes it possible to at least partially remove traces of the pre-coating 5 on the weld edge 14, and more particularly on the side surface 13, resulting from the mechanical cutting operation and / or from the removal of the pre-coating 5 at the weld edge 14.

[0086] By chamfering or beveling the weld edge 14 , it is possible to increase the amount of filler material added without resulting in a thickened weld joint 22 .

[0087] Machining of the weld edge 14 is performed when the shape of the weld edge 14 before machining is not straight enough for laser welding.

[0088] The method further comprises the step of butt-welding the precoated sheet 2 using a filler wire 20, after optional preparation of the weld edge 14, to obtain a welded steel blank 1.

[0089] 3 and 4 show two stages of the welding step to produce a welded steel blank 1.

[0090] In the example shown in Figures 3 and 4, the two precoated sheets 2 are precoated sheets as shown in Figure 1, with a removal zone 18 at each weld edge 14, where the metal alloy layer 11 has been removed throughout its thickness, but the intermetallic alloy layer 9 remains integral.

[0091] As a result of the welding operation, a molten metal zone is formed at the joint between the two sheets 2 which subsequently solidifies to form a weld joint 22 .

[0092] The welding step is in particular a laser welding step in which a laser beam 24 is directed at the joint between the two sheets 2. This laser beam 24 is configured to melt the filler wire 20 at the impact point 26 of the laser beam 24.

[0093] The laser welding step is performed, for example, using a CO2 laser or a solid state laser.

[0094] The laser source is preferably a high-power laser source, which may be chosen, for example, from among a CO2 laser with a wavelength of approximately 10 micrometers, a solid-state laser source with a wavelength of approximately 1 micrometer, or a semiconductor laser source, for example a diode laser with a wavelength comprised between approximately 0.8 and 1 micrometer.

[0095] The power of the laser source is selected depending on the thickness of the sheet 2. In particular, the power is selected so as to allow fusion of the filler wire 20 with the weld edge 14 of the sheet 2 and sufficient mixing in the weld joint 22. In the case of a CO2 laser, the laser power is, for example, comprised between 3 kW and 12 kW. In the case of a solid-state or semiconductor laser, the laser power is, for example, comprised between 2 kW and 8 kW.

[0096] The diameter of the laser beam 24 at the point of impact 26 on the sheet 2 may be equal to about 600 μm for both types of laser sources.

[0097] During the welding step, the welding is carried out, for example, under a protective atmosphere, which in particular prevents oxidation and decarburization of the area where the welding is performed, the formation of boron nitride in the weld joint 22, as well as cold cracking that may result from hydrogen absorption.

[0098] The protective atmosphere is for example an inert gas or a mixture of inert gases, which may be helium or argon or a mixture of these gases.

[0099] During this welding step, the distance between the opposing side surfaces 13 of the two sheets 1 is, for example, equal to or less than 0.3 mm, more particularly equal to or less than 0.1 mm. Providing such a clearance between the opposing side surfaces 13 of the two sheets 1 facilitates the deposition of filler metal during the welding operation and prevents the formation of thick walls at the weld joint 22. The deposition of filler metal and the prevention of thick walls are also improved if, during the preparation step, a chamfered or beveled edge is produced at the weld edge 14 of the sheets 2.

[0100] In particular, the average aluminum content in the weld joint 22, Al WJ is between 0.1% and 1.2% by weight. More specifically, the average aluminum content Al in the weld joint 22 is WJ The average aluminum content in the weld joint 22 is 0.15% by weight or more. WJ is, for example, 0.8% by weight or less.

[0101] This average aluminum content Al WJ originates from the parts of the pre-coating 5 that may remain in the removal zone 18 after removal of the removal rate F, as well as from traces of aluminum present on the side(s) 13 of the weld edge(s) 14 resulting from the removal and / or cutting operations. In the weld joint 22 it is mixed with the steel of the substrate 3 and the filler wire 20.

[0102] During the welding step, the proportion of filler wire 20 added to the weld pool is for example comprised between 10% and 50%, more particularly between 10% and 40%.

[0103] In accordance with the present invention, the composition of the filler wire 20 and the rate at which the filler wire 20 is added to the weld pool are such that the resulting weld joint 22 is (a)

[0104]

number

[0105] Among the substrates 3 of the precoat sheet 2, the substrate 3 with the lowest hardening property is the substrate 3 with the lowest carbon content.

[0106] In fact, the inventors of the present invention have surprisingly shown that, if the above criteria C1, C2 and C3 are cumulatively fulfilled, a part obtained from such a welded steel blank 1 after a heat treatment including an austenitizing step (hot press-forming in a press tool and cooling) has a welded joint 22 with a thermal conductivity of 25 J / cm at 20° C. 2 It was found that the precoated sheet 2 exhibited a Charpy energy equal to or greater than the above and an ultimate tensile strength equal to or greater than the ultimate tensile strength of the weakest substrate among the substrates 3 of the precoated sheet 2.

[0107] The weakest substrate 3 is the substrate that has the smallest product of thickness and ultimate tensile strength after hot press forming and cooling.

[0108] In particular, the Charpy energy of welded joint 22 at 20°C is 25 J / cm 2 If this is the case, complete brittle fracture of the welded joint can be avoided.

[0109] Thus, when the above criteria C1, C2 and C3 are cumulatively met, the presence of the welded joint 22 does not reduce the properties of the welded steel part obtained by hot press forming and cooling from a welded blank compared to the properties after hot press forming and cooling of the weakest of the substrates 3 of the precoated sheet 2, even if the welded joint 22 contains a relatively high aluminum content.

[0110] The method according to the invention therefore makes it possible to obtain a part having a satisfactory crash performance, despite the possibly relatively high aluminum content in the weld joint 22 .

[0111] Preferably, the composition of the filler wire 20 and the proportion of filler wire 20 added to the weld pool is less than the average nickel content of the weld joint 22, Ni WJ is further selected to be comprised between 0.1% and 13.7% by weight, more particularly between 0.2% and 12.0% by weight.

[0112] For example, the composition of the filler wire 20 and the percentage of filler wire 20 added to the weld pool may be adjusted to account for the average chromium content, Cr, of the weld joint 22. WJ is further selected to be equal to or greater than 0.05 weight percent. Such a chromium content in the weld joint is advantageous because it improves the corrosion resistance and hardenability of the weld joint 22.

[0113] Preferably, the composition of the weld joint 22 is such that it has a predominantly martensite microstructure after hot pressing and cooling, where "predominantly" means at least 95% martensite, and more particularly 100% martensite.

[0114] The filler wire 20 has a carbon content in particular comprised between 0.01% and 0.45% by weight. According to one example, the carbon content of the filler wire 20 is equal to or greater than the carbon content of the least hardenable of the substrates 3 of the two precoat sheets 2.

[0115] Indeed, the inventors of the present invention have found in a surprising manner that in order to reduce the risk of carbon segregation and therefore the occurrence of hardness peaks in the weld joint 22 after hot pressing in a press tool and cooling, the carbon content in the filler wire should be comprised between 0.01% and 0.45% by weight, especially when significant amounts of aluminum are present in the weld joint 22. By using such a filler wire 20, the risk of embrittlement of the weld joint 22 is thus reduced, which is responsible for avoiding fractures in the weld joint 22 of the part obtained after hot pressing in a press tool under tension perpendicular to the weld joint 22 and cooling.

[0116] In particular, the inventors of the present invention have observed that in order to be able to obtain a welded joint 22, the carbon content in the filler wire should be comprised between 0.01% and 0.45% by weight, and the maximum hardness change ΔHV(WJ) across the welded joint 22 is 0.01% by weight and 0.45% by weight, and the maximum hardness change ΔHV(WJ) across the welded joint 22 is 0.01% by weight and 0.45% by weight, respectively. 平均 (WJ) less than 20%. In other words,

[0117]

number

[0118] Preferably, the filler wire 20 has a manganese content strictly less than the manganese content of the substrate 3 of the precoat sheet 2 .

[0119] For example, the filler wire 20 may have the following composition, by weight: 0.01%≦C≦0.45%, and for example 0.02%≦C≦0.45% 0.001%≦Mn≦0.45%, and for example 0.05%≦Mn≦0.45%, even more particularly 0.05%≦Mn≦0.20%, 0.001%≦Si≦1% 0.02%≦Ni≦56%, and for example 0.2%≦Ni≦10.0%; 0.001%≦Cr≦30% 0.001%≦Mo≦5% 0.001%≦Al≦0.30% 0.001%≦Cu≦1.80% 0.001%≦Nb≦1.50% 0.001%≦Ti≦0.30% 0.001%≦N≦10% 0.001%≦V≦0.1% 0.001%≦Co≦0.20% The balance is iron and inevitable impurities.

[0120] In the above exemplary filler wire composition, the contents of Mn, Si, Cr, Mo, Al, Cu, Nb, Ti, N, V ​​and Co equal to about 0.001% correspond to traces of these elements at the level of impurities resulting from the fusion of the raw materials and from the sophistication or accuracy of the measuring equipment for very low contents, which may result in the fact that elements that are completely absent in the analyzed steel are measured as being present at very low contents or that elements that are present at very low contents may be measured as being absent in the steel.

[0121] For example, the filler wire 20 may be comprised of the elements listed above.

[0122] The filler wire 20 may be, for example, a solid wire or a flux cored wire.

[0123] The invention also relates to a welded steel blank 1 which can be obtained using the method described above.

[0124] An example of such a welded steel blank is shown in FIG.

[0125] The welded steel blank 1 comprises two precoated sheets 2, each precoated sheet 2 comprising a steel substrate 3 having a precoating 5 on each of its major surfaces 4, the precoating 5 comprising an intermetallic alloy layer 9 comprising at least iron and aluminium and optionally a metal alloy layer 11 extending on top of the intermetallic alloy layer 9, the metal alloy layer 11 being a layer of aluminium, an aluminium alloy or an aluminium-based alloy, and the precoated sheets 2 being joined by a weld joint 22.

[0126] The welded steel blank 1 is provided, on both sides of the weld joint 22, with an intermediate zone 28 from which the pre-coating 5 has been removed over the removal zone F as defined above.

[0127] Additionally, as can be seen in FIG. 5, each intermediate zone 28 includes an inner edge 30 located at the weld joint 22 and an outer edge 32 located away from the weld joint 22 .

[0128] The width W of each intermediate zone 28, ie the distance between the inner edge 30 and the outer edge 32, measured from the edge of the weld joint 22, is comprised between 5 μm and 2000 μm, more particularly between 5 μm and 1500 μm.

[0129] Preferably, in the intermediate zone 28 the precoating 5 has been removed over a removal rate F which is strictly less than 100%. In particular, the metal alloy layer 11 has been removed, while the intermetallic alloy layer 9 remains intact.

[0130] According to an alternative, in the intermediate zone 28, the precoating 5 has been removed over a removal rate F equal to 100%, ie over its entire thickness.

[0131] Thus, in the intermediate zone 28 the thickness of the precoating 5 is strictly smaller than in the zones of the precoat sheet 2 located further away from the weld joint 22 or even absent.

[0132] The intermediate zone 28 arises from the removal zone 18 on the corresponding precoat sheet 2 .

[0133] The precoated sheet 2 and the welded joint 22 have the characteristics disclosed above with respect to the method for producing a welded steel blank 1 .

[0134] Therefore, the weld joint 22 complies with the criteria C1, C2 and C3 defined above.

[0135] Furthermore, the average aluminum content of the weld joint 22, Al WJ is between 0.1% and 1.2% by weight. More specifically, the average aluminum content Al of the weld joint 22 is WJ The average aluminum content of the welded joint 22 is 0.15% by weight or more. WJ is, for example, 0.8% by weight or less.

[0136] The weld joint 22 may, for example, be hot pressed in a press tool and cooled, such that the Charpy energy of the weld joint 22 at 20° C. is 25 J / cm 2 That's all.

[0137] Furthermore, after hot press forming and cooling, the ultimate tensile strength of the hot press formed and cooled welded steel blank is equal to or greater than the ultimate tensile strength of the weakest substrate among the substrates 3 of the precoated sheet 2. In this context, the weakest substrate is defined as above.

[0138] For example, the average nickel content of weld joint 22, Ni WJ is comprised between 0.1% and 13.6% by weight, more particularly between 0.2% and 12.0% by weight.

[0139] For example, the average chromium content of welded joint 22, Cr WJ is 0.05% by weight or more.

[0140] The weld joint 22, for example, after hot pressing in a press tool and cooling, has a maximum hardness change ΔHV(WJ) across the weld joint 22 that is less than the average hardness HV 平均 (WJ) is less than 20%. In other words,

[0141]

number

[0142] The weld joint 22 has an average hardness HV of, for example, 0.01 mm after hot pressing in a press tool and cooling. 平均 (WJ) is 700HV or less.

[0143] Preferably, the composition of the weld joint 22 is such that it has a predominantly martensite microstructure after hot pressing and cooling, where "predominantly" means at least 95% martensite, and more particularly 100% martensite.

[0144] The present invention also provides a method for producing a welded, hot press formed and cooled steel part, comprising the steps of: - producing a welded steel blank 1 using the method described above; - heating the welded steel blank 1 in order to obtain a fully austenitic structure in the substrate 3 of the precoated sheet 2 constituting the welded blank 1; - hot press forming the welded steel blank 1 in a press tool to obtain a steel part; - cooling the steel part in the press tool; The present invention relates to a method comprising the steps of:

[0145] More specifically, during the heating step, the welded steel blank 1 is heated up to the austenitizing temperature. It is then held at the austenitizing temperature for a holding time depending on the thickness of the steel sheet 2 forming the welded steel blank 1. The holding time is chosen depending on the austenitizing temperature so that the welded blank 1 is austenitized and an alloyed intermetallic layer of a predetermined thickness is formed by alloying between the substrate 3 and the pre-coating 5. For example, the holding time is equal to about 5 minutes.

[0146] Prior to hot press forming, the thus heated welded steel blank 1 is transferred to a hot forming press tool. The transfer time is advantageously comprised between 5 and 10 seconds. The transfer time is selected to be as short as possible in order to avoid metallurgical deformations in the welded steel blank 1 prior to hot press forming.

[0147] During the cooling step, the cooling rate is equal to or greater than the critical martensite or bainite cooling rate of at least one of the substrates 3 of the two steel sheets 2 and, for example, of the most hardenable steel sheet 1, i.e. the steel sheet having the lowest critical cooling rate.

[0148] After cooling, the weld joint 22 has a predominantly martensite microstructure, where "predominantly" means at least 95% martensite, and more specifically, 100% martensite.

[0149] The invention also relates to a welded, hot-pressed and cooled steel part obtained using the method described above.

[0150] More specifically, the steel part comprises a first coated steel part portion and a second coated steel part portion resulting respectively from hot press-forming in a press tool and cooling of two pre-coated steel sheets 2 .

[0151] More specifically, each coated steel component portion comprises a steel substrate having a coating comprising iron and aluminum on each of its major surfaces, and the first and second steel component portions are joined by a weld joint 22 as described above.

[0152] In particular, the coating of the first and second steel component parts results from at least partial alloying of the pre-coating 5 during hot press forming.

[0153] The substrates of the first and second steel component parts have the composition described above for the precoated sheet 2. They result from hot press forming and cooling of the substrate 3 of the precoated sheet 2.

[0154] Each steel part comprises an intermediate zone on each face of the steel part adjacent to the weld joint 22. This intermediate zone arises from the intermediate zone 28 described with reference to the welded blank 1. In the intermediate zone, the thickness of the coating is strictly smaller than in the remainder of the steel part, or the coating is even absent.

[0155] The weld joint 22 complies with the criteria C1, C2 and C3 defined above.

[0156] Furthermore, the weld joint 22 has an average aluminum content Al comprised between 0.1% and 1.2% by weight. WJ The average aluminum content of the weld joint 22 is, for example, 0.15% by weight or more. For example, the average aluminum content of the weld joint 22 is 0.8% by weight or less.

[0157] The Charpy energy of welded joint 22 at 20°C is 25 J / cm 2 and the ultimate tensile strength of the part is equal to or greater than the ultimate tensile strength of the weakest of the substrates 3 of the coated steel part portion.

[0158] For example, the weld joint 22 has an average nickel content Ni comprised between 0.1% and 13.6% by weight, more particularly between 0.2% and 12.0% by weight. WJ has.

[0159] For example, the weld joint 22 has an average chromium content of 0.05% by weight or more. WJ has.

[0160] For example, the maximum hardness change ΔHV(WJ) of the entire weld joint 22 is 1 / 2 the average hardness HV 平均 (WJ) is less than 20%. In other words,

[0161]

number

[0162] Average hardness of welded joints 22 HV 平均 (WJ) is, for example, 700HV or less.

[0163] Preferably, the weld joint 22 has a predominantly martensite microstructure, where "predominantly" means at least 95% martensite, and more specifically, 100% martensite.

[0164] The inventors have carried out experiments in which a welded steel blank 1 is produced by butt laser welding together two pre-coated sheets A and B using a filler wire W.

[0165] The experimental conditions for each of the experiments E1 to E22 are shown in Table 1 below.

[0166] Precoated Sheets A and B as initially provided had precoatings 5 ​​on both of their major surfaces 4 that were approximately 25 micrometers thick.

[0167] For all tested precoated sheets A and B, the precoating 5 was obtained by hot-dip coating in a molten metal bath and comprised a metallic alloy layer 11 and an intermetallic alloy layer 9 .

[0168] The metal alloy layer 11 of the pre-coating 5 comprises, by weight: Si: 9% Fe: 3%, the balance consisted of aluminum and possible impurities resulting from refining.

[0169] The average total thickness of the metal alloy layer 11 was 20 μm.

[0170] The intermetallic alloy layer 9 is made of Fe x -Aly type intermetallic compounds and mainly Fe2Al3. Fe2AI5 and Fe x AI y S z Its average thickness is 5 μm.

[0171] For all of the tested precoated sheets A and B, removal zones 18 were created on both major faces by removal of the metal alloy layer 11 while leaving the intermetallic alloy layer intact. Removal was achieved by laser ablation using the method disclosed in prior application WO 2007 / 118939.

[0172] [Table 1]

[0173] In the above table, experiments not according to the invention are underlined.

[0174] In the above table, a "0" in the "Percentage of filler wire added to weld pool" column means that no filler wire was added.

[0175] The steel substrates used in the different experiments described in Table 1 have the compositions listed in Table 2 below, the contents being expressed in weight percent.

[0176] [Table 2]

[0177] For all substrates, the balance of the composition is iron, possible impurities, and unavoidable elements resulting from manufacturing.

[0178] In Table 2 above, "-" means that the substrate contains the maximum trace of the element being considered.

[0179] The filler wire W used in the different experiments described in Table 1 has the composition listed in Table 3 below, the contents being expressed in weight percent.

[0180] [Table 3]

[0181] For all welding wires, the balance of the composition is iron, possible impurities, and unavoidable elements resulting from manufacturing.

[0182] Unless otherwise stated, these filler wires may contain Al, Cu, Nb, Ti, N, V ​​and Co in trace amounts corresponding to these elements with contents equal to about 0.001%.

[0183] Next, the inventors measured the composition of the obtained welded joint 22 for each of Experiments E1 to E21 using a conventional measuring method.

[0184] The average manganese, aluminum, nickel, chromium and silicon contents of the weld joint 22 were determined by averaging over the entire surface of the weld analyzed using an energy dispersive spectroscopy detector integrated into a scanning electron microscope. The average carbon content was determined using a casting electron microprobe on cross sections of the specimens taken perpendicular to the weld joint 22. The results of these measurements are shown below in Table 4.

[0185] [Table 4]

[0186] Furthermore, the inventors have subjected the welded steel blank 1 thus produced to a heat treatment, including austenitization, followed by quenching in order to obtain a heat-treated part, which has the same properties as a hot press-formed and cooled part.

[0187] The inventors then carried out measurements to determine the mechanical properties of these parts (ultimate tensile strength of hot pressed and cooled parts and Charpy energy of the welded joints).

[0188] They are hot press formed parts and cooled parts (UTS 部品 The measured ultimate tensile strength of the weakest substrate after heat treatment (UTS 最も弱い基材 ) was further compared with the ultimate tensile strength of

[0189] The mechanical properties thus determined are given in Table 5 below.

[0190] [Table 5]

[0191] In the table above, "nd" means "to be determined."

[0192] Tensile tests were performed at ambient temperature (approx. 20 °C) using the methods disclosed in the following standards: NF EN ISO 4136 and NF ISO 6892-1 on transversely welded tensile specimens of type EN 12.5 × 50 (240 × 30 mm) and extracted perpendicular to the laser welding direction. For each experiment (E1-E21), five tensile tests were performed.

[0193] The Charpy energy was measured using a standard Charpy impact test using a test specimen with a V-shaped notch in the weld joint 22, 2 mm deep and 8 mm wide overall, the notch being located in the weld joint to an accuracy of 0.2 mm or better, and having a width that is the dimension of the test specimen parallel to the depth of the notch. The test was performed at 20°C.

[0194] Based on the measured composition of the welded joint 22, the inventors determined whether each of the experiments E1 to E22 met the criteria C1 to C3 defined above.

[0195] The results of this decision are summarized in Table 6 below.

[0196] [Table 6] Underlined parts: not applicable to the present invention

[0197] As can be seen from table 6, the experiments referred to E1, E2, E5 to E14, E17 and E18 are examples according to the invention: in these experiments criteria C1 to C3 are fulfilled.

[0198] In contrast, the experiments referred to E3, E4, E15 and E20 to E22 are not according to the invention: in these experiments at least one of the criteria C1 to C3 is not met.

[0199] As can be seen from Table 5 above, in experiments E1, E2, E5-E14, E17 and E18, which meet criteria C1-C3, the hot press-formed parts and cooled parts obtained from the welded blank 1 had excellent mechanical properties, in particular the ultimate tensile strength of the weakest of the two substrates of the welded blank 1 after heat treatment, which was 25 J / cm2 at 20 °C, and the ultimate tensile strength of the welded joint 22 was 25 J / cm2 at 20 °C. 2 Meet the Charpy energy above.

[0200] Thus, in the parts obtained from the blank 1 according to the invention, the presence of the welded joints 22 does not reduce the properties of the welded steel part compared to the properties of the weakest substrate 3 after hot press forming and cooling. These parts therefore have a satisfactory crash performance despite the presence of aluminum in the welded joints.

[0201] In contrast, in the non-inventive experiments E3, E4, E14-E15 and E20-E22, at least one of the criteria C1-C3 is not met, so that at least one of the ultimate tensile strengths of the hot pressed and cooled parts or the Charpy energy of the welded joints is too low and therefore unsatisfactory, and therefore there is a risk of the parts breaking in the welded joints, for example in a crash situation.

[0202] The method according to the invention is therefore particularly advantageous since, after hot pressing in a press tool and cooling, it makes it possible to obtain a part including the weld joint 22, which has excellent mechanical properties, despite the presence of aluminum in the weld joint.

[0203] It is therefore particularly suitable for the manufacture of anti-intrusion, structural or energy absorbing parts which contribute to the safety of motor vehicles.

[0204] According to a particular embodiment of the method for producing a welded blank 1 according to the invention, the step of producing the two precoated sheets 2 comprises: - providing two initial precoat sheets 2'; - arranging these two initial precoat sheets 2' adjacent to each other leaving a predetermined gap between them; a step of simultaneously removing the precoating 5 on two adjacent initial precoat sheets 2' by laser ablation over a removal rate F so as to simultaneously generate removal zones 18 on the adjacent faces of these two initial precoat sheets 2', the laser beam overlapping the two adjacent initial precoat sheets 2' during said removal step; Includes.

[0205] During the welding step, the two adjacent pre-coated sheets 2 thus prepared are welded together with the laser beam spot overlapping the two pre-coated sheets 2. Preferably, the time between the end of the laser ablation and the start of the welding is less than or equal to 10 seconds.

[0206] Each intermediate zone 28 of the welded blank 1 thus obtained contains solidification streaks resulting from laser ablation.

[0207] Because one laser beam overlapping both initial precoat sheets 2 is used to simultaneously remove the precoating 5 over a removal rate F, the solidification striations on adjacent major surfaces 4 of the two precoat sheets 2 are symmetrical with respect to the vertical midplane M between the two precoat sheets 2.

[0208] Additionally, as discussed above with respect to FIG. 5, each intermediate zone 28 includes an inner edge 30 located at the weld joint 22 and an outer edge 32 located away from the weld joint 22 .

[0209] In this particular embodiment, the simultaneous ablation method results in a distance between the outer edges of the adjacent intermediate zones of the two precoated sheets being substantially constant along the longitudinal direction of the weld joint 22. By substantially constant, it is meant that the distance between the outer edges 32 of the adjacent intermediate zones of the two precoated sheets 2 varies by up to 5% along the weld joint 22, i.e. in the longitudinal direction of the weld joint 22.

[0210] The invention also relates to a part obtained by hot pressing and cooling of a welded blank 1 obtained using the method according to the particular embodiment.

[0211] The hot pressed and cooled part has the same characteristics as described above.

[0212] Furthermore, in this part, each intermediate zone includes solidification striations, and the solidification striations on adjacent major surfaces 4 of the two coated steel part portions are symmetrical about a vertical mid-plane between the two coated steel part portions.

[0213] Preferably, each intermediate zone comprises an inner edge located at the weld joint 22 and an outer edge located away from the weld joint 22, and the distance between the outer edges of adjacent intermediate zones of the two coated steel part portions is substantially constant along the longitudinal direction of the weld joint 22. By substantially constant is meant that the distance between the outer edges of adjacent intermediate zones of the two coated steel part portions varies along the weld joint 22, i.e. in the longitudinal direction of the weld joint 22, by a maximum of 5%.

Claims

1. A method for producing a welded steel blank (1), comprising the steps of: providing two precoat sheets (2), each precoat sheet (2) comprising a steel substrate (3) having a precoating (5) on each of its two main faces (4), said precoating (5) comprising an intermetallic alloy layer (9) comprising at least iron and aluminium and a metal alloy layer (11) extending on the upper surface of said intermetallic alloy layer (9), said metal alloy layer (11) being a layer of aluminium, a layer of an aluminium alloy or a layer of an aluminium-based alloy; each precoat sheet (2) comprises on each of its main faces (4) a removal zone (18) in which, at least partially at the weld edge (14) intended to be incorporated in the weld joint (22), said metal alloy layer (11) has been removed over its entire thickness, but said intermetallic alloy layer (9) remains intact; - butt welding the precoated sheets (2) using a filler wire (20) to produce a weld joint (22) at the junction between the precoated sheets (2), the average aluminum content (Al ＷＪ ) is included between 0.1% and 1.2% by weight; Including, the composition of the filler wire (20) and the proportion of filler wire (20) added to the weld pool such that the weld joint (22) thus obtained (a) [0010] The hardening factor FT of the welded joint (22) is ＷＪ (Criteria C1), Where: -FT ＢＭ is the hardening coefficient of the least hardenable steel substrate (3) of the two precoated sheets (2); said hardening factor FT ＷＪ and F.T. ＢＭ is expressed by the following formula: FT=128+1553xC+55xMn+267xSi+49xNi+5xCr-79xAl-2xNi ２ -1532xC ２ -5xMn ２ -127xSi ２ -40xCxNi-4xNixMn, where Al, Cr, Ni, C, Mn and Si are the average aluminum, chromium, nickel, carbon, manganese and silicon contents, respectively, in weight percent, of the region for which the hardenability factor is to be determined, and this region is determined using the FT ＷＪ In the case of , it is in the weld joint (22), and FT ＢＭ In the case of , the substrate is the most difficult to cure. (b) The following relationship: ＷＪ ≦14-3.4xAl ＷＪ The average nickel content (Ni ＷＪ ), where Al ＷＪ is the average aluminum content of the weld joint (22) (Criterion C2); and (c) The following relationship: Cr ＷＪ ≦5-2xAl ＷＪ The average chromium content (Cr ＷＪ ), where Al ＷＪ is the average aluminum content of the weld joint (22) (Criterion C3) The method of claim 1, wherein the first and second chromium atoms are selected to be characterized by:

2. The steel of the substrate (3) of at least one of the precoated sheets (2) has, by weight: 0.10%≦C≦0.5% 0.5%≦Mn≦4.5% 0.1%≦Si≦1% 0.01%≦Cr≦1% Ti≦0.2% Al≦0.1% S≦0.05% P≦0.1% B≦0.010% and the balance being iron and impurities due to manufacturing.

3. The average aluminum content (Al ＷＪ 3. The method according to claim 1, wherein the amount of the sintered material is 0.15% by weight or more.

4. The average aluminum content (Al ＷＪ 4. The method according to claim 1, wherein the amount of the sintered material is 0.8% by weight or less.

5. The average nickel content of the weld joint (22) (Ni ＷＪ 5. The method according to claim 1, wherein the amount of glycerol in the aqueous solution is between 0.1% and 13.6% by weight, more particularly between 0.2% and 12.0% by weight.

6. The welded steel blank, after hot press forming and cooling, The Charpy energy of the welded joint (22) at 20°C is 25 J / cm ２ That's all. the ultimate tensile strength of the hot press-formed and cooled steel welded steel blank is equal to or greater than the ultimate tensile strength of the weakest substrate among the substrates (3) of the precoated sheet (2), the weakest substrate (3) being the substrate (3) with the lowest product of thickness and ultimate tensile strength after hot press-formed and cooling; The method according to any one of claims 1 to 5, wherein

7. The method according to any one of the preceding claims, wherein the filler wire (20) has a carbon content comprised between 0.01% and 0.45% by weight.

8. 8. The method according to claim 1, further comprising, prior to said providing step, a step of producing said two precoated sheets (2) from respective initial precoated sheets (2'), said step comprising a sub-step of removing said metal alloy layer (11) by laser ablation at said welded edges (14) of said precoated sheets (2) to obtain said removal zones (18) on each main surface (4) of each precoated sheet (2).

9. The step of producing the two precoated sheets (2) comprises: - providing two initial precoat sheets (2'), - arranging these two initial precoat sheets (2') adjacent to each other leaving a predetermined gap between them; - simultaneously removing said pre-coating (5) on two adjacent initial pre-coated sheets (2') by laser ablation so as to simultaneously generate said removal zones (18) on the adjacent faces of these two initial pre-coated sheets (2'), the laser beam overlapping said two adjacent initial pre-coated sheets (2') during said removal step; The method of claim 8 , comprising:

10. The method according to any one of claims 1 to 9, further comprising preparing the weld edge (14) of at least one of the precoated sheets (2) prior to butt welding using at least one of the following processing steps: brushing, machining, chamfering and / or beveling.

11. The method according to any one of the preceding claims, wherein the welding step is performed using a laser beam.

12. For at least one of the precoated sheets (2), the steel of the substrate (3) 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% and the remainder being iron and impurities resulting from production.

13. For one of the precoated sheets (2), the steel of the substrate (3) comprises, by weight: 0.040%≦C≦0.100% 0.80%≦Mn≦2.00% Si≦0.30% 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≦0.100% Mo≦0.100% Ca≦0.006%, and the remainder being iron and impurities resulting from production.

14. For one of the precoated sheets (2), the steel of the substrate (3) 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% wherein the titanium and nitrogen contents satisfy the following relationships: Ti / N>3.42 and the contents of carbon, manganese, chromium and silicon satisfy the following relationships: [0025] The steel optionally comprises 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 12, wherein the balance is iron and impurities which inevitably arise from production.

15. The method according to any one of the preceding claims, wherein the welding step is carried out using a protective gas.

16. A method for producing a welded, subsequently hot press formed and cooled steel part, comprising the successive steps: - carrying out the method according to any one of claims 1 to 15 in order to obtain a welded steel blank (1); - heating the welded steel blank (1) in order to obtain a fully austenitic structure in the substrate (3) of the precoat sheet (2); - hot press forming said welded steel blank (1) in a press tool to obtain a steel part; - cooling the steel part in the press tool; A method comprising:

17. 17. The method according to claim 16, wherein during the cooling step the cooling rate is equal to or greater than the bainite or martensite cooling rate of the most hardenable of the substrates (3) of the precoat sheet (2).

18. A welded steel blank (1) comprising two precoated sheets (2), each precoated sheet (2) comprising a steel substrate (3) having a precoating (5) on each of its main surfaces (4), said precoating (5) comprising an intermetallic alloy layer (9) comprising at least iron and aluminium, and a metal alloy layer (11) extending on a top surface of said intermetallic alloy layer (9), said metal alloy layer (11) being a layer of aluminium, a layer of an aluminium alloy or a layer of an aluminium-based alloy, The precoated sheets (2) are joined by welded joints (22), the welded joints (22) having an average aluminum content (Al ＷＪ ), and the weld joint (22) has (a) [0030] The hardening factor FT of the welded joint (22) is ＷＪ (Criteria C1), Where: -FT ＢＭ is the hardening coefficient of the least hardenable steel substrate (3) of the two precoated sheets (2); said hardening factor FT ＷＪ and F.T. ＢＭ is expressed by the following formula: FT=128+1553xC+55xMn+267xSi+49xNi+5xCr-79xAl-2xNi ２ -1532xC ２ -5xMn ２ -127xSi ２ -40xCxNi-4xNixMn, where Al, Cr, Ni, C, Mn and Si are the average aluminum, chromium, nickel, carbon, manganese and silicon contents, respectively, in weight percent, of the region for which the hardenability factor is to be determined, and this region is determined using the FT ＷＪ In the case of , it is in the weld joint (22), and FT ＢＭ In the case of , the substrate is the most difficult to cure. (b) The following relationship: ＷＪ ≦14-3.4xAl ＷＪ The average nickel content (Ni ＷＪ ), where Al ＷＪ is the average aluminum content of the weld joint (22) (Criterion C2); and (c) The following relationship: Cr ＷＪ ≦5-2xAl ＷＪ The average chromium content (Cr ＷＪ ), where Al ＷＪ is the average aluminum content of the weld joint (22) (Criterion C3) is further characterized by A welded steel blank (1), in which each precoated sheet (2) comprises on each of its main faces (4) and adjacent to said weld joint (22) an intermediate zone (28) in which said metal alloy layer (11) has been removed over its entire thickness, but said intermetallic alloy layer (9) remains integral.

19. The steel of the substrate (3) of at least one of the precoated sheets (2) has, by weight: 0.10%≦C≦0.5% 0.5%≦Mn≦4.5% 0.1%≦Si≦1% 0.01%≦Cr≦1% Ti≦0.2% Al≦0.1% S≦0.05% P≦0.1% B≦0.010% 19. The welded steel blank (1) according to claim 18, comprising:

20. Welded steel blank (1) according to claim 18 or 19, wherein for each precoat sheet (2), the width of the intermediate zone (28) is comprised between 5 μm and 2000 μm from the edge of the welded joint (22).

21. The nickel content (Ni ＷＪ 21. The welded steel blank (1) according to any one of claims 18 to 20, wherein the Cr content of the welded steel blank (1) is comprised between 0.1% and 13.6% by weight, more particularly between 0.2% and 12.0% by weight.

22. The welded steel blank (1), after hot press forming and cooling, The Charpy energy of the welded joint (22) at −20° C. is 25 J / cm ２ and the ultimate tensile strength of the hot press-formed and chill-welded steel blank is equal to or greater than the ultimate tensile strength of the weakest substrate among the substrates (3) of the precoated sheet (2), the weakest substrate (3) being the substrate with the smallest product of thickness and ultimate tensile strength after hot press-formed and chilled A welded steel blank (1) according to any one of claims 18 to 21, such that

23. After the weld joint (22) is hot-pressed and cooled, the maximum hardness change ΔHV(WJ) of the entire weld joint (22) is greater than or equal to the average hardness HV 平均 The welded steel blank (1) according to any one of claims 18 to 22, wherein the welded steel blank (1) is 20% or less of (WJ).

24. 24. The welded steel blank (1) according to any one of claims 18 to 23, wherein each intermediate zone (28) comprises solidification striations, said solidification striations on adjacent main faces (4) of the two precoated sheets (2) being symmetrical with respect to a vertical mid-plane (M) between the two precoated sheets (2).

25. 25. The welded steel blank (1) according to any one of claims 18 to 24, wherein each intermediate zone (28) comprises an inner edge (30) located at the weld joint (22) and an outer edge (32) located away from the weld joint (22), and the distance between the outer edges (32) of adjacent intermediate zones (28) of the two precoat sheets (2) is constant along the longitudinal direction of the weld joint (22).

26. A welded steel component comprising a first coated steel component portion and a second coated steel component portion, each coated steel component portion comprising a steel substrate (3) having on at least one of its major surfaces a coating comprising at least iron and aluminum; The first and second coated steel part portions are joined by a weld joint (22), the weld joint (22) having an average aluminum content (Al ＷＪ ) The weld joint (22) is (a) [0045] The hardening factor FT of the welded joint (22) is ＷＪ (Criteria C1), Where: -FT ＢＭ is the hardening coefficient of the least hardenable steel substrate (3) of the two precoated sheets (2); said hardening factor FT ＷＪ and F.T. ＢＭ is expressed by the following formula: FT=128+1553xC+55xMn+267xSi+49xNi+5xCr-79xAl-2xNi ２ -1532xC ２ -5xMn ２ -127xSi ２ -40xCxNi-4xNixMn, where Al, Cr, Ni, C, Mn and Si are the average aluminum, chromium, nickel, carbon, manganese and silicon contents, respectively, in weight percent, of the region for which the hardenability factor is to be determined, and this region is determined using the FT ＷＪ In the case of , it is in the weld joint (22), and FT ＢＭ In the case of , the substrate is the most difficult to cure. (b) The following relationship: ＷＪ ≦14-3.4xAl ＷＪ The average nickel content (Ni ＷＪ ), where Al ＷＪ is the average aluminum content of the weld joint (22) (Criterion C2); and (c) The following relationship: Cr ＷＪ ≦5-2xAl ＷＪ The average chromium content (Cr ＷＪ ), where Al ＷＪ is the average aluminum content of the weld joint (22) (Criterion C3) is further characterized by each coated steel part comprises on each of its main faces an intermediate zone (28) adjacent to said weld joint (22), in which the thickness of said coating is strictly less than in adjacent zones of said coated steel part which are located at a greater distance from said weld joint (22) than in said intermediate zone; and The Charpy energy of the welded joint (22) at 20°C is 25 J / cm ２ That's it, welded steel parts.

27. The steel of the substrate (3) of at least one of the first and second steel component portions comprises, by weight: 0.10%≦C≦0.5% 0.5%≦Mn≦4.5% 0.1%≦Si≦1% 0.01%≦Cr≦1% Ti≦0.2% Al≦0.1% S≦0.05% P≦0.1% B≦0.010% 27. The welded steel component of claim 26, comprising:

28. 28. The welded steel component according to claim 26 or 27, wherein each intermediate zone comprises solidification striations, said solidification striations on adjacent main faces (4) of the two coated steel component parts being symmetrical with respect to a vertical mid-plane between the two coated steel component parts.

29. 29. The welded steel component according to any one of claims 26 to 28, wherein each intermediate zone comprises an inner edge located at the weld joint (22) and an outer edge located away from the weld joint (22), and the distance between the outer edges of adjacent intermediate zones of the two coated steel component portions is constant along the longitudinal direction of the weld joint (22).

30. Average hardness HV in the welded joint (22) 平均 The welded steel part according to any one of claims 26 to 29, wherein (WJ) is 700HV or less.

31. The average nickel content of the weld joint (22) (Ni ＷＪ 31. The welded steel part according to any one of claims 26 to 30, wherein the content of Cd is between 0.1% and 13.6% by weight, more particularly between 0.2% and 12.0% by weight.

32. the ultimate tensile strength of the welded steel part is equal to or greater than the ultimate tensile strength of the weakest substrate of the substrates (3) of the coated steel part, the weakest substrate (3) being the substrate with the lowest product of thickness and ultimate tensile strength; A welded steel part according to any one of claims 26 to 31.

33. The maximum hardness change ΔHV(WJ) of the entire welded joint (22) is the average hardness HV 平均 The welded steel part according to any one of claims 26 to 32, wherein the welded steel part has a welded joint strength of 20% or less of (WJ).

34. The steel of the substrate (3) of at least one of the first and second coated steel component portions 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% and the balance being iron and impurities due to manufacturing.

35. The steel of the substrate (3) of one of the first and second coated steel part portions comprises, by weight: 0.040%≦C≦0.100% 0.80%≦Mn≦2.00% Si≦0.30% 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≦0.100% Mo≦0.100% Ca≦0.006%, and the balance being iron and impurities due to manufacturing.

36. The steel of the substrate (3) of one of the first and second coated steel part portions 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% wherein the titanium and nitrogen contents satisfy the following relationships: Ti / N>3.42 and the contents of carbon, manganese, chromium and silicon satisfy the following relationships: [0050] The steel optionally comprises one or more of the following elements: 0.05%≦Mo≦0.65% 0.001%≦W≦0.30% 0.0005%≦Ca≦0.005% A welded steel part according to any one of claims 26 to 34, wherein the balance is iron and impurities which inevitably arise from production.

37. Use of a welded steel part according to any one of claims 26 to 36 for producing an anti-intrusion or energy absorbing part for a motor vehicle.

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