Laser cutting of pre-coated steel blanks and associated blanks

Abstract

A method for producing a pre-coated steel blank, the method comprising the following consecutive steps: providing a pre-coated steel strip, the pre-coated steel strip comprising a steel substrate having a pre-coating layer on at least one of its main faces, the pre-coating layer comprising an intermetallic alloy layer and a metal layer extending above the intermetallic alloy layer, the metal layer being a layer of aluminum, a layer of an aluminum alloy or a layer of an aluminum-based alloy; laser cutting the pre-coated steel strip to obtain at least one pre-coated steel blank, the pre-coated steel blank comprising a laser-cut edge surface resulting from the laser cutting operation, the laser-cut edge surface comprising a substrate portion and a pre-coating layer portion, wherein the laser cutting is implemented so that the substrate portion of the laser-cut edge directly resulting from the cutting operation has an oxygen content greater than or equal to 15% by weight.

Description

Technical Field

[0001] The invention relates to a method for producing a pre-coated steel blank from a pre-coated steel strip, the pre-coated steel strip comprising a steel substrate having a pre-coating layer on at least one of its faces, the pre-coating layer comprising an intermetallic alloy layer and a metal layer extending over the intermetallic alloy layer, the metal layer being a layer of aluminium, a layer of an aluminium alloy or a layer of an aluminium-based alloy. Background Art

[0002] Steel parts for motor vehicles can be produced using the following method. First, a pre-coated steel strip, usually obtained by hot dip coating, is provided and cut into blanks by laser cutting. The individual blanks are then prepared for welding by removing the metal layer in the removal zone adjacent to the cut edge by laser ablation, and the blanks thus prepared are laser welded together to produce welded blanks. The welded blanks are then hot stamped and press hardened to obtain the final component.

[0003] Such steel components are used in particular in the automotive industry and more particularly for the production of anti-intrusion components, structural components or components contributing to the safety of motor vehicles.

[0004] The use of laser cutting to prepare individual steel blanks offers many industrial advantages, such as very good cut surface quality, the possibility of obtaining cut shapes with very high geometric accuracy and the possibility of processing very high-strength steels. Laser cutting offers more flexibility than mechanical cutting, since it is not necessary to produce new cutting dies for changing the blank shape.

[0005] Laser cutting can be performed on blanks that have been mechanically cut from coils. Laser cutting can also be performed directly on coils, in which case laser cutting is also known as laser blanking. Due to the advent of high-powered industrial lasers for laser cutting, laser blanking is becoming a viable option in industry, advantageously eliminating the intermediate mechanical cutting step performed on the coils.

[0006] However, laser cutting also has some limitations. One of these limitations is the presence of aluminum on the cut edge in the case of laser cutting of aluminized press-hardened steel.

[0007] This aluminum, which is not removed by the subsequent laser ablation process, is introduced into the weld metal zone during laser welding and leads to an increase in the aluminum content in the weld metal zone. Aluminum is a ferrite former in solid solution in the matrix and therefore prevents transformation to austenite from occurring during the steps prior to hot forming. Therefore, it is no longer possible to obtain a fully martensitic or bainitic structure in the weld joint during cooling after hot forming, and the weld joint will contain ferrite.

[0008] Therefore, after press hardening, the welded joint of the steel components exhibits a lower hardness and mechanical strength than the hardness and mechanical strength of the two adjacent blanks.

[0009] The edges can be cleaned mechanically to remove the aluminum contamination. However, the inventors have found that it is difficult to mechanically remove aluminum contamination from the cut edges when using laser cutting parameters known from the prior art, because the aluminum particles exhibit a high degree of adhesion to the cut edges. Therefore, it is necessary to implement costly measures, such as rubbing or grinding the edges, to ensure effective removal of the aluminum contamination. This means that manufacturers need to invest in specific tools and need to dedicate part of their production time and workshop space to this specific task.

[0010] The term "brushing" refers to the process of cleaning an edge using a brush equipped with stiff bristles. A rotating brush that moves along the edge is usually used. The term "rubbing" refers to the process of cleaning an edge using an abrasive belt that rubs the edge with a certain amount of normal force on the edge to achieve the desired result. The term "grinding" refers to the process of cleaning an edge by milling the edge to a depth of, for example, 0.1 mm to 0.3 mm. Brushing is easier to implement and maintain than rubbing and grinding, and has lower implementation and maintenance costs in terms of processing costs and maintenance costs.

[0011] Publication PCT / EP2016 / 082267 discloses a method for minimizing the amount of aluminum deposited on the laser cut edge by adjusting the laser cutting parameters. The disadvantage of this method is that it reduces the cutting productivity, thereby increasing the production cost. Summary of the invention

[0012] One of the objects of the present invention is to provide a method of producing a pre-coated steel blank having a cut edge surface that can be easily cleaned to remove aluminum contaminants to a level acceptable for subsequent welding operations. Another object of the present invention is to provide a method of producing a pre-coated steel blank having a cut edge surface that can be directly used for laser welding without the need for additional cleaning.

[0013] For this purpose, the present invention relates to a method for producing a pre-coated steel blank, comprising the following successive steps:

[0014] - providing a pre-coated steel strip comprising a steel substrate having a pre-coating layer on at least one of its main faces, the pre-coating layer comprising an intermetallic alloy layer and a metal layer extending over the intermetallic alloy layer, the metal layer being a layer of aluminium, a layer of an aluminium alloy or a layer of an aluminium-based alloy,

[0015] - laser cutting the pre-coated steel strip so as to obtain at least one pre-coated steel blank, the pre-coated steel blank comprising a laser cut edge surface resulting from the laser cutting operation, the laser cut edge surface comprising a substrate portion and a pre-coating layer portion,

[0016] Therein, the laser cutting is performed in such a way that a substrate portion of the laser cut edge resulting directly from the cutting operation has an oxygen content of greater than or equal to 15% by weight.

[0017] The inventors have found that the method described above allows the production of laser cut pre-coated steel blanks for which aluminum contamination on the edges resulting from the cutting process is easily removed in a subsequent brushing operation. The inventors have also found that such pre-coated steel blanks can be used directly for laser welding without the need to brush the edges prior to welding.

[0018] According to other optional features of the present invention, considered alone or in any possible technical combination:

[0019] - Laser cutting is performed using an assist gas comprising at least 10% by weight of oxygen, and most preferably at least 18% by weight of oxygen.

[0020] -Laser cutting is performed using pure oxygen as assist gas.

[0021] The product of the linear energy of the laser used for the laser cutting operation and the oxygen content of the auxiliary gas in volume % is greater than or equal to 0.09 kJ / cm.

[0022] - after laser cutting, performing a brushing operation on at least a portion of the laser cut edge to form a brushed cut edge, the brushed cut edge comprising a brushed substrate portion and at least one brushed pre-coating layer portion.

[0023] The aluminum content of the brushed substrate portion is less than 6.0% by weight.

[0024] The brushed cutting edge extends over the entire length of the laser cut edge.

[0025] The brushed cut edge only extends over a portion of the laser cut edge.

[0026] The product of the linear energy of the laser used for the laser cutting operation and the oxygen content of the auxiliary gas in volume % is greater than or equal to 0.03 kJ / cm.

[0027] The invention also relates to a method for producing a welding blank, the method comprising the following steps:

[0028] - producing a first pre-coated steel blank and a second pre-coated steel blank, producing at least one of the first pre-coated steel blank and the second pre-coated steel blank using the method described above to form a laser cut edge or a brushed cut edge on at least one of the pre-coated steel blanks;

[0029] - butt welding the welding edge of the first pre-coated steel blank to the welding edge of the second pre-coated steel blank so as to produce a weld joint between said pre-coated steel blanks, thereby obtaining a welded blank, whereby the butt welding step comprises the step of arranging the first pre-coated steel blank and the second pre-coated steel blank such that the laser cut edge of at least one of the pre-coated steel blanks is the welding edge.

[0030] According to other optional features of the present invention, considered alone or in any possible technical combination:

[0031] - The welding operation is a laser welding operation.

[0032] Prior to the butt welding step, there is a step of removing, for at least one of the first pre-coated steel blank and the second pre-coated steel blank, the metal layer in a removal zone adjacent to the welding edge of the pre-coated steel blank.

[0033] - The removal of the metal layer is performed using a laser beam.

[0034] - During the removal step, the intermetallic alloy layer remains in the removal zone with at least a portion of its height.

[0035] -Laser welding is performed using filler wire or powder additions.

[0036] -The filler wire or powder contains austenite forming alloying elements.

[0037] The invention also relates to a method for producing a press-hardened steel component, the method comprising the following consecutive steps:

[0038] - carrying out the method described above in order to obtain a welded blank;

[0039] - heating the welding blank in order to obtain at least a partially austenitic structure in the welding blank;

[0040] - hot forming the welded blank in a press to obtain a press-formed steel part; and

[0041] - Cooling the steel component in the press in order to obtain a press-hardened steel component.

[0042] According to other optional features of the present invention, considered alone or in any possible technical combination:

[0043] - The cooling rate is equal to or greater than the critical martensite or bainite cooling rate of the steel blank.

[0044] The present invention also relates to a pre-coated steel blank, comprising:

[0045] a steel substrate portion having a pre-coating layer portion on at least one of its faces, the pre-coating layer portion comprising an intermetallic alloy layer portion and a metal layer portion extending over the intermetallic alloy layer portion, the metal layer portion being a layer of aluminium, a layer of an aluminium alloy or a layer of an aluminium-based alloy, the thickness of the pre-coated steel blank being comprised between 0.5 mm and 5 mm, and

[0046] at least one laser cut edge surface extending between the faces of the pre-coated steel blank and comprising a substrate region and at least one pre-coating region,

[0047] Therein, the substrate region of the laser cut edge has an oxygen content greater than or equal to 15% by weight and an aluminum content less than or equal to 6.0% by weight.

[0048] The present invention also relates to a pre-coated steel blank, comprising:

[0049] a steel substrate portion having a pre-coating layer portion on at least one of its faces, the pre-coating layer portion comprising an intermetallic alloy layer portion and a metal layer portion extending over the intermetallic alloy layer portion, the metal layer portion being a layer of aluminium, a layer of an aluminium alloy or a layer of an aluminium-based alloy, the thickness of the pre-coated steel blank being comprised between 0.5 mm and 5 mm, and

[0050] at least one brushed cutting edge surface extending between the faces of the pre-coated steel blank and comprising a brushed substrate area and at least one brushed pre-coating area,

[0051] The oxygen content of the brushed substrate region by weight is greater than or equal to 0.5%, and the aluminum content of the brushed substrate region by weight is less than 6.0%.

[0052] The present invention also relates to a welding blank, which comprises at least:

[0053] a first pre-coated steel blank and a second pre-coated steel blank, each pre-coated steel blank comprising a steel substrate portion having a pre-coating layer portion on at least one of its faces, the pre-coating layer portion comprising an intermetallic alloy layer portion and a metal layer portion extending over the intermetallic alloy layer portion, the metal layer portion being a layer of aluminum, a layer of an aluminum alloy or a layer of an aluminum-based alloy, the thickness of the pre-coated steel blank being comprised between 0.5 mm and 5 mm, and

[0054] a welding zone connecting the first pre-coated steel blank to the second pre-coated steel blank,

[0055] wherein the weld zone has an aluminum content by weight of less than or equal to 0.3%, and wherein the weld zone comprises at least 0.2% by volume of aluminum oxide particles having a diameter of less than or equal to 2 microns.

[0056] According to other optional features:

[0057] The weld zone contains at least 0.4% by volume of aluminum oxide particles having a diameter less than or equal to 2 micrometers.

[0058] The invention also relates to a press-hardened component which is produced by press-hardening a welded blank according to the method described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Other aspects and advantages of the invention will become apparent on reading the following description given by way of example and made with reference to the accompanying drawings, in which:

[0060] - Figure 1 is a schematic cross-sectional view of a pre-coated steel strip taken perpendicularly to the longitudinal direction of the strip;

[0061] - Figure 2 is a schematic perspective view of a pre-coated steel blank;

[0062] - Figure 3 yes Figure 2 A schematic side view of a pre-coated steel blank;

[0063] - Figure 4 is a schematic side view of a pre-coated steel blank subjected to a brushing operation after a laser cutting step;

[0064] - Figure 5 is a schematic perspective view of a pre-coated steel blank including a removal zone;

[0065] - Fig. 6A and Figure 6B Observations of a cross section of a laser cut edge of a pre-coated steel blank according to the present invention after the laser cutting operation and before any further processing of the steel blank. Fig. 6Ashows the aluminum pattern on the laser cut edge surface, Figure 6B shows the oxygen profile on the laser cut edge surface;

[0066] - Fig. 7A and Figure 7B is a cross-sectional observation of the brushed cut edge of a pre-coated steel blank according to the present invention after it has been brushed. Fig. 7A shows the aluminum pattern on the laser cut edge surface, Figure 7B shows the oxygen profile on the laser cut edge surface;

[0067] - Figure 8 is a cross-sectional observation result of a welded blank according to the present invention comprising a weld joint and two pre-coated steel blanks, which shows the hardness evolution across the weld joint;

[0068] - Fig. 9 is a cross-sectional observation result of an aluminum map of a weld joint portion having welded blanks, wherein a laser cut edge surface of each of the pre-coated steel blanks is not brushed between a laser cutting step and a welding step;

[0069] - Fig.10 is a cross-sectional observation result of an aluminum map of a welded joint portion having welded blanks according to the present invention, wherein a laser cut edge surface of each of the pre-coated steel blanks is brushed between a laser cutting step and a welding step;

[0070] - Fig.11 is a graph of aluminum weight content of a laser cut edge without brushing after laser cutting;

[0071] - Fig.12 is a graph of the aluminum weight content of a brushed cut edge where brushing is performed after laser cutting. DETAILED DESCRIPTION

[0072] The invention relates to a method for producing a pre-coated steel blank 1 .

[0073] The method comprises providing Figure 1 The first step of pre-coating a steel strip 2 is shown in cross section.

[0074] like Figure 1 As shown in FIG. 1 , the pre-coated steel strip 2 includes a metal substrate 3 having a pre-coating layer 5 on at least one of its faces. The pre-coating layer 5 is superposed on the substrate 3 and in contact with the substrate 3 .

[0075] The metal substrate 3 is more particularly a steel substrate.

[0076] The steel of the substrate 3 is more particularly a steel having a ferrite-pearlite microstructure.

[0077] The substrate 3 is advantageously made of a steel intended for heat treatment, more particularly a press-hardenable steel, and for example of a manganese-boron steel, such as type 22MnB5.

[0078] According to one embodiment, the steel of the substrate 3 comprises by weight:

[0079] 0.10%≤C≤0.5%

[0080] 0.5%≤Mn≤3%

[0081] 0.1%≤Si≤1%

[0082] 0.01%≤Cr≤1%

[0083] Ti≤0.2%

[0084] Al≤0.1%

[0085] S≤0.05%

[0086] P≤0.1%

[0087] B≤0.010%

[0088] The rest is iron and impurities from manufacturing.

[0089] More particularly, the steel of the substrate 3 comprises by weight:

[0090] 0.15%≤C≤0.25%

[0091] 0.8%≤Mn≤1.8%

[0092] 0.1%≤Si≤0.35%

[0093] 0.01%≤Cr≤0.5%

[0094] Ti≤0.1%

[0095] Al≤0.1%

[0096] S≤0.05%

[0097] P≤0.1%

[0098] B≤0.005%

[0099] The rest is iron and impurities from manufacturing.

[0100] According to an alternative, the steel of the substrate 3 comprises by weight:

[0101] 0.040%≤C≤0.100%

[0102] 0.80%≤Mn≤2.00%

[0103] Si≤0.30%

[0104] S≤0.005%

[0105] P≤0.030%

[0106] 0.010%≤Al≤0.070%

[0107] 0.015%≤Nb≤0.100%

[0108] Ti≤0.080%

[0109] N≤0.009%

[0110] Cu≤0.100%

[0111] Ni≤0.100%

[0112] Cr≤0.100%

[0113] Mo≤0.100%

[0114] Ca≤0.006%,

[0115] The rest is iron and impurities from manufacturing.

[0116] According to an alternative, the steel of the substrate 3 comprises by weight:

[0117] 0.24%≤C≤0.38%

[0118] 0.40%≤Mn≤3%

[0119] 0.10%≤Si≤0.70%

[0120] 0.015%≤Al≤0.070%

[0121] 0%≤Cr≤2%

[0122] 0.25%≤Ni≤2%

[0123] 0.015%≤Ti≤0.10%

[0124] 0%≤Nb≤0.060%

[0125] 0.0005%≤B≤0.0040%

[0126] 0.003%≤N≤0.010%

[0127] 0.0001%≤S≤0.005%

[0128] 0.0001%≤P≤0.025%

[0129] Among them, the titanium content and nitrogen content satisfy the following relationship:

[0130] Ti / N>3.42,

[0131] And the carbon content, manganese content, chromium content and silicon content satisfy the following relationship:

[0132]

[0133] The steel may optionally include one or more of the following elements:

[0134] 0.05%≤Mo≤0.65%

[0135] 0.001% ≤ W ≤ 0.30% %

[0136] 0.0005%≤Ca≤0.005%

[0137] The rest is iron and impurities inevitably produced during manufacture.

[0138] Depending on the desired thickness of the substrate 3 , the substrate 3 may be obtained by hot rolling and / or by cold rolling followed by annealing, or by any other suitable method.

[0139] The substrate 3 typically has a thickness comprised between 0.5 mm and 5 mm.

[0140] The pre-coating layer 5 is obtained by hot-dip coating, ie by dipping the substrate 3 into a molten metal bath. The pre-coating layer 5 comprises an intermetallic alloy layer 9 in contact with the substrate 3 and a metal layer 11 extending over the intermetallic alloy layer 9.

[0141] Intermetallic alloy layer 9 is formed by a reaction between substrate 3 and the molten metal of the molten pool. Intermetallic alloy layer 9 includes an intermetallic compound including at least one element from metal layer 11 and at least one element from substrate 3.

[0142] The thickness of the intermetallic alloy layer 9 is generally of the order of a few micrometers. In particular, the average thickness of the intermetallic alloy layer 9 is typically comprised between 2 and 7 micrometers.

[0143] The composition of the metal layer 11 is close to the composition of the molten metal in the bath.The metal layer 11 is formed by molten metal carried away by the strip as it travels through the molten metal bath during hot dip coating.

[0144] The thickness of the metal layer 11 is, for example, comprised between 19 μm and 33 μm or between 10 μm and 20 μm.

[0145] The metal layer 11 is a layer of aluminum, a layer of an aluminum alloy, or a layer of an aluminum-based alloy.

[0146] In this context, an aluminum alloy refers to an alloy comprising more than 50% by weight of aluminum. An aluminum-based alloy is an alloy in which aluminum is the main element by weight.

[0147] The intermetallic alloy layer 9 includes an intermetallic compound of the Fex-Aly type, and more particularly includes an intermetallic compound of Fe2Al5.

[0148] Patent EP 2 007 545 discloses in particular a specific structure of the pre-coating layer 5 obtained by hot-dip coating.

[0149] According to one embodiment, the metal layer 11 is a layer of an aluminum alloy also containing silicon.

[0150] According to one example, the metal layer 11 comprises by weight:

[0151] -8%≤Si≤11%,

[0152] -2%≤Fe≤4%,

[0153] The rest is aluminum and possible impurities.

[0154] Advantageously, if Figure 1 As shown in the figure, the substrate 3 is provided with the pre-coating layer 5 as described above on both sides thereof.

[0155] The method for producing pre-coated steel blanks 1 further comprises a step of cutting said pre-coated steel strip 2 by laser cutting in order to obtain at least one pre-coated steel blank 1 .

[0156] Figure 2 FIG. 1 is a schematic perspective view of such a pre-coated steel blank 1 .

[0157] The pre-coated steel blank 1 comprises a substrate portion 3 ′ and at least one pre-coating layer portion 5 ′, wherein the pre-coating layer portion 5 ′ comprises an intermetallic alloy layer portion 9 ′ and a metal layer portion 11 ′.

[0158] The pre-coated steel blank 1 also comprises two opposite main faces 4' and a peripheral edge 12 extending between the faces 4' around the periphery of the blank 1. The length of the peripheral edge 12 is equal to the circumference of the blank 1. The height h of the peripheral edge 12 is equal to the thickness of the blank 1.

[0159] In the context of the present patent application, the height of an element is the dimension of the element taken along the direction of the thickness of the pre-coated blank 1 (z direction in the drawings).

[0160] The peripheral edge 12 extends substantially perpendicularly to the face 4'. In this context, "substantially" means that the peripheral edge 12 extends at an angle comprised between 50° and 90° relative to one of the faces 4'. The angle of the peripheral edge 12 relative to the face 4' may vary along the periphery of the blank 1.

[0161] exist Figure 2 In the example shown in , the peripheral edge 12 has a substantially rectangular profile comprising four straight sides. However, any other profile may be used depending on the application.

[0162] The peripheral edge 12 includes a laser cut edge surface 13 resulting from the laser cutting operation.

[0163] The laser cut edge surface 13 extends from one face 4 ′ to the other face 4 ′ between the faces 4 ′ of the pre-coated steel blank 1 . The laser cut edge surface 13 extends over the entire height h of the peripheral edge 12 .

[0164] Advantageously, the pre-coated steel blank 1 is obtained by laser cutting along its entire contour. In this case, the peripheral edge 12 comprises a laser cut edge surface 13. The laser cut edge surface 13 thus extends around the entire periphery of the blank 1. According to an alternative, the cut edge surface 13 extends only over a portion of the length of the peripheral edge 12. In this case, the rest of the peripheral edge 12 may coincide with the original side edge of the strip 2.

[0165] In the context of this patent application, the length of an element is its dimension in the plane of a given face 4' of the precoated steel strip 2. The length of the laser cut edge surface 13 thus corresponds in particular to its dimension along the path of the laser beam during laser cutting.

[0166] As in Figure 2 and Figure 3 As can be seen in the figure, the laser cutting edge surface 13 includes a substrate portion 14 and at least one pre-coating layer portion 15. The substrate portion 14 corresponds to the surface of the substrate 3' located at the laser cutting edge surface 13. The pre-coating layer portion 15 corresponds to the surface of the pre-coating layer 5' located at the laser cutting edge surface 13. The pre-coating layer portion 15 is substantially composed of the material of the pre-coating layer 5'.

[0167] The thickness of the pre-coated steel blank 1 is the same as the thickness of the pre-coated steel strip 2. The thickness of the pre-coated steel blank 1 is comprised between 1.0 mm and 5 mm, more particularly between 1.0 mm and 3.0 mm, more particularly between 1.0 mm and 2.5 mm, and even more particularly between 1.2 mm and 2.5 mm.

[0168] During the laser cutting step, a laser beam of a laser cutting device is applied to the steel strip 2 along a predetermined path in order to obtain a laser cut edge surface 13. The predetermined path extends in the plane of the face 4' of the blank 1 .

[0169] In a particular embodiment, the laser used for laser cutting is advantageously a continuous laser.

[0170] According to the invention, the substrate portion 14 of the laser cut edge 13 shows an oxygen content by weight of greater than 15%.

[0171] It should be noted that the oxygen weight content on the edge is defined as the weight content measured by conventional measurement using an Energy Dispersive Spectroscopy detector integrated on a Scanning Electron Microscope. This measurement technique typically measures the element concentration to a depth of about 1 micron below the surface. In the subsequent description, the same definition is used for the aluminum content on the edge.

[0172] In this context, "directly generated" means in particular that the aluminum fraction or ratio is measured immediately after the laser beam of the laser cutting device has cut the pre-coated steel blank 1 from the pre-coated steel strip 2, and in particular before any further steps are performed on the cut edge surface 13 of the pre-coated steel blank 1, for example before possible finishing steps of the cut edge surface 13 such as brushing, machining, milling, sandblasting or peeling.

[0173] In a particular embodiment, the aluminum content of the substrate portion 14 of the laser cut edge 13 is less than or equal to 6.0% by weight.

[0174] Advantageously, the laser cut edge surface 13 extends over a length equal to at least 3 mm, and more particularly at least 10 mm.

[0175] For example, in Figure 2 In the example shown in , in which the pre-coated steel blank 1 has a rectangular contour, the laser cut edge surface 13 extends on one or more sides of the rectangle.

[0176] Laser cutting is performed using an auxiliary gas containing at least 10% oxygen by absolute volume. For example, laser cutting is performed using air as auxiliary gas, the air containing between 19% and 21% oxygen by weight, the balance being mainly nitrogen. For example, laser cutting is performed using pure oxygen as auxiliary gas. Advantageously, by using an auxiliary gas containing oxygen, the productivity of the laser cutting operation can be increased compared to laser cutting processes using an inert gas, such as pure nitrogen or argon as auxiliary gas. This is due to the exothermic reaction that occurs between oxygen and iron and possibly between oxygen and aluminum.

[0177] It should be noted that in the present context pure oxygen is defined as a gas having an oxygen content higher than 99% by absolute volume.

[0178] During the cutting operation, the aluminum contained in the pre-coating layer 5 of the strip 2 is heated and melted by the heat generated by the laser. Therefore, the molten metal aluminum has a tendency to flow onto the laser cut edge 13, thereby contaminating the laser cut edge 13 with aluminum, which, as previously explained, is potentially detrimental to the subsequent strength of the weld in the event that the laser cut edge 13 is incorporated into a weld joint.

[0179] According to the implementation mode, Figure 4 As depicted in , after the laser cutting operation, a brushing operation is performed on at least a portion of the laser cut edge 13 to form a brushed cut edge 17. For example, the brushing operation can be performed using the following parameters:

[0180] -Brush rotation speed: 1180rpm

[0181] -Brush reference: NH-S 80

[0182] The brushed cutting edge 17 comprises a brushed base material portion 18 and at least one brushed pre-coating layer portion 19. For example, in the case when the pre-coated steel blank 1 has a rectangular profile, the brushed cutting edge 17 may extend only on some of the sides of the rectangle, and for example only on one side of the rectangle.

[0183] According to an embodiment, the brushed cutting edge 17 extends over the entire length of the laser cut edge 13 , in which case the length of the brushed cutting edge 17 is equal to the length of the laser cut edge 13 .

[0184] The purpose of the brushing operation is to remove contaminants deposited on the laser cut edge 13 and directly generated by the laser cutting operation. In particular, the purpose of the brushing operation is to remove aluminum contaminants on the laser cut edge 13 generated by the laser cutting operation.

[0185] According to an embodiment, the aluminum content by weight of the brushed substrate portion 18 is less than 6.0%. Due to the brushing operation, aluminum contaminants present on the edge and generated by the laser cutting operation can be at least partially removed. Surprisingly, the inventors have found that it is easier to remove aluminum contaminants from the laser cut edge 13 by brushing when an auxiliary gas containing at least 10% oxygen by volume is used than when an inert gas is used as an auxiliary gas to perform the laser cutting operation.

[0186] Without wishing to be bound by any theory, the use of a significant amount of oxygen in the auxiliary gas may result in the formation of aluminum oxide on the laser cut edge 13, which has a lower adhesion to the edge than metallic aluminum, which would be present on the edge due to cutting under inert gas, during which the molten aluminum produced by the heat generated by the cutting operation is not exposed to oxygen and thus remains in an unoxidized or only slightly oxidized form of metal. Due to the lower adhesion of aluminum oxide on the laser cut edge 13, in the case of a pre-coated steel blank 1 obtained by laser cutting according to the invention, the brushing operation is more effective.

[0187] The inventors have also observed that the laser cut edge 13 produced by the laser cutting operation according to the invention presents a distinct visual appearance, which is different from the visual appearance of a laser cut pre-coated steel blank using an inert gas as auxiliary gas. In particular, the laser cut edge 13 has a bluish or even black hue, which is due to the presence of oxidized metal particles, such as aluminum oxide from the pre-coating layer 5 of the steel strip 2 and iron oxide and other oxidized metal elements, such as manganese, from the substrate 3 of the steel strip 2. This particular visual appearance may be interpreted by a casual observer as a sign of poor quality, which would prevent the application of the described process of the invention to obtain a good quality pre-coated steel blank 1. However, as previously explained, laser cutting using an auxiliary gas containing more than 10% oxygen by volume to produce a laser cut edge 13 having a substrate region 14 containing at least 15% oxygen by weight actually allows the laser cut edge 13 to be effectively brushed so as to form a brushed cut edge 17 with a low content of aluminum.

[0188] The inventors have also observed that the laser cut edge 13 obtained by applying the present invention surprisingly has better corrosion resistance than the laser cut edge obtained by the laser cutting process using an inert auxiliary gas. The pre-coated steel blanks produced according to the present invention were placed in a paste at a humidity of 100% and maintained at a temperature of 70°C. The pre-coated steel blanks laser cut using pure nitrogen as auxiliary gas were placed in the same paste as the reference pre-coated steel blanks. After 96 hours, the first group of pre-coated steel blanks were removed from the paste and the appearance of the laser cut edges was observed. No red rust was found on the laser cut edges of the pre-coated steel blanks that had been produced according to the present invention, while red rust was observed on the laser cut edges of the reference pre-coated steel blanks. After 1 week, the second group of pre-coated steel blanks were removed from the paste and the appearance of the laser cut edges was observed. No red rust was found on the laser cut edges of the pre-coated steel blanks that had been produced according to the present invention, while red rust was observed on the laser cut edges of the reference pre-coated steel blanks.

[0189] While not wishing to be bound by any theory, the presence of the above-mentioned metal oxides on the laser cut edge 13 may form a barrier to atmospheric corrosion. The observed improved corrosion resistance presents a significant industrial advantage, since the pre-coated steel blanks 1 can thus be stored without the need to take costly measures to prevent rust from forming on the edges before the pre-coated steel blanks 1 are used to form laser welded blanks.

[0190] According to an embodiment, the amount of oxygen by weight on the brushed cutting edge 17 is higher than 0.5%. In fact, the brushing operation makes it possible to remove a part of the aluminum contamination present at the surface of the laser cut edge 13 and a part of the oxide. However, the inventors have found that, after the brushing operation has been performed, a significant part of the oxygen obtained by the laser cutting process according to the invention is still visible at the surface. It should be noted that the oxygen content measurement on the edge is preferably performed immediately after the brushing step and before the pre-coated steel blank is stored. In fact, during storage, the oxygen present in the air will oxidize the edge and thereby increase the measured oxygen content of the edge.

[0191] The inventors have also observed that, in addition to the above-mentioned effect of reducing aluminum edge contaminants, the brushing operation applied on at least a portion of the laser cut edge 13 also has the beneficial effect of removing some or all of the burrs generated by the laser cutting operation. In fact, as compared to laser cutting performed using an inert assist gas, a cutting operation performed using an oxygen-enriched assist gas will more frequently result in the formation of burrs at the bottom of the laser cut edge 13. Such burrs can be easily separated from the laser cut edge 13, and such burrs can be largely removed by the brushing operation.

[0192] According to an embodiment, the laser cut edge 13 forms at least the edge of the pre-coated steel blank 1 intended to be welded to another pre-coated steel blank 1. In this case, at least a portion of the laser cut edge 13 is intended to be incorporated into the weld joint. In this embodiment, the laser cut edge 13 is used as a welding edge as is, without a subsequent brushing operation.

[0193] According to an embodiment, the brushed cut edge 17 forms at least the edge of the pre-coated steel blank 1 intended to be welded to another pre-coated steel blank 1. In this case, at least a part of the brushed cut edge 17 is intended to be incorporated into a weld joint.

[0194] According to an embodiment, the brushed cut edge 17 extends only over the edge of the pre-coated steel blank 1 intended to be welded to another pre-coated steel blank 1. Advantageously, this allows optimizing the productivity of the production process of the pre-coated steel blanks 1. In practice, the brushing operation has a cost associated with the length of the laser cut edge 13 to be brushed. By limiting this length to the portion of the laser cut edge 13 intended to be welded, the brushing operation will be performed only where it actually benefits the final quality of the welded blank, by reducing aluminum contamination and improving edge quality overall.

[0195] Turning to the laser cutting parameters, the inventors have found that a specific combination of linear energy and the oxygen content of the auxiliary gas can produce favorable results. The laser cutting linear energy corresponds to the amount of energy emitted by the laser beam during the laser cutting of a unit length. The laser cutting linear energy can be calculated by dividing the power of the laser beam by the cutting speed. The inventors have found that by using a composite parameter of linear energy and the amount of oxygen in the auxiliary gas, the process window for obtaining a satisfactory amount of aluminum on the edge after laser cutting can be defined. This parameter is the product of the linear energy and the oxygen content of the auxiliary gas. Due to the exothermic oxidation of iron and possibly aluminum, the oxygen of the auxiliary gas plays a role in the energy balance of the cutting operation, so it can be understood that the amount of oxygen contained in the auxiliary gas multiplied by the linear energy of the laser is a form of measuring the cutting energy, and can therefore be used to define the process window.

[0196] Laser cutting can be advantageously performed using linear energy and an auxiliary gas selected such that the product of the linear energy of the laser used for the laser cutting operation and the oxygen content of the auxiliary gas in volume % is greater than or equal to 0.09 kJ / cm. As illustrated in Table 1 of the examples described below, this minimum value consistently enables a laser cut edge 13 having a substrate portion 14 having an oxygen content greater than or equal to 15% in weight % and an aluminum content less than or equal to 6.0% in weight %.

[0197] Laser cutting can be advantageously performed using linear energy and an auxiliary gas selected such that the product of the linear energy of the laser used for the laser cutting operation and the oxygen content of the auxiliary gas in % by volume is greater than or equal to 0.03 kJ / cm. As illustrated in Table 1 of the examples described below, this minimum value consistently enables a brushed cut edge 17 having a substrate portion having an oxygen content in % by weight greater than or equal to 0.5% and an aluminum content in % by weight less than or equal to 6.0%.

[0198] In other words, the brushing operation can be used to widen the process window for the laser cutting operation by reducing the minimum value of the composite parameter defined above from 0.09 kJ / cm to 0.03 kJ / cm while maintaining an acceptable level of aluminum on the edge (less than 6.0% by weight).

[0199] According to one embodiment, the laser cutting step is carried out using a CO2 laser. The CO2 laser is advantageously a continuous laser.

[0200] A CO 2 laser has, for example, a power comprised between 2 kW and 10 kW.

[0201] According to another embodiment, the laser cutting step is performed using a solid-state laser, such as a Nd:YAG (neodymium-doped yttrium aluminum garnet) laser, a fiber laser, a diode laser or a disk laser.

[0202] The solid-state laser has, for example, a power comprised between 2 kW and 20 kW.

[0203] The present invention also relates to a pre-coated steel blank 1 obtainable using the method disclosed above. Figure 2 , Figure 3 and Figure 4 Such a pre-coated steel blank 1 is described.

[0204] The pre-coated steel blank 1 has an oxygen content by weight of more than 15% in the base material region 14 .

[0205] In a particular embodiment, the pre-coated steel blank 1 has an aluminum weight content in the brushed substrate region of less than 6.0% and an oxygen surface fraction in the brushed substrate region of greater than 0.5%.

[0206] Furthermore, the pre-coated steel blank 1 comprises a heat affected zone at the cut edge surface 13. The heat affected zone is produced by heating of the cut edge surface 13 during laser cutting. The presence of the heat affected zone can be observed by conventional means for detecting it, for example by microhardness or nanohardness measurement or by metallographic observation after appropriate etching.

[0207] The invention also relates to a method for producing a welding blank, the method comprising the following steps:

[0208] - producing a first pre-coated steel blank and a second pre-coated steel blank 1, using the method as described above to produce at least one of the first pre-coated steel blank and the second pre-coated steel blank 1, and preferably producing the first pre-coated steel blank and the second pre-coated steel blank 1;

[0209] - Butt welding the first pre-coated steel blank and the second pre-coated steel blank 1 in order to produce a weld joint between said steel blanks 1 and obtain a welded blank.

[0210] The butt welding step includes the step of arranging the first and second pre-coated steel blanks 1 so that the laser cut edge 13 of at least one of the pre-coated steel blanks 1 faces the edge of the other pre-coated steel blank 1 .

[0211] In a particular embodiment, the butt welding step comprises the step of arranging the first and second precoated steel blanks 1 so that the brushed cut edge 17 of at least one of the precoated steel blanks 1 faces the edge of the other precoated steel blank 1 .

[0212] The weld joint between the first pre-coated steel blank and the second pre-coated steel blank 1 is obtained by melting the facing edges of the first pre-coated steel blank and the second pre-coated steel blank, in particular by melting at least one of the laser cut edges 13. In a particular embodiment, the weld joint is obtained by melting at least one of the brushed cut edges 17 of at least one of the pre-coated steel blanks 1.

[0213] The welding is advantageously laser welding.

[0214] The welding may be autogenous welding, ie without the addition of filler material, for example in the form of welding wire or powder.

[0215] According to an alternative, welding is performed using a suitable filler material, such as a filler wire or powder. The filler wire or powder may contain austenite forming elements in particular in order to balance the ferrite forming effect and / or intermetallic compound forming effect of the aluminum contamination from the pre-coating layer.

[0216] Advantageously, if Figure 5As shown in , before butt welding, for at least one of the pre-coated steel blanks 1, the metal layer 11' on at least one face 4' of the pre-coated steel blank 1 is removed on a removal zone 25 adjacent to the laser cut edge 13 of the pre-coated steel blank 1 under consideration, and during the butt welding step, the pre-coated steel blank 1 is welded at least along one edge from which the metal layer 11' has been removed. Preferably, before butt welding, the metal layer 11' is removed from each of the first pre-coated steel blank and the second pre-coated steel blank 1.

[0217] The removal of the metal layer 11 ′ is advantageously performed by laser ablation, as disclosed in the prior application WO 2007 / 118939.

[0218] The width of the removed zone 25 on each of the steel blanks 1 is, for example, comprised between 0.2 mm and 2.2 mm.

[0219] Preferably, if Figure 5 As shown in , the removal step is performed so as to remove only the metal layer 11 ', while leaving the intermetallic alloy layer 9'. Thus, the intermetallic alloy layer 9' remains in the removal zone over at least a portion of its height. In this case, the residual intermetallic alloy layer 9' protects the area of ​​the welded blank immediately adjacent to the weld joint from oxidation and decarburization during the subsequent hot forming step and from corrosion during use of the hot formed steel part.

[0220] Optionally, the method for manufacturing a welded blank comprises the following step before performing the welding step: brushing the edges of at least one of the first pre-coated steel blank and the second pre-coated steel blank 1 to be welded, preferably both of the first pre-coated steel blank and the second pre-coated steel blank 1 .

[0221] If the method comprises removing the metal layer 11 'before welding, brushing is preferably carried out after this removal step. In this case, brushing can remove traces of aluminum that have been spattered onto the edges of the blanks 1 to be welded during the removal operation. Such spattering can occur in particular when the removal is performed by laser ablation. Such spatter has a relatively low adhesion to the edges and can therefore be removed relatively easily by brushing. Brushing can therefore further reduce the aluminum content in the weld joint.

[0222] The inventors have found that by applying the present invention, using a pre-coated steel blank 1, for which the two edges to be welded are laser cut edges 13, on which no brushing operation has been performed before welding, a laser welded blank is formed, the weld joint has an aluminum content of less than 0.3% by weight and presents a characteristic inclusion population of aluminum oxide having a diameter less than 4 microns and covering at least 0.4% by volume of the weld joint.

[0223] In the present context, the diameter of a particle is defined as the diameter of the smallest possible sphere within which the particle can be enclosed.

[0224] The inventors have found that by applying the present invention, using a pre-coated steel blank 1, for which the two edges to be welded are brushed laser cut edges 17, a laser welded blank is formed, the weld joint having an aluminum content of less than 0.3% by weight and presenting a characteristic inclusion population of aluminum oxide having a diameter less than 2 microns and covering at least 0.2% by volume of the weld joint.

[0225] Surprisingly, the inventors have found that despite the presence of oxygen on the edge before welding due to the presence of oxygen in the auxiliary gas and despite the presence of aluminum oxide in the weld joint, the weld joint exhibits good mechanical strength and toughness, as will be described later in the examples. It is known in the literature that the presence of oxygen, and in particular the presence of aluminum oxide, in a weld joint may have a negative impact on the plasticity and toughness of the weld joint.

[0226] The invention also relates to a method for producing a press-hardened steel component, the method comprising the following steps:

[0227] - producing a welding blank using the method as disclosed above;

[0228] - heating the welding blank so as to obtain at least a partially austenitic structure in the steel blank 1 forming the welding blank;

[0229] - hot forming the welded blank in a press so as to obtain a press-formed steel part; and

[0230] - Cooling the steel component in the press in order to obtain a press-hardened steel component.

[0231] More specifically, the welding blank is heated to a temperature higher than the upper austenite transformation temperature Ac3 of the steel blank 1 .

[0232] During the cooling step, the cooling rate is advantageously equal to or greater than the critical martensitic or bainitic cooling rate of the steel blank.

[0233] Since the above-described aluminum oxide inclusions in the weld joint are stable at the temperature used to heat the laser welded blank prior to hot forming, the resulting press-hardened steel component will retain the same aluminum oxide inclusions at the location where the weld joint existed on the original laser welded blank prior to the press forming operation. The location of the weld joint within the press-hardened steel component is a volume that includes at least a portion of the surface of each face of the press-hardened steel component and that extends between at least two edges of the press-hardened steel component.

[0234] More specifically, in the case of forming a press-hardened steel component by a laser-welded blank using a pre-coated steel blank 1, for which the two edges to be welded are laser-cut edges 13 on which no brushing operation is performed before welding, the position where the weld joint of the laser-welded blank initially exists has a characteristic inclusion group of aluminum oxide having a diameter of less than 4 microns and covering at least 0.4% by volume of the weld joint.

[0235] In the case of forming a press-hardened steel part by a laser-welded blank using a pre-coated steel blank 1, for which the two edges to be welded are brushed laser-cut edges 17, the location where the weld joint of the laser-welded blank initially exists has a characteristic inclusion group of aluminum oxide having a diameter of less than 4 microns and covering at least 0.2% of the weld joint by volume.

[0236] The inventors of the present invention carried out the following experiments. The first set of experiments focused on analyzing the laser cut edges 13 and brushed cut edges 17 of pre-coated steel blanks according to the present invention. The second set of experiments focused on analyzing press hardened steel parts according to the present invention.

[0237] In a first set of experiments, a pre-coated steel blank 1 was cut from a pre-coated steel strip 2 by laser cutting using a CO2 laser with pure oxygen and air as auxiliary gases and using different laser cutting speeds and energies. The pre-coated steel blank 1 had a rectangular shape. Steel strips 2 of different thicknesses were used. A portion of the pre-coated steel strip 1 thus produced was then observed as is with a laser cut edge 13 that was not subsequently processed by brushing. Another portion of the pre-coated steel strip 1 thus produced was subjected to a brushing operation to form a brushed cut edge 17 before being observed.

[0238] The pre-coated steel strip 2 is a steel strip having the composition and pre-coating layer as disclosed above.

[0239] More particularly, the steel of band 2 comprises, in % by weight:

[0240] C: 0.22%

[0241] Mn: 1.16%

[0242] Al: 0.03%

[0243] Si: 0.26%

[0244] Cr: 0.17%

[0245] B: 0.003%

[0246] Ti: 0.035%

[0247] S: 0.001%

[0248] N: 0.005%

[0249] The remainder is iron and possible impurities resulting from processing.

[0250] This steel is known under the trade name 1500 known.

[0251] The pre-coating layer 5 has been obtained by hot-dip coating of the steel strip 2 in a bath of molten metal.

[0252] The metal layer of the pre-coating layer 5 comprises by weight:

[0253] Si: 9%,

[0254] Fe: 3%,

[0255] The remainder consists of aluminum and possible impurities resulting from processing.

[0256] The average total thickness of the metal layers was 20 μm.

[0257] The intermetallic alloy layer includes intermetallic compounds of the Fex-Aly type and mainly Fe2Al3, Fe2Al5 and FexAlySiz. The average thickness of the intermetallic alloy layer is 5 μm.

[0258] For each pre-coated steel blank 1 thus produced, the inventors measured the weight contents of aluminum and oxygen on the substrate area 14 of the laser cut edge 13 of the unbrushed sample and the weight contents of aluminum and oxygen on the brushed substrate area 19 .

[0259] Based on images of the considered edge surface taken with a scanning electron microscope, the measurements were performed using the following parameters:

[0260] -Magnification: x60;

[0261] -Analysis length: 3mm;

[0262] -Electron beam energy: between 15keV and 25keV.

[0263] Experiments were carried out using a CO2 laser with a nominal power of 4 kW, in which different power levels between 1.9 kW and 3.8 kW were used. The auxiliary gas pressure was comprised between 3 bar and 15 bar. The cutting speed was comprised between 3 meters per minute and 20 meters. In the case of pure oxygen, the nozzle diameter for blowing the auxiliary gas was 0.8 mm, and in the case of air the nozzle diameter was 1.4 mm. The spacing distance separating the nozzle from the laser beam impact point was 0.7 mm. The thickness of the steel strip 2 used was comprised between 0.8 mm and 1.6 mm.

[0264] The brushing operation was performed using 7 brushes, which were powered by a motor applying a torque of 0.3 Newton-meter and rotating at a speed of 1180 RPM and traveled at a speed of 10 meters per minute. The brushes used were of commercial reference NH-S 80.

[0265] The test results are reported in Table 1. The results are expressed in terms of weight content of oxygen and aluminum on the substrate region 14 of the laser cut edge 13 or on the brushed substrate portion 19 of the brushed cut edge 17 and edge quality. Fig.11 and Fig.12 The results in Table 1 are shown below. Fig.11 and Fig.12 The % Al on the substrate portion 14 of the laser cut edge 13 and the % Al on the brushed substrate region 19 of the brushed cut edge 18 are plotted as a function of the product of the linear energy and the amount of oxygen in the assist gas, respectively.

[0266] It should be noted here that in the present context, edge quality is classified into one of the following three categories: very good, small burr, large burr. By very good, it is meant that no burrs are observed on the bottom of the laser cut edge 13 or the brushed cut edge 17. By small burr, it is meant that the height of the burr produced by the laser cutting process is strictly less than 0.1 mm. By large burr, it is meant that the height of the burr produced by the laser cutting process is 0.1 mm or more. The presence of burrs on the edge of the pre-coated steel blank intended for welding may lead to defects on the weld joint, which are detrimental to the mechanical strength of the weld joint.

[0267] Modes 1 to 14 are before brushing, while modes 1b to 14b are after brushing.

[0268] As shown in Table 1 and Fig.11 It can be seen from FIG. 1 that when the product of the linear energy and the amount of oxygen in the auxiliary gas is greater than or equal to 0.09 kJ / cm, the amount of aluminum on the substrate region 14 is less than 6.0%.

[0269] As shown in Table 1 and Fig.12It can be seen from FIG. 1 that when the product of the linear energy and the amount of oxygen in the auxiliary gas is greater than or equal to 0.03 kJ / cm, the amount of aluminum on the brushed substrate area 19 is less than 6.0%.

[0270] As can be seen in Table 1, the amount of oxygen generated directly from the laser cutting operation on the substrate portion 14 is above 15%, except for Mode 7, which shows an oxygen content of 9% at the edge. Mode 7 was performed using a very low linear energy times the oxygen percentage in the assist gas of 0.02 kJ / cm.

[0271] As can be seen in Table 1, when pure oxygen is used as the assist gas, the weight content of oxygen on the substrate portion 14 of the laser cut edge 13 is always higher than 15%.

[0272] Table 1 also reports the edge quality of the laser cut edge 13 or the brushed cut edge 17. As can be seen, the quality of the laser cut edge 13 varies from very good to large burrs. The laser cut edge 13 produced according to the invention is itself very suitable for brushing in order to improve the edge quality. In fact, as can be seen in Table 1, the edge quality after brushing is always considered very good after the brushing operation.

[0273] Fig. 6A and Figure 6B 1 is a cross-sectional observation of the laser cut edge 13 of the pre-coated steel blank corresponding to Mode 1 of Table 1, i.e., the pre-coated steel blank thickness is 0.8 mm, the laser power used for the cutting operation is 1.9 kW, the cutting speed is 3 meters per minute, and the pressure of pure oxygen as the auxiliary gas is 18 bar. Fig. 6A A map of aluminum on the surface of the laser cut edge 13 is shown, with aluminum pixels appearing in white on a gray background. Figure 6B The oxygen map on the surface of the laser cut edge 13 is shown, with oxygen pixels corresponding to the overall grey background, while the black dots on the grey background are non-oxygen pixels. A burr 20 resulting from the laser cutting process can be seen on the bottom of the cross sections 6A and 6B.

[0274] Fig. 7A and Figure 7B 1 is a cross-sectional observation of a brushed cut edge 17 of a pre-coated steel blank corresponding to Mode 1b of Table 1. The laser cutting parameters are the same as those of Mode 1 detailed above, but in the case of Mode 1b, the laser cut edge 13 is brushed using the brushing parameters detailed above to obtain a brushed cut edge 17. Fig. 7A A map of aluminum on the brushed cutting edge 17 surface is shown, with aluminum pixels appearing in white on a gray background. Figure 7B The oxygen spectrum on the surface of the brushed cutting edge 17 is shown, and the oxygen pixels correspond to the gray pixels on the black background. It can also be seen that Fig. 6A and Figure 6B The glitch observed in 20 Fig. 7A and Figure 7B This confirms that the brushing operation performed on the pre-coated steel blank according to the invention is able to remove the burrs directly generated by the cutting operation.

[0275] In a second set of experiments, press-hardened steel components produced according to the invention were analyzed.In a first step, a pre-coated steel blank 1 was produced according to the invention.

[0276] Pre-coated steel blanks 1 were made of pre-coated steel strips having a thickness of 0.8 mm and 1.6 mm and having the same chemical composition as the first set of experiments detailed above. The laser cutting operation was performed using a product of linear energy and oxygen content of the auxiliary gas greater than 0.09 kJ / cm in the case of cutting pre-coated steel blanks and greater than 0.03 kJ / cm in the case of cutting brushed pre-coated steel blanks.

[0277] The pre-coated steel blank 1 is then subjected to the following steps: the metal layer is removed in the removal zone adjacent to the weld edge on both faces 4' of the pre-coated steel blank 1 using a pulsed laser and applying the following parameters:

[0278] -Spot size: 0.5*2mm

[0279] -Travel speed: 2.5m / min

[0280] -Frequency: 6kHz

[0281] -Power: 450W

[0282] -Laser source: Rofin DQx 45s

[0283] A brushing operation was performed on a portion of the pre-coated steel blank 1. The brushing operation was performed using the following parameters: brushing was performed with 7 brushes, the brushes were powered by a motor applying a torque of 0.3 Newton-meter and having a speed of 1180 RPM and traveled at a speed of 10 meters per minute, and the brushes were made using a commercial reference NH-S 80 brush.

[0284] The pre-coated steel blanks 1 thus prepared are then laser welded, the first pre-coated steel blank and the second pre-coated steel blank 1 being arranged so that the laser cut edge 13 is the welding edge, or in the case of brush grinding, the brushed cut edge 17 is the welding edge. The laser welding is performed using a filler wire. The following laser welding parameters are used for all modes:

[0285] -Filling wire diameter: 1mm

[0286] - Collimation / focusing: 200 / 200

[0287] -Fiber diameter: 600mm

[0288] -Shielding gas: Helium (flow rate 15 liters per minute).

[0289] The filler wire used had the following composition, expressed in % by weight:

[0290] 0.65%≤C≤0.75%

[0291] 1.95%≤Mn≤2.05%

[0292] 0.35%≤Si≤0.45%

[0293] 0.95%≤Cr≤1.05%

[0294] 0.15%≤Ti≤0.25%

[0295] The balance is iron and unavoidable impurities resulting from processing.

[0296] The welded blanks thus produced are then processed into press-hardened steel components by heating them above the austenitizing temperature and then quenching them in a tool at a rate above the critical martensite cooling rate of the pre-coated steel blank 1 .

[0297] The detailed laser welding parameters specific to each mode, as well as the results of the second set of experiments are reported in Table 2.

[0298] The first set of results relates to the presence or absence of a drop in hardness within the weld metal zone compared to the hardness of the portion of the press-hardened steel component corresponding to the substrate of the pre-coated steel blank. As can be seen in Table 2, in all cases, no drop in hardness was observed in the weld metal zone, regardless of whether brushing was performed or not. This indicates that the weld metal zone will have good mechanical behavior on the component and will not constitute a weak area of ​​the component that could lead to premature failure of the component.

[0299] The hardness was measured using the Vickers hardness test according to standard NF EN ISO 6507-1. The test was performed transversely to the weld joint using a test force of 0.5 kgf (HV0.5).

[0300] Figure 8 Depicted are hardness measurements performed on mode 15b in which two pre-coated steel blanks each having a thickness of 1.6 mm were prepared and welded according to the parameters detailed in Table 1 and the description above. Figure 8The top portion of is a cross-sectional micrograph of a welded sample including a weld in the middle identified by the letter W and two pre-coated steel blanks 1 on either side of the weld identified by the letter P. The three horizontal black dashed lines on the micrograph correspond to the areas where the microhardness test was performed, and the black dots are traces left by the indentation performed to measure the microhardness. These lines are identified by the letters T, M, and B, representing the top, middle, and bottom, respectively. Figure 8 The bottom part of depicts the results of microhardness measurements along lines T, M and B. As can be seen, there is no drop in hardness within the weld zone compared to the pre-coated steel blank 1 .

[0301] The second set of results involved measuring the amount of aluminum dissolved in the weld metal zone using an energy dispersive spectroscopy detector integrated into a scanning electron microscope.

[0302] As can be seen in Table 2, the amount of aluminum dissolved in the weld zone by weight is always below 0.3%. Due to this low level of aluminum, the weld can undergo a metallurgical transformation resulting in a fully martensitic microstructure, which, as seen in the hardness measurements explained above, does not exhibit a lower hardness than the surrounding substrate.

[0303] The third set of results - corresponding to observations made on the weld metal zones of samples of Patterns 15 and 15b - relates to the inclusion population of the weld metal zone, and more particularly to the characteristics of the small alumina particles in the weld metal zone. Fig. 9 and Fig.10 are aluminum spectra on cross sections of the weld metal zones corresponding to modes 15 and 15b, respectively. These images were obtained using a scanning electron microscope at 10,000 times magnification and using an energy dispersive X-ray analysis probe set to detect aluminum and oxygen. Fig. 9 and Fig.10 As can be seen above, the shape of the detected alumina particles is generally spherical and the diameter of the alumina particles does not exceed 2 microns. The bulk density of the alumina is reported in Table 2. For mode 15, the measured bulk density is 0.6% on average, and for mode 15b, the measured bulk density is 0.3% on average. The decrease in density between modes 15 and 15b can be explained by the fact that some aluminum is removed from the edge by the brushing operation, so that less aluminum is dissolved in the weld. It should be noted that these small alumina particles are not observed on welds performed on pre-coated steel billets cut using mechanical cutting or laser cutting using an inert assist gas.

[0304] Without wishing to be bound by theory, the inventors propose the following reasons to explain why the aluminum oxide particles observed in the weld metal zone are not detrimental to the overall mechanical strength of the weld metal zone. The first reason is that the aluminum present in these oxides cannot be dissolved in the iron matrix of the weld metal zone and therefore does not affect the metallurgical phenomena occurring during the hot stamping process. More specifically, it does not affect the austenitization temperature nor the hardenability of the weld metal zone. The second reason is that the aluminum oxide particles are small enough not to have any significant effect on the mechanical resistance of the weld metal zone. Due to the small size of these particles, these particles do not represent areas of significant stress concentration and therefore do not become the cause of microcracks that would lead to failure of the weld.

[0305]

[0306] Table 1

[0307]

[0308] Table 2

Claims

1. A method for producing a pre-coated steel blank, the method comprising the following consecutive steps: - providing a pre-coated steel strip comprising a steel substrate having a pre-coating layer on at least one of its main faces, the pre-coating layer comprising an intermetallic alloy layer and a metal layer extending over the intermetallic alloy layer, the metal layer being a layer of aluminium, a layer of an aluminium alloy or a layer of an aluminium-based alloy, - laser cutting the pre-coated steel strip to obtain at least one pre-coated steel blank, the pre-coated steel blank comprising a laser cut edge surface resulting from the laser cutting, the laser cut edge surface comprising a substrate portion and a pre-coating layer portion, in, The laser cutting is performed using an auxiliary gas containing at least 10% oxygen by weight, the product of the linear energy of the laser used for the laser cutting operation and the oxygen content of the auxiliary gas by volume % is greater than or equal to 0.09 kJ / cm, and is implemented so that the substrate portion of the laser cut edge directly produced by the laser cutting has an oxygen content greater than or equal to 15% by weight.

2. The method according to claim 1, wherein: The laser cutting is performed using an assist gas comprising at least 18% by weight oxygen.

3. The method according to claim 1 or 2, wherein: The laser cutting is performed using pure oxygen as an assist gas.

4. The method according to claim 1 or 2, wherein: After the laser cutting, a brushing operation is performed on at least a portion of the laser cut edge to form a brushed cut edge, the brushed cut edge including a brushed substrate portion and at least one brushed pre-coating layer portion.

5. The method according to claim 4, wherein: The brushed substrate portion has an aluminum content by weight of less than 6.0%.

6. The method according to claim 4, wherein: The brushed cut edge extends the entire length of the laser cut edge.

7. The method according to claim 4, wherein: The brushed cut edge extends over only a portion of the laser cut edge.

8. A method for manufacturing a welding blank, the method comprising the steps of: - producing a first pre-coated steel blank and a second pre-coated steel blank, producing at least one of the first pre-coated steel blank and the second pre-coated steel blank using the method according to any one of claims 1 to 7 to form a laser cut edge or a brushed cut edge on at least one of the pre-coated steel blanks; - butt welding the welding edge of the first pre-coated steel blank to the welding edge of the second pre-coated steel blank so as to produce a weld joint between the pre-coated steel blanks and thereby obtain a welded steel blank, whereby the butt welding comprises the step of arranging the first pre-coated steel blank and the second pre-coated steel blank so that the laser cut edge or the brushed cut edge of at least one of the pre-coated steel blanks is the welding edge.

9. The method according to claim 8, wherein: The butt welding is a laser welding operation.

10. The method according to claim 8 or 9, further comprising the following step before the butt welding: for at least one of the first pre-coated steel blank and the second pre-coated steel blank, removing the metal layer in the removal area adjacent to the welding edge of the pre-coated steel blank.

11. The method according to claim 10, wherein: The removal of the metal layer is performed using a laser beam.

12. The method according to claim 11, wherein: During the removing, the intermetallic alloy layer remains in the removal zone with at least a portion of its height.

13. The method according to claim 9, wherein: The laser welding is performed using a filler wire or powder addition.

14. The method according to claim 13, wherein: The filler wire or powder contains austenite forming alloying elements.

15. A method for manufacturing a press-hardened steel component, the method comprising the following consecutive steps: - carrying out a method according to any one of claims 8 to 14 in order to obtain a welded blank; - heating the welding blank in order to obtain at least a partially austenitic structure in the welding blank; - hot forming the welded blank in a press to obtain a press-formed steel part; and - Cooling the steel component in the press in order to obtain the press-hardened steel component.

16. The method for manufacturing a steel component according to claim 15, wherein: The cooling rate is equal to or greater than the critical martensite or bainite cooling rate of the steel blank.

17. A pre-coated steel blank produced according to the method of any one of claims 1 to 7, comprising: a steel substrate portion having a pre-coating layer portion on at least one of its faces, the pre-coating layer portion comprising an intermetallic alloy layer portion and a metal layer portion extending over the intermetallic alloy layer portion, the metal layer portion being a layer of aluminum, a layer of an aluminum alloy or a layer of an aluminum-based alloy, the thickness of the pre-coated steel blank being comprised between 0.5 mm and 5 mm, and at least one laser cut edge extending between the faces of the pre-coated steel blank and comprising a substrate portion and at least one pre-coating layer portion, Wherein the substrate portion of the laser cut edge has an oxygen content greater than or equal to 15% by weight and an aluminum content less than or equal to 6.0% by weight.

18. A pre-coated steel blank produced according to the method of any one of claims 1 to 7, comprising: a steel substrate portion having a pre-coating layer portion on at least one of its faces, the pre-coating layer portion comprising an intermetallic alloy layer portion and a metal layer portion extending over the intermetallic alloy layer portion, the metal layer portion being a layer of aluminum, a layer of an aluminum alloy or a layer of an aluminum-based alloy, the thickness of the pre-coated steel blank being comprised between 0.5 mm and 5 mm, and at least one brushed cutting edge extending between said faces of said pre-coated steel blank and comprising a brushed substrate portion and at least one brushed pre-coating layer portion, The oxygen content of the brushed substrate portion is greater than or equal to 0.5% by weight, and the aluminum content of the brushed substrate portion is less than 6.0% by weight.

19. A welding blank, comprising at least: - at least one first and second pre-coated steel blank produced by the method according to any one of claims 1 to 7, each pre-coated steel blank comprising a steel substrate portion having a pre-coating layer portion on at least one of its faces, the pre-coating layer portion comprising an intermetallic alloy layer portion and a metal layer portion extending over the intermetallic alloy layer portion, the metal layer portion being a layer of aluminum, a layer of an aluminum alloy or a layer of an aluminum-based alloy, the thickness of the pre-coated steel blank being comprised between 0.5 mm and 5 mm, a welding zone connecting the first pre-coated steel blank to the second pre-coated steel blank, wherein the weld zone has an aluminum content by weight of less than or equal to 0.3%, and wherein the weld zone comprises at least 0.2% by volume of aluminum oxide particles having a diameter of less than or equal to 2 micrometers.

20. The welding blank according to claim 19, wherein: The weld zone comprises at least 0.4% by volume of alumina particles having a diameter less than or equal to 2 microns.

21. A press-hardened component produced by press-hardening a welded blank according to claim 19 or 20.

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