Methods for producing welded steel billets and related welded billets

Abstract

This application relates to a method for producing a weld blank (1), comprising: providing two pre-coated plates (2), butt-welding the pre-coated plates (2) using filler wire, wherein during butt welding, a pre-coated layer (5) completely covers at least one face (4) of each plate (2), the filler wire (20) having a carbon content of 0.01% to 0.45% by weight, and the composition of the filler wire (20) and the proportion of filler wire (20) added to the weld pool are selected such that the weld joint (22) has (a) a quenching factor FT. WJ FT WJ -0.9FT BM ≥0, where: FT BM The quenching factor of the least hardenable matrix (3), and FT WJ and FT BM The value is determined to be: FT = 128 + 1553xC + 55xMn + 267xSi + 49xNi + 5xCr - 79xAl - 2xNi 2 -1532xC 2 -5xMn 2 -127xSi 2 -40xCxNi-4xNixMn, and (b) carbon content C WJ <0.15% by weight, or, if C WJ ≥0.15% by weight, then the softening factor FA WJ Make FA WJ >5000, where FA=10291+4384.1xMo+3676.9Si-522.64xAl-2221.2xCr-118.11xNi-1565.1xC-246.67xMn. This application also relates to welded steel billets and steel components formed by welding, hot pressing and cooling.

Description

[0001] This invention patent application is a divisional application of the invention patent application filed on November 26, 2018, with application number 201880075840.0 and invention title "Method for producing welded steel billets, related welded billets, welded components and their uses".

[0002] The present invention relates to a method for producing welded steel billets, the welded steel billets obtained therefrom, a method for producing steel parts from the welded steel billets that are welded, hot-pressed and cooled, and the steel parts obtained therefrom that are welded, hot-pressed and cooled.

[0003] Methods for manufacturing welded components from steel plates of different compositions and / or thicknesses, butt-welded to each other, are known from the prior art. More specifically, the weld blank is typically heated to a temperature that allows for austenitization of the steel, then hot-formed in a hot-pressing tool and cooled. The composition of the steel can be selected to allow for subsequent heating and forming operations and to impart high mechanical strength, high impact strength, and good corrosion resistance to the welded steel components.

[0004] This type of steel component is used particularly in the automotive industry, and even more so in the manufacture of intrusion prevention components, structural components, or components that contribute to the safety of motor vehicles.

[0005] To prevent corrosion, the steel sheet is pre-coated with an aluminum-based pre-coating by hot-dip immersion in an aluminum bath. If the steel sheet is welded without any prior preparation, the aluminum-based pre-coating will be diluted by the steel substrate within the molten metal during the welding operation. Then, within the range of aluminum content in the pre-coating, two phenomena may occur.

[0006] If the aluminum content in the molten metal is locally high, intermetallic compounds will form in the weld joint. This is caused by the dilution of a portion of the pre-coating in the molten metal and by alloying that occurs during subsequent heating of the weld joint before the thermoforming step. These intermetallic compounds are the most likely sites for initial cracking.

[0007] Furthermore, aluminum tends to increase the austenitizing temperature (Ac3) of the weld joint, and this change in the austenitic domain is even more significant due to the high aluminum levels in the weld joint. In some cases, this may prevent complete austenitization of the weld joint, which should occur during heating before forming and is the first step required to obtain a martensitic structure in the weld joint after hot stamping and hot pressing and cooling.

[0008] In addition, aluminum has an adverse effect on the hardenability of welded joints because it increases the critical cooling rate required to obtain martensitic or bainitic structures in the welded joint during cooling.

[0009] Therefore, martensite or bainite can no longer be obtained during the cooling process after hot forming, and the resulting weld joint will contain ferrite. The weld joint then exhibits lower hardness and mechanical strength compared to the two adjacent plates, and thus constitutes the weakest area of ​​the component.

[0010] Publication EP2007545 describes a solution involving the removal of a surface layer of metallic 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 performed by brushing or using a laser beam. The intermetallic compound alloy layer is retained to ensure corrosion resistance and prevent decarburization and oxidation during heat treatment prior to forming operations. The impact of aluminum is then significantly reduced by locally removing the surface layer of the coating.

[0011] However, removing the pre-coating is a supplementary step and therefore increases production costs.

[0012] EP 2 737 971, US 2016 / 0144456 and WO 2014075824 attempt to overcome this problem by providing a method for welding pre-coated plates using filler wires containing austenitic stabilizing elements such as carbon, manganese or nickel, with the aim of obtaining a fully martensitic structure in the weld joint after hot pressing and cooling, despite the presence of aluminum in the weld due to the melting of the pre-coating.

[0013] However, these methods are not entirely satisfactory because they only address one of the problems related to the presence of aluminum in the weld pool: compensation for the austenitizing temperature (Ac3), and in some cases, the use of high-carbon filler wire can cause segregation in the weld joint. In fact, the inventors of this invention have found that the methods disclosed in the aforementioned documents do not allow for satisfactory mechanical properties in the parts obtained after hot pressing and cooling, particularly for weld joints with an aluminum content greater than or equal to 0.7% by weight, and even more so for those with an aluminum content greater than or equal to 2.1%. Specifically, for such parts, there is a high risk of weld joint failure under tensile testing in the transverse direction of the weld.

[0014] The methods disclosed in WO 2015 / 086781 and EP 2 942 143 also address this problem and describe a method for welding pre-coated steel plates using specific filler materials and specific welding methods.

[0015] More specifically, WO 2015 / 086781 recommends the use of two-point laser welding, while supplying a filler material in the form of metal powder having the following composition by weight percentage: C: 0 wt% to 0.03 wt%, Mo: 2.0 wt% to 3.0 wt%, Ni: 10 wt% to 14 wt%, Mn: 1.0 wt% to 2.0 wt%, Cr: 16 wt% to 18 wt%, and Si: 0.0 wt% to 1.0 wt%, with the balance being iron.

[0016] EP 2 942 143 recommends the use of hybrid laser / arc welding with an arc welding torch located in front of the laser beam, while supplying filler material in the form of filler wire having the following composition: C: 0 wt% to 0.3 wt%, Mo: 0 wt% to 0.4 wt%, Ni: 6 wt% to 20 wt%, Mn: 0.5 wt% to 7 wt%, Cr: 5 wt% to 22 wt%, and Si: 0 wt% to 1.3 wt%, Nb: 0 wt% to 0.7 wt%, with the remainder being iron.

[0017] These methods are also unsatisfactory. In fact, the inventors of this invention observed that the use of filler wire described herein leads to a high risk of component failure in the region immediately adjacent to the weld after hot pressing and cooling.

[0018] Furthermore, the use of hybrid laser-arc welding is undesirable because hybrid laser / arc welding does not allow for the same welding speed as laser welding, thus resulting in a reduction in the overall productivity of the process.

[0019] Furthermore, powder addition is generally more difficult to implement in large-scale industrial environments compared to filler wire.

[0020] All the methods mentioned above based on filler material addition only specify the range of chemical composition of the filler material. Furthermore, because welding parameters and conditions affect the filler material ratio, a single filler wire can result in a very different chemical composition in the weld joint. Therefore, simply describing the composition of the filler wire seems insufficient to address the aforementioned issues.

[0021] Therefore, one object of the present invention is to provide a method for producing welded steel billets from two pre-coated plates at a relatively low cost, which allows for the production of parts with satisfactory impact performance characteristics after hot pressing and cooling, even for relatively high aluminum content in the weld joint.

[0022] For this purpose, it is desirable that the weld joint does not constitute the weakest zone of the component obtained after hot pressing and cooling of the weld blank. Therefore, when subjected to tension in a direction perpendicular to the weld joint, such a component should not fail in the weld joint or in the region adjacent to the weld joint corresponding to the heat-affected zone generated by the welding operation.

[0023] For this purpose, the present invention relates to a method for producing welded steel billets, comprising the following sequential steps:

[0024] - Two pre-coated plates are provided, each pre-coated plate comprising a steel substrate having a pre-coated layer on at least one of its main surfaces, the pre-coating layer comprising an intermetallic compound alloy layer comprising at least iron and aluminum, and optionally a metal alloy layer extending on top of the intermetallic compound alloy layer, the metal alloy layer being an aluminum layer, an aluminum alloy layer, or an aluminum-based alloy layer.

[0025] - Use filler wire to butt weld the pre-coated plates to form a weld joint at the junction between the pre-coated plates, wherein the pre-coating completely covers at least one main surface of each pre-coated plate during butt welding.

[0026] in:

[0027] - The carbon content of the filler wire is from 0.01% to 0.45% by weight.

[0028] -So that the resulting welded joint is characterized by selecting the composition of the filler wire and the proportion of filler wire added to the weld pool in the following manner:

[0029] (a) Quenching factor FT of welded joint WJ This makes FT WJ -0.9FT BM ≥0 (Standard C2),

[0030] in:

[0031] -FT BM The quenching factor of the least hardenable steel matrix in the two pre-coated plates, and

[0032] -Quenching factor FT WJ and FT BM Use the following formula to determine:

[0033] FT=128+1553xC+55xMn+267xSi+49xNi+5xCr-79xAl-2xNi 2 -1532xC 2 -5xMn 2 -127xSi 2 -40xCxNi-4xNixMn, where Al, Cr, Ni, C, Mn, and Si are the average aluminum, chromium, nickel, carbon, manganese, and silicon contents (in weight percentage) of the region for which the quenching factor is to be determined, in FT. WJ In the case of welded joints, and in FT BM In the case of the least hardenable matrix, and

[0034] (b) Carbon content C of the welded joint WJ Strictly less than 0.15% by weight, or if the carbon content C of the welded joint is less than 0.15% by weight. WJ If the softening factor FA of the welded joint is greater than or equal to 0.15% by weight, then the softening factor FA of the welded joint is... WJ Make FA WJ >5000 (Standard C3), where the softening factor FA of the welded joint WJ The following formula is used to calculate the average aluminum, chromium, nickel, molybdenum, carbon, manganese, and silicon content of the welded joint, expressed as a weight percentage:

[0035] FA=10291+4384.1xMo+3676.9Si-522.64xAl-2221.2xCr-118.11xNi-1565.1xC-246.67xMn.

[0036] According to a particular implementation, the method may include one or more of the following features, either alone or in any technically possible combination:

[0037] - Carbon content (C) of the welded joint, by weight percentage WJ Make 1.25×C BM(最可硬化) -C WJ ≥0 (Standard C4), where C BM The carbon content of the most hardenable matrix in the two pre-coated plates, expressed as a percentage by weight.

[0038] - Nickel content (Ni) of the welded joint WJ It ranges from 2.0% to 11.0% by weight (Standard C5).

[0039] - The pre-coated plate provided in the providing step has a pre-coating on both of its main surfaces.

[0040] - During butt welding, the pre-coating layer remains intact on both main surfaces of at least one of the pre-coated plates, and preferably, it remains intact on both main surfaces of both pre-coated plates.

[0041] The method further includes preparing a weld edge of at least one of the pre-coated plates to be at least partially incorporated into the weld joint by using at least one of the following processing steps prior to butt welding: brushing, machining, chamfering, beveling, and / or removing at least a portion of the pre-coating, thereby preparing the pre-coating in such a manner that the pre-coating is completely retained on at least one main surface of each of the two pre-coated plates.

[0042] The welding process is performed using a laser beam.

[0043] -For at least one of the pre-coated plates, the steel substrate comprises, by weight:

[0044] 0.10% ≤ C ≤ 0.5%

[0045] 0.5% ≤ Mn ≤ 3%

[0046] 0.1% ≤ Si ≤ 1%

[0047] 0.01% ≤ Cr ≤ 1%

[0048] Ti≤0.2%

[0049] Al≤0.1%

[0050] S≤0.05%

[0051] P≤0.1%

[0052] B≤0.010%

[0053] The remainder consists of iron and impurities produced during manufacturing.

[0054] -For at least one of the pre-coated plates, the steel substrate comprises, by weight:

[0055] 0.15% ≤ C ≤ 0.25%

[0056] 0.8% ≤ Mn ≤ 1.8%

[0057] 0.1% ≤ Si ≤ 0.35%

[0058] 0.01% ≤ Cr ≤ 0.5%

[0059] Ti≤0.1%

[0060] Al≤0.1%

[0061] S≤0.05%

[0062] P≤0.1%

[0063] B≤0.005%

[0064] The remainder consists of iron and impurities produced during manufacturing.

[0065] -For at least one of the pre-coated plates, the steel substrate comprises, by weight:

[0066] 0.040% ≤ C ≤ 0.100%

[0067] 0.80% ≤ Mn ≤ 2.00%

[0068] Si≤0.30%

[0069] S≤0.005%

[0070] P≤0.030%

[0071] 0.010% ≤ Al ≤ 0.070%

[0072] 0.015% ≤ Nb ≤ 0.100%

[0073] Ti≤0.080%

[0074] N≤0.009%

[0075] Cu≤0.100%

[0076] Ni≤0.100%

[0077] Cr≤0.100%

[0078] Mo≤0.100%

[0079] Ca ≤ 0.006%.

[0080] The remainder consists of iron and impurities produced during manufacturing.

[0081] -For at least one of the pre-coated plates, the steel substrate comprises, by weight:

[0082] 0.24% ≤ C ≤ 0.38%

[0083] 0.40% ≤ Mn ≤ 3%

[0084] 0.10% ≤ Si ≤ 0.70%

[0085] 0.015% ≤ Al ≤ 0.070%

[0086] 0% ≤ Cr ≤ 2%

[0087] 0.25% ≤ Ni ≤ 2%

[0088] 0.015% ≤ Ti ≤ 0.10%

[0089] 0% ≤ Nb ≤ 0.060%

[0090] 0.0005% ≤ B ≤ 0.0040%

[0091] 0.003% ≤ N ≤ 0.010%

[0092] 0.0001% ≤ S ≤ 0.005%

[0093] 0.0001% ≤ P ≤ 0.025%

[0094] The titanium and nitrogen contents satisfy the following relationship:

[0095] Ti / N > 3.42,

[0096] The carbon, manganese, chromium, and silicon contents satisfy the following relationship:

[0097]

[0098] Steel may optionally contain one or more of the following elements:

[0099] 0.05% ≤ Mo ≤ 0.65%

[0100] 0.001% ≤ W ≤ 0.30%

[0101] 0.0005% ≤ Ca ≤ 0.005%

[0102] The remainder consists of iron and impurities unavoidably produced during manufacturing, as well as

[0103] Welding is performed using shielding gases, particularly helium and / or argon.

[0104] The present invention also relates to a method for producing steel parts that are welded, hot-pressed, and cooled, comprising the following sequential steps:

[0105] - The method described above is used to obtain welded steel billets;

[0106] - The welded steel billet is heated to obtain a fully austenitic structure in the matrix of the pre-coated plate;

[0107] - To hot-press a welded steel billet into a steel component using a pressing tool; and

[0108] - Allow the steel parts to cool in the pressing tool.

[0109] According to a particular embodiment of a method for producing steel parts that are welded, hot-pressed, and cooled, during the cooling step, the cooling rate is greater than or equal to the cooling rate of the most hardenable bainitic or martensitic material in the matrix of the pre-coated plate.

[0110] The present invention also relates to a welded steel billet comprising two pre-coated plates, each pre-coated plate comprising a steel substrate having a pre-coated layer on at least one main surface thereof, the pre-coating layer comprising an intermetallic compound alloy layer comprising at least iron and aluminum, and optionally a metal alloy layer extending on top of the intermetallic compound alloy layer, the metal alloy layer being an aluminum layer, an aluminum alloy layer, or an aluminum-based alloy layer, the pre-coated plates being joined by a welded joint characterized in that:

[0111] (a) Quenching factor FT of welded joint WJ This makes FT WJ -0.9FT BM ≥0 (Standard C2),

[0112] in:

[0113] -FT BM It is the quenching factor of the least hardenable steel matrix in the two pre-coated plates, and

[0114] -Quenching factor FT WJ and FT BM The following formula is used to determine: FT = 128 + 1553xC + 55xMn + 267xSi + 49xNi + 5xCr - 79xAl - 2xNi 2 -1532xC 2 -5xMn 2 -127xSi 2 -40xCxNi-4xNixMn, where Al, Cr, Ni, C, Mn, and Si are the average aluminum, chromium, nickel, carbon, manganese, and silicon contents (in weight percentage) of the region for which the quenching factor is to be determined, in FT. WJ In the case of welded joints, and in FT BM In the case of the least hardenable matrix; and

[0115] (b) Carbon content C of the welded joint WJ Strictly less than 0.15% by weight, or if the carbon content C of the welded joint is less than 0.15% by weight. WJ If the softening factor FA of the welded joint is greater than or equal to 0.15% by weight, then the softening factor FA of the welded joint is... WJ Make FA WJ >5000 (Standard C3), where the softening factor FA of the welded joint WJ The following formula is used to calculate the average aluminum, chromium, nickel, molybdenum, carbon, manganese, and silicon content of the welded joint, expressed as a weight percentage:

[0116] FA=10291+4384.1xMo+3676.9Si-522.64xAl-2221.2xCr-118.11xNi-1565.1xC-246.67xMn,

[0117] And the welded joint is designed such that the maximum hardness change ΔHV (WJ) across the welded joint after hot pressing and cooling is less than or equal to the average hardness HV of the welded joint. 平均 20% of (WJ).

[0118] According to a specific implementation scheme for the steel billet, the carbon content C of the welded joint, by weight percentage, is... WJ To make 1.25x C BM(最可硬化) -C WJ ≥0 (Standard C4), of which, by weight percentage, C BMThe carbon content of the most hardenable steel matrix in the two pre-coated plates, and the nickel content (Ni) of the weld joint. WJ It ranges from 2.0% to 11.0% by weight (Standard C5).

[0119] The present invention also relates to a steel component that has been welded, hot-pressed, and cooled, comprising a first coated steel component portion and a second coated steel component portion, each coated steel component portion comprising a steel substrate having a coating comprising at least iron and aluminum on at least one main surface, the first coated steel component portion and the second coated steel component portion being joined by a welded joint, the welded joint being characterized in that:

[0120] (a) Quenching factor FT of welded joint WJ This makes FT WJ -0.9FT BM ≥0 (Standard C2),

[0121] in:

[0122] -FT BM It is the quenching factor of the least hardenable steel matrix in the two pre-coated plates, and

[0123] -Quenching factor FT WJ and FT BM The following formula is used to determine: FT = 128 + 1553xC + 55xMn + 267xSi + 49xNi + 5xCr - 79xAl - 2xNi 2 -1532xC 2 -5xMn 2 -127xSi 2 -40xCxNi-4xNixMn, where Al, Cr, Ni, C, Mn, and Si are the average aluminum, chromium, nickel, carbon, manganese, and silicon contents (in weight percentage) of the region for which the quenching factor is to be determined, in FT. WJ In the case of a welded joint, in FT BM In the case of the least hardenable matrix; and

[0124] (b) Carbon content C of the welded joint WJ Strictly less than 0.15% by weight, or if the carbon content C of the welded joint is less than 0.15% by weight. WJ If the softening factor FA of the welded joint is greater than or equal to 0.15% by weight, then the softening factor FA of the welded joint is... WJ Make FA WJ >5000 (Standard C3), where the softening factor FA of the welded joint WJ The following formula is used to calculate the average aluminum, chromium, nickel, molybdenum, carbon, manganese, and silicon content of the welded joint, expressed as a weight percentage:

[0125] FA=10291+4384.1xMo+3676.9xSi-522.64xAl-2221.2xCr-118.11xNi-1565.1xC-246.67xMn,

[0126] And the maximum hardness variation ΔHV (WJ) across the weld joint is less than or equal to the average hardness HV of the weld joint. 平均 20% of (WJ).

[0127] According to a specific implementation, the steel component that has been welded, hot-pressed, and cooled may include one or more of the following characteristics, either alone or in any possible combination:

[0128] - The hardness decrease in the heat-affected zone is less than or equal to 8% relative to the base metal adjacent to the heat-affected zone in the first and second coated steel component portions.

[0129] - Average hardness HV of welded joint 平均 (WJ) less than or equal to 600 HV,

[0130] -Carbon content (C) in welded joints, by weight percentage WJ To achieve 1.25xC BM -C WJ ≥0 (Standard C4), of which, by weight percentage, C BM The carbon content of the most hardenable steel matrix in the steel matrix of the first and second coated steel component portions.

[0131] - Nickel content in welded joints WJ It ranges from 2.0% to 11.0% by weight (Standard C5).

[0132] -The steel in the base of at least one of the first and second coated steel component portions comprises, by weight:

[0133] 0.10% ≤ C ≤ 0.5%

[0134] 0.5% ≤ Mn ≤ 3%

[0135] 0.1% ≤ Si ≤ 1%

[0136] 0.01% ≤ Cr ≤ 1%

[0137] Ti≤0.2%

[0138] Al≤0.1%

[0139] S≤0.05%

[0140] P≤0.1%

[0141] B≤0.010%

[0142] The remainder consists of iron and impurities produced during manufacturing.

[0143] -The steel in the base of at least one of the first and second coated steel component portions comprises, by weight:

[0144] 0.15% ≤ C ≤ 0.25%

[0145] 0.8% ≤ Mn ≤ 1.8%

[0146] 0.1% ≤ Si ≤ 0.35%

[0147] 0.01% ≤ Cr ≤ 0.5%

[0148] Ti≤0.1%

[0149] Al≤0.1%

[0150] S≤0.05%

[0151] P≤0.1%

[0152] B≤0.005%

[0153] The remainder consists of iron and impurities produced during manufacturing.

[0154] -The steel in the base of at least one of the first and second coated steel component portions comprises, by weight:

[0155] 0.040% ≤ C ≤ 0.100%

[0156] 0.80% ≤ Mn ≤ 2.00%

[0157] Si≤0.30%

[0158] S≤0.005%

[0159] P≤0.030%

[0160] 0.010% ≤ Al ≤ 0.070%

[0161] 0.015% ≤ Nb ≤ 0.100%

[0162] Ti≤0.080%

[0163] N≤0.009%

[0164] Cu≤0.100%

[0165] Ni≤0.100%

[0166] Cr≤0.100%

[0167] Mo≤0.100%

[0168] Ca≤0.006%,

[0169] The remainder consists of iron and impurities produced during manufacturing.

[0170] -The steel in the base of at least one of the first and second coated steel component portions comprises, by weight:

[0171] 0.24% ≤ C ≤ 0.38%

[0172] 0.40% ≤ Mn ≤ 3%

[0173] 0.10% ≤ Si ≤ 0.70%

[0174] 0.015% ≤ Al ≤ 0.070%

[0175] 0% ≤ Cr ≤ 2%

[0176] 0.25% ≤ Ni ≤ 2%

[0177] 0.015% ≤ Ti ≤ 0.10%

[0178] 0% ≤ Nb ≤ 0.060%

[0179] 0.0005% ≤ B ≤ 0.0040%

[0180] 0.003% ≤ N ≤ 0.010%

[0181] 0.0001% ≤ S ≤ 0.005%

[0182] 0.0001% ≤ P ≤ 0.025%

[0183] The titanium and nitrogen contents satisfy the following relationship:

[0184] Ti / N > 3.42,

[0185] The carbon, manganese, chromium, and silicon contents satisfy the following relationship:

[0186]

[0187] Steel may optionally contain one or more of the following elements:

[0188] 0.05% ≤ Mo ≤ 0.65%

[0189] 0.001% ≤ W ≤ 0.30%

[0190] 0.0005% ≤ Ca ≤ 0.005%

[0191] The remainder consists of iron and impurities that are unavoidable during manufacturing.

[0192] The present invention also relates to the use of steel components that have been welded, hot-pressed and cooled as described above for the production of anti-intrusion components or energy-absorbing components for motor vehicles.

[0193] The invention will be better understood after reading the following detailed description, given only by way of example and with reference to the accompanying drawings, in which:

[0194] - Figure 1 Perspective view of the pre-coated panel;

[0195] - Figure 2 This is a perspective view of a pre-coated plate, which includes a removal area in the pre-coating at the periphery of the plate;

[0196] - Figure 3 This is a schematic cross-sectional view showing the start of the welding step according to the method of the present invention.

[0197] - Figure 4 A schematic cross-sectional view showing the end of the welding step according to the method of the present invention, and

[0198] - Figure 5 This is a schematic diagram of the test location used for hardness testing.

[0199] Throughout the patent application, the content of elements is expressed as a weight percentage (wt%).

[0200] In the context of this invention, the term "heat-affected zone" is used to refer to the heat-affected zone in a welded steel billet resulting from the welding operation, and also by extension to the area obtained in a welded, hot-pressed and cooled steel component by hot-pressing and cooling the heat-affected zone of the welded steel billet.

[0201] The heat-affected zone extends from the weld joint on each side of the weld joint, for example, with a width of 150 micrometers to 500 micrometers.

[0202] The base metal is the portion of the substrate that is a pre-coated sheet or coated steel component located adjacent to the heat-affected zone generated by the welding operation.

[0203] The present invention relates to a method for producing welded steel billets 1.

[0204] The method includes a first step of providing two pre-coated plates 2.

[0205] like Figure 1 As shown, each pre-coated plate 2 includes two main surfaces 4 and at least one side surface 13, the side surface 13 extending from one main surface 4 to the other between the two main surfaces 4. Figure 1 In the example shown, the pre-coated panel 2 includes four sides 13. For example, the sides 13 form an angle of 60° to 90° with one of the main surfaces 4.

[0206] Each pre-coated plate 2 includes a metal substrate 3 having a pre-coating 5 on at least one of its main surfaces. The pre-coating 5 is overlaid on and in contact with the substrate 3.

[0207] The metal matrix 3 is more specifically a steel matrix.

[0208] The steel in matrix 3 is more specifically steel with a ferrite-pearlite microstructure.

[0209] Preferably, the matrix 3 is made of heat-treatable steel, more particularly compressible hardening steel, and, for example, manganese-boron steel such as 22MnB5 type steel.

[0210] According to one embodiment, the steel of the base 3 comprises, by weight, the following, and is composed of, for example, the following:

[0211] 0.10% ≤ C ≤ 0.5%

[0212] 0.5% ≤ Mn ≤ 3%

[0213] 0.1% ≤ Si ≤ 1%

[0214] 0.01% ≤ Cr ≤ 1%

[0215] Ti≤0.2%

[0216] Al≤0.1%

[0217] S≤0.05%

[0218] P≤0.1%

[0219] B≤0.010%

[0220] The remainder consists of iron and impurities produced during manufacturing.

[0221] More specifically, the steel in base 3 comprises, by weight:

[0222] 0.15% ≤ C ≤ 0.25%

[0223] 0.8% ≤ Mn ≤ 1.8%

[0224] 0.1% ≤ Si ≤ 0.35%

[0225] 0.01% ≤ Cr ≤ 0.5%

[0226] Ti≤0.1%

[0227] Al≤0.1%

[0228] S≤0.05%

[0229] P≤0.1%

[0230] B≤0.005%

[0231] The remainder consists of iron and impurities produced during manufacturing.

[0232] According to an alternative, the steel of the base 3 comprises, by weight, the following, and is composed of, for example, the following:

[0233] 0.040% ≤ C ≤ 0.100%

[0234] 0.80% ≤ Mn ≤ 2.00%

[0235] Si≤0.30%

[0236] S≤0.005%

[0237] P≤0.030%

[0238] 0.010% ≤ Al ≤ 0.070%

[0239] 0.015% ≤ Nb ≤ 0.100%

[0240] Ti≤0.080%

[0241] N≤0.009%

[0242] Cu≤0.100%

[0243] Ni≤0.100%

[0244] Cr≤0.100%

[0245] Mo≤0.100%

[0246] Ca≤0.006%,

[0247] The remainder consists of iron and impurities produced during manufacturing.

[0248] According to an alternative, the steel of the base 3 comprises, by weight, the following, and is composed of, for example, the following:

[0249] 0.24% ≤ C ≤ 0.38%

[0250] 0.40% ≤ Mn ≤ 3%

[0251] 0.10% ≤ Si ≤ 0.70%

[0252] 0.015% ≤ Al ≤ 0.070%

[0253] 0% ≤ Cr ≤ 2%

[0254] 0.25% ≤ Ni ≤ 2%

[0255] 0.015% ≤ Ti ≤ 0.10%

[0256] 0% ≤ Nb ≤ 0.060%

[0257] 0.0005% ≤ B ≤ 0.0040%

[0258] 0.003% ≤ N ≤ 0.010%

[0259] 0.0001% ≤ S ≤ 0.005%

[0260] 0.0001% ≤ P ≤ 0.025%

[0261] The titanium and nitrogen contents satisfy the following relationship:

[0262] Ti / N > 3.42,

[0263] The carbon, manganese, chromium, and silicon contents satisfy the following relationship:

[0264]

[0265] The steel optionally comprises one or more of the following elements:

[0266] 0.05% ≤ Mo ≤ 0.65%

[0267] 0.001% ≤ W ≤ 0.30%

[0268] 0.0005% ≤ Ca ≤ 0.005%

[0269] The remainder consists of iron and impurities that are unavoidable during manufacturing.

[0270] In one example, the substrates 3 of the two pre-coated plates 2 have the same composition.

[0271] According to another example, the substrates 3 of the two pre-coated plates 2 have different compositions. In particular, the two substrates 3 each have a different composition selected from the four compositions described above. For example, the steel of the substrate 3 of one pre-coated plate 2 has the first composition described above, while the steel of the substrate 3 of the other pre-coated plate 2 has a composition selected from the second, third, or fourth composition described above.

[0272] Depending on its desired thickness, the matrix 3 can be obtained by hot rolling and / or by cold rolling followed by annealing, or by any other suitable method.

[0273] The thickness of the substrate 3 is advantageously 0.8 mm to 5 mm, and more particularly 1.0 mm to 2.5 mm.

[0274] The pre-coating 5 is obtained by hot-dip coating, that is, by immersing the substrate 3 in a molten metal bath.

[0275] The pre-coating 5 includes at least an intermetallic compound alloy layer 9 in contact with the substrate 3. The intermetallic compound alloy layer 9 comprises at least iron and aluminum. The intermetallic compound alloy layer 9 is formed specifically by a reaction between the substrate 3 and a molten metal bath. More specifically, the intermetallic compound alloy layer 9 comprises Fe. x -Al y It is a type, and more specifically an intermetallic compound of Fe2Al5.

[0276] exist Figure 1 In the example shown, the pre-coating 5 also includes a metal alloy layer 11 extending on top of the intermetallic compound 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 carrying away molten metal with the plate as it travels through the molten metal bath during hot-dip coating. The metal alloy layer 11 is an aluminum layer, an aluminum alloy layer, or an aluminum-based alloy layer.

[0277] In this article, aluminum alloy refers to an alloy containing more than 50% by weight of aluminum. An aluminum-based alloy is an alloy in which aluminum is the main element by weight.

[0278] For example, metal alloy layer 11 is an aluminum alloy layer that also contains silicon. More specifically, metal alloy layer 11 comprises, by weight:

[0279] -8%≤Si≤11%,

[0280] -2%≤Fe≤4%,

[0281] The remainder consists of aluminum and possible impurities.

[0282] The thickness of the metal alloy layer 11 is, for example, 19 μm to 33 μm or 10 μm to 20 μm.

[0283] exist Figure 1 In the example shown where the pre-coating 5 includes a metal alloy layer 11, the thickness of the intermetallic compound alloy layer 9 is typically a few micrometers. In particular, its average thickness is typically between 2 and 8 micrometers.

[0284] The specific structure of the pre-coating layer 5, which includes an intermetallic compound alloy layer 9 and a metal alloy layer 11, obtained by hot-dip coating, is specifically disclosed in patent EP 2 007 545.

[0285] According to another embodiment, the pre-coating 5 comprises only the intermetallic compound alloy layer 9 as described above. In this case, the thickness of the intermetallic compound alloy layer 9 is, for example, 10 μm to 40 μm. Such a pre-coating 5 composed of intermetallic compound alloy 9 can be obtained, for example, by subjecting the pre-coating 5 comprising the aforementioned intermetallic compound alloy layer 9 and metal alloy layer 11 to a pre-alloying treatment. Such a pre-alloying treatment is performed at a temperature and holding time selected such that the pre-coating 5 is alloyed with the substrate 3 over at least a portion of the thickness of the pre-coating 5. More specifically, the pre-alloying treatment may include the following steps: heating the plate to a pre-alloying temperature of 700°C to 900°C, and holding the pre-alloyed plate at this temperature for 2 minutes to 200 hours. In this case, the intermetallic compound alloy layer 9 may be composed of different intermetallic compound sublayers, such as Fe2Al5, FeAl3, FeAl, Fe6Al. 12 Si5 and FeAl3 sublayers.

[0286] Advantageously, such as Figure 1 As shown, the substrate 3 has a pre-coating 5 as described above on both of its main surfaces.

[0287] Optional, such as Figure 2 As shown, the method also includes the step of preparing the welding edge 14 of at least one of the pre-coated plates 2 (e.g., both pre-coated plates 2).

[0288] The weld edge 14 includes the peripheral portion of the pre-coated plate 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 and a portion of the pre-coated plate 2 extending from the side 13 and including a portion of the pre-coating 5 and a portion of the substrate 3.

[0289] More specifically, the preparation of the weld edge 14 may include at least one of the following processing steps:

[0290] -like Figure 2 As shown, at least a portion of the pre-coating 5 is removed from the removal area 18 at the weld edge 14.

[0291] - Brush and clean the welded edge 14.

[0292] - Machining is performed on the welded edge 14.

[0293] - Chamfer the weld edge 14, and / or

[0294] - Beveling is performed on the weld edge 14.

[0295] An example of a pre-coated plate 2 including removal zone 18 is shown. Figure 2In the process of removing at least a portion of the pre-coating 5 at the weld edge 14, a laser beam is preferably used.

[0296] The removal area 18 can extend from the side 13 of the plate 2 in a width of 0.5 mm to 2 mm.

[0297] Advantageously, in the removal zone 18, the metal alloy layer 11 is removed, while the intermetallic compound alloy layer 9 is retained at least a portion of its thickness. In this case, the retained intermetallic compound alloy layer 9 protects the area of ​​the weld blank 1 adjacent to the weld joint 22 from oxidation and decarburization during subsequent hot pressing steps, and from corrosion during its service life.

[0298] According to one embodiment, during the removal step, the intermetallic compound alloy layer 9 retains its integrity or only a portion of its initial thickness, such as only 60%, 80%, or 90% of its initial thickness.

[0299] According to one embodiment, during the preparation step, the weld edge 14 is prepared in such a way that the pre-coating 5 is completely retained on at least one main surface 4 of each of the two pre-coated plates 2.

[0300] Specifically, the weld edge 14 is prepared in such a way that the aluminum content of the weld joint 22 obtained by joining the two pre-coated plates 2 by butt welding is greater than or equal to 0.7% by weight, and more particularly greater than or equal to 1.0% by weight, or even more particularly greater than or equal to 1.5% by weight, for example greater than or equal to 2.0% by weight, or greater than or equal to 2.1% by weight.

[0301] For example, for a pre-coating layer 5 comprising an aluminum alloy layer as a metal alloy layer 11 and having a thickness greater than or equal to 25 μm, and for a typical weld width (0.8 mm to 1.8 mm), if the pre-coating layer 5 is completely retained on at least one side 4 of each of the two pre-coated plates 2 after preparation, the aluminum content in the weld joint 22 will be greater than or equal to 0.7 wt%.

[0302] The brushing step allows for at least partial removal of traces of the pre-coating 5 on the weld edge 14 and more particularly on the side 13, resulting from mechanical cutting operations and / or the possible removal of the pre-coating 5 at the weld edge 14.

[0303] Chamfering or beveling the weld edge 14 allows for an increase in the amount of filler material added without resulting in excessive thickness at the weld joint 22.

[0304] If the shape of the weld edge 14 before machining is not straight enough for laser welding, the weld edge 14 is machined.

[0305] The method further includes the following steps: after optionally preparing the weld edge 14, butt welding the pre-coated plate 2 with filler wire 20 to obtain the welded steel billet 1.

[0306] Figure 3 and Figure 4 The two stages of the welding process for forming the welded steel billet 1 are shown.

[0307] exist Figure 3 and Figure 4 In the example shown, the pre-coated plate 2 is not subjected to any removal of its pre-coating layer 5 before welding. In this example, the pre-coating layer 5 remains intact on both main surfaces 4 of the pre-coated plate 2 before welding. In this example, both main surfaces of the pre-coated plate 2 are completely covered by the pre-coating layer 5 during butt welding.

[0308] The welding operation results in the formation of a molten metal zone at the joint between the two plates 2, which subsequently solidifies to form a welded joint 22.

[0309] The welding step is specifically a laser welding step, in which a laser beam 24 is guided toward the joint between the two plates 2. The laser beam 24 is configured to melt the filler wire 20 at the impact point 26 of the laser beam 24.

[0310] Laser welding steps can be performed using, for example, a CO2 laser or a solid-state laser.

[0311] The laser source is preferably a high-power laser source. It can be, for example, selected from CO2 lasers with a wavelength of about 10 micrometers, solid-state laser sources with a wavelength of about 1 micrometer, or semiconductor laser sources such as diode lasers with a wavelength of about 0.8 to 1 micrometer.

[0312] The power of the laser source is selected based on the thickness of plate 2. Specifically, the power is selected to allow for the melting of the filler wire 20 and the welding edge 14 of plate 2, as well as adequate mixing within the weld joint 22. For CO2 lasers, the laser power is, for example, 3 kW to 12 kW. For solid-state or semiconductor lasers, the laser power is, for example, 2 kW to 8 kW.

[0313] For both types of laser sources, the diameter of the laser beam 24 on plate 2 at its impact point 26 can be approximately 600 μm.

[0314] 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.

[0315] The protective atmosphere is, for example, an inert gas or a mixture of inert gases. The inert gas can be helium or argon or a mixture of these gases.

[0316] During this welding step, the distance between the facing sides 13 of the two plates 1 is, for example, less than or equal to 0.3 mm, and more particularly less than or equal to 0.1 mm. Providing such a gap between the facing sides 13 of the two plates 1 facilitates the deposition of filler metal during the welding operation and prevents excessive thickness from forming at the weld joint 22. This also improves filler metal deposition and prevents excessive thickness in cases where a chamfer or bevel is formed at the weld edge 14 of the plate 2 during the preparation step.

[0317] During the welding step, the proportion of filler wire 20 added to the weld pool is, for example, 10% to 50%, and more specifically, 10% to 40%.

[0318] According to the present invention, the carbon content of the filler wire 20 is from 0.01% to 0.45% by weight.

[0319] Furthermore, the resulting welded joint 22 is characterized by selecting the composition of the filler wire 20 and the proportion of filler wire 20 added to the weld pool in the following manner:

[0320] (a) Quenching factor FT of welded joint 22 WJ This makes FT WJ -0.9FT BM ≥0 (Standard C2),

[0321] in:

[0322] -FT BM It is the quenching factor of the most unhardenable steel substrate 3 among the two pre-coated plates 2, and

[0323] -Quenching factor FT WJ and FT BM Use the following formula to determine:

[0324] FT=128+1553xC+55xMn+267xSi+49xNi+5xCr-79xAl-2xNi 2 -1532xC 2 -5xMn 2 -127xSi 2 -40xCxNi-4xNixMn, where Al, Cr, Ni, C, Mn, and Si are the average aluminum, chromium, nickel, carbon, manganese, and silicon contents (in weight percentage) of the region for which the quenching factor is to be determined, in FT. WJ In the case of welded joint 22, and in FT BM In the case of matrix 3, which is the least hardenable matrix; and

[0325] (b) Carbon content C of welded joint 22WJ Strictly less than 0.15% by weight, or if the carbon content C of weld joint 22 is... WJ If the softening factor FA of welded joint 22 is greater than or equal to 0.15% by weight, then the softening factor FA of welded joint 22 is greater than or equal to 0.15% by weight. WJ Make FA WJ >5000 (Standard C3),

[0326] The softening factor FA of welded joint 22 WJ The following formula is used to calculate the average aluminum, chromium, nickel, molybdenum, carbon, manganese, and silicon content of welded joint 22 as a weight percentage:

[0327] FA=10291+4384.1xMo+3676.9xSi-522.64xAl-2221.2xCr-118.11xNi-1565.1xC-246.67xMn.

[0328] The most non-hardening substrate 3 in the substrate 3 of the pre-coated plate 2 is the substrate 3 with the lowest carbon content.

[0329] In fact, the inventors of the present invention discovered in an unexpected way that when the above-mentioned criteria C1, C2 and C3 are met, even if the weld joint 22 contains an aluminum content of greater than or equal to 0.7% by weight and even greater than or equal to 2.1%, when subjected to a tensile test perpendicular to the weld joint 22, the part obtained from such a welded steel billet 1 after heat treatment including an austenitizing step (hot pressing and cooling in a pressing tool) exhibits a metallurgical guarantee that it will not fail in the weld joint 22 or in the heat-affected zone adjacent to the weld joint 22.

[0330] Therefore, although the aluminum content in the welded joint 22 may be relatively high, a component with satisfactory impact performance can still be obtained at a relatively low cost by means of the method according to the invention.

[0331] In particular, compared to methods that require removing the pre-coating layer 5 from both main surfaces 4 of the pre-coated board 2, production costs are reduced because it is no longer necessary to remove the pre-coating layer 5 from both surfaces of the pre-coated board 2. More precisely, for a board 2 coated on both main surfaces 4, satisfactory properties can be obtained by removing the pre-coating layer 5 from only one main surface 4 of the pre-coated board 2, or even without removing the pre-coating layer 5 from any of the main surfaces 4 of the pre-coated board 2.

[0332] More specifically, the inventors of this invention have unexpectedly discovered that using filler wire 20 with a carbon content of 0.01% to 0.45% by weight allows for the prevention of carbon segregation and thus hardness peaks in the weld joint 22 after hot pressing and cooling in a pressing tool, especially in the presence of a large amount of aluminum in the weld joint 22. Therefore, using such filler wire 20 reduces the brittleness of the weld joint 22 and helps prevent weld joint 22 failure of the part obtained after hot pressing and cooling in a pressing tool under tension perpendicular to the weld joint 22. In particular, the inventors of this invention have observed that when using filler wire 20 with a carbon content of 0.01% to 0.45% by weight, the maximum hardness change ΔHV(WJ) across the weld joint 22 is less than or equal to the average hardness HV of the weld joint 22. 平均 (WJ) 20%. In other words, Where ΔHV(WJ) is the difference between the maximum and minimum hardness measured in welded joint 22, HV 平均 (WJ) represents the average hardness measured in welded joint 22.

[0333] Furthermore, the inventors of this invention have unexpectedly discovered that when the composition of the welded joint 22 meets standard C2, after hot pressing and cooling in a pressing tool, the minimum hardness HV of the welded joint 22 is... 最小 (WJ) is greater than or equal to the average hardness HV of the most unhardenable of the two substrates 3 of the pre-coated plate 2. 平均 (BM 最不可硬化 Therefore, when standard C2 is met and it is assumed that the mixture is uniformly mixed in the weld joint 22, tensile failure is unlikely to occur in the weld joint 22 in a direction perpendicular to the weld joint 22 of the part obtained after hot pressing and cooling in the pressing tool.

[0334] Finally, the inventors unexpectedly observed that when the carbon content C of the welded joint 22... WJ When strictly less than 0.15% by weight (Standard C3, First Alternative), a decrease in hardness of less than or equal to 8% occurs in the heat-affected zone relative to the base metal adjacent to the heat-affected zone of the part obtained after hot pressing and cooling in a pressing tool.

[0335] When the carbon content C of welded joint 22 WJ When the softening factor FA is greater than or equal to 0.15% by weight, WJ If the hardness is less than or equal to 5000, the inventors observed that in the part obtained after hot pressing and cooling in a pressing tool, the hardness of the heat-affected zone relative to the adjacent base metal decreased by more than or equal to 10%. Conversely, when the softening factor FA of the weld joint 22 is less than or equal to 5000, the hardness of the heat-affected zone decreased by more than or equal to 10%. WJWhen the hardness is strictly greater than 5000 (Standard C3, Second Alternative), the inventors observed that in the parts obtained after hot pressing and cooling in the pressing tool, the hardness of the heat-affected zone relative to the adjacent base metal decreases by less than or equal to 8%.

[0336] In this paper, hardness decrease is defined as follows:

[0337] In the context of this invention, it is desirable to avoid a decrease in hardness of the heat-affected zone relative to the adjacent base metal of the part obtained after hot pressing and cooling in a pressing tool, which is strictly greater than 8%, because such a decrease in hardness increases the risk of failure of the heat-affected zone under tension perpendicular to the weld joint.

[0338] Therefore, when standard C3 is met, the risk of failure in the heat-affected zone is significantly reduced.

[0339] Therefore, by utilizing the method according to the invention in which the cumulative satisfaction of standards C1, C2 and C3 is achieved, failure under tension perpendicular to the weld joint 22 is unlikely to occur in the heat-affected zone or weld joint 22.

[0340] Advantageously, the aluminum content in the welded joint 22 is greater than or equal to 0.7% by weight, more particularly greater than or equal to 1.0% by weight, more particularly greater than or equal to 1.5% by weight, and even more particularly greater than or equal to 2.0% by weight, for example greater than or equal to 2.1% by weight.

[0341] Advantageously, it also allows the carbon content C in the welded joint 22 to be reduced. WJ The carbon content C of the most hardenable matrix 3 in the pre-coated plate 2 forming the weld blank 1 is strictly less than or equal to that of the matrix 3. BM The composition of filler wire 20 and the proportion of filler wire 20 added to the weld pool are selected in a manner that is 1.25 times (standard C4). In other words, 1.25 × C BM(最可硬化) -C WJ ≥0.

[0342] The most hardenable component in the substrate 3 of the pre-coated plate 2 that forms the welding blank 1 is the substrate 3 with the highest carbon content.

[0343] In fact, the inventors of this invention have discovered that when standard C4 is followed, the risk of failure of the welded joint 22 after heat treatment is even further reduced.

[0344] Preferably, the nickel content (Ni) of the weld joint 22 is further increased. WJ The composition of filler wire 20 and the proportion of filler wire 20 added to the weld pool are selected in a manner ranging from 2.0% to 11.0% by weight (standard C5).

[0345] In fact, the inventors have observed that the hardness of the weld joint 22 is particularly stable after heat treatment when standard C5 is met. More specifically, in this case, even for a carbon content in the weld joint 22 greater than or equal to 0.15% by weight, a hardness difference ΔHV (WJ) across the weld joint 22 after hot pressing and cooling in a pressing tool is observed to be less than or equal to 80 HV. This improved stability is advantageous because it further reduces the risk of failure in the weld joint 22 due to a more uniform strain distribution under tension.

[0346] For example, by weight, filler wire 20 has the following composition:

[0347] 0.001% ≤ C ≤ 0.45%, and more specifically 0.02% ≤ C ≤ 0.45%.

[0348] 0.001% ≤ Mn ≤ 30%, and more specifically 0.05% ≤ Mn ≤ 20%.

[0349] 0.001% ≤ Si ≤ 1%

[0350] 0.001% ≤ Ni ≤ 56%

[0351] 0.001% ≤ Cr ≤ 30%

[0352] 0.001% ≤ Mo ≤ 5%

[0353] 0.001% ≤ Al ≤ 0.30%

[0354] 0.001% ≤ Cu ≤ 1.80%

[0355] 0.001% ≤ Nb ≤ 1.50%

[0356] 0.001% ≤ Ti ≤ 0.30%

[0357] 0.001% ≤ N ≤ 10%

[0358] 0.001% ≤ V ≤ 0.1%

[0359] 0.001% ≤ Co ≤ 0.20%,

[0360] The remainder consists of iron and impurities produced during manufacturing.

[0361] For example, the filler wire 20 is composed of the elements described above.

[0362] According to one example, the filler wire 20 has the composition defined above, and the nickel content is from 0.001% to 7% by weight.

[0363] According to an alternative example, the filler wire 20 has the composition defined above, and the nickel content is from 7% to 56% by weight.

[0364] According to a specific example, the filler wire 20 has the following composition by weight:

[0365] 0.02% ≤ C ≤ 0.45%,

[0366] 0.05% ≤ Mn ≤ 20%

[0367] 0.001% ≤ Si ≤ 1%

[0368] 7% ≤ Ni ≤ 56%

[0369] 0.001% ≤ Cr ≤ 30%

[0370] 0.001% ≤ Mo ≤ 5%

[0371] 0.001% ≤ Al ≤ 0.30%

[0372] 0.001% ≤ Cu ≤ 1.80%

[0373] 0.001% ≤ Nb ≤ 1.50%

[0374] 0.001% ≤ Ti ≤ 0.30%

[0375] 0.001% ≤ N ≤ 10%

[0376] 0.001% ≤ V ≤ 0.1%

[0377] 0.001% ≤ Co ≤ 0.20%,

[0378] The remainder consists of iron and impurities produced during manufacturing.

[0379] For example, the filler wire 20 is composed of the elements described above.

[0380] The filler wire 20 is, for example, a solid wire or a flux-cored wire.

[0381] The present invention also relates to a welded steel billet 1 that can be obtained using the above method.

[0382] Such a welded steel billet 1 includes two pre-coated plates 2, each pre-coated plate 2 including a steel substrate 3 having a pre-coating layer 5 on at least one of its main surfaces 4, the pre-coating layer 5 including an intermetallic compound alloy layer 9 comprising at least iron and aluminum, and optionally a metal alloy layer 11 extending on top of the intermetallic compound alloy layer 9, the metal alloy layer 11 being an aluminum layer, an aluminum alloy layer, or an aluminum-based alloy layer, the pre-coated plates 2 being joined by a weld joint 22.

[0383] The pre-coated plate 2 and the welded joint 22 have the features disclosed above regarding the method for producing welded steel billets 1.

[0384] Specifically, the welded joint 22 is designed such that, after hot pressing and cooling in the pressing tool, the maximum hardness change ΔHV (WJ) across the welded joint 22 is less than or equal to the average hardness HV of the welded joint 22. 平均 20%. In other words,

[0385] Welded joint 22 is also designed to ensure that the minimum hardness HV of welded joint 22 after hot pressing and cooling in the pressing tool is [not specified]. 最小 (WJ) is greater than or equal to the average hardness HV of the two least hardenable substrates 3 of the pre-coated plate 2 after hot pressing and cooling in a pressing tool. 平均 (BM 最不可硬化 ).

[0386] The welded joint 22 is further designed so that, after hot pressing and cooling in a pressing tool, the hardness of the heat-affected zone relative to its adjacent base metal decreases by less than or equal to 8%. In other words,

[0387]

[0388] Advantageously, the welded joint 22 is configured such that, after hot pressing and cooling in the pressing tool, the hardness difference ΔHV (WJ) across the welded joint 22 is less than or equal to 80HV.

[0389] Advantageously, the welded joint 22 is designed such that, after hot pressing and cooling in the pressing tool, the average hardness HV of the welded joint 22 is [value missing]. 平均 (WJ) less than or equal to 600HV.

[0390] The present invention also relates to a method for producing steel parts that are welded, hot-pressed, and cooled, comprising:

[0391] - Use the above method to produce welded steel billet 1;

[0392] - Heat and weld the steel billet 1 to obtain a fully austenitic structure in the matrix 3 of the pre-coated plate 2 constituting the welded billet 1;

[0393] - The welded steel billet 1 is hot-pressed in a pressing tool to obtain a steel component; and

[0394] - Allow the steel parts to cool in the pressing tool.

[0395] More specifically, during the heating step, the weld billet 1 is heated to the austenitizing temperature. It is then held at the austenitizing temperature for a holding time depending on the thickness of the plate 2 forming the weld billet 1. The holding time is selected according to the austenitizing temperature in such a way that the weld billet 1 is austenitized and that an alloyed metal interlayer of a predetermined thickness is formed through the alloying of the substrate 3 and the pre-coating 5. For example, the holding time is approximately 5 minutes.

[0396] Before hot pressing, the heated welded steel billet 1 is transferred to a hot forming press. The transfer time is advantageously 5 to 10 seconds. The transfer time is chosen to be as short as possible to avoid metallographic transformation in the welded steel billet 1 before hot pressing.

[0397] During the cooling step, the cooling rate is greater than or equal to the critical martensite or bainite cooling rate of at least one of the two steel plates 2 matrix 3 (e.g., the most hardenable steel plate 1, i.e. the plate with the lowest critical cooling rate).

[0398] The present invention also relates to steel components obtained by the above-described method after welding, hot pressing and cooling.

[0399] More specifically, the steel component includes a first coated steel component portion and a second coated steel component portion, which are respectively produced by hot pressing and cooling of two pre-coated steel plates 2 in a pressing tool.

[0400] More specifically, each coated steel component portion includes a steel substrate having a coating comprising iron and aluminum on at least one of its main surfaces, and the first steel component portion and the second steel component portion are joined by a weld joint 22 as described above.

[0401] Specifically, the coatings on the first and second steel components are produced by at least partial alloying of the pre-coating 5 during hot pressing.

[0402] The substrates of the first steel component and the second steel component have the composition described above for the pre-coated plate 2. They are produced by hot pressing and cooling of the substrate 3 of the pre-coated plate 2.

[0403] For welded joint 22, the maximum hardness variation ΔHV (WJ) across welded joint 22 is less than or equal to the average hardness HV of welded joint 22. 平均 (WJ) 20%. In other words,

[0404]

[0405] The minimum hardness HV of welded joint 22 after hot pressing and cooling in a pressing tool. 最小 (WJ) is greater than or equal to the average hardness (HV) of the most unhardenable of the two substrates 3 of the pre-coated plate 2. 平均 (BM最不可硬化 )).

[0406] Furthermore, for each of the first and second coated steel component portions, the decrease in hardness of the heat-affected zone relative to the adjacent base metal is less than or equal to 8%. In other words,

[0407]

[0408] Advantageously, the hardness difference ΔHV (WJ) across weld joint 22 is less than or equal to 80HV.

[0409] Advantageously, the average hardness HV of welded joint 22 平均 (WJ) less than or equal to 600HV.

[0410] The inventors of this invention have conducted an experiment in which welded steel billet 1 was produced by laser welding two pre-coated plates A and B together using filler wire W.

[0411] Table 1 below lists the experimental conditions for each experiment, E1 to E21.

[0412] The initially supplied pre-coated plates A and B have a pre-coating 5 with a thickness of approximately 25 micrometers on their two main surfaces 4.

[0413] For all the pre-coated plates A and B tested, the pre-coating layer 5 was obtained by hot-dip coating in a molten metal bath and included a metal alloy layer 11 and an intermetallic compound alloy layer 9.

[0414] By weight, the metal alloy layer 11 of the pre-coated layer 5 comprises:

[0415] Si: 9%

[0416] Fe: 3%,

[0417] The remainder consists of aluminum and possible impurities resulting from refining.

[0418] The average total thickness of the metal alloy layer 11 is 20 μm.

[0419] Intermetallic compound alloy layer 9 contains Fe x -Al y Type, and mainly Fe2Al3, Fe2Al5 and Fe x Al y Si z An intermetallic compound. Its average thickness is 5 μm.

[0420] As can be seen in the column titled "Removal of Pre-coating at Weld Edges" in Table 1, some weld blanks 1 were obtained after removing the intermetallic alloy 11 of the pre-coating 5 from one main surface 4 of each of the pre-coated plates A and B ("Removal of One Surface") before butt welding, while others were obtained by welding the pre-coated plates 2 with the pre-coating 5 intact on both main surfaces 4 ("No"). The removal was performed by laser ablation using the method disclosed in earlier application WO2007 / 118939.

[0421]

[0422] Table 1: List of Experimental Conditions

[0423] In the table above, experiments that are not based on the invention are marked with an underline.

[0424] The steel matrix used in the different experiments mentioned in Table 1 has the composition listed in Table 2 below, with the content expressed as % by weight.

[0425] ％C %Mn ％Al ％Cr %Si %Ti ％B %Nb ％P ％S S1 0.06 1.57 0.02 0.02 0.02 0.07 - 0.048 ＜0.020 ＜0.005 S2 0.22 1.17 0.04 0.17 0.25 0.040 0.003 - ＜0.025 ＜0.005 S3 0.06 1.6 0.02 0.02 0.02 0.07 - 0.048 ＜0.020 ＜0.005 S4 0.23 1.19 0.04 0.18 0.26 0.040 0.003 - ＜0.025 ＜0.005 S5 0.24 1.2 0.03 0.2 0.27 0.040 0.003 - ＜0.025 ＜0.005 S6 0.22 1.16 0.04 0.2 0.25 0.040 0.003 - ＜0.025 ＜0.005 S7 0.22 1.15 0.04 0.18 0.26 0.040 0.003 - ＜0.025 ＜0.005 S8 0.22 1.15 0.04 0.19 0.27 0.040 0.003 - ＜0.025 ＜0.005 S9 0.22 1.18 0.03 0.17 0.25 0.040 0.003 - ＜0.025 ＜0.005 S10 0.22 1.18 0.03 0.17 0.26 0.040 0.003 - ＜0.025 ＜0.005 S11 0.22 1.2 0.05 0.19 0.26 0.040 0.003 - ＜0.025 ＜0.005 S12 0.24 1.24 0.04 0.17 0.27 0.040 0.003 - ＜0.025 ＜0.005

[0426] --------------------------------------------------------------------

[0427] Table 2: Composition of the matrix

[0428] For all matrices, the remainder of the composition consists of iron, possible impurities resulting from manufacturing, and unavoidable elements.

[0429] In Table 2 above, "-" means that the matrix contains at most trace amounts of the elements under consideration.

[0430] The filler wire W used in the different experiments mentioned in Table 1 has the composition listed in Table 3 below, with the content expressed in weight % .

[0431]

[0432] Table 3: Composition of filler wire W

[0433] For all welding wires, the remainder of the composition consists of iron, possible impurities resulting from manufacturing, and unavoidable elements.

[0434] Then, for each experiment E1 to E21, the inventors used conventional measurement methods to measure the composition of the obtained weld joint 22.

[0435] The contents of manganese, aluminum, nickel, chromium, molybdenum, and silicon in the weld joint 22 were determined using an energy-dispersive spectroscopy detector integrated into a scanning electron microscope, on a cross-section of the sample taken perpendicular to weld joint 22. The carbon content was determined using a Castaing electron microprobe on a cross-section of the sample taken perpendicular to weld joint 22. The results of these measurements are shown in Table 4 below.

[0436] experiment ％C %Mn ％Al %Ni ％Cr %Mo %Si E1 0.05 1.32 1.13 2.70 4.49 0.03 0.26 E2 0.05 1.60 1.18 5.53 4.17 1.03 0.19 E3 0.22 1.25 2.05 7.48 0.23 0.00 0.66 E4 0.23 1.10 1.89 9.62 0.20 0.00 0.61 E5 0.20 1.20 2.14 5.96 0.21 0.00 0.61 E6 0.19 1.21 1.93 8.23 0.23 0.00 0.67 E7 0.22 2.45 1.86 5.60 0.25 0.00 0.59 E8 0.23 2.89 1.80 8.05 0.24 0.00 0.60 <![CDATA[ E9 ]]> 0.07 1.52 1.68 0.33 0.04 0.15 0.15 <![CDATA[ E10 ]]> 0.21 1.20 1.68 0.29 0.19 0.07 0.37 <![CDATA[ E11 ]]> 0.23 3.91 2.00 11.40 0.12 0.00 0.22 <![CDATA[ E12 ]]> 0.26 1.00 1.17 0.00 0.31 0.02 0.34 <![CDATA[ E13 ]]> 0.38 1.17 1.04 0.01 0.33 0.02 0.53 <![CDATA[ E14 ]]> 0.33 3.47 1.12 0.45 0.28 0.02 0.63 <![CDATA[ E15 ]]> 0.32 3.28 1.25 0.42 0.15 0.02 0.35 <![CDATA[ E16 ]]> 0.31 2.84 1.07 0.33 0.06 0.02 0.35 <![CDATA[ E17 ]]> 0.31 2.95 1.11 0.35 0.16 0.02 0.34 <![CDATA[ E18 ]]> 0.32 3.28 1.25 0.42 0.15 0.02 0.35 <![CDATA[ E19 ]]> 0.31 2.87 1.46 0.33 0.15 0.02 0.37 <![CDATA[ E20 ]]> 0.19 1.33 1.49 2.29 3.95 0.03 0.44 <![CDATA[ E21 ]]> 0.18 1.32 1.28 5.41 4.42 1.06 0.34

[0437] ---------------------------------------------------------------------

[0438] Table 4: Measured content in welded joints

[0439] Based on these measurements, the inventors determined whether each welded steel billet 1 conformed to the standards C1, C2, C3 and optional additional standards C4 and C5 according to experiments E1 to E21. The results of these determinations are summarized in Table 5 below.

[0440]

[0441] Table 5: Standard values ​​for welded joints

[0442] Underlined values: do not conform to this invention.

[0443] As can be seen from Table 5, the experiments with reference to E1 to E8 are embodiments of the present invention: in these experiments, standards C1 to C3 are satisfied.

[0444] Conversely, the experiments referred to in E9 to E21 are not according to the invention: in these experiments, at least one of the criteria C1 to C3 is not met.

[0445] Finally, the inventors subjected the welded steel billet 1 thus manufactured to austenitizing heat treatment, followed by rapid cooling to obtain a heat-treated component. Such a heat-treated component possesses the same properties as a component formed by hot pressing and cooling. The inventors then performed measurements to determine the mechanical properties of these components. The results of these measurements are shown in Table 6 below.

[0446]

[0447]

[0448] Table 6: Results of hardness measurement after heat treatment

[0449] Underlined values: do not conform to this invention.

[0450] Tensile tests were performed at ambient temperature (approximately 20°C) on transverse weld tensile specimens of type EN 12.5×50 (240×30 mm) extracted perpendicular to the laser welding direction using the methods disclosed in NF EN ISO 4136 and NF ISO 6892-1. Five tensile tests were performed for each experiment (E1 to E21). For each experiment (E1 to E21), the percentage shown in the column titled “Failure Location” corresponds to the percentage of tensile tests in which failure occurred in the stated area (base metal, HAZ, or weld joint).

[0451] Hardness was measured using the Vickers hardness test according to standard NF EN ISO 6507-1. A test force of 0.5 kgf (HV0.5) was applied transversely relative to the weld joint. The locations for hardness measurement of each heat-treated component are as follows: Figure 5 As shown in the figure, hardness was measured along three lines located at 1 / 4, 1 / 2, and 3 / 4 of the thickness of the heat-treated component. For each line, measurements were taken following standard procedures, starting from the central axis of weld joint 22, according to standard NF EN ISO 6507-1.

[0452] After etching with a nitric acid alcohol solution (Nital, a known reagent), the location of test points in the weld joint 22 or the base metal is determined by metallographic examination of the test surface. The heat-affected zone is determined along three test lines, including the area containing two test points immediately adjacent to the weld joint 22.

[0453] Minimum hardness HV of welded joint 最小 (WJ) corresponds to the lowest hardness value measured in welded joint 22.

[0454] Maximum hardness HV of welded joint 最大 (WJ) corresponds to the maximum hardness value measured in welded joint 22.

[0455] Average hardness HV of welded joint 平均 (WJ) corresponds to the average of all hardness values ​​measured in welded joint 22.

[0456] Minimum hardness HV of the heat-affected zone 最小 (HAZ) corresponds to the lowest hardness value measured in the heat-affected zone.

[0457] The average hardness HV of base metals 平均 (BM) corresponds to the average of all hardness values ​​measured in the base metal.

[0458] As can be seen from Table 6 above, in experiments E1 to E8 that meet standards C1 to C3, 100% of the failures occurred outside the weld joint 22 or the heat-affected zone during the tensile test.

[0459] also:

[0460] -The maximum hardness variation ΔHV (WJ) across weld joint 22 is less than or equal to the average hardness HV of weld joint 22. 平均 20% of (WJ);

[0461] -Minimum hardness HV of welded joint 22 最小 (WJ) is higher than or equal to the average hardness HV of the most indurable base metal. 平均 (BM 最不可硬化 );as well as

[0462] - The hardness decrease of the heat-affected zone relative to the base metal is less than or equal to 8%.

[0463] Conversely, in experiments E9 to E21 not according to the invention, failure occurs in the weld joint 22 or the heat-affected zone because at least one of the standards C1 to C3 is not met.

[0464] More specifically, when standard C1 is not met but standards C2 and C3 are met (experiments E13 to E19), the maximum hardness change ΔHV (WJ) across weld joint 22 is strictly greater than the average hardness HV of weld joint 22. 平均 (WJ) 20%. Therefore, in this case, the weld joint 22 includes a localized region of high hardness, which has lower ductility and increases the risk of failure in the weld joint 22.

[0465] Furthermore, in the case where standard C1 and C3 are met but standard C2 is not met (experiments E9 to E12), the minimum hardness HV of welded joint 22 is... 最小 (WJ) is strictly less than the average hardness HV of the most inhardenable base metal. 平均 (BM 最不可硬化 In this case, 100% of the failures occur in welded joint 22.

[0466] Finally, when C1 and C2 are met but C3 is not satisfied (Experiments E20 and E21), the hardness decrease of the heat-affected zone relative to the base metal is strictly greater than 8%. In this case, at least 20% of the failures occur in the heat-affected zone. These results confirm that the risk of failure in the heat-affected zone increases when C3 is not met.

[0467] Furthermore, it was observed that in experiments meeting standard C5, even when the carbon content in welded joint 22 was greater than or equal to 0.15% by weight, the hardness change ΔHV (WJ) within the welded joint was less than or equal to 80 HV (experiments E1 to E8, and E19 and E20). Conversely, if standard C5 was not met, for a carbon content in welded joint 22 greater than or equal to 0.15% by weight, the hardness change ΔHV (WJ) within the welded joint was strictly greater than 80 HV (experiments E10 to E18).

[0468] Therefore, the method according to the invention is particularly advantageous because it allows for the acquisition of a component with excellent mechanical properties, including the welded joint 22, after hot pressing and cooling in a pressing tool, without having to remove the pre-coating 5 before welding.

[0469] Therefore, it is particularly suitable for manufacturing anti-intrusion components, structural components, or energy-absorbing components that contribute to the safety of motor vehicles.

Claims

1. A method for producing welded steel billets (1), comprising the following sequential steps: - Two pre-coated plates (2) are provided, each pre-coated plate (2) comprising a steel substrate (3) having a pre-coating layer (5) on at least one of its main surfaces (4), the pre-coating layer (5) comprising an intermetallic compound alloy layer (9) comprising at least iron and aluminum and optionally a metal alloy layer (11) extending on top of the intermetallic compound alloy layer (9), the metal alloy layer (11) being an aluminum layer or an aluminum alloy layer, the steel substrate (3) being made of compressible hardening steel, the steel substrate (3) having a thickness of 0.8 mm to 5 mm. in, For at least one of the pre-coated plates (2), the steel substrate (3) comprises, by weight: The remainder consists of iron and impurities produced during manufacturing. - The pre-coated plates (2) are butt-welded using filler wire (20) to form a weld joint (22) at the joint between the pre-coated plates (2), wherein the pre-coating layer (5) completely covers at least one main surface (4) of each pre-coated plate (2) during butt welding. Its characteristics are: - The carbon content of the filler wire (20) is from 0.01% to 0.45% by weight. - Select the composition of the filler wire (20) and the proportion of filler wire (20) added to the weld pool such that the obtained weld joint (22) has the following characteristics: (a) The quenching factor FT of the welded joint (22) WJ Satisfying FT WJ -0.9FT BM ≥0 in: - FT BM The quenching factor of the most non-hardenable steel substrate (3) among the two pre-coated plates (2), and - Quenching Factor FT WJ and FT BM Use the following formula to determine: Wherein Al, Cr, Ni, C, Mn, and Si are the average aluminum, chromium, nickel, carbon, manganese, and silicon contents, expressed as weight percentages, for the region where the quenching factor is to be determined, in FT. WJ In the case of the welded joint (22), and in FT BM In the case of the least hardenable steel matrix, and (b) The carbon content C of the welded joint (22) WJ Strictly less than 0.15% by weight, or if the carbon content C of the welded joint (22) is... WJ If the softening factor FA of the welded joint (22) is greater than or equal to 0.15% by weight, then the softening factor FA of the welded joint (22) is greater than or equal to 0.15% by weight. WJ Satisfy FA WJ >5000, wherein the softening factor FA of the welded joint (22) is... WJ The following formula is used as a function of the average aluminum, chromium, nickel, molybdenum, carbon, manganese and silicon content of the welded joint (22) expressed as a weight percentage: FA=10291 + 4384.1xMo + 3676.9xSi - 522.64xAl - 2221.2xCr - 118.11xNi , The carbon content C of the welded joint (22) is calculated as a percentage by weight. WJ Make 1.25×C BM(最可硬化) -C WJ ≥0, where C BM The carbon content of the most hardenable steel substrate (3) of the two pre-coated plates (2) by weight percentage.

2. The method according to claim 1, wherein the nickel content (Ni) of the welded joint (22) is... WJ It ranges from 2.0% by weight to 11.0% by weight.

3. The method according to claim 1 or claim 2, wherein the pre-coated plate (2) provided in the providing step has a pre-coating (5) on its two main surfaces (4).

4. The method according to claim 3, wherein, During butt welding, the pre-coating layer (5) is completely retained on the two main surfaces (4) of at least one of the pre-coated plates (2).

5. The method according to claim 1 or claim 2, further comprising preparing at least one of the pre-coated plates (2) to be at least partially incorporated into the weld edge (14) of the weld joint (22) by using at least one of the following processing steps prior to butt welding: brushing and / or machining and / or chamfering and / or beveling and / or removing at least a portion of the pre-coating (5), thereby preparing the pre-coating (5) in such a way that the pre-coating (5) is completely retained on at least one main surface (4) of each of the two pre-coated plates (2).

6. The method according to claim 1 or claim 2, wherein the welding step is performed using a laser beam.

7. The method according to claim 1 or claim 2, wherein, For at least one of the pre-coated plates (2), the steel substrate (3) comprises, by weight: The remainder consists of iron and impurities produced during manufacturing.

8. The method according to claim 1 or claim 2, wherein a shielding gas is used for welding.

9. A method for producing steel parts that are welded, then hot-pressed and cooled, comprising the following sequential steps: - Perform the method according to claim 1 or claim 2 to obtain a welded steel billet (1); - The welded steel billet (1) is heated to obtain a fully austenitic structure in the steel matrix (3) of the pre-coated plate (2); - The welded steel billet (1) is hot-pressed in a pressing tool to obtain a steel component; and - Cool the steel component in the pressing tool.

10. The method according to claim 9, wherein, During the cooling step, the cooling rate is greater than or equal to the cooling rate of the most hardenable bainite or martensite in the steel matrix (3) of the pre-coated plate (2).

11. A weldable steel billet (1) comprising two pre-coated plates (2), each pre-coated plate (2) comprising a steel substrate (3) having a pre-coating layer (5) on at least one of its main surfaces (4), the pre-coating layer (5) comprising an intermetallic compound alloy layer (9) comprising at least iron and aluminum and optionally a metal alloy layer (11) extending on top of the intermetallic compound alloy layer (9), the metal alloy layer (11) being an aluminum layer or an aluminum alloy layer, the steel substrate (3) being made of compressible hardening steel, the thickness of the steel substrate (3) being 0.8 mm to 5 mm. in, For at least one of the pre-coated plates (2), the steel substrate (3) comprises, by weight: The remainder consists of iron and impurities produced during manufacturing. The pre-coated plate (2) is joined by a welded joint (22), which is characterized in that: (a) The quenching factor FT of the welded joint (22) WJ Satisfying FT WJ -0.9FT BM ≥0, in: - FT BM The quenching factor of the most non-hardenable steel substrate (3) among the two pre-coated plates (2), and - Quenching Factor FT WJ and FT BM The following formula is used to determine: FA = 10291 + 4384.1xMo + 3676.9xSi - 522.64xAl - 2221.2xCr - 118.11xNi Wherein Al, Cr, Ni, C, Mn, and Si are the average aluminum, chromium, nickel, carbon, manganese, and silicon contents, expressed as weight percentages, for the region where the quenching factor is to be determined, in FT. WJ In the case of the welded joint (22), and in FT BM In the case of the least hardenable steel matrix, and (b) The carbon content C of the welded joint (22) WJ Strictly less than 0.15% by weight, or if the carbon content C of the welded joint (22) is... WJ If the softening factor FA of the welded joint (22) is greater than or equal to 0.15% by weight, then the softening factor FA of the welded joint (22) is greater than or equal to 0.15% by weight. WJ Make FA WJ >5000, wherein the softening factor FA of the welded joint (22) is... WJ The following formula is used as a function of the average aluminum, chromium, nickel, molybdenum, carbon, manganese and silicon content of the welded joint (22) expressed as a weight percentage: ， The welded joint (22) ensures that, after hot pressing and cooling, the maximum hardness change ΔHV(WJ) across the welded joint (22) is less than or equal to the average hardness HV of the welded joint (22). 平均 20% of (WJ). The carbon content C of the welded joint (22) is calculated as a percentage by weight. WJ Make 1.25×C BM(最可硬化) -C WJ ≥0, where C BM The carbon content of the most hardenable steel substrate (3) of the two pre-coated plates (2) by weight percentage.

12. The welded steel billet (1) according to claim 11, wherein the nickel content (Ni) of the welded joint (22) is... WJ It ranges from 2.0% by weight to 11.0% by weight.

13. A steel component that is welded, hot-pressed, and cooled, 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 comprising at least iron and aluminum on at least one main surface thereof, said steel substrate (3) being made of compressible hardening steel, said steel substrate (3) having a thickness of 0.8 mm to 5 mm, The steel substrate (3) of at least one of the first coated steel component portion and the second coated steel component portion comprises, by weight: The remainder consists of iron and impurities produced during manufacturing. The first coated steel component portion and the second coated steel component portion are joined by a welded joint (22), the welded joint (22) being characterized in that: (a) The quenching factor FT of the welded joint (22) WJ Satisfying FT WJ -0.9FT BM ≥0, in: - FT BM The quenching factor of the most non-hardenable steel substrate (3) in the steel substrate (3) of the first coated steel component portion and the second coated steel component portion, and - Quenching Factor FT WJ and FT BM Use the following formula to determine: Wherein Al, Cr, Ni, C, Mn, and Si are the average aluminum, chromium, nickel, carbon, manganese, and silicon contents, expressed as weight percentages, for the region where the quenching factor is to be determined, in FT. WJ In the case of the welded joint (22), and in FT BM In the case of the least hardenable steel matrix, and (b) The carbon content C of the welded joint (22) WJ Strictly less than 0.15% by weight, or if the carbon content C of the welded joint (22) is... WJ If the softening factor FA of the welded joint (22) is greater than or equal to 0.15% by weight, then the softening factor FA of the welded joint (22) is greater than or equal to 0.15% by weight. WJ Make FA WJ >5000 The softening factor FA of the welded joint (22) mentioned above WJ The following formula is used as a function of the average aluminum, chromium, nickel, molybdenum, carbon, manganese and silicon content of the welded joint (22) expressed as a weight percentage: ， And the maximum hardness variation ΔHV(WJ) across the welded joint (22) is less than or equal to the average hardness HV of the welded joint (22). 平均 20% of (WJ), The carbon content C in the welded joint (22) is calculated as a percentage by weight. WJ This makes 1.25 C BM -C WJ ≥0, where C BM The carbon content of the most hardenable steel substrate (3) in the first coated steel component portion and the second coated steel component portion, expressed as a percentage by weight.

14. The steel component of claim 13, which is welded, hot-pressed and cooled, wherein the hardness decrease in the heat-affected zone is less than or equal to 8% relative to the base metal adjacent to the heat-affected zone of the first coated steel component portion and the second coated steel component portion.

15. The steel component that has been welded, hot-pressed and cooled according to claim 13 or claim 14, wherein the average hardness HV of the welded joint (22) is... 平均 (WJ) less than or equal to 600 HV.

16. The steel component that has been welded, hot-pressed and cooled according to claim 13 or claim 14, wherein the nickel content (Ni) in the welded joint (22) is... WJ It ranges from 2.0% by weight to 11.0% by weight.

17. The steel component according to claim 13 or claim 14, which is welded, hot-pressed, and cooled, wherein, The steel substrate (3) of at least one of the first coated steel component portion and the second coated steel component portion comprises, by weight: The remainder consists of iron and impurities produced during manufacturing.

18. Use of the welded, hot-pressed and cooled steel component according to claim 13 or claim 14 for the production of intrusion prevention components or energy-absorbing components for motor vehicles.

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