Press-hardened laser-welded steel components and their production method

Abstract

A method for producing press-hardened laser-welded steel parts and the resulting press-hardened laser-welded steel parts are provided. The method includes: providing a first pre-coated plate (1) and a second pre-coated plate (2); butt-welding the first and second pre-coated plates to obtain a blank (15); heating the blank to a temperature at least 10°C lower than the full austenitization temperature of the weld joint (22) and lower than the minimum temperature T. min A heat treatment temperature at least 15°C higher, Equation (I), where Ac3(WJ) is the complete austenitizing temperature of the welded joint, Equation (II), where Ts1 and Ts2 are the ultimate tensile strengths of the strongest and weakest matrices after compressive hardening, C FW β is the carbon content of the filler material, ρ is the proportion of the filler material, and ρ is the ratio of the thickness of the weakest matrix to the thickness of the strongest matrix. The blank is held at this heat treatment temperature for 2 to 10 minutes. The blank is then pressed into a part and cooled.

Description

[0001] This application is a divisional application of the invention patent application filed on February 26, 2019, with application number 201980015662.7 (PCT / IB2019 / 051528) and entitled "Method for producing pressure-hardened laser-welded steel parts and pressure-hardened laser-welded steel parts".

[0002] This invention relates to a method for producing pressure-hardened laser-welded steel components and to the pressure-hardened laser-welded steel components obtained therefrom.

[0003] This type of steel component is used particularly in the automotive industry, and even more specifically in the manufacture of collision management components, such as intrusion prevention components or shock absorbers, structural components, or components that contribute to the safety of motor vehicles.

[0004] For this type of component, motor vehicle manufacturers stipulate that welded joints should not constitute the weakest area of ​​the welded steel component.

[0005] To prevent corrosion, the steel sheet used to manufacture such welded steel components is pre-coated with an aluminum-based pre-coating by hot-dip coating in an aluminum-containing bath. If the steel sheet is welded without any prior preparation, the aluminum-based pre-coating will be diluted by the steel matrix within the molten metal during the welding operation. Aluminum tends to raise the complete austenitizing temperature of the molten metal and thus prevents complete transformation to austenite during hot forming at conventional heat treatment temperatures. Therefore, a fully martensitic or bainitic microstructure may no longer be obtained in the weld joint during the press-cooling process that is possible during hot forming.

[0006] Furthermore, it is not possible to use higher heat treatment temperatures that allow for full austenitization of the weld joint, as this would lead to over-alloying of the coating and potential negative impacts on coating adhesion and / or the spot weldability of the press-hardened component.

[0007] In response to this situation, when manufacturing parts from such pre-coated steel sheets, two types of solutions have been developed in the prior art to enable the formation of a fully martensitic structure in the weld joint after hot forming and quenching at conventional heat treatment temperatures.

[0008] In particular, EP2007545 describes a first solution which involves removing the surface layer of the metal alloy at the weld edge of the pre-coated steel plate to significantly reduce the total aluminum content in the weld joint and thus obtain a full austenitization temperature close to the full austenitization temperature of the pre-coated steel plate substrate.

[0009] In addition, EP 2 737 971, US 2016 / 0144456 and WO 2014075824 describe a second solution, which involves welding a pre-coated steel plate with filler wire containing austenitic stabilizing elements (such as carbon, manganese or nickel) to compensate for the presence of aluminum in the weld joint and lower its full austenitization temperature, so that a fully martensitic structure can be obtained in the weld joint after hot forming and quenching at conventional heat treatment temperatures.

[0010] However, these methods are not entirely satisfactory.

[0011] In practice, the first method is relatively time-consuming. Furthermore, the second method may require the addition of a relatively large amount of austenite-forming elements to achieve a fully martensitic structure in the weld joint after heat treatment. This addition increases production costs and may also lead to problems such as unsatisfactory weld joint geometry or uneven mixing of material from the pre-coated steel plate with material from the filler wire, and the risk of locally retained austenite.

[0012] Therefore, one object of the present invention is to provide a method for producing welded steel billets from two such pre-coated plates at a relatively low cost, which allows for the production of parts with satisfactory impact performance characteristics after press hardening, even with relatively high aluminum content in the weld joint.

[0013] For this purpose, the present invention relates to a method for producing press-hardened laser-welded steel components, the method comprising the following sequential steps:

[0014] - Provide a first pre-coated steel sheet and a second pre-coated steel sheet, each of the first and second pre-coated steel sheets comprising a steel substrate, and at least one of the first and second pre-coated steel sheets having an aluminum-containing pre-coating comprising at least 50% by weight of aluminum on at least one main surface.

[0015] The first pre-coated steel plate has a first thickness and the second pre-coated steel plate has a second thickness.

[0016] The ultimate tensile strength of the substrate of the first pre-coated steel plate after compression hardening is significantly greater than that of the substrate of the second pre-coated steel plate after compression hardening.

[0017] The product of the first thickness of the first pre-coated steel plate and its ultimate tensile strength after press hardening is strictly greater than the product of the second thickness of the second pre-coated steel plate and its ultimate tensile strength after press hardening.

[0018] - If the theoretical average aluminum content in the weld joint obtained by butt welding the first and second pre-coated steel plates provided in the providing step (possibly using a filler material containing up to 0.05% by weight of aluminum) is strictly greater than 1.25% by weight, then the aluminum-containing pre-coating within at least a portion of the thickness of the aluminum-containing pre-coating on at least one main surface at the weld edge or the edge to be welded of at least one of the first and second pre-coated steel plates is removed, such that the theoretical average aluminum content in the weld joint obtained by butt welding the first and second pre-coated steel plates thus prepared (possibly using a filler material containing up to 0.05% by weight of aluminum) is between 0.5% by weight and 1.25% by weight.

[0019] - Butt welding of a first pre-coated steel plate and a second pre-coated steel plate is performed using laser welding to obtain a weld joint between the two pre-coated steel plates, thereby obtaining a weld blank. The welding steps may include the use of filler material.

[0020] - Heat the weld blank to a heat treatment temperature that is at least 10°C lower than the complete austenitization temperature of the weld joint and lower than the minimum temperature T. min At least 15°C higher, of which

[0021]

[0022] in

[0023] Ac3(WJ) is the complete austenitizing temperature of the welded joint, expressed in °C, and Al is the aluminum content in the welded joint, expressed in weight %

[0024] as well as The maximum intercritical ferrite content of the welded joint is calculated using the following formula.

[0025]

[0026] in

[0027] Ts1 is the ultimate tensile strength of the strongest matrix after compression hardening, expressed in MPa.

[0028] Ts2 is the ultimate tensile strength of the weakest matrix after compression hardening, expressed in MPa.

[0029] C FW Carbon content of the filler material, expressed as a percentage by weight.

[0030] β is the proportion of filler material added to the weld pool, ranging from 0 to 1.

[0031] ρ is the ratio of the thickness of the pre-coated steel plate including the weakest substrate to the thickness of the pre-coated steel plate including the strongest substrate (ρ=t2 / t1).

[0032] The weldment blank is then held at this heat treatment temperature for 2 to 10 minutes.

[0033] - Pressing welded blanks into steel components; and

[0034] - The steel component thus formed is cooled at a cooling rate greater than or equal to the critical martensite or bainite cooling rate of the most hardenable matrix in the matrix of the first pre-coated steel plate and the second pre-coated steel plate to obtain a press-hardened welded steel component.

[0035] According to a specific implementation of this method:

[0036] - Perform the step of removing the aluminum-containing pre-coating:

[0037] - If the theoretical average aluminum content in the weld joint obtained by butt welding the first and second pre-coated steel plates provided in the providing step (possibly using filler material containing up to 0.05% by weight of aluminum) is strictly greater than 1.25% by weight.

[0038] Optionally, if the theoretical average aluminum content in the weld joint obtained by butt welding the first and second pre-coated steel plates provided in the providing step (possibly using a filler material containing up to 0.05% by weight of aluminum) is between 0.5% by weight and 1.25% by weight, and more particularly strictly greater than 0.5% by weight.

[0039] This step is then performed such that the theoretical average aluminum content in the weld joint obtained by butt welding the first and second pre-coated steel plates thus prepared (possibly using a filler material containing up to 0.05% by weight of aluminum) is 0.5% to 1.25% by weight;

[0040] - At the end of the heating step, the microstructure of the matrix of the first pre-coated steel plate and the second pre-coated steel plate is fully austenitic; - The ratio of the ultimate tensile strength of the matrix of the first pre-coated steel plate after compression hardening to the ultimate tensile strength of the matrix of the second pre-coated steel plate after compression hardening is greater than or equal to 1.2.

[0041] - The carbon content of the substrate of the first pre-coated steel plate is at least 0.05% by weight higher than that of the substrate of the second pre-coated steel plate.

[0042] - The first and second pre-coated steel plates provided in the providing step each include an aluminum-containing pre-coating comprising at least 50% by weight of aluminum on at least one of their main surfaces;

[0043] - The first and second pre-coated steel plates provided in the providing step include an aluminum-containing pre-coating on both of their main surfaces, comprising at least 50% by weight of aluminum;

[0044] - During butt welding, the aluminum pre-coating is intact on at least one of the first pre-coated steel plate and the second pre-coated steel plate, and for example, on both main surfaces of each of the first pre-coated steel plate and the second pre-coated steel plate;

[0045] The method further includes, prior to butt welding, a step of preparing a weld edge of at least one of the first and second pre-coated steel plates by removing at least a portion of the thickness of the aluminum-containing pre-coating on at least one main surface of at least one of the first and second pre-coated steel plates, prior to butt welding, even if the theoretical average aluminum content in the weld joint obtained by butt welding the first and second pre-coated steel plates provided in the providing step (possibly using a filler material containing up to 0.05 wt% aluminum), said weld edge being intended to be at least partially incorporated into the weld joint.

[0046] - The method further includes, prior to butt welding, even if the theoretical average aluminum content in the weld joint obtained by butt welding the first pre-coated steel plate and the second pre-coated steel plate provided in the providing step (possibly using a filler material containing up to 0.05 wt% aluminum) is 0.5 wt% to 1.25 wt%, a step of preparing a weld edge of at least one of the first pre-coated steel plate and the second pre-coated steel plate by removing at least a portion of the thickness of the aluminum-containing pre-coating on at least one main surface of the first pre-coated steel plate and the second pre-coated steel plate, the weld edge being intended to be at least partially incorporated into the weld joint, the removal step being carried out in such a manner that the theoretical average aluminum content in the weld joint obtained by butt welding the first pre-coated steel plate and the second pre-coated steel plate thus prepared (possibly using a filler material containing up to 0.05 wt% aluminum) remains at 0.5 wt% to 1.25 wt%.

[0047] - For at least one of the first pre-coated steel sheet and the second pre-coated steel sheet, the steel of the base material comprises, by weight:

[0048]

[0049] The remainder consists of iron and impurities produced during manufacturing;

[0050] - For at least one of the first pre-coated steel sheet and the second pre-coated steel sheet, the steel of the base material comprises, by weight:

[0051]

[0052] The remainder consists of iron and impurities produced during manufacturing;

[0053] - For at least one of the first pre-coated steel sheet and the second pre-coated steel sheet, the steel of the base material comprises, by weight:

[0054]

[0055] The remainder consists of iron and impurities produced during manufacturing;

[0056] - For at least one of the first pre-coated steel sheet and the second pre-coated steel sheet, the steel of the base material comprises, by weight:

[0057]

[0058] The remainder consists of iron and impurities produced during manufacturing;

[0059] - For at least one of the first pre-coated steel sheet and the second pre-coated steel sheet, the steel of the base material comprises, by weight:

[0060]

[0061] The contents of titanium and nitrogen satisfy the following relationship:

[0062] Ti / N > 3.42,

[0063] The contents of carbon, manganese, chromium, and silicon satisfy the following relationship:

[0064] ,

[0065] The steel optionally comprises one or more of the following elements:

[0066]

[0067] The remainder consists of iron and impurities unavoidably produced during manufacturing;

[0068] Laser welding is performed using shielding gases, particularly helium and / or argon; and

[0069] - The first pre-coated steel plate and the second pre-coated steel plate have different thicknesses.

[0070] The present invention also relates to a pressure-hardened laser-welded steel component, the steel component comprising a first coated steel component portion and a second coated steel component portion.

[0071] Each coated steel component includes a steel substrate, and at least one of the first coated steel component and the second coated steel component has an aluminum-containing coating comprising at least 30% by weight of aluminum on at least one of its main surfaces.

[0072] The first coated steel component portion has a first thickness and the second coated steel component portion has a second thickness. The ultimate tensile strength of the substrate of the first coated steel component portion is strictly greater than the ultimate tensile strength of the substrate of the second coated steel component portion, and the product of the first thickness and the ultimate tensile strength of the first coated steel component portion is strictly greater than the product of the second thickness and the ultimate tensile strength of the second coated steel component portion.

[0073] The first coated steel component and the second coated steel component are joined by a welded joint, wherein the aluminum content of the welded joint is from 0.5% to 1.25% by weight, and the microstructure of the welded joint comprises martensite and / or bainite, and the sub-thermal ferrite fraction is from 15% to a maximum sub-thermal ferrite fraction -5%, the maximum sub-thermal ferrite fraction being determined using the following formula:

[0074] ,

[0075] in

[0076] Ts1 is the ultimate tensile strength of the strongest matrix after compression hardening, expressed in MPa.

[0077] Ts2 is the ultimate tensile strength of the weakest matrix after compression hardening, expressed in MPa.

[0078] β is the proportion of filler material added to the weld pool, ranging from 0 to 1.

[0079] C FW Carbon content of the filler material, expressed as a percentage by weight.

[0080] ρ is the ratio of the thickness of the coated steel component including the weakest substrate to the thickness of the coated steel component including the strongest substrate (ρ=t2 / t1).

[0081] as well as

[0082] The matrix of at least one of the first coated steel component and the second coated steel component has a microstructure that is primarily martensitic and / or bainitic.

[0083] According to a specific implementation of the steel component, the ratio of the ultimate tensile strength of the substrate of the first coated steel component portion to the ultimate tensile strength of the substrate of the second coated steel component portion is greater than or equal to 1.2;

[0084] - For at least one of the first coated steel component portion and the second coated steel component portion, the steel of the base material comprises, by weight:

[0085]

[0086] The remainder consists of iron and impurities produced during manufacturing;

[0087] - For at least one of the first coated steel component portion and the second coated steel component portion, the steel of the base material comprises, by weight:

[0088]

[0089] The remainder consists of iron and impurities produced during manufacturing;

[0090] - For at least one of the first coated steel component portion and the second coated steel component portion, the steel of the base material comprises, by weight:

[0091]

[0092] The remainder consists of iron and impurities produced during manufacturing;

[0093] - For at least one of the first coated steel component portion and the second coated steel component portion, the steel of the base material comprises, by weight:

[0094]

[0095] The contents of titanium and nitrogen satisfy the following relationship:

[0096] Ti / N > 3.42,

[0097] The contents of carbon, manganese, chromium, and silicon satisfy the following relationship:

[0098] ,

[0099] The steel optionally comprises one or more of the following elements:

[0100]

[0101] The remainder consists of iron and impurities unavoidably produced during manufacturing; and

[0102] - For at least one of the first coated steel component portion and the second coated steel component portion, the steel of the base material comprises, by weight:

[0103]

[0104] The remainder consists of iron and impurities produced during manufacturing.

[0105] The invention will be better understood after reading the following detailed description, which is given by way of example only and with reference to the accompanying drawings, in which:

[0106] - Figure 1 This is a schematic cross-sectional view showing the start of the welding step according to the method of the present invention.

[0107] - Figure 2It is a schematic cross-sectional view of the welded blank obtained at the end of the welding process, and

[0108] - Figure 3 This is a perspective view of the pre-coated steel plate after the preparation steps.

[0109] Throughout the patent application, the content of elements is expressed as a weight percentage (wt%).

[0110] This invention relates to a method for producing laser-welded steel components that have undergone pressure hardening.

[0111] More specifically, the method includes a first step of providing a first pre-coated steel plate 1 and a second pre-coated steel plate 2.

[0112] Each pre-coated steel plate 1, 2 includes two opposing main surfaces 5, 6, and at least one side surface 13 extending from one main surface 5, 6 to the other between the two opposing main surfaces 5, 6. Figure 3 In the example shown, the pre-coated steel plates 1 and 2 include four sides 13. For example, the sides 13 form an angle of 60° to 90° with one of the main surfaces 5 and 6.

[0113] like Figure 1 As shown, each pre-coated steel plate 1, 2 includes a metal substrate 3, 4 having an aluminum-containing pre-coating 7, 8 on at least one of its main surfaces. The pre-coating 7, 8 is overlaid on and in contact with the substrate 3, 4.

[0114] Metal substrates 3 and 4 are more specifically steel substrates.

[0115] The steels in matrix 3 and 4 are more specifically steels with a ferrite-pearlite microstructure.

[0116] Preferably, the matrix 3, 4 is made of steel intended for heat treatment, more particularly of press-hardenable steel, such as manganese-boron steel (e.g., 22MnB5 type steel).

[0117] According to one implementation scheme, the steel in bases 3 and 4 comprises, by weight:

[0118]

[0119] The remainder consists of iron and impurities produced during manufacturing.

[0120] More specifically, the steel in bases 3 and 4 comprises, by weight:

[0121]

[0122] The remainder consists of iron and impurities produced during manufacturing.

[0123] According to an alternative, the steel in bases 3 and 4 comprises, by weight:

[0124]

[0125] Si ≤ 0.50%, and more specifically, Si ≤ 0.30%.

[0126] S≤0.009%, and more specifically, S≤0.005%.

[0127]

[0128] The remainder consists of iron and impurities produced during manufacturing.

[0129] According to an alternative, the steel in bases 3 and 4 comprises, by weight:

[0130]

[0131] The contents of titanium and nitrogen satisfy the following relationship:

[0132] Ti / N > 3.42,

[0133] The contents of carbon, manganese, chromium, and silicon satisfy the following relationship:

[0134] ,

[0135] The steel optionally comprises one or more of the following elements:

[0136]

[0137] The remainder consists of iron and impurities that are unavoidable during manufacturing.

[0138] According to an alternative, the steel in bases 3 and 4 comprises, by weight:

[0139]

[0140] The remainder consists of iron and impurities produced during manufacturing.

[0141] Depending on the desired thickness of the substrates 3 and 4, the substrates 3 and 4 can be obtained by hot rolling and / or by cold rolling followed by annealing, or by any other suitable method.

[0142] The thickness of the substrates 3 and 4 is advantageously 0.6 mm to 5 mm, more particularly 0.8 mm to 5 mm, and even more particularly 1.0 mm to 2.5 mm.

[0143] In one example, the thickness of the substrate 3 of the first pre-coated steel plate 1 is different from the thickness of the substrate 4 of the second pre-coated steel plate 2.

[0144] According to an alternative, the substrates 3 and 4 of the first pre-coated steel plate 1 and the second pre-coated steel plate 2 have the same thickness.

[0145] According to the present invention, the ultimate tensile strength Ts1 of the substrate 3 of the first pre-coated steel plate 1 after pressing and hardening is strictly greater than the ultimate tensile strength Ts2 of the substrate 4 of the second pre-coated steel plate 2 after pressing and hardening.

[0146] In this context, "after pressing hardening" means hot pressing after heating to a temperature greater than or equal to the full austenitizing temperature Ac3 of the steel matrix under consideration, followed by cooling to achieve hardening compared to the initial state.

[0147] For example, the ultimate tensile strength Ts1 of the substrate 3 of the first pre-coated steel plate 1 after pressing and hardening is 1400 MPa to 1600 MPa or 1700 MPa to 2000 MPa.

[0148] For example, the ultimate tensile strength Ts2 of the substrate 3 of the second pre-coated steel plate 2 after pressing and hardening is 500 MPa to 700 MPa or 1000 MPa to 1200 MPa.

[0149] For example, the ratio of the ultimate tensile strength Ts1 of the substrate 3 of the first pre-coated steel plate 1 after press hardening to the ultimate tensile strength Ts2 of the substrate 4 of the second pre-coated steel plate 2 after press hardening ( () is greater than or equal to 1.2, and more specifically greater than or equal to 1.4.

[0150] Furthermore, the first pre-coated steel plate 1 has a first thickness t1. The second pre-coated steel plate 1 has a second thickness t2.

[0151] The thicknesses t1 and t2 are, for example, 0.6 mm to 5 mm, more particularly 0.8 mm to 5 mm, and even more particularly 1.0 mm to 2.5 mm.

[0152] In one implementation, thicknesses t1 and t2 are the same. In an alternative implementation, thicknesses t1 and t2 are different.

[0153] The product of the first thickness t1 of the first pre-coated steel plate 1 and the ultimate tensile strength Ts1 is strictly greater than the product of the second thickness t2 of the second pre-coated steel plate 1 and the ultimate tensile strength Ts2.

[0154] Specifically, the substrates 3 and 4 of the first pre-coated steel plate 1 and the second pre-coated steel plate 2 are selected from the above-mentioned compositions.

[0155] For example, the steel in the substrate 3 of the first pre-coated steel plate 1 comprises, by weight:

[0156]

[0157] The remainder consists of iron and impurities produced during manufacturing.

[0158] According to another example, the steel in the substrate 3 of the first pre-coated steel plate 1 comprises, by weight:

[0159]

[0160] The contents of titanium and nitrogen satisfy the following relationship:

[0161] Ti / N > 3.42,

[0162] The contents of carbon, manganese, chromium, and silicon satisfy the following relationship:

[0163] ,

[0164] The steel optionally comprises one or more of the following elements:

[0165]

[0166] The remainder consists of iron and impurities that are unavoidable during manufacturing.

[0167] For example, the steel in the substrate 4 of the second pre-coated steel plate 2, by weight, comprises:

[0168]

[0169] Si ≤ 0.50%, and more specifically Si ≤ 0.30%.

[0170] S≤0.009%, and more specifically S≤0.005%.

[0171]

[0172] The remainder consists of iron and impurities produced during manufacturing.

[0173] According to another example, the steel in the substrate 4 of the second pre-coated steel plate 2 comprises, by weight:

[0174]

[0175] The remainder consists of iron and impurities produced during manufacturing.

[0176] Preferably, the carbon content of the substrate 3 of the first pre-coated steel plate 1 is at least 0.05% greater by weight than the carbon content of the substrate 4 of the second pre-coated steel plate 2.

[0177] According to the present invention, for at least one of the first pre-coated steel plate 1 and the second pre-coated steel plate 2, the aluminum-containing pre-coating 7 and 8 contain at least 50% by weight of aluminum.

[0178] Preferably, the pre-coatings 7 and 8 are obtained by hot-dip coating, i.e., by immersing the substrates 3 and 4 in a molten metal bath. In this case, as... Figure 1 As shown, the pre-coating layers 7 and 8 include at least intermetallic compound alloy layers 9 and 10 that are in contact with the substrates 3 and 4.

[0179] Intermetallic compound alloy layers 9 and 10 comprise intermetallic compounds containing at least iron and aluminum. Specifically, intermetallic compound alloy layers 9 and 10 are formed through a reaction between the substrates 3 and 4 and a molten metal bath. More specifically, intermetallic compound alloy layers 9 and 10 contain Fe. x -Al y It is a type, and more specifically an intermetallic compound of Fe2Al5.

[0180] exist Figure 1 In the examples shown, the pre-coatings 7 and 8 also include metal alloy layers 11 and 12 extending on top of the intermetallic compound alloy layers 9 and 10. The metal alloy layers 11 and 12 have compositions similar to those of the molten metal in the bath. They are formed from the molten metal carried away by the plate as it travels through the molten metal bath during hot-dip coating.

[0181] The metal alloy layers 11 and 12 are, for example, aluminum layers, aluminum alloy layers, or aluminum-based alloy layers.

[0182] In this article, aluminum alloy refers to an alloy containing more than 50% by weight of aluminum. Aluminum-based alloys are alloys in which aluminum is the main element by weight.

[0183] For example, metal alloy layers 11 and 12 are layers of aluminum alloy that also contain silicon. More specifically, metal alloy layers 11 and 12, by weight, comprise:

[0184]

[0185] The remainder consists of aluminum and possible impurities.

[0186] The thickness of the metal alloy layers 11 and 12 is, for example, 19 μm to 33 μm or 10 μm to 20 μm.

[0187] exist Figure 1 In the example shown, the pre-coatings 7 and 8 include metal alloy layers 11 and 12, and the intermetallic compound alloy layers 9 and 10 typically have a thickness of about a few micrometers. In particular, their average thickness is typically between 2 and 7 micrometers.

[0188] The specific structure of the pre-coating layers 7 and 8, which are obtained by hot-dip coating and include intermetallic compound alloy layers 9 and 10 and metal alloy layers 11 and 12, is specifically disclosed in patent EP 2 007 545.

[0189] According to another embodiment, the aluminum-containing pre-coatings 7 and 8 comprise only the intermetallic compound alloy layers 9 and 10 as described above. In this case, the thickness of the intermetallic compound alloy layers 9 and 10 is, for example, 10 μm to 40 μm. Such pre-coatings 7 and 8 consisting of intermetallic compound alloy layers 9 and 10 can be obtained, for example, by subjecting the pre-coatings 7 and 8 comprising the intermetallic compound alloy layers 9 and 10 and the metal alloy layers 11 and 12 as described above to a pre-alloying treatment. Such pre-alloying treatment is performed at a temperature and holding time selected such that the pre-coatings 7 and 8 are alloyed with the substrates 3 and 4 at least a portion of the thickness of the pre-coatings 7 and 8.

[0190] More specifically, the pre-alloying process includes the following steps: heating the plate to a pre-alloying temperature of 620°C to 1000°C, and holding the pre-alloyed plate at that temperature for a period of several minutes to several hours, depending on the processing temperature used. In this case, the intermetallic compound alloy layers 9 and 10 themselves can be composed of different intermetallic sublayers such as Fe2Al5, FeAl3, FeAl, and Fe6Al. 12 It consists of Si5 and FeAl3 sublayers.

[0191] Advantageously, such as Figure 1 As shown, the substrates 3 and 4 have aluminum pre-coatings 7 and 8 as described above on both of their main surfaces.

[0192] The first pre-coated steel plate 1 and the second pre-coated steel plate 2 may have the same pre-coatings 7 and 8.

[0193] Alternatively, the pre-coatings 7 and 8 of the first pre-coated steel plate 1 and the second pre-coated steel plate 2 may have different compositions.

[0194] Then, the theoretical average aluminum content in the weld joint 22 obtained by butt welding (possibly using filler material) between the first pre-coated steel plate 1 and the second pre-coated steel plate 2 is determined. .

[0195] When a filler material is intended to be used, the filler material is preferably a steel-based filler material with an aluminum content of less than or equal to 0.05% by weight.

[0196] This determination shall be made in any manner known to a technician.

[0197] For example, the theoretical average aluminum content in welded joint 22 can be determined using the following formula:

[0198] ,

[0199] in

[0200] The theoretical average aluminum content in welded joint 22, expressed as a percentage by weight.

[0201] The average aluminum content in aluminum pre-coatings 7 and 8, expressed as a percentage by weight.

[0202] M c The weight per unit area of ​​the aluminum-containing pre-coatings 7 and 8 on each of the two pre-coated steel plates 1 and 2, in g / m².

[0203] β is the proportion of steel-based filler material optionally added to the weld pool, ranging from 0 to 1, where β equals zero when no filler material is added to the weld pool.

[0204] t1 is the thickness of the first pre-coated steel plate 1, in mm, and

[0205] t2 is the thickness of the second pre-coated steel plate 2, in mm.

[0206] The above formula can be used even when a filler material is used, as long as the filler material contains less than or equal to 0.05% by weight of aluminum.

[0207] Even if the substrates 3 and 4 contain aluminum, the above formula can still be used, as long as the aluminum content of the substrates 3 and 4 is less than or equal to 0.05% by weight.

[0208] When expressed as a percentage, the proportion β of the steel-based filler material optionally added to the weld pool is, for example, 0 to 0.5, i.e., 0% to 50%.

[0209] Theoretical average aluminum content in welded joint 22 When the content is strictly greater than 1.25% by weight, the method according to the invention further includes ensuring that the theoretical average content of aluminum in the welded joint after preparation is... The step of preparing the weld edge 14 of at least one of the pre-coated steel plates 1 and 2 in a manner ranging from 0.5% to 1.25% by weight.

[0210] More specifically, the weld edges 14 of the pre-coated steel plates 1 and 2 under consideration are the edges of the pre-coated steel plates 1 and 2 that are expected to be welded to another pre-coated steel plate 1 and 2.

[0211] like Figure 3 As shown more specifically, the weld edge 14 includes the peripheral portion of the pre-coated steel plates 1, 2 that is intended to be at least partially incorporated into the weld joint 22 during butt welding. More specifically, the weld edge 14 includes the side 13 of the pre-coated steel plates 1, 2 and portions of the pre-coated steel plates 1, 2 extending from the side 13 and including a portion of the pre-coated coatings 7, 8 and a portion of the substrates 3, 4.

[0212] More specifically, the step of preparing the weld edge 14 includes removing the aluminum-containing pre-coating 7, 8 from at least a portion of the thickness of at least one main surface 5, 6 of the first pre-coated steel plate 1 and the second pre-coated steel plate 2. The pre-coating 7, 8 is removed in a removal area 18 extending from the side surface 13 of the pre-coated steel plates 1, 2 at the weld edge 14. The removal area 18 may extend from the side surface 13 of the pre-coated steel plates 1, 2 in a width of 0.5 mm to 2 mm. An example of the pre-coated steel plate 1 thus prepared is shown in… Figure 3 middle.

[0213] Removal is preferably performed using a laser beam.

[0214] Advantageously, in the removal zone 18, the metal alloy layers 11 and 12 are removed, while the intermetallic compound alloy layers 9 and 10 are retained with at least a portion of their thickness.

[0215] More specifically, in removal zone 18, metal alloy layers 11 and 12 are removed, while intermetallic compound alloy layers 9 and 10 are completely retained.

[0216] The remaining intermetallic compound alloy layers 9 and 10 protect the area of ​​the weld blank immediately adjacent to the weld joint 22 from oxidation and decarburization during subsequent hot forming steps, and from corrosion during service.

[0217] exist Figure 3 In the example shown, the metal alloy layers 11 and 12 have been removed from the removal area 18 at the weld edge 14, leaving the intermetallic compound alloy layers 9 and 10 intact.

[0218] Specifically, the number of pre-coating portions 7 and 8 to be removed, and the number of main surfaces of the pre-coated steel plates 1 and 2 to be removed, such that after removal, the theoretical average aluminum content in the weld joint 22 is reduced. It ranges from 0.5% to 1.25% by weight.

[0219] Specifically, at least a portion of the thickness of the pre-coatings 7 and 8 on the following main surfaces can be removed:

[0220] - Only one main surface 5, 6 of the first pre-coated steel plate 1 or the second pre-coated steel plate 2, or

[0221] - On a total of two main surfaces, for example, on only one main surface 5, 6 of each of the first pre-coated steel plate 1 and the second pre-coated steel plate 2, or on both main surfaces 5, 6 of only one of the first pre-coated steel plate 1 and the second pre-coated steel plate 2; or

[0222] - On a total of three main surfaces 5 and 6, namely on two main surfaces 5 and 6 of one of the first pre-coated steel plate 1 and the second pre-coated steel plate 2, and on only one main surface 5 and 6 of the other pre-coated steel plate 1 and 2; or

[0223] - On a total of four main surfaces 5 and 6, namely on the two main surfaces 5 and 6 of the first pre-coated steel plate 1 and the second pre-coated steel plate 2.

[0224] The theoretical average aluminum content in the weld joint 22 obtained by butt welding between the first pre-coated steel plate 1 and the second pre-coated steel plate 2 provided in the providing step, possibly using filler material with an aluminum content of less than or equal to 0.05% by weight. In cases where the amount is between 0.5% and 1.25% by weight, the first pre-coated steel plate 1 and the second pre-coated steel plate 2 are welded specifically without prior removal of the pre-coatings 7 and 8. More specifically, in this case, the first pre-coated steel plate 1 and the second pre-coated steel plate 2, on which the pre-coatings 7 and 8 are at least intact, are used for welding.

[0225] Alternatively, even in the weld joint 22 obtained by butt welding between the first pre-coated steel plate 1 and the second pre-coated steel plate 2 provided in the providing step, the theoretical average aluminum content may be less than or equal to 0.05% by weight of filler material. In cases where the amount is from 0.5% to 1.25% by weight, and more particularly strictly greater than 0.5% by weight, the pre-coating 7, 8 may also be removed from at least a portion of the thickness of the pre-coating 7, 8 on at least one main surface 5, 6 of at least one of the first pre-coated steel plate 1 and the second pre-coated steel plate 2 (and, for example, on only one main surface 5, 6 of at least one of the two pre-coated steel plates 1, 2) at the weld edge 14. For example, the pre-coating 7, 8 on only one main surface 5, 6 of each of the two pre-coated steel plates 1, 2, at at least a portion of the thickness of the pre-coating 7, 8 at the weld edge 14 may be removed. This is to ensure that the theoretical average aluminum content in the weld joint 22 obtained by welding the first pre-coated steel plate 1 and the second pre-coated steel plate 2 thus prepared by possibly using a filler material containing at most 0.05% by weight of aluminum is reduced. Optional removal steps are performed at a rate of 0.5% to 1.25% by weight.

[0226] In particular, such removal can be performed in order to even further reduce the heat treatment temperature T used for subsequent heat treatment. t Heat treatment temperature T t As determined later. In fact, the austenitizing temperature Ac3 (WJ) of weld joint 22 decreases with decreasing aluminum content. Specifically, the heat treatment temperature T, which can be determined without removal, is... tThis optional removal step should be performed at temperatures strictly above 950°C. In practice, to maintain good paintability and weldability, a heat treatment temperature T below or equal to 950°C is preferred. t .

[0227] Determining the theoretical average aluminum content in welded joint 22 Following the preparation steps (as needed or desired), the method further includes the step of butt welding the first pre-coated steel plate 1 to the second pre-coated steel plate 2 using laser welding to obtain a welded joint 22 between the first pre-coated steel plate 1 and the second pre-coated steel plate 2, thereby obtaining a welded steel billet 15.

[0228] The aluminum content of welded joint 22 is 0.5% to 1.25% by weight.

[0229] According to one implementation, the welding step includes the use of filler material.

[0230] Advantageously, the filler material is a steel-based filler material with an aluminum content of less than or equal to 0.05% by weight. The filler material has a low aluminum content to dilute the aluminum in the coating.

[0231] For example, the filler material also contains austenite-forming elements to partially balance the ferrite formation and / or intermetallic compound formation effects from the pre-coated aluminum 7, 8.

[0232] The filler material is, for example, filler wire or powder.

[0233] The proportion of filler material added to the weld pool is, for example, 0 to 0.5.

[0234] Based on one example, the filler material has the following composition by weight:

[0235]

[0236] Optional:

[0237] Trace ≤Mo≤1%

[0238] Trace amount ≤ Ti ≤ 0.1%

[0239] Trace amount ≤ V ≤ 0.1%

[0240] Trace amount ≤ B ≤ 0.01%

[0241] Trace amounts ≤ Nb ≤ 0.1%

[0242] Trace amount ≤Al ≤ 0.05%

[0243] The remainder consists of iron and impurities that are unavoidable during manufacturing.

[0244] Depending on the specific instance, the filler material may have one of the compositions W1, W2 or W3 as described in Table 1 below.

[0245]

[0246] In all these components, the content is expressed as a weight percentage.

[0247] In addition, for each component, the remainder consists of iron and unavoidable impurities.

[0248] In Table 1 above, "-" means that the composition contains at most trace amounts of the element.

[0249] According to one variation, the welding step is a self-fusion welding step, which means that welding is performed without the use of filler material. In this case, the composition of the weld joint 22 depends only on the composition of the substrates 3 and 4 of the first pre-coated steel plate 1 and the second pre-coated steel plate 2, and the amount of the pre-coating layers 7 and 8 incorporated into the weld joint 22.

[0250] The welding operation resulted in the formation of a weld joint 22 at the junction between the two plates 1 and 2.

[0251] The welding step is a laser welding step, in which the laser beam 24 is guided toward the joint between the two pre-coated steel plates 1 and 2.

[0252] Laser welding steps can be performed using, for example, CO2 lasers, solid-state lasers, or semiconductor lasers.

[0253] The laser source is preferably a high-power laser source. It can be, for example, selected from a CO2 laser with a wavelength of 10 micrometers, a solid-state laser source with a wavelength of 1 micrometer, or a semiconductor laser source such as a diode laser with a wavelength of 0.8 micrometers to 1 micrometer.

[0254] The laser power is selected based on the thickness of the first pre-coated steel plate 1 and the second pre-coated steel plate 2. Specifically, the power is selected to allow for melting of the weld edges 14 of the pre-coated steel plates 1 and 2, and adequate mixing within the weld joint 22. For CO2 lasers, the laser power is, for example, 3 kW to 12 kW. For solid-state lasers or semiconductor lasers, the laser power is, for example, 2 kW to 8 kW.

[0255] For both types of laser sources, the diameter of the laser beam 24 on the pre-coated steel plates 1 and 2 at its impact point 26 can be approximately 600 μm.

[0256] During the welding process, welding is performed, for example, under a protective atmosphere. This protective atmosphere specifically prevents oxidation and decarburization in the areas where welding is carried out, prevents the formation of boron nitride in the weld joint 22, and prevents possible cold cracking caused by hydrogen absorption.

[0257] The protective atmosphere is formed, for example, by an inert gas or a mixture of inert gases. The inert gas can be helium or argon, or a mixture of these gases.

[0258] Welding can be performed using a laser beam as the sole heat source.

[0259] Alternatively, in addition to the laser beam, the laser welding step may include an additional heat source, such as an electric arc or induction heating. This additional heat source helps to melt the edges of the first pre-coated steel plate 1 and the second pre-coated steel plate 2 to form a weld joint 22.

[0260] Optionally, the welding step includes using filler wire 20, such as Figure 1 As shown by the dashed line. In this case, the laser beam 24 is further configured to melt the filler wire 20 at the impact point 26 of the laser beam 24.

[0261] During the welding step, the distance between the welding-facing edges 14 of the two pre-coated steel plates 1, 2 is, for example, less than or equal to 0.3 mm, and more specifically less than or equal to 0.1 mm. Providing such a gap between the welding-facing edges 14 of the two plates 1, 2 facilitates the deposition of material from the possible filler wire 20 during the welding operation and prevents excessive thickness from forming at the weld joint 22.

[0262] At the end of the welding step, the following is obtained: Figure 2 The welded steel billet 15 shown in the figure.

[0263] Following the welding step, the method according to the invention includes the step of heating the welded steel billet 15 thus obtained in a heat treatment furnace.

[0264] More specifically, the heating step includes heating the welded steel billet 15 to the heat treatment temperature T. t .

[0265] According to the present invention, the heat treatment temperature T t It is at least 10°C lower than the full austenitization temperature Ac3 (WJ) of weld joint 22.

[0266] The complete austenitizing temperature Ac3 (WJ) (in °C) of welded joint 22 is determined, for example, by the composition of welded joint 22 using the following formula:

[0267] Al, C, Mn and Si refer to the contents of aluminum, carbon, manganese and silicon in the welded joint 22, respectively, expressed as % by weight.

[0268] The above formula for Ac3 (WJ) can be used within the content range shown in Table 2 below:

[0269]

[0270] In Table 2 above:

[0271] - All contents are expressed as a percentage by weight.

[0272] - "-" means there is no lower limit.

[0273] According to the present invention, the heat treatment temperature T t It's also lower than the lowest temperature T min At least 15°C higher. In this article, the lowest temperature T... min The following limitations apply:

[0274]

[0275] in

[0276] Ac3(WJ) is the complete austenitizing temperature of weld joint 22, expressed in °C.

[0277] Al represents the aluminum content in welded joint 22, expressed as a percentage by weight.

[0278]

[0279] in

[0280] Ts1 is the ultimate tensile strength of the strongest matrix 3 after compression hardening, expressed in MPa.

[0281] Ts2 is the ultimate tensile strength of the weakest matrix 4 after compression hardening, expressed in MPa.

[0282] β is the proportion of filler material added to the weld pool, ranging from 0 to 1.

[0283] C FW Carbon content of the filler material, expressed as a percentage by weight.

[0284] ρ is the ratio of the thickness of the pre-coated steel plate 2 including the weakest substrate 4 to the thickness of the pre-coated steel plate 1 including the strongest substrate 3 (ρ=t2 / t1).

[0285] In this paper, if the matrix has a lower ultimate tensile strength Ts after compression hardening, it is weaker than the other.

[0286] Therefore, the lowest temperature T min The calculation can be based on the following:

[0287] - Chemical composition of weld joint 22

[0288] - Characteristics of the substrates 3 and 4 of the pre-coated steel plates 1 and 2.

[0289] - When using filler materials, the proportion and composition of the filler materials.

[0290] The step of heating and welding the blank 15 also includes heating the welding steel blank 15 at a heat treatment temperature T. t Follow the steps to maintain the position for 2 to 10 minutes.

[0291] At the end of the heating step, since the welded steel billet 15 has been heated to a temperature at least 10°C lower than the full austenitization temperature Ac3 (WJ) of the weld joint 22, the microstructure of the weld joint 22 is not entirely austenitic. The fraction of sub-thermal ferrite in the weld joint 22 depends on the heat treatment temperature T of the weld joint 22. t The temperature difference between the temperature and the complete austenitizing temperature Ac3 (WJ). Specifically, the fraction of sub-temperature ferrite in the weld joint 22 at the end of the heating step. Greater than or equal to 15%, and greater than the maximum sub-temperature ferrite fraction At least 5% (15% ≤) ≤ -5%).

[0292] Maximum sub-temperature ferrite fraction (In percentage terms) can be determined using the following formula:

[0293] ,

[0294] in

[0295] Ts1 is the ultimate tensile strength of the strongest matrix 3 after compression hardening, expressed in MPa.

[0296] Ts2 is the ultimate tensile strength of the weakest matrix 4 after compression hardening, expressed in MPa.

[0297] β is the proportion of filler material added to the weld pool, ranging from 0 to 1.

[0298] C FW Carbon content of the filler material, expressed as a percentage by weight.

[0299] ρ is the ratio of the thickness of the pre-coated steel plate 2 including the weakest substrate 4 to the thickness of the pre-coated steel plate 1 including the strongest substrate 3 (ρ=t2 / t1).

[0300] As is known to those skilled in the art, the sub-temperature ferrite fraction can be determined, for example, by heating to a heat treatment temperature T. t The welded blank 15 was then directly quenched for measurement. After etching with a suitable nitric acid ethanol etchant (Nital), the sub-temperature ferrite appeared as a light-colored component on a light gray martensitic matrix.

[0301] The sub-thermal ferrite fraction in weld joint 22 can also be determined by analyzing a manganese elemental mapping image of a sample illustrating the distribution of manganese content in the sample. Such a mapping image can be obtained, for example, by analyzing the sample via electron probe microanalysis (EPMA). In this Mn mapping image, the region showing the minimum Mn content coincides with the sub-thermal ferrite region, while regions with higher Mn content correspond to the phase resulting from the austenite transformation formed during critical zone annealing. Therefore, the surface fraction of sub-thermal ferrite in this image corresponds to the surface fraction of the region with the minimum Mn content. This method is described, for example, in the following literature: Hanlon, D, Rijkenberg, A, Leunis, E, et al.: Quantitative phase analysis of multi-phase steels, PHAST (2007), ISBN 92-79-02658-5, pp. 77-79. In fact, it is known that during critical zone annealing, manganese partitioning occurs between austenite and ferrite, with manganese migrating from ferrite to austenite. This results in the Mn content of sub-critical ferrite being strictly less than that of austenite at the end of critical zone annealing. During subsequent cooling, phases formed from austenite (e.g., martensite, transformed ferrite, and / or bainite) inherit the Mn content from austenite, while sub-critical ferrite retains its lower Mn content resulting from partitioning. Therefore, on the Mn elemental mapping diagram, sub-critical ferrite can be distinguished from other phases, and particularly from other types of ferrite, and corresponds to the region where the Mn content is minimum.

[0302] In the context of this patent application, all fractions relating to microstructure are expressed as surface percentages.

[0303] At the end of the heating step, the microstructure of the substrates 3 and 4 of the first pre-coated steel plate 1 and the second pre-coated steel plate 2 is entirely austenitic. In particular, during welding, due to the presence of aluminum from the pre-coatings 5 ​​and 6 on the weld edges 14 of the pre-coated steel plates 1 and 2, the complete austenitizing temperature Ac3 of the substrates 3 and 4 is strictly lower than the complete austenitizing temperature Ac3 (WJ) of the weld joint 22.

[0304] At the end of the heating step, the welded steel billet 15 is hot-formed into a steel part under pressure in a pressing forming tool. For example, the welded steel billet 15 is formed into a steel part by hot stamping using a suitable hot stamping tool.

[0305] Preferably, the transfer time between the heat treatment furnace and the pressing tool is less than or equal to 10 seconds. For example, it is 5 to 10 seconds. The transfer time is chosen to be as short as possible to avoid metallographic transformation in the weld blank 15 before hot forming, especially the formation of ferrite.

[0306] The steel component thus formed is then cooled at a cooling rate greater than or equal to the critical martensite or bainite cooling rate of the most hardenable matrix 3, 4 among the first pre-coated steel plate 1 and the second pre-coated steel plate 2.

[0307] Advantageously, the cooling step is performed in a compression molding tool, for example by using a molding tool equipped with a cooling system (including, for example, cooling channels formed in the compression molding tool).

[0308] According to the present invention, at the end of the cooling step, the weld joint 22 has a microstructure comprising martensite and / or bainite and a fraction greater than or equal to 15% and exceeding the maximum sub-temperature ferrite fraction. Less than 5% of the sub-temperature ferrite fraction (15%≤) ≤ -5%). Maximum sub-temperature ferrite fraction. This can be determined as described above.

[0309] At the end of the cooling step, at least one of the matrices 3 and 4 has a predominantly martensitic and / or bainitic microstructure. The martensite and / or bainite are generated by the transformation of austenite formed during the heating step during the cooling step.

[0310] In one example, both matrix 3 and 4 have a predominantly martensitic and / or bainitic structure.

[0311] In this paper, "major" means that the microstructure consists of martensite and / or bainite and up to 5% ferrite.

[0312] The present invention also relates to laser-welded steel components that have undergone pressure hardening using the above-described method.

[0313] This component is specifically a collision control component, such as an intrusion prevention component or a shock absorption component, a structural component, or a component that contributes to the safety of a motor vehicle.

[0314] The press-hardened laser-welded steel component includes a first coated steel component portion and a second coated steel component portion connected by a weld joint 22 as described above.

[0315] More specifically, the first coated steel component and the second coated steel component are respectively produced by hot pressing and cooling of the first pre-coated steel plate 1 and the second pre-coated steel plate 2 in a pressing tool.

[0316] More specifically, each coated steel component includes a steel substrate having an aluminum-containing coating comprising iron and at least 30% by weight of aluminum on at least one of its main surfaces.

[0317] Specifically, the aluminum-containing coatings of the first steel component and the second steel component are produced by at least partial alloying of the pre-coatings 7 and 8 during hot pressing.

[0318] The substrates of the first and second steel components have the composition described above for pre-coated steel plates 1 and 2. They are produced by hot pressing and cooling of the substrates 3 and 4 of the pre-coated steel plates 1 and 2.

[0319] The ultimate tensile strength Ts1 of the substrate of the first coated steel component is significantly greater than the ultimate tensile strength Ts2 of the substrate of the second coated steel component.

[0320] For example, the first coated steel component portion has a first thickness and the second coated steel component portion has a second thickness, and the product of the first thickness of the first coated steel component portion and the ultimate tensile strength is strictly greater than the product of the second thickness of the second coated steel component portion and the ultimate tensile strength Ts2.

[0321] The aluminum content of welded joint 22 is 0.5% to 1.25% by weight.

[0322] Welded joint 22 has a microstructure comprising martensite and / or bainite and a fraction greater than or equal to 15% and exceeding the maximum sub-temperature ferrite fraction. Less than 5% of the sub-temperature ferrite fraction (15%≤) ≤ -5%).

[0323] Maximum sub-temperature ferrite fraction This can be determined as described above.

[0324] In press-hardened laser-welded steel components, the proportion β of filler material added to the weld pool during the welding operation can be measured by any suitable method, specifically the aluminum content in the weld joint 22. To determine. The aluminum content in the coating of the welded steel plate is known. Furthermore, considering that the aluminum content in the filler material is negligible, based on the above formula... The proportion β can be calculated using the following formula: Then, the carbon content C of the filler material. FW The carbon content of the substrates 3 and 4 can be determined based on the following: the proportion β of the filler material determined based on the aluminum content of the weld joint 22, and the measured carbon content in the weld joint 22.

[0325] After compression hardening, the ultimate tensile strength of welded joint 22 is greater than or equal to the ultimate tensile strength of the weakest matrix 4.

[0326] The steel on at least one side of the welded joint 22 (corresponding to the steel of at least the first matrix 3) has a predominantly martensitic and / or bainitic microstructure. For example, the steel on either side of the welded joint 22 (corresponding to the steel of the first matrix 3 and the second matrix 4) has a predominantly martensitic and / or bainitic microstructure.

[0327] The inventors of this invention unexpectedly discovered that when the weld blank 15 is subjected to heat treatment under the above conditions, the ultimate tensile strength of the weld joint 22 is significantly greater than the ultimate tensile strength of the substrate 4 of the second pre-coated steel plate 2 (i.e., the substrate with the lowest ultimate tensile strength). Therefore, when subjected to a tensile test in a direction perpendicular to the weld joint 22, the component obtained after the above heat treatment will not fail in the weld joint 22, even if the microstructure of the heat-treated weld joint 22 is not entirely martensite or bainite.

[0328] Therefore, the method according to the invention is particularly advantageous because it allows for satisfactory mechanical properties at a reduced cost. In fact, when pre-coated steel plates, including an aluminum pre-coating, are welded together, it is no longer necessary to adjust the composition of the weld joint in a manner that makes the full austenitization temperature of the weld joint less than or equal to that of the base material, for example, by removing the pre-coating from both sides of the pre-coated steel plate or by adding a large amount of austenite-forming elements to the weld using filler materials such as filler wire. In particular, avoiding the removal of the pre-coating from both sides of the steel plate reduces the total processing time. Furthermore, reducing the amount of austenite-forming elements that must be added via filler material, or even completely avoiding the use of filler material, reduces production costs and prevents problems arising from adding a high proportion of filler material, particularly concerning the geometry of the weld joint and obtaining a uniform mixture between the material from the pre-coated steel plate and the material from the filler material in the weld joint.

[0329] The inventors of this invention conducted experiments E1 to E36, in which pre-coated steel plates 1 and 2 were used to produce welded steel billets 15. Each pre-coated steel plate 1 and 2 has a substrate 3 and 4 having the following composition (see Table 5), and has a pre-coating layer 7 and 8 formed by hot-dip coating on two main surfaces. The pre-coating layer 7 and 8 includes a metal alloy layer 11 and 12 containing 88% by weight aluminum, 10% by weight silicon and 2% iron.

[0330] Prior to any removal step, the total weight per unit area of ​​the pre-coating 7 and 8 on the two main surfaces of each pre-coated steel plate 1 and 2 is 150 g / m². 2 .

[0331] After removing the metal alloy layers 11 and 12 from only one of the main surfaces 5 and 6 of the pre-coated steel plates 1 and 2, leaving the intermetallic compound alloy layers 9 and 10 intact, the total weight per unit area of ​​the remaining pre-coated layers 7 and 8 on each pre-coated steel plate 1 and 2 is 100 g / m².2 .

[0332] The composition of the substrate used in the experiment is disclosed in Table 3 below. The composition of the filler wire used in the experiment is disclosed in Table 4 below.

[0333]

[0334] In Tables 3 and 4 above, the composition is expressed as a weight percentage.

[0335] Furthermore, for each component in Tables 3 and 4, the remainder consists of iron and unavoidable impurities.

[0336] The "-" sign indicates that the element is composed of at most trace amounts.

[0337] The complete austenitization temperature Ac3 and ultimate tensile strength Ts of the above matrices S1, S2 and S3 are as follows:

[0338] S1: 834℃; Ts = 1500 MPa

[0339] S2: 858℃; Ts = 1050 MPa

[0340] S3: 806℃; Ts = 700 MPa

[0341] Butt welding of pre-coated steel plates 1 and 2 was performed using a disk laser with a power of 5.6 kW or a YAG laser with a power of 4 kW.

[0342] In all embodiments, a protective atmosphere consisting of helium or argon is used to prevent oxidation and decarburization in the areas where welding is performed, as well as the formation of boron nitride in the weld joint and possible cold cracking due to hydrogen absorption. The gas flow rate is greater than or equal to 15 l / min.

[0343] Then the welded blank 1 is subjected to a heat treatment temperature T, including heating to 920°C. t The blank is then heat-treated at this temperature for six minutes, and the transfer time between the heating furnace and the hot forming tool is selected to prevent the formation of ferrite. The blank is then transferred to the hot pressing tool and cooled in the pressing tool at a cooling rate of greater than or equal to 30°C / second for one minute to obtain a press-hardened blank.

[0344] The experimental conditions used for experiments E1 to E36 are summarized in Tables 5 and 6 below.

[0345] Tensile specimens are then cut from the heat-treated blank obtained in a direction perpendicular to the weld joint.

[0346] Tensile tests were performed on longitudinal tensile specimens of type EN 12,5×50 (240×30 mm) extracted parallel to the rolling direction at ambient temperature (approximately 20°C) using the methods disclosed in the following standards: NF EN ISO 4136 and NF ISO 6892-1. Five tensile tests were performed for each heat-treated welded blank.

[0347] The results of the tensile test are shown in the column titled "Failure Location" in Table 6 below, which shows the location where failure occurred during the tensile test.

[0348] In this column:

[0349] - "BM" refers to failure in the base metal (i.e., in the matrix of one of the pre-coated plates);

[0350] - "Weld" refers to failure in a welded joint; and

[0351] - "Mix" refers to a situation where some of the tensile specimens fail in the welded joint while others fail in the base metal.

[0352]

[0353]

[0354] In Table 5 above, 150g / m 2 The pre-coating weight corresponds to the following situation: no preparation step is performed before welding, that is, the pre-coating on both main surfaces of the pre-coated steel plate remains intact during welding; while the pre-coating weight of 100 g / m2 corresponds to the following situation: the pre-coated steel plate is prepared by removing the metal alloy layers 11 and 12 on only one main surface of each of the pre-coated steel plates 1 and 2 before welding, leaving the intermetallic compound alloy layers 9 and 10 intact.

[0355]

[0356]

[0357] In Tables 5 and 6 above, no lines are drawn according to embodiments of the present invention.

[0358] These results show that when the weld blank 15 is heated to a heat treatment temperature within the above-mentioned temperature range before pressing and cooling, and held at that heat treatment temperature for 2 to 10 minutes, the failure occurs in the weakest base metal of the component (the “substrate of the second pre-coated steel plate” in Tables 5 and 6 above), rather than in the weld joint 22 (experiments E1, E2, E5, E10, E12, E13, E16, E18, E22 and E29 to E32).

[0359] Conversely, for temperatures strictly below the minimum heat treatment temperature T min A heat treatment temperature of +15°C and a holding time of 2 to 10 minutes at that heat treatment temperature were observed to result in either failure in welded joint 22 (experiments E3, E4, E6 to E8, E14, E15, E17, E19, E21, E23, E27 to E27 and E33 to E36), or failure in at least some of the welded joints 22 in the tensile specimens for the experiments considered (experiments E9, E11, E20, E24 and E28, referred to as “mixed” in the table).

[0360] The inventors also noted that, in all experiments according to the invention, the welded joint 22 had a sub-temperature ferrite fraction. 15% to -5% of the microstructure.

[0361] These results demonstrate that when the weld blank 15 is heat-treated using the heat treatment conditions according to the invention, the ultimate tensile strength of the weld joint 22 is significantly greater than the ultimate tensile strength of the weakest substrate corresponding to the matrix 4 of the second pre-coated steel plate 2. Therefore, the matrix 4 forms the weakest region of the component, not the weld joint 22. Consequently, failure occurs in the matrix 4 of the second pre-coated steel plate 2, rather than in the weld joint 22 itself. These results are unexpected, as they were obtained even though the weld joint 22 is not fully austenitized after heat treatment and therefore does not possess a predominantly martensitic and / or bainitic microstructure.

[0362] Therefore, the method according to the invention is particularly advantageous because it allows the determination of optimal process parameters (including the minimum heat treatment temperature and the amount of filler material to be added) in order to obtain parts with satisfactory properties while minimizing production costs and time.

Claims

1. A method for producing press-hardened laser-welded steel components, comprising the following sequential steps: - Provide a first pre-coated steel plate (1) and a second pre-coated steel plate (2), each of the first pre-coated steel plate (1) and the second pre-coated steel plate (2) comprising a substrate, and at least one of the first pre-coated steel plate (1) and the second pre-coated steel plate (2) having an aluminum-containing pre-coating (7, 8) comprising at least 50% by weight of aluminum on at least one of its main surfaces. The first pre-coated steel plate (1) has a first thickness (t1) and the second pre-coated steel plate (2) has a second thickness (t2). The ultimate tensile strength (Ts1) of the substrate (3) of the first pre-coated steel plate (1) after compression hardening is strictly greater than the ultimate tensile strength (Ts2) of the substrate (4) of the second pre-coated steel plate (2) after compression hardening, and The product of the first thickness (t1) of the first pre-coated steel plate (1) and the ultimate tensile strength (Ts1) after press hardening is strictly greater than the product of the second thickness (t2) of the second pre-coated steel plate (2) and the ultimate tensile strength (Ts2) after press hardening. The thickness of the first pre-coated steel plate and the second pre-coated steel plate (2) is 0.6 mm to 5 mm. Then - At least in the theoretical average aluminum content in the weld joint (22) obtained by butt welding the first pre-coated steel plate (1) and the second pre-coated steel plate (2) provided in the providing step, possibly using a filler material containing up to 0.05% by weight of aluminum. In cases where the aluminum content is strictly greater than 1.25% by weight, the aluminum pre-coating (7, 8) is removed from at least a portion of the thickness of the aluminum pre-coating (7, 8) on at least one main surface (5, 6) at the weld edge (14) of at least one of the first pre-coated steel plate (1) and the second pre-coated steel plate (2), such that the theoretical average aluminum content in the weld joint (22) obtained by butt welding the first pre-coated steel plate (1) and the second pre-coated steel plate (2) thus prepared by using a filler material containing at most 0.05% by weight of aluminum is ( ). The content ranges from 0.5% to 1.25% by weight. - The first pre-coated steel plate (1) and the second pre-coated steel plate (2) are butt-welded using laser welding to obtain a weld joint (22) between the first pre-coated steel plate (1) and the second pre-coated steel plate (2), thereby obtaining a weld blank (15). This welding step may include the use of filler material (20). - Heat the welded blank (15) to the heat treatment temperature (T t The heat treatment temperature (T) t The temperature is at least 10°C lower than the full austenitization temperature (Ac3(WJ)) of the weld joint (22), and lower than the lowest temperature T. min At least 15°C higher, of which in Ac3(WJ) is the complete austenitizing temperature of the welded joint (22) in °C, and Al is the aluminum content in the welded joint (22) in % by weight. as well as The maximum sub-temperature ferrite content of the welded joint (22) is calculated using the following formula. ， in Ts1 is the ultimate tensile strength of the strongest matrix after compression hardening, expressed in MPa. Ts2 is the ultimate tensile strength of the weakest matrix after compression hardening, expressed in MPa. C FW The carbon content of the filler material, expressed as a percentage by weight. β is the proportion of filler material added to the weld pool, ranging from 0 to 1. ρ is the ratio of the thickness of the pre-coated steel plate (2) including the weakest substrate to the thickness of the pre-coated steel plate (1) including the strongest substrate (ρ=t2 / t1). The weld blank (15) is held at the heat treatment temperature (Tt) for 2 to 10 minutes. - Press the welded blank (15) into a steel component; and - The steel component thus formed is cooled at a cooling rate greater than or equal to the critical martensite or bainite cooling rate of the most hardenable matrix in the matrix of the first pre-coated steel plate (1) and the second pre-coated steel plate (2) to obtain a press-hardened welded steel component.

2. The method according to claim 1, wherein, After pressing and hardening, the ratio of the ultimate tensile strength (Ts1) of the substrate (3) of the first pre-coated steel plate (1) to the ultimate tensile strength (Ts2) of the substrate (4) of the second pre-coated steel plate (2) is greater than or equal to 1.

2.

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

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

5. The method according to claim 1 or 2, wherein the first pre-coated steel plate (1) and the second pre-coated steel plate (2) provided in the providing step include an aluminum-containing pre-coating (7, 8) comprising at least 50% by weight of aluminum on their two main surfaces (5, 6).

6. The method according to claim 1 or 2, wherein, During butt welding, the aluminum pre-coating (7, 8) remains intact on at least one of the two main surfaces (5, 6) of the first pre-coated steel plate (1) and the second pre-coated steel plate (2).

7. The method according to claim 1 or 2, further comprising the following step before butt welding: Even if the theoretical average aluminum content in the weld joint (22) obtained by butt welding the first pre-coated steel plate (1) and the second pre-coated steel plate (2) provided in the providing step with a filler material containing up to 0.05% by weight of aluminum is used ( The content is 0.5% to 1.25% by weight, and the weld edge (14) of the at least one of the first pre-coated steel plate (1) and the second pre-coated steel plate (2) is also prepared by removing the aluminum pre-coating (7, 8) from at least a portion of the thickness of the aluminum pre-coating (7, 8) on at least one of the main surfaces (5, 6) of the first pre-coated steel plate (1) and the second pre-coated steel plate (2).

8. The method according to claim 1 or 2, wherein, For at least one of the first pre-coated steel plate (1) and the second pre-coated steel plate (2), the steel of the substrate comprises, by weight: The remainder consists of iron and impurities produced during manufacturing.

9. The method according to claim 8, wherein, For at least one of the first pre-coated steel plate (1) and the second pre-coated steel plate (2), the steel of the substrate comprises, by weight: The remainder consists of iron and impurities produced during manufacturing.

10. The method according to claim 1 or 2, wherein, For at least one of the first pre-coated steel plate (1) and the second pre-coated steel plate (2), the steel of the substrate comprises, by weight: The remainder consists of iron and impurities produced during manufacturing.

11. The method according to claim 1 or 2, wherein, For at least one of the first pre-coated steel plate (1) and the second pre-coated steel plate (2), the steel of the substrate comprises, by weight: The remainder consists of iron and impurities produced during manufacturing.

12. The method according to claim 1 or 2, wherein, For at least one of the first pre-coated steel plate (1) and the second pre-coated steel plate (2), the steel of the substrate comprises, by weight: The contents of titanium and nitrogen satisfy the following relationship: Ti / N > 3.42, The contents of carbon, manganese, chromium, and silicon satisfy the following relationship: ， The steel optionally comprises one or more of the following elements: The remainder consists of iron and impurities that are unavoidable during manufacturing.

13. The method according to claim 1 or 2, wherein the laser welding is performed using a shielding gas.

14. The method of claim 13, wherein the protective gas is helium and / or argon.

15. The method according to claim 1 or 2, wherein the first pre-coated steel plate (1) and the second pre-coated steel plate (2) have different thicknesses.

16. The method of claim 1 or 2, wherein the welding is performed without the use of filler material.

17. The method of claim 1 or 2, wherein the welding is performed using a filler material.

18. The method of claim 17, wherein the filler material has the following composition by weight: Optional location: Trace ≤Mo≤1% Trace amount ≤ Ti ≤ 0.1% Trace amount ≤ V ≤ 0.1% Trace amount ≤ B ≤ 0.01% Trace amounts ≤ Nb ≤ 0.1% Trace amount ≤Al ≤ 0.05% The remainder consists of iron and impurities that are unavoidable during manufacturing.

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