Welded steel blanks and related methods for producing welded steel blanks - Patents.com

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 that are butt-welded together are known in the prior art. More specifically, the welded blank is typically heated to a temperature that allows the steel to austenitize, then hot-formed in a hot press tool and cooled. The composition of the steel can be selected to both allow for the subsequent heating and forming operations and to provide the welded steel part with 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, more particularly for the manufacture of anti-intrusion parts, structural parts or parts that contribute to the safety of motor vehicles.

[0004] To prevent corrosion, steel sheets are pre-coated with an aluminum-based pre-coating by hot-dipping in an aluminum-containing bath. If the steel sheets are welded without any prior preparation, the aluminum-based pre-coating is diluted with the steel substrate in the molten metal during the welding process. Depending on the aluminum content of the pre-coating, two phenomena can 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. In some cases, this can prevent the complete austenitization of the weld joint that would occur during hot stamping and pre-forming heating, 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, martensite or bainite can no longer be obtained during cooling after hot forming, and the resulting weld joint contains ferrite, which then exhibits a hardness and mechanical strength less than those of the two adjacent sheets and thus constitutes the weakest area of ​​the part.

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

[0012] However, these methods are not entirely satisfactory because they address only one of the problems related to the presence of aluminum in the weld pool: the compensation of the austenitizing temperature (Ac3) and, in some cases, the use of high-carbon filler wire, which can induce segregation in the weld joint. Indeed, the inventors of the present invention have found that the methods disclosed in the above-mentioned documents do not allow for 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, such parts are at high risk of weld joint failure under tensile tests in the transverse weld direction.

[0013] The methods disclosed in WO2015 / 086781 and EP2942143 also address this issue and describe how to weld pre-coated steel sheets using a specific welding method with a specific filler material.

[0014] More specifically, WO 2015 / 086781 proposes using twin-spot laser welding while 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 filler material in the form of a filler wire having the following composition: C: 0-0.3 wt%, Mo: 0-0.4 wt%, Ni: 6-20 wt%, Mn: 0.5-7 wt%, Cr: 5-22 wt% and Si: 0-1.3 wt%, Nb: 0-0.7 wt%, the remainder 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 immediately 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 addition is generally more difficult to implement in a large scale industrial setting than filler wire.

[0019] All of the above-mentioned methods based on the addition of filler materials only specify the chemical composition range of the filler material, and since welding parameters and conditions affect the filler material velocity, a single filler wire may induce very different chemical compositions in the weld joint. Therefore, describing 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 pre-coated sheets, which, after hot press-forming and cooling, makes it possible to obtain a part with satisfactory crush performance properties, even with a relatively high aluminum content in the weld 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 precoated sheets, each precoated sheet comprising a steel substrate (3) having a precoating on each of its two main surfaces, the precoating comprising an intermetallic alloy layer comprising at least iron and aluminum and, optionally, a metal alloy layer extending on top of the intermetallic alloy layer, the metal alloy layer being a layer of aluminum, an aluminum alloy or an aluminum-based alloy; each precoated sheet is provided on each of its main faces with a removal zone in which the precoating has been removed over a removal rate comprised between 30% and 100% of the thickness of the precoating, at the weld edge intended to be at least partially incorporated into the weld joint; - butt-welding the precoated sheets using a filler wire to produce a weld joint at the junction between the precoated sheets, wherein the average aluminum content of the weld joint is less than or equal to 100% by weight of the filler wire. WJ is contained between 0.1% by weight and 1.2% by weight, and 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 the quality of the welded joint obtained. (a)

[0024]

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[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 resulting from manufacturing.

[0026] - The substrate of each of the pre-coated sheets is made of press-hardenable steel; - the average aluminum content of the welded joint is not less than 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 particularly between 0.2% and 12.0% by weight; - Welded steel blanks, after hot press forming and cooling, have a Charpy energy of 25 J / cm at 20°C. 2 or more, and the ultimate tensile strength of the hot-pressed and cooled steel 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 with the lowest product of thickness and ultimate tensile strength after hot-pressed and cooled; - the filler wire has a carbon content comprised between 0.01% and 0.45% by weight; - for at least one precoated sheet, and for example for both precoated sheets, the removal rate is strictly less than 100% of the thickness of the precoating; - for at least one precoated sheet, the precoating comprises a metal alloy layer extending on top of the intermetallic alloy layer, the metal alloy layer being a layer of aluminum, an aluminum alloy or an aluminum-based alloy, and for at least one precoated sheet, and for example for both precoated sheets, the metal alloy layer has been removed throughout its thickness, while the intermetallic alloy layer remains intact in removal zones on each major surface of the precoated sheet; - for at least one precoated sheet provided in the providing step, and for example for both precoated sheets, in a removal zone on each main surface of the precoated sheet, the removal rate is equal to 100% such that the precoating is removed over its entire thickness; The method further comprises, before 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 pre-coated sheets includes: - 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 precoating on two adjacent initial precoated sheets by laser ablation so as to simultaneously generate removal zones on adjacent faces of these two initial precoated sheets, wherein during said removing step the laser beam overlaps the two adjacent initial precoated sheets, and optionally during a welding step the two adjacent precoated sheets thus prepared are welded together so that the laser beam spot overlaps the two adjacent precoated sheets, the time between the end of the laser ablation and the start of the welding being preferably 10 seconds or less; - the method further comprises, prior to butt welding, preparing a weld edge of at least one of the precoated sheets 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 pre-coated sheets have the same thickness; - The two pre-coated sheets have different thicknesses; - for at least one of the pre-coated sheets, and for example for both pre-coated 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.

[0028] - For one of the pre-coated 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.

[0029] - For one of the pre-coated 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 relationship:

[0030]

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[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-pressed 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 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, including:

[0033] According to a particular embodiment of this method for producing a welded, hot-pressed 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 in the base material of the pre-coated 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 aluminum, and optionally a metal alloy layer extending on top of the intermetallic alloy layer, the metal alloy layer being a layer of aluminum, an aluminum alloy, or an aluminum-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, and the weld joint (a)

[0035]

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[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 pre-coated sheets, and for example both pre-coated 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 manufacturing. - The substrate of each of the pre-coated 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 precoated sheet, and for example for both precoated sheets, the removal rate is equal to 100% of the thickness of the precoating; - for at least one precoated sheet, and for example for both precoated sheets, the removal rate is strictly less than 100% of the thickness of the precoating; - for at least one precoat sheet, and e.g., for both precoat sheets, the precoating comprises a metal alloy layer extending on top of the intermetallic alloy layer, the metal alloy layer being a layer of aluminum, an aluminum alloy, or an aluminum-based alloy, and for at least one precoat sheet, and e.g., for both precoat sheets, the metal alloy layer has been removed throughout its thickness, while 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 particularly between 0.2% and 12.0% by weight; - Welded steel blanks, after hot press forming and cooling, have a Charpy energy of 25 J / cm at 20°C. 2or greater, and the ultimate tensile strength of the hot-pressed and cooled 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 the substrate with the lowest product of thickness and ultimate tensile strength after hot-pressed and cooled; - 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 striations, and the solidification striations on the adjacent major surfaces of the two precoated sheets are symmetrical with respect to the vertical median plane between the two precoated 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 pre-coated sheets, and for example for both pre-coated 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 pre-coated 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 pre-coated 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 relationship:

[0037]

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[0038] The present 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, and the weld joint comprises: (a)

[0039]

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[0040] According to particular embodiments 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 characteristics, taken alone or according to any possible combination: the steel of the substrate of at least one of the first and second steel part portions, and for example the steel of the first and second steel part portions, 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 manufacturing; - the substrate of each of the first and second steel component portions is made of press-hardenable steel; - each intermediate zone includes solidification striations, and the solidification striations on adjacent major surfaces of the two coated steel part portions are 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, and the distance between the outer edges of adjacent intermediate zones of the two coated steel part portions is constant along the longitudinal direction of the weld joint; - Average hardness HV of the welded joint 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 particularly between 0.2% and 12.0% by weight; - The Charpy energy of the welded joint at 20°C is 25J / cm 2 the ultimate tensile strength of the welded, hot-pressed and cooled steel part is equal to or greater than the ultimate tensile strength of the weakest substrate among the substrates of the coated steel part portion, the weakest substrate being the substrate with 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 portions, and e.g. the substrate steel of the first and second coated steel part portions, 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 parts 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 parts 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 relationship:

[0041]

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[0042] The invention further relates to the use of the above-described 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 from reading the following specification, given by way of example only, with reference to the accompanying drawings, in which: [Brief explanation 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. [Figure 2] FIG. 2 is a perspective view of an initial precoated sheet. [Figure 3] 1 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; [Figure 5] 1 is a schematic cross-sectional view of a welded steel blank according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0045] Throughout the patent application, elemental contents are expressed in weight percent (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 the two main surfaces 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 between 60° and 90° with one of the main surfaces 4.

[0049] Each precoat sheet 2 comprises a metal substrate 3 having on each of its major surfaces a precoating 5. The precoatings 5 ​​are superimposed on and 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 with a ferritic-pearlitic microstructure.

[0052] Preferably, the substrate 3 is made of a heat-treatable steel, 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 is iron and impurities resulting from manufacturing.

[0054] More specifically, 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% The remainder is iron and impurities resulting from manufacturing.

[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 is iron and impurities resulting from manufacturing.

[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 relationship:

[0057]

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

[0059] According to another example, the substrates 3 of the two precoated sheets 2 have different compositions. In particular, the two substrates 3 have different compositions selected from the four compositions described above. For example, the steel of the substrate 3 of one precoated sheet 2 has the first composition described above, and the steel of the substrate 3 of the other precoated 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, more particularly comprised 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 molten metal bath.

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

[0064] In the example shown in FIG. 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 similar to that of the molten metal in the bath. It is formed by the molten metal carried away by the sheet as it moves through the molten metal bath during hot-dip coating. The metal alloy layer 11 is a layer of aluminum, or an aluminum alloy, or an aluminum-based alloy.

[0065] In this context, an aluminum alloy refers to an alloy containing more than 50% aluminum by weight. An aluminum-based alloy is an alloy in which aluminum is the major 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 a layer of aluminum alloy further containing silicon, by weight. - 8%≦Si≦11%, - 2%≦Fe≦4%; the balance being aluminum and possible impurities.

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

[0068] 1, where the pre-coating 5 comprises a metal alloy layer 11, the thickness of the intermetallic alloy layer 9 is generally on 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 above-mentioned intermetallic alloy layer 9. In this case, the thickness of the intermetallic alloy layer 9 is, for example, comprised between 10 μm and 40 μm. Such a pre-coating 5 consisting of an intermetallic alloy 9 can be obtained, for example, by subjecting a pre-coating 5 comprising the above-disclosed intermetallic alloy layer 9 and the metal alloy layer 11 to a pre-alloying treatment. Such a pre-alloying treatment is carried out at a temperature and for 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 Fe2Al5, FeAl3, FeAl, Fe6Al 12 It may be composed of different intermetallic sublayers such as Si5 and FeAl3 sublayers.

[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] Additionally, as shown in Figure 1, for each precoated sheet 2, the precoating 5 is removed at the weld edge 14 of the precoated sheet 2 on each major surface 4 of the precoated sheet 2 to create a removal zone 18 at the weld edge 14. More specifically, the precoating 5 is removed over a removal rate 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 side 13 of precoated sheet 2 and a portion of precoated sheet 2 extending from side 13 and comprising a portion of precoated sheet 2 including a portion of precoating 5 and a portion of substrate 3.

[0074] The removal of the removal rate F of the pre-coating 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 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] 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 its 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, before 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 geometry 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 a sub-step of obtaining removal zones 18 on each main surface 4 of each precoated sheet 2 by removing the precoating 5 over a removal rate F at the weld edges 14 by laser ablation.

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

[0084] Preparation of the weld edge 14 may include at least one of the following processing steps: - brushing of the weld edges 14, - machining of the weld 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 introducing a thick wall at the 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 removal zones 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 intact.

[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 point of impact 26 of the laser beam 24.

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

[0094] The laser source is preferably a high-power laser source, which may be selected from among, for example, 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 of approximately 0.8 to 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 at 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 source.

[0097] During the welding step, the welding is carried out, for example, under a protective atmosphere, which prevents, in particular, oxidation and decarburization of the area where the welding is being carried out, the formation of boron nitride in the weld joint 22, and 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, 0.3 mm or less, more particularly 0.1 mm or less. Providing such a clearance between the opposing side surfaces 13 of the two sheets 1 facilitates filler metal deposition during the welding operation and prevents the formation of thick walls at the weld joint 22. Filler metal deposition and prevention of thick walls are also improved if a chamfered or beveled edge is produced at the weld edge 14 of the sheets 2 during the preparation step.

[0100] In particular, the average aluminum content in 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 welded 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 arises 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: (a)

[0104]

number

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

[0106] In fact, the inventors of the present invention have surprisingly discovered that, if the above criteria C1, C2 and C3 are cumulatively fulfilled, the 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 the above Charpy energy and 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 with 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] Therefore, when the above criteria C1, C2 and C3 are cumulatively met, the presence of the welded joint 22, even if the welded joint 22 contains a relatively high aluminum content, does not reduce the properties of the welded steel part obtained by hot press forming and cooling from the welded blank compared to the properties after hot press forming and cooling of the weakest substrate 3 among the substrates 3 of the precoated sheet 2.

[0110] The method according to the invention therefore makes it possible to obtain a part with 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, 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 proportion 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 wt. %. 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 comprised between 0.01% and 0.45% by weight, in particular, and in one example is equal to or greater than the carbon content of the substrate 3 of the two precoated sheets 2 that is the least curable.

[0115] Indeed, the inventors of the present invention have surprisingly found 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 the press tool and cooling, the carbon content in the filler wire should be comprised between 0.01% and 0.45% by weight, especially when a significant amount of aluminum is present in the weld joint 22. The use of such a filler wire 20 therefore reduces the risk of embrittlement of the weld joint 22 and is responsible for avoiding fractures in the weld joint 22 of the part obtained after hot pressing in the 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 weld joint 22, the carbon content in the filler wire should be comprised between 0.01% by weight and 0.45% by weight, and the maximum hardness change ΔHV(WJ) across the weld joint 22 is ΔHV(WJ) / ... 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 unavoidable 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 incorporation of 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 may be measured as being present at very low contents or 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 made of the elements listed above.

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

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

[0124] An example of such a welded steel blank is shown in Figure 5.

[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 aluminum 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 aluminum, an aluminum alloy or an aluminum-based alloy, and the precoated sheets 2 being joined by a weld joint 22.

[0126] The welded steel blank 1 comprises, on either side of the weld joint 22, 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 measured from the edge of the weld joint 22, i.e. the distance between the inner edge 30 and the outer edge 32, 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 that 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 precoated 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 the 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 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 wt % or more. WJ is, for example, 0.8% by weight or less.

[0136] The weld joint 22 may be, for example, 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 is 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 may be, for example, hot pressed in a press tool and cooled such that the maximum hardness change ΔHV(WJ) across the weld joint 22 is less than the average hardness HV 平均 (WJ) is less than 20%. In other words,

[0141]

number

[0142] The weld joint 22 may be, for example, hot pressed in a press tool and cooled to an average hardness HV of the weld joint 22. 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, including:

[0145] More specifically, during the heating step, the welded steel blank 1 is heated to the austenitizing temperature and then held at the austenitizing temperature for a holding time that depends on the thickness of the steel sheet 2 forming the welded steel blank 1. The holding time is selected 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 welded steel blank 1 thus heated is transferred to a hot press tool, the transfer time being advantageously comprised between 5 and 10 seconds, and is chosen to be as short as possible in order to avoid metallurgical deformation of 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 plates 2, and for example the most hardenable steel plate 1, i.e. the steel plate with 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 and cooling of two pre-coated steel sheets 2 in a press tool.

[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 portions 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 compositions described above for the precoated sheet 2. They result from hot pressing 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 the weld joint 22. This intermediate zone arises from the intermediate zone 28 described with respect to the welded blank 1. In the intermediate zone, the coating thickness is strictly less 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 substrate 3 of the coated steel part portion.

[0158] For example, the weld joint 22 may have 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 It has.

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

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

[0161]

number

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

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

[0164] The inventors have carried out experiments in which a welded steel blank 1 is produced by butt laser welding two pre-coated sheets A and B together 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 metal 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%, and 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 Si z Its average thickness is 5 μm.

[0171] For all tested precoated sheets A and B, removal zones 18 were created on both major surfaces by removal of 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 table above, experiments not according to the present invention are underlined.

[0174] In the table above, 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 had the compositions listed in Table 2 below, the contents being expressed in wt. %.

[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 considered.

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

[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 contents equal to about 0.001% corresponding to trace amounts of these elements.

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

[0184] The average manganese, aluminum, nickel, chromium, and silicon contents of 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 a cross section of the sample taken perpendicular to 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 to obtain a heat-treated part, which has the same properties as a hot-pressed 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 welded joints).

[0188] They are hot stamped and cooled parts (UTS 部品 ) is 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 shown in Table 5 below.

[0190] [Table 5]

[0191] In the table above, "nd" means "undecided."

[0192] Tensile tests were performed at ambient temperature (approximately 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), extracted perpendicular to the laser welding direction. Five tensile tests were performed per experiment (E1–E21).

[0193] Charpy energy was measured using a standard Charpy impact test using a specimen with a V-shaped notch 2 mm deep and 8 mm overall width in the weld joint 22, the notch positioned in the weld joint to an accuracy of 0.2 mm or better, and having a width that was the dimension of the specimen parallel to the depth of the notch. Testing 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 determination are summarized in Table 6 below.

[0196] [Table 6] Underlined: 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 fulfilled.

[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-pressed and cooled parts obtained from welded blank 1 exhibited excellent mechanical properties, especially the ultimate tensile strength of the weakest of the two substrates of welded blank 1 after heat treatment, which was 25 J / cm at 20°C. 2 Meets or exceeds the Charpy energy.

[0200] Therefore, in parts obtained from the blank 1 according to the invention, the presence of the welded joint 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 satisfactory crash performance despite the presence of aluminum in the welded joint.

[0201] In contrast, in experiments E3, E4, E14-E15 and E20-E22 not according to the invention, 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 part breaking at the welded joint, for example in a crash situation.

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

[0203] It is therefore particularly suitable for the production of anti-intrusion, structural or energy absorbing parts that 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 two precoated sheets 2 comprises: - providing two initial precoated sheets 2'; - arranging these two initial precoat sheets 2' adjacent to each other while leaving a predetermined gap between them; a removal 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 laser ablation and the start of welding is 10 seconds or less.

[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, we mean 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 a method according to a particular embodiment.

[0211] This 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 with respect to a vertical midplane between the two coated steel part portions.

[0213] Preferably, each intermediate zone has 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, it is meant that the distance between the outer edges of adjacent intermediate zones of the two coated steel part portions varies by a maximum of 5% along the weld joint 22, i.e., in the longitudinal direction of the weld joint 22.

Claims

1. 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 a coating on at least one of its major surfaces 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 ) comprised between 0.1% and 1.2% by weight. ＷＪ ) and an average chromium content of 0.05 wt.% or more (Cr ＷＪ ) The weld joint (22) (a) [Equation 1] The hardening factor FT of the welded joint (22) is ＷＪ (Criterion C1), where: -FT ＢＭ is the hardening coefficient of the least hardenable steel substrate (3) among the steel substrates (3) of the first and second coated steel component portions (2), - said hardening factor FT ＷＪ and FT ＢＭ is calculated by the following formula: FT = 128 + 1553 x C + 55 x Mn + 267 x Si + 49 x Ni + 5 x Cr - 79 x Al - 2 x Ni 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), FT ＢＭ In this case, the substrate is the most difficult to cure. (b) The following relationship: Ni 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 the weld joint (22), in which the thickness of the coating is strictly less than that of adjacent zones of the coated steel part that are located at a greater distance from the weld joint (22) than the intermediate zone; and The Charpy energy of the welded joint (22) at 20 ° C. is 25 J / cm ２ That's all, A welded steel component, wherein the weld joint has a microstructure comprising at least 95% martensite.

2. 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% 10. The welded steel part of claim 1, comprising:

3. 3. The welded steel component according to claim 1 or 2, wherein each intermediate zone comprises solidification striations, and wherein the solidification striations on adjacent main surfaces (4) of the two coated steel component portions are symmetrical with respect to a vertical mid-plane between the two coated steel component portions.

4. 4. The welded steel component according to claim 1, 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).

5. Average hardness HV in the welded joint (22) 平均 The welded steel part according to any one of claims 1 to 4, wherein (WJ) is 700 HV or less.

6. The average nickel content of the weld joint (22) (Ni ＷＪ 6. The welded steel part according to claim 1, wherein the content of Cr is between 0.1% and 13.6% by weight, more particularly between 0.2% and 12.0% by weight.

7. the ultimate tensile strength of the welded steel part is equal to or greater than the ultimate tensile strength of the weakest substrate (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 1 to 6.

8. The maximum hardness change ΔHV (WJ) of the entire weld joint (22) is the average hardness HV of the weld joint (22). 平均 The welded steel part according to any one of claims 1 to 7, wherein the welded steel part has a melting point of 20% or less of (WJ).

9. 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% The welded steel part according to any one of claims 1 to 8, comprising:

10. The steel of the substrate (3) of one of the first and second coated steel part portions has, 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 welded steel part according to any one of claims 1 to 9, comprising:

11. The steel of the substrate (3) of one of the first and second coated steel part portions has, 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: [Equation 2] 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 1 to 9, wherein the balance is iron and impurities which inevitably arise from the production.

12. Use of a welded steel part according to any one of claims 1 to 11 for producing an anti-intrusion or energy absorbing part for a motor vehicle.

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