Crack-containing hot-stamped coated steel component with excellent spot weldability and excellent spray adhesion

Abstract

A hot-stamped coated steel component including a steel substrate and an aluminum alloy coating, the coating including an interdiffusion layer and an outer layer in order from the steel substrate outward, the total thickness ecoating of the coating and the thickness eIDL of the interdiffusion layer satisfying the following condition: 40 < = Epc < = 80 wherein the hot-stamped coated steel component includes an undeformed portion having a thickness ePflag of 0.6 mm to 3.5 mm and at least one deformed portion. The crack linear density dC in the coating in the undeformed portion is higher than or equal to a minimum crack linear density dCmin (ePflag), the minimum crack linear density being defined as:.

Description

[0001] This invention patent application is a divisional application of the invention patent application filed on February 9, 2024, with application number 202480001267.4 and invention title "Cracked Hot Stamped Coated Steel Parts with Excellent Spot Weldability and Excellent Spray Coating Adhesion". Technical Field

[0002] This invention relates to hot-stamped coated steel parts comprising a steel substrate and an aluminum alloy coating on at least one side of the steel substrate, the coating having an optimized crack density, and the parts exhibiting excellent paint adhesion and excellent spot weldability. The invention also relates to a method for manufacturing hot-stamped coated steel parts. Background Technology

[0003] This type of component can also be used, for example, in the automotive industry, to manufacture structural elements for intrusion prevention or energy absorption functions.

[0004] In this type of application, steel components that combine high mechanical strength, high impact resistance, good corrosion resistance, and dimensional accuracy are desired. These properties are particularly important for automotive components such as front or rear guide rails, roof rails, and B-pillars, as well as chassis components such as lower control arms and engine mounts.

[0005] To meet these requirements, these parts are now typically produced using a hot stamping process (also known as press hardening). In the hot stamping process, as disclosed in particular in FR 2 780 984 and FR 2 807 447, a pre-coated blank, cut from a sheet of steel and pre-coated with a metal or metal alloy, is heated in a furnace to a temperature at which the ferrite and cementite microstructure of the low-carbon steel is at least partially transformed into austenite, and then the blank is hot-stamped in a die. During stamping, the part is held in the die for rapid cooling, resulting in the formation of the desired hardened microstructure and the acquisition of the desired mechanical properties. The pre-coating can be aluminum or an aluminum alloy. During heating in the furnace, the pre-coated alloy forms a compound with the steel substrate that protects the steel surface from decarburization and oxide scale formation.

[0006] Recently, the focus has been on the coating of hot-stamped parts and how the coating affects the performance of the parts in use.

[0007] In WO 2008 / 053273A1, it is proposed to limit the pre-coating thickness at each location to between 20 μm and 33 μm and to control the hot stamping process, particularly the heating rate and austenitizing parameters, in order to achieve favorable continuity and morphology of the continuous layer in the component coating, thereby resulting in improved weldability.

[0008] However, it is still desirable to provide a hot-stamped steel part with further improved spot weldability and improved paint adhesion that can be produced with a wider range of pre-coating thicknesses.

[0009] In particular, the inventors have discovered that even though coating adhesion can be improved in some cases, this improvement comes at the cost of compromised spot weldability. Therefore, it is still desirable to provide a component that combines excellent coating adhesion and excellent spot weldability. Summary of the Invention

[0010] Therefore, the present invention aims to provide a hot-stamped coated steel component and a method for manufacturing the hot-stamped coated steel component, the hot-stamped coated steel component comprising a steel substrate and an aluminum alloy coating on at least one side of the steel substrate, the hot-stamped coated steel component having both excellent spray coating adhesion and excellent spot weldability.

[0011] Specifically, it is desirable to provide a hot-stamped coated steel component comprising at least one undeformed portion having a weld range greater than 1 kA as measured according to standard SEP 1220-2 (2011) and excellent paint adhesion. Paint adhesion is evaluated by performing dry paint adhesion tests and wet paint adhesion tests according to standard ISO 2409:2013. Paint adhesion is considered excellent if the result of the dry paint adhesion test is strictly below 1 and the result of the wet paint adhesion test is below or equal to 1.

[0012] For this purpose, the present invention relates to a hot-stamped coated steel component according to claim 1.

[0013] The hot-stamped coated steel component preferably has one or more of the features of claims 2 to 13.

[0014] The present invention also relates to a method for producing hot-stamped coated steel parts according to claim 14.

[0015] The method preferably includes one or more of the features of claims 15 to 24.

[0016] The present invention also relates to the use of a hot-stamped coated steel component according to the present invention, or a hot-stamped coated steel component produced by the method according to the present invention, for manufacturing chassis or body-in-white components or suspension arms of motor vehicles. Attached Figure Description

[0017] The invention will now be described and illustrated in detail by way of example, with reference to the accompanying drawings, without introducing any limitation, in which:

[0018] Figure 1 An example of a hot-stamped coated steel component according to the present invention is illustrated schematically.

[0019] Figure 2 It is a cross-section of the coating in the undeformed portion of the component according to the invention, as observed in a given field of view.

[0020] Figure 3 An example of a cross-section of the coating in an undeformed portion of a comparison component is shown, where no cracks are present. Detailed Implementation

[0021] This invention relates to hot-stamped coated steel parts.

[0022] Hot-stamped coated steel parts are non-planar parts produced from hot-stamped blanks.

[0023] Steel plate refers to a flat steel sheet. Here, steel plate means hot-rolled or cold-rolled steel sheet in coil form or cut from such coil.

[0024] The steel plate has a top surface and a bottom surface, also referred to as a top side and a bottom side, or simply a top surface and a bottom surface. The distance between these surfaces is specified as the thickness of the plate. This thickness can be measured, for example, using a micrometer, with the micrometer's spindle and anvil positioned on the top and bottom surfaces, the axis between the spindle and the anvil perpendicular to the plate surface. Similarly, this thickness can also be measured on formed parts. Likewise, it can be measured on blanks and parts.

[0025] A steel billet refers to a flat steel plate that has been cut into any shape suitable for its use, or a billet made by cutting two or more steel plates of different thicknesses or compositions into the desired shape and assembling them together, in particular by welding them together.

[0026] The average thickness of a component, or the average thickness of a portion of a component, refers to the overall average thickness of the material that makes up the component after it has been formed from an initial flat plate into a three-dimensional component.

[0027] Uniform thickness refers to a constant thickness of the blank, component, sheet, or region or portion thereof, with a maximum variation of at most 0.1 mm above or below the average thickness of the blank, component, sheet, or region or portion. Specifically, uniform thickness means that no thickness variation is automatically introduced during production, particularly during the hot rolling and / or cold rolling of sheets and during the forming operations of component production.

[0028] In the following text, the thickness of a blank, component, plate or region or portion thereof having a uniform thickness is defined as the average thickness of the blank, component, plate or region or portion thereof.

[0029] Furthermore, the term "thickness" is used to refer to the thickness of a blank, component, plate or area or portion thereof of uniform thickness, and thus "average thickness" is more generally used to refer to the average thickness of a blank, component, plate or area or portion thereof, whether the thickness is uniform or variable.

[0030] Custom welded blanks are made by assembling several steel plates or cut steel blanks, referred to as sub-blanks, together, for example by laser welding, to optimize the performance of the component in different areas, thereby reducing the overall component weight, lowering the overall component cost, and reducing material waste. The sub-blanks forming the custom welded blanks can be assembled with or without overlap; for example, these sub-blanks can be laser butt-welded (without overlap) or they can be spot-welded to each other (with overlap).

[0031] A flexible preform is a custom-made welded preform that includes areas where at least partial connections between different sub-preforms are not rigid, thereby allowing the sub-preforms to move in different directions during forming operations in the corresponding areas.

[0032] In contrast to custom welded blanks, integral blanks refer to blanks that include a single sub-blank, rather than a combination of several sub-blanks.

[0033] Custom-rolled billets are billets with varying thicknesses, that is, billets that vary along the thickness of the billet, obtained during the steel plate production process through differential rolling.

[0034] Hot stamping is a forming technique that involves heating a blank to a temperature at which the microstructure of steel is at least partially transformed into austenite, thereby forming the blank at a high temperature by stamping, and then quenching the formed part to obtain a microstructure with high strength. Hot stamping allows for the production of very high-strength parts with complex shapes and offers many technical advantages.

[0035] The integral component is a hot-stamped component produced from an integral blank.

[0036] The integral component is made, for example, from an integral blank with uniform thickness, or from an integral custom-rolled blank.

[0037] Hot-stamped welded steel components or hot-stamped laser-welded steel components are hot-stamped parts produced from custom-made welding blanks, such as flexible blanks. Therefore, hot-stamped welded steel components include two or more hot-stamped sub-components and one or more hot-stamped welded sections that join the hot-stamped sub-components together.

[0038] In an embodiment, the hot-stamped coated steel component of the present invention is an integral component.

[0039] In another embodiment, the hot-stamped coated steel component is a hot-stamped welded steel component.

[0040] The hot-stamped coated steel component of the present invention preferably has an average thickness e between 0.6 mm and 3.5 mm. P .

[0041] The range of 0.6 mm to 3.5 mm is a common thickness used in the manufacture of structural or reinforcing components for the automotive industry. This thickness range is also suitable for industrial press-hardening tools, particularly hot stamping presses or dies.

[0042] Furthermore, as detailed below, heat treatment applied to a given steel sheet thickness affects the coating, particularly the formation of cracks in the coating of the flat, undeformed portion desired by the present invention.

[0043] Preferably, the average thickness e of the hot-stamped coated steel component P Within the range of 0.7 mm to 3.0 mm.

[0044] In this embodiment, the hot-stamped coated steel component has a uniform thickness of 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm. P .

[0045] In another embodiment, the hot-stamped coated steel part has a variable thickness (and therefore non-uniformity). In this case, the hot-stamped coated steel part comprises parts with different uniform thicknesses e. Pi Two or more regions with different uniform thicknesses e Pi Preferably, the thickness is 0.6 mm to 3.5 mm, and more preferably 0.7 mm to 3.0 mm.

[0046] As an example, hot-stamped coated steel parts with variable thickness can be produced from custom rolled blanks with varying thicknesses obtained through differential rolling during the steel sheet production process.

[0047] As another example, hot-stamped coated steel parts with variable thickness can be hot-stamped welded steel parts produced by custom-welded blanks made from blanks with different thicknesses.

[0048] Hot-stamped coated steel parts consist of a steel substrate (also known as a steel base) with two main surfaces.

[0049] The steel in the base material is steel used for hot stamping, that is, steel that can be hardened after austenitization and rapidly cooled by quenching.

[0050] In this implementation, the component is produced from a single blank, and the steel substrate is made from a single piece of steel.

[0051] In another embodiment, the component is a hot-stamped welded steel component, the steel substrate comprising two or more regions (or sub-components) that may be made of the same or different steel and may have the same or different microstructures.

[0052] In the following text, the microstructure and composition of the steel substrate refers to the microstructure and composition of the steel substrate (if the substrate is made of a single piece of steel) or the microstructure or composition of one or more regions or sub-parts of the steel substrate.

[0053] The composition of the steel depends on the desired mechanical properties of the component. However, preferably, in the steel substrate or in each region of the steel substrate, the steel has a composition comprising, by weight percent, the following:

[0054] 0.062% ≤ C ≤ 0.4%

[0055] 0.4% ≤ Mn ≤ 3.9%

[0056] 0.10% ≤ Si ≤ 1.5%

[0057] 0.005% ≤ Al ≤ 1.0%

[0058] 0.001% ≤ Cr ≤ 2.0%

[0059] 0.001% ≤ Ti ≤ 0.2%

[0060] 0.0005% ≤ B ≤ 0.010%

[0061] Ni ≤ 2%

[0062] Nb ≤ 0.1%

[0063] Mo ≤ 0.65%

[0064] W ≤ 0.30%

[0065] N ≤ 0.010%

[0066] 0.0001% ≤ S ≤ 0.05%

[0067] 0.0001% ≤ P ≤ 0.1%

[0068] Ca ≤ 0.005%,

[0069] The balance of this composition includes iron and unavoidable impurities produced by refining.

[0070] The level of impurities produced by the refining process will depend on the production route used. For example, when using a blast furnace route with low levels of scrap steel (recycled steel), impurity levels will remain very low. On the other hand, when refining steel using an electric arc furnace with a very high ratio of recycled scrap steel, impurity levels will increase significantly. For example, in the case of refining steel using an electric arc furnace with a very high ratio of recycled scrap steel, the level of Cu can rise to 0.25%, the level of Ni can rise to 0.25%, the level of Sn can rise to 0.05%, the level of As can rise to 0.03%, the level of Sb can rise to 0.03%, and the level of Pb can rise to 0.03%.

[0071] Therefore, in the embodiments, the steel contains up to 0.25% Cu, up to 0.05% Sn, up to 0.03% As, up to 0.03% Sb and / or up to 0.03% Pb as unavoidable impurities.

[0072] The above composition is conducive to achieving high mechanical properties, especially tensile strength TS in the range of 950 MPa to 2100 MPa.

[0073] Tensile strength was measured according to ISO standard NF EN ISO 6892-1, published in October 2009. The tensile test specimen was cut from a flat portion of the hot-stamped part.

[0074] In the following text, unless otherwise stated, the content of elements is expressed as a weight percentage.

[0075] The carbon content depends on the expected tensile strength (TS) of the hot-stamped coated steel component.

[0076] With a carbon content below 0.062%, it is difficult to obtain a tensile strength of at least 950 MPa after hot stamping under any cooling conditions. Above 0.4%, combined with other elements in the composition, the adhesion of the coating after hot stamping may be unsatisfactory, and the steel's resistance to delayed cracking and toughness will decrease. In this embodiment, the carbon content is at most 0.38%.

[0077] The carbon (C) content depends on the desired tensile strength (TS) of the hot-stamped part produced by hot stamping the steel sheet. In this embodiment, the C content is between 0.062% and 0.095%. If a higher tensile strength, approximately 1500 MPa, is desired, the C content can be increased to the range of 0.15% to 0.30%. If a further increase in tensile strength to at least 1800 MPa is required, the C content can be added at a maximum of 0.4%.

[0078] Besides its deoxidizing effect, manganese has a significant impact on hardenability, especially when the manganese content is at least 0.4%. Above 3.9%, Mn may be too important for the stability of austenite, leading to the formation of overly pronounced banded structures. Preferably, the Mn content is at most 3.0%.

[0079] Silicon is added at a content of at least 0.10% to help deoxidize the liquid steel and to aid in its hardening through precipitation in the solid solution. However, the silicon content is generally limited to avoid excessive silicon oxide formation, which could impair the coatability of the steel. Therefore, the silicon content is typically less than or equal to 1.5%, for example, less than or equal to 0.80%.

[0080] Aluminum can be added as a deoxidizer at a content of at least 0.005%. Additionally, if the titanium content is insufficient, aluminum can protect boron by combining with nitrogen. The Al content is preferably at least 0.01%. The Al content is typically less than or equal to 1.0% to avoid oxidation problems and to prevent the formation of ferrite during hot stamping. Preferably, the Al content is at most 0.1%.

[0081] Cr can be added to increase the hardenability of the steel and to help achieve the desired tensile strength after hot stamping. When Cr is added, the Cr content is greater than or equal to 0.01%, preferably greater than or equal to 0.1%, and at most 2.0%. If Cr is not actively added, the Cr content may be present as an impurity at a level as low as 0.001%.

[0082] When titanium is added, its content is preferably at least 0.008% and at most 0.2%. When the Ti content is between 0.008% and 0.2%, precipitation occurs at very high temperatures as TiN, and then at lower temperatures as fine TiC in austenite, resulting in hardening. Furthermore, when titanium is added in addition to boron, it prevents the combination of boron and nitrogen, which binds to titanium. Therefore, the titanium content is preferably higher than 3.42*N, where N is the N content in the composition expressed as a weight percentage. However, the Ti content should preferably be kept below or equal to 0.2%, preferably below or equal to 0.1%, more preferably at most 0.05%, to avoid the precipitation of coarse TiN precipitates. If active Ti addition is not performed, Ti exists as an impurity at a content of at least 0.001%.

[0083] Boron is added at a content of at least 0.0005% and at most 0.010% to increase the hardenability of the steel. Preferably, the B content is at most 0.004%.

[0084] In implementations, Ni may be added at a content of up to 2%, typically at least 0.25%, and preferably up to 0.5%, to reduce susceptibility to delayed fracture by concentrating it on the surface of the component. If not added, Ni may be present as an impurity at a content as low as 0.001%. Depending on the production method used, the Ni content as an impurity may be as high as 0.25% (e.g., when producing steel using a high proportion of recycled scrap steel) or as high as 0.1% (e.g., when using a lower level of steel scrap).

[0085] Optionally, up to 0.1% niobium is added to provide precipitation hardening and microstructure refinement, such as the previously mentioned austenite grain size. Nb further improves the ductility of the steel. When Nb is added, the Nb content is preferably at least 0.01%. The Nb content is preferably at most 0.06% to avoid the formation of coarse (Ti,Nb)(C,N) precipitates.

[0086] Molybdenum can be added at a maximum content of 0.65%. When Mo is added, the Mo content is preferably at least 0.05%. Mo is preferably added together with Nb and Ti to form a coprecipitate that is very stable at high temperatures. Mo can also be added to increase the toughness of the steel, thereby acting as a grain boundary strengthener in the solid solution state. Optimal results are obtained when the Mo content is between 0.15% and 0.25%.

[0087] W can be added to improve the hardenability and hardenability of steel by forming tungsten carbide. When W is added, the W content is greater than or equal to 0.001% and less than or equal to 0.30%.

[0088] Sulfur, phosphorus, and nitrogen are typically present as impurities in steel composition.

[0089] The nitrogen content is typically at least 0.0005%. The N content is typically at most 0.010%, preferably at most 0.005%, to prevent the precipitation of crude TiN precipitates.

[0090] When in excess, sulfur and phosphorus reduce ductility. Therefore, the content of sulfur and phosphorus is limited to 0.05% and 0.1%, respectively.

[0091] In particular, the presence of sulfur (S) in liquid steel leads to the formation of MnS precipitates, which are detrimental to performance. Preferably, the S content is at most 0.01%, and most preferably at most 0.005%. Achieving very low S content, i.e., below 0.0001%, is very expensive and offers no benefit. Therefore, the S content is typically greater than or equal to 0.0001%.

[0092] Preferably, the phosphorus content is at most 0.05%, more preferably at most 0.02%. Achieving very low P content, i.e., below 0.0001%, is very expensive. Therefore, the P content is typically higher than or equal to 0.0001%.

[0093] The steel can undergo a spheroidizing treatment using calcium sulfides, which improves the bending angle due to the spheroidization of MnS. Therefore, the steel composition can contain at least 0.0001% and at most 0.005% Ca.

[0094] The remaining components of steel are iron and impurities resulting from the refining process. As mentioned above, impurities resulting from the refining process may include 0.25% or less Cu, 0.05% or less Sn, 0.03% or less As, 0.03% or less Sb, and / or 0.03% or less Pb.

[0095] The composition of steel can be selected based on the desired mechanical properties, particularly in terms of strength and ductility.

[0096] Specifically, when a tensile strength in the range of 950 MPa to 1200 MPa and a bending angle greater than 75° (measured according to the VDA 238-100 bending standard of July 2020) are desired, the steel substrate or the steel in at least one region of the steel substrate preferably has a composition according to the first preferred composition comprising, by weight percent, the following:

[0097] 0.062% ≤ C ≤ 0.095%

[0098] 1.4% ≤ Mn ≤ 1.9%

[0099] 0.2% ≤ Si ≤ 0.5%

[0100] 0.020% ≤ Al ≤ 0.070%

[0101] 0.02% ≤ Cr ≤ 0.1%

[0102] Of which, 1.5% ≤ (C + Mn + Si + Cr) ≤ 2.7%

[0103] 0.0035% ≤ Ti ≤ 0.072%

[0104] 0.0002% ≤ B ≤ 0.004%

[0105] 0.04% ≤ Nb ≤ 0.06%

[0106] Among them, 0.044% ≤ (Nb+Ti) ≤ 0.09%

[0107] 0.001% ≤ N ≤ 0.009%

[0108] 0.0005% ≤ S ≤ 0.003%

[0109] 0.0001% ≤ P ≤ 0.020%

[0110] Ca ≤ 0.005%,

[0111] The balance of this composition includes iron and unavoidable impurities produced by refining.

[0112] On the other hand, when a tensile strength of at least 1400 MPa is required, the steel substrate or the steel in at least one region of the steel substrate preferably has a composition according to the second preferred composition comprising, by weight percent, the following:

[0113] 0.15% ≤ C ≤ 0.30%

[0114] 0.5% ≤ Mn ≤ 3.0%

[0115] 0.10% ≤ Si ≤ 0.50%

[0116] 0.005% ≤ Al ≤ 0.1%

[0117] 0.01% ≤ Cr ≤ 1.0%

[0118] 0.001% ≤ Ti ≤ 0.2%

[0119] 0.0002% ≤ B ≤ 0.010%

[0120] 0.0005% ≤ N ≤ 0.010%

[0121] 0.0001% ≤ S ≤ 0.05%

[0122] 0.0001% ≤ P ≤ 0.1%

[0123] Ca ≤ 0.005%,

[0124] The remainder consists of Fe and unavoidable impurities produced during refining.

[0125] If a tensile strength of 1800 MPa or higher, or even higher, is required, the composition of the steel substrate or at least one region thereof preferably comprises, by weight percent, the following according to the third preferred composition:

[0126] 0.3% ≤ C ≤ 0.4%

[0127] 0.5% ≤ Mn ≤ 1.0%

[0128] 0.40% ≤ Si ≤ 0.80%

[0129] 0.01% ≤ Al ≤ 0.1%

[0130] 0.1% ≤ Cr ≤ 1.0%

[0131] 0.008% ≤ Ti ≤ 0.03%

[0132] 0.0005% ≤ B ≤ 0.003%

[0133] Ni ≤ 0.5%

[0134] 0.01% ≤ Nb ≤ 0.1%

[0135] 0.1% ≤ Mo ≤ 0.5%

[0136] N ≤ 0.005%

[0137] 0.0001% ≤ S ≤ 0.004%

[0138] 0.0001% ≤ P ≤ 0.02%

[0139] Ca ≤ 0.0010%,

[0140] The balance of this composition includes iron and unavoidable impurities produced by refining.

[0141] The steel substrate of hot-stamped coated steel parts typically has a microstructure comprising, by volume fraction, at least 60% martensite, up to 20% bainite, up to 5% ferrite, and up to 15% austenite.

[0142] The martensite fraction can be as high as 100%, while the bainite, ferrite, and austenite fractions are each as low as 0%.

[0143] This microstructure description applies to most steel substrates, meaning that this microstructure exists in at least 95% of the volume of the steel substrate, preferably in the entire volume of the steel substrate.

[0144] The microstructure was determined by cutting a sample from a hot-stamped coated steel component, polishing it as detailed below, and etching it with Nital 2% (10 s) to reveal the microstructure. The section was then examined by an optical microscope at 500x magnification, and, if differentiation between martensite and bainite was required, by scanning electron microscopy (SEM) (backscattered electron mode, 500x magnification, EHT (electron high voltage) = 15.00 kV, scale 10 μm). The volume fraction of each component (martensite, bainite, ferrite, austenite) was determined using image analysis via methods known per se.

[0145] In the implementation, the austenite fraction is up to 5% by volume, and / or the bainite fraction is up to 10% by volume.

[0146] In this embodiment, the microstructure comprises, by volume, at least 80% martensite, at most 10% bainite, at most 5% austenite, and at most 5% ferrite.

[0147] In a preferred embodiment, the microstructure is substantially martensite, i.e., comprising at least 95% martensite and at most 5% bainite and / or ferrite by volume.

[0148] More preferably, the microstructure is fully martensitic.

[0149] Hot-stamped coated steel parts include an undeformed portion and at least one deformed portion.

[0150] In one embodiment, the hot-stamped coated steel component comprises two or more undeformed portions.

[0151] In practice, during stamping, particularly hot stamping, to produce parts, one or more portions of the blank remain undeformed, while other portions deform to achieve the final non-planar geometry of the part. The undeformed portion, or each undeformed portion, is the part that did not undergo deformation during hot stamping or, if performed, during the blank's prior cold pre-deformation.

[0152] Regarding the undeformed portion, it must be understood that during the stamping process, the undeformed portion undergoes an equivalent deformation of at most 0.01. The equivalent deformation is limited to ,in, and It is the main transformation.

[0153] For example, given the main deformation of the molded part and The procedure described below is an example of a method for determining the principal deformation, and is by no means limiting—other methods exist:

[0154] - Use a 3D camera to acquire a digital model of the physical component. The output of this first operation is a CAD file representing the physical component.

[0155] - The CAD file is then processed by reverse forming software, such as Pamstamp® Onestep, which calculates the deformation field required to stamp the part shape starting from the flat blank.

[0156] Then, using any of the commercial software listed above (such as Pamstamp®, Abaqus®, or LS-Dyna®), the deformation field is represented by the corresponding principal deformation.

[0157] When the above methods cannot be applied, for example because only a portion of the fully formed part is available, or to assess very local deformation in a specific area, such as at an edge, scanning electron microscopy (SEM) observation can be used to perform electron backscatter diffraction (EBSD) measurements. It relies on the correlation between the deformation and the local crystal orientation error. For example, the following reference gives an example of such a measurement: "Ultramicroscopy July 2011; 111(8): 1189-99. doi: 10.1016 / j.ultramic.2011.02.004. Europe February 21, 2011. PMID: 21763236 Kamaya M. Assessment of local deformation using EBSD: quantification of accuracy of measurement and definition of local gradient."

[0158] Another method that can be applied to determine the main deformation of a molded part is to measure the thickness of the deformed material in the molded area and compare that thickness with the thickness of the undeformed area.

[0159] The undeformed portion, or each undeformed portion, is a flat part of the component.

[0160] The undeformed portion, or each undeformed portion, is, for example, a flange of the component or a flat portion located between two deformed portions of the component. As an example, if the component is cap-shaped, the flat portion may also include the flat top portion of the cap-shaped stamped component.

[0161] Figure 1 An example of this component is illustrated schematically.

[0162] Figure 1 A hot-stamped coated steel component 1 has been welded to a flat component 2 by multiple spot welds 3. In this example, the spot welds 3 are located at the flange 8 of the hot-stamped coated steel component 1.

[0163] An exemplary hot-stamped coated steel component 1 is a cap-shaped component (or "Ω"-shaped component) comprising a flat top portion 4, two first curved portions 5 (or radii) extending from two opposite longitudinal edges of the flat top portion 4, two sidewalls 6 each extending from the longitudinal edge of the first curved portion 5, two second curved portions 7 each extending outward from the longitudinal edge of the sidewall 6, and two flat flanges 8 each extending from the longitudinal edge of the second curved portion 7 to the outer edge of the component 1. Thus, the flanges 8 form the edges of the component 1.

[0164] In this example, flange 8 and flat top portion 4 are flat, undeformed portions of hot-stamped coated steel component 1.

[0165] Undeformed portions can be distinguished from deformed portions in terms of their shape and / or by observing cracks in the coating of these portions.

[0166] In fact, as detailed below, the coating in the deformed portion contains wide cracks that are absent or very rare in the coating in the undeformed portion.

[0167] Although each undeformed part has not been deformed, it has undergone the same thermal cycling as one or more deformed parts.

[0168] The hot-stamped coated steel component of the present invention differs from a flat blank produced by austenitization and quenching without any deformation, not only in that the hot-stamped coated steel component of the present invention includes at least one deformed portion produced by hot stamping, but also in that the entire hot-stamped coated steel component undergoes thermal cycling through hot stamping, which is different from the thermal cycling experienced by an undeformed plate.

[0169] In particular, although each undeformed part is not deformed, it undergoes the same thermal cycle as one or more deformed parts during hot stamping, during heating, transfer to the mold and holding in the mold.

[0170] Therefore, in hot-stamped coated steel parts, the deformed portion or each deformed portion deforms during hot stamping, and the undeformed portion or each undeformed portion, although not deformed, undergoes the same thermal cycle as one or more deformed portions during hot stamping.

[0171] The hot-stamped coated steel parts of the present invention, obtained through an in-mold hot stamping process, also differ from samples produced by heating in a test furnace or any non-surface-contact technology, such as in a Gleeble machine, and undergoing uniaxial deformation within that furnace. In fact, such samples have no undeformed portions and undergo uniaxial deformation (tensile deformation), thus, in hot stamping, the deformation is non-uniform and three-dimensional throughout the part. Furthermore, the thermal cycling experienced by the hot-stamped coated steel parts differs from that experienced in a test furnace, which does not involve any transfer or stamping in a die.

[0172] The flat, undeformed portion of the component, or each flat, undeformed portion, has a uniform thickness of 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm. pflat The part.

[0173] For example, if the flange includes different thicknesses e pflat(1) and e pflat(2) If there are two regions, then each of these regions is a different undeformed part.

[0174] If the hot-stamped coated steel part has a uniform thickness, then the thickness of the flat, undeformed portion (or each undeformed portion) is equal to the thickness e of the hot-stamped coated steel part. P .

[0175] If a hot-stamped coated steel part has a variable thickness and includes two or more undeformed portions, the undeformed portion is denoted as e. pflat(i) The thicknesses of the undeformed portions can be different from each other or the same, where i=1……n are exponents associated with the corresponding undeformed portions, and n is the number of undeformed portions.

[0176] Thickness e pflat For example, a micrometer can be used to measure it.

[0177] Hot-stamped coated steel parts have a coating on at least one side of a steel substrate.

[0178] The coating is an aluminum alloy coating.

[0179] The coating contains aluminum and iron, and preferably also contains silicon.

[0180] The coating advantageously contains more than 50% aluminum.

[0181] For example, the coating contains 8 wt% to 12 wt% Si and 3 wt% to 5 wt% Fe, with the balance being aluminum and unavoidable impurities.

[0182] The coating is the result of interdiffusion between the aluminum alloy pre-coating and the steel during hot stamping.

[0183] The coating includes an interdiffusion layer located on the innermost side of the coating, i.e., in contact with the steel substrate.

[0184] The interdiffusion layer typically has an Fe content of at least 80% and at most 95%, an Al content of 4% to 20%, and at most 2% Si.

[0185] In one embodiment, the interdiffusion layer has a composition comprising 86% to 95% Fe, 4% to 12% Al, and 0% to 2% Si.

[0186] The coating also includes an outer layer that extends from the diffusion layer to the surface of the coating.

[0187] The outer layer typically comprises or consists of intermetallic compounds of Fe, Al, and possibly Si.

[0188] The outer layer may be a single layer, or it may itself consist of sublayers of different intermetallic compounds.

[0189] For example, the outer layer may consist of one to four sub-layers.

[0190] However, in this invention, the inventors have discovered that the object of the invention is achieved if the linear density of the cracks meets the conditions defined below, regardless of the layers in the coating, and especially regardless of their composition and number.

[0191] In the hot-stamped coated steel component according to the present invention, the total coating thickness e coating and the thickness e of the interdiffusion layer IDL The following conditions must be met:

[0192] 40 ≤ E pc ≤ 80

[0193] in,

[0194]

[0195] If E pc If the value is below 40, the coating may not adequately protect the component for its intended use and may not guarantee excellent paint adhesion and spot weldability. If E pc If the value is greater than 80, the coating may become brittle, resulting in powder formation.

[0196] In the hot-stamped coated steel part according to the invention, the coating is applied to at least one undeformed portion of the hot-stamped coated steel part, and it is assumed that 40 ≤ E pc ≤ 80, the coating has a thickness e greater than or equal to that of the undeformed portion. pflat Minimum crack line density dC min (e) pflatThe crack linear density dC is defined as follows:

[0197]

[0198] In this expression, dC min (e pflat ) is the minimum crack line density expressed as the number of cracks per mm, and e pflat This represents the thickness of the undeformed portion, expressed in mm. The crack linear density dC is also the number of cracks per mm. "e" indicates an exponential function.

[0199] The “crack linear density” here, which indicates the number of cracks per millimeter, is not a bulk density, but a linear density, because it measures how many cracks exist in a cross-section of the coating over a given length in a direction parallel to the surface of the steel substrate.

[0200] In fact, the inventors conducted in-depth research to address the issues of improving spot weldability and paint adhesion, and surprisingly discovered that, contrary to the established view that cracks would be detrimental to the properties of hot-stamped coated steel parts, spot weldability and paint adhesion increased dramatically together if the crack line density exceeded a threshold.

[0201] The inventors also discovered that the portion of the component relating to crack line density is the undeformed portion. Although the undeformed portion has undergone the same thermal cycling as the deformed portion, it has not been deformed because the undeformed portion is located on the flange of the hot-stamped component, or because the undeformed portion is located in a flat region between two deformed portions of the component (e.g., as shown in the image). Figure 1 (in the flat top portion of the hat-shaped component shown in the diagram above).

[0202] In practice, during the deformation process, the coating undergoes deformation, which leads to cracks in the coating due to the difference in expansion between the substrate and the coating, and depending on the deformation temperature and the rate of deformation applied at each location. However, the inventors have found that a high number of cracks in some deformation sections is insufficient to ensure excellent spot weldability and excellent spray coating adhesion, at least because such properties cannot be guaranteed in other parts of the hot-stamped coated steel parts.

[0203] Preferably, the undeformed portion is located in the flange of the component, which is the area most likely to be spot welded, making excellent spot weldability (and excellent paint adhesion) in this portion particularly desirable.

[0204] The inventors also discovered that the threshold for crack line density is not an absolute value, but depends on the thickness of the undeformed portion, and that as the thickness of the undeformed portion decreases, a higher crack line density is required to achieve the desired spot weldability and coating adhesion.

[0205] As a result of these studies, the inventors found that, assuming the coating of the component is such that 40 ≤ E pc If ≤ 80, then the crack line density in the coating in the undeformed portion is higher than or equal to dC. min At the same time, it achieved excellent spot welding performance and coating adhesion.

[0206] Preferably, the crack line density in the coating is maintained at a maximum of 4*dC. min Preferably, at most 3*dC min .

[0207] Preferably, the hot-stamped coated steel component comprises two or more undeformed portions, wherein the crack line density dC(i) in each undeformed portion is greater than or equal to the thickness e of the undeformed portion. pflat (i) The associated marker is The minimum crack line density.

[0208] Therefore, the undeformed portions each have a thickness e ranging from 0.6 mm to 3.5 mm. pflat (i), where i = 1...n is an index associated with one of the undeformed portions, n ≥ 2 is the number of undeformed portions, and the thickness is e. pflat (i) The crack line density dC(i) in the coating of each undeformed portion is greater than or equal to 0. ,in:

[0209]

[0210] Where, dC(i) and Expressed as the number of cracks per mm, and e pflat (i) indicates the thickness of the undeformed portion under consideration, expressed in mm.

[0211] The formula is valid regardless of whether all undeformed parts have the same thickness or different thicknesses.

[0212] The crack in the coating extends from the surface of the coating in a direction generally orthogonal to the surface of the steel substrate (i.e., the interface between the steel substrate and the coating) to a depth of at least 5 μm. The crack has a width of less than 2 μm (in a direction parallel to the surface of the steel substrate).

[0213] Therefore, cracks are different from possible gaps in the coating with a width greater than 2 μm, different from cracks in deformed parts that can also have a width greater than 2 μm, and different from pores or coating defects that affect the coating at a depth of less than 5 μm.

[0214] Furthermore, as mentioned above, in the undeformed portion, cracks typically have a width of at most 1 μm. Specifically, the average width of the largest cracks is less than 1 μm; the largest cracks are those with a width greater than 90% of the cracks in the coating. In other words, among the observed cracks, 10% of the cracks with the largest widths have an average crack width of less than 1 μm. The average value here represents the average calculated based on the widths of all the largest cracks.

[0215] In contrast, in the deformed portion, the coating comprises more cracks with a width greater than 1 μm, which can be as large as 2 μm or even 3 μm, such that the average width of the cracks is typically greater than 1 μm. In any case, the deformed portion, or each deformed portion, results in an average width greater than 1 μm for cracks with a width greater than 90% of the crack widths in the overall structure.

[0216] Preferably, the coating in the undeformed portion does not include any gaps or cracks with a width of 2 μm or greater.

[0217] Crack line density is determined as the ratio between the number of cracks observed in a cross-section of an undeformed portion using a bright-field optical microscope over a total observation length of at least 5 mm (in a direction parallel to the surface of the steel substrate) and that total length (i.e., the number of cracks divided by the total length).

[0218] Specifically, the crack line density in the coating was determined by observing the cross-sections of the coating of two samples obtained from the undeformed portion of a hot-stamped coated steel part in several fields of view using a bright-field microscope with 500x magnification, such that the total length of the field of view (in the direction parallel to the surface of the steel substrate) was at least 5 mm. The crack line density was then determined as the ratio between the number of observed cracks and the total length of the field of view.

[0219] In fact, the inventors have discovered that, in order to ensure excellent spot weldability and excellent spray coating adhesion, only a small portion of the coating needs to have at least dC. min The crack line density is insufficient. The inventors also discovered that, in order to achieve these properties, a crack line density of at least dC measured over a length of at least 5 mm is required. min Crack line density is necessary.

[0220] The average width of the largest crack is determined by measuring the width of all cracks observed along the observation length, identifying the largest crack based on these measurements, and calculating the average width of these largest cracks.

[0221] In detail, the characteristics of the coating described above are determined as follows.

[0222] First, samples are obtained from the components and prepared.

[0223] For this purpose, the component was cut in the middle of its length to produce a sample of suitable size of 20×30 mm in the undeformed portion of the hot-stamped coated steel component.

[0224] The cutting process is carefully performed using a hard iron cutting wheel (e.g., Struers 60A25) or preferably a micro-cutting device to avoid excessive stress that could damage the sample.

[0225] The sample is then (preferably by ultrasonic bath) cleaned and dried with ethanol and compressed air.

[0226] The samples are then cold-mounted with resin. Cold mounting is chosen over hot mounting because the gap between the resin and the sample is very small or non-existent. This is important because gaps can cause several preparation problems such as etching issues, scratches, or coating damage. The resin is preferably Liquid EpoFix® blended with Epofix Hardener®. Polymerization lasts for 10 hours. Two protective metal plates (guards) are placed on opposite sides of the samples to protect the coating from surface damage during polishing. Each time, the two samples are mounted together to ensure that both samples subsequently undergo the same polishing process.

[0227] The samples are then carefully polished. Polishing is a crucial step in evaluating the coating's characteristics, as improper polishing can introduce defects. These defects include deposited diamond particles, damage to the coating such as peeling, and, more importantly, cracks, particularly longitudinal cracks. Longitudinal cracks extending generally parallel to the surface of the steel substrate can reflect fabrication problems and must be avoided during polishing to ensure reliable results when counting cracks.

[0228] Polishing consists of three main stages: disc polishing, diamond polishing, and oxide polishing. After each polishing step, the sample must be cleaned and dried to remove abrasive contamination and must be examined under an optical microscope.

[0229] First, the sample was polished with SiC paper P320 abrasive disc for 120 seconds using a rotating sample holder at 150 RPM and a disc at 100 RPM, rotating in the same direction. A force of 15 N was applied. The sample was then rinsed with water.

[0230] The material is then removed from the surface using a 9 μm diamond polisher without introducing scratches or deformation. This step is important because it ensures a smooth polished surface. The rotation speed is the same as in the previous step, but the sample rotates in the opposite direction to the disk rotation. This stage is set for 300 s and a force of 20 N is used. The polishing disk is damped with an optimal amount of lubricant (0.5 mL / 30 s) to ensure better material removal. The sample is then rinsed with water.

[0231] The final step is oxide polishing using a colloidal silica solution (1 mL / 5 s). This step is performed for 90 s at a rotational speed of 150 RPM for the sample holder and 60 RPM for the disk, both rotating in the same direction. The applied force is 20 N. The sample is first washed with water, followed by ethanol.

[0232] The samples were then etched with Nital (2% for 5 to 10 seconds) to reveal the steel microstructure and the steel / coating interface.

[0233] To determine the total coating thickness e coating and the thickness e of the interdiffusion layer IDL The sample was imaged using a scanning electron microscope (SEM) (backscattered electron mode, magnification 500x, working distance WD preferably 10 mm, EHT (electron high voltage) = 15.00 kV, scale 10 micrometers) to show the cross-section of the coating and at least a portion of the base steel.

[0234] Based on the image, the interdiffusion layer can be identified as the light layer closest to the steel substrate.

[0235] The total thickness of the coating (including the interdiffusion layer) and the thickness of the interdiffusion layer were measured at five points spaced 15 μm horizontally apart.

[0236] The total coating thickness and the thickness of the interdiffusion layers are then calculated as the average of the obtained values. The total coating thickness e coating and the thickness e of the interdiffusion layer IDL Therefore, it is the average thickness.

[0237] Furthermore, the composition of the interdiffusion layer can be determined as follows.

[0238] The sample was observed using a scanning electron microscope (SEM) to show the cross-section of the coating and at least a portion of the base steel.

[0239] By using energy-dispersive spectroscopy (EDS), the composition was determined at five different horizontal positions, considering two vertically spaced points in the interdiffusion layer, and at ten points.

[0240] The composition of the interdiffusion layers is then calculated as the average of the obtained values.

[0241] If needed, the composition of the entire coating can be measured using EDS.

[0242] The crack line density in the undeformed portion of the coating is determined as follows.

[0243] Two samples embedded in the same resin were imaged using a bright-field optical microscope (500x magnification) to show cross-sections of the coating at several locations on the samples and at least a portion of the steel substrate.

[0244] Then, for each sample, ten different and non-overlapping fields of view were randomly selected and observed. Each field of view had a length of at least 250 μm (in a direction parallel to the surface of the steel substrate).

[0245] The total observation length, i.e. the total length of the field of view for the two samples (in the direction parallel to the surface of the steel substrate), is therefore 5 mm or greater (> 250 μm * 2 * 10).

[0246] In each field of view, firstly, it was verified that there were no longitudinal cracks that would extend in a direction generally parallel to the surface of the steel substrate. If a longitudinal crack was present, the field of view was ignored and replaced, because the presence of such a crack meant that the sample preparation (cutting, polishing) was not performed correctly at that location.

[0247] Then, in each field of view, cracks extending from the uppermost surface of the coating, which is approximately orthogonal to the steel substrate, along the direction of the steel substrate are identified.

[0248] To distinguish cracks from potential gaps in the coating, only cracks with a width of less than 2 μm are identified as cracks. Cracks with a width of 2 μm can be easily identified at 500x magnification, but if necessary, the crack width can be measured as detailed below. Additionally, to distinguish cracks from porosity or coating defects, the crack length is measured, and only cracks with a depth of at least 5 μm (in a direction approximately orthogonal to the surface of the steel substrate) are considered cracks.

[0249] The number of cracks in each field of view is counted, and the total number of cracks observed in the twenty fields of view is calculated.

[0250] The crack line density in the coating was then determined as the ratio between the total number of cracks and the total length of the field of view used for both samples.

[0251] The determination of the average width of the maximum crack, or more generally the determination of the width of any crack, can be performed as follows.

[0252] Cross sections used for assessing crack line density were observed using a bright-field microscope with a 500x magnification and a camera with a resolution of 3072*2048 pixels. The images were displayed such that one pixel of the camera was displayed as one pixel in the image. The imaging magnification was 3020x.

[0253] Determine the width of each crack in all fields of view.

[0254] Therefore, for each crack, the crack width was measured at three locations: one at the center of the crack and two at a distance of 2 μm on each side of the center. The average of the three widths was determined as the crack width.

[0255] Then, the cracks with the largest width, i.e., those wider than 90% of the cracks in the coating, are identified. These cracks are therefore the 10% of the cracks with the largest width. The average width of these cracks is then calculated as the sum of the individual widths of these cracks divided by the number of these cracks.

[0256] If the number of cracks makes 90% of the number of cracks not an integer, then the result of 90% of the number of cracks is rounded to the nearest integer (for example, 90% of 108 cracks is 97 cracks, and 90% of 135 cracks is considered to be 122 cracks).

[0257] The hot-stamped coated steel component according to the invention has a weld range greater than 1 kA in at least the undeformed portion, as measured according to standard SEP 1220-2 (2011).

[0258] Hot-stamped coated steel parts also exhibit excellent paint adhesion, because at least in the undeformed portion, when subjected to dry paint adhesion and wet paint adhesion tests according to standard ISO 2409:2013, the dry paint adhesion is strictly below 1 and the wet paint adhesion is below or equal to 1.

[0259] A method for producing hot-stamped coated steel parts according to the present invention will now be disclosed.

[0260] The method includes providing a steel billet with an average thickness typically from 0.6 mm to 3.5 mm, preferably from 0.7 mm to 3.0 mm. This thickness is typically specified as the average thickness e of the hot-stamped coated steel part produced from the billet. P same.

[0261] The blank includes an aluminum or aluminum alloy pre-coating on at least one side, the pre-coating having an average thickness between 19.91 μm and 40 μm.

[0262] Preferably, the blank includes an aluminum or aluminum alloy pre-coating on each of its two main surfaces, the pre-coating having an average thickness between 19.91 μm and 40 μm.

[0263] The pre-coating can be aluminum or an aluminum alloy (containing more than 50% aluminum).

[0264] Advantageously, the pre-coating is an aluminum-silicon alloy containing 7% to 15% silicon, 2% to 4% iron, and optionally 0.0015% to 0.0030% calcium by weight, with the remainder being aluminum and unavoidable impurities from refining.

[0265] Preferably, the pre-coating comprises 8% to 11% Si, 2% to 4% Fe, and optionally 0.0015% to 0.0030% Ca by weight, with the remainder being Al and impurities produced by smelting.

[0266] Pre-coating is typically obtained by hot-dip coating in an Al or Al alloy bath.

[0267] In this implementation, the blank is an integral blank, that is, a blank that includes a single sub-blank obtained by cutting a pre-coated steel plate.

[0268] Integral blanks are, for example, custom-rolled blanks, which are blanks with variable thicknesses obtained by differential rolling during the steel plate production process.

[0269] In another embodiment, the blank is a custom-welded blank, which is produced by assembling, in particular welding together, for example by laser welding, several blanks called sub-blanks cut from different pre-coated steel sheets.

[0270] In an embodiment, the blank has a uniform thickness of 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm.

[0271] In another embodiment, the blank is a custom-rolled blank or a custom-welded blank, and the blank has a variable thickness.

[0272] In this case, the blanks are made of different thicknesses e, each ranging from 0.6 mm to 3.5 mm, preferably from 0.7 mm to 3.0 mm. Bi It is made from two or more regions. The blank has a given thickness e. Bi Each region corresponds to a thickness of e for the final hot-stamped coated steel component. Pi The area.

[0273] In any case, the blank includes a uniform thickness e BflatThe flat portion, specified as the undeformed portion of the hot-stamped coated steel part after the hot stamping process. Thickness e Bflat It is also 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm.

[0274] In other words, thickness e Bflat It is the thickness of the corresponding undeformed portion of the blank that is transformed into a hot-stamped coated steel part during hot stamping.

[0275] The thickness e of this part of the blank Bflat Equal to the thickness e of the corresponding undeformed part of the component pflat .

[0276] In this embodiment, the blanks each have a uniform thickness e. Bflat(i) Two or more portions (where i = 1...n, and n is the number of such portions) are designated as two or more undeformed portions of a hot-stamped coated steel part after the hot stamping process.

[0277] Each thickness e Bflat(i) The thickness is between 0.6 mm and 3.5 mm, preferably between 0.7 mm and 3.0 mm. Additionally, each thickness e... Bflat(i) Equal to the thickness e of the corresponding undeformed part of the component Pflat(i) .

[0278] If the thickness of the blank is uniform, then the thickness e Bflat Or, where applicable, for each thickness e Bflat(i) equal to the thickness e of the blank B .

[0279] The blank, or each sub-blank in the case of a custom-welded blank, is preferably made of steel having the composition disclosed above, particularly according to a first preferred composition, a second preferred composition, or a third preferred composition. The sub-blanks may have the same steel composition or different steel compositions.

[0280] The blank or each sub-blank is produced, for example, as follows.

[0281] Semi-finished products in the form of slabs, thin slabs, or ingots, which can be further hot-rolled, are provided, preferably having the steel composition described above. The thickness of the semi-finished product typically includes between 50 mm and 250 mm.

[0282] If necessary, the semi-finished product is heated to a temperature typically between 1100°C and 1300°C, and then hot-rolled at a finishing temperature preferably between 880°C and 950°C to obtain a hot-rolled steel sheet. The hot-rolled steel sheet is coiled at a temperature Tc below or equal to 750°C and typically above or equal to the Ms temperature of the steel.

[0283] At this stage, the thickness of the hot-rolled steel sheet can typically range from 1.5 mm to 4 mm. Depending on the thickness of the hot-rolled steel sheet and the desired thickness of the sheet, the sheet can be pickled and further cold-rolled under normal conditions or directly annealed via the process described below.

[0284] If the blank to be produced is a custom rolled blank, the steel sheet can be produced by differential rolling (or continuous flexible rolling), that is, by a process in which the thickness of the sheet obtained after rolling is variable in the rolling direction, depending on the load applied to the sheet by the rolls during the rolling process.

[0285] After hot or cold rolling, the steel sheet is annealed in preparation for coating at temperatures typically between Ac1 and Ac3, usually between 700°C and 850°C, and in the case of cold rolling, to allow the grains to recrystallize. The sheet is then hot-dipped in an Al or Al alloy bath at a temperature typically between about 670°C and 680°C, the exact temperature depending on the composition of the bath.

[0286] The preferred pre-coating is Al-Si obtained by hot immersing the plate in a bath containing 7% to 15% Si, 2% to 4% Fe, optionally 0.0015% to 0.0030% Ca by weight, with the remainder being Al and impurities produced by melting.

[0287] Preferably, the bath contains 8% to 11% Si, 2% to 4% Fe, and optionally 0.0015% to 0.0030% Ca by weight, with the remainder being Al and impurities produced by smelting.

[0288] The pre-coated steel sheet is then cooled to room temperature.

[0289] The pre-coated steel sheet is cut to obtain a blank (or sub-blank), the geometry of which is related to the final geometry of the hot-stamped coated steel part.

[0290] If the blanks used to produce the parts are custom-made welded blanks, then the sub-blanks produced therefrom are welded together as disclosed above.

[0291] In one embodiment, the sub-blanks have the same composition. In another embodiment, the sub-blanks have different compositions. This is particularly true when different mechanical properties are required at different locations in the final part. As an example, the steel composition in the first sub-blank is selected from the three preferred compositions described above, and the composition of the second sub-blank is selected from two other preferred compositions or from the same preferred compositions as the first sub-blank but with a different specific composition.

[0292] In one embodiment, the sub-blanks have the same uniform thickness. In another embodiment, the sub-blanks have different thicknesses.

[0293] Alternatively, the blank can be cold-formed to obtain a pre-deformed blank before the heating and hot stamping steps in the die. This cold pre-deformation allows for a reduction in the amount of deformation in the subsequent hot stamping step. In any case, the portion of the blank designated as the undeformed part of the hot-stamped coated steel component after the hot stamping process remains undeformed during this cold pre-deformation. Therefore, this portion of the blank remains flat. On the other hand, the portion of the blank that subsequently undergoes deformation in the hot stamping process is only partially deformed during the cold pre-deformation. Thus, it is ensured that the cold pre-deformation has no impact on the target coating properties.

[0294] Then, the blank (flat or cold-pre-deformed) is heated in a furnace to a temperature T between 850°C and 970°C. heat .

[0295] Heating includes maintaining the temperature of the blank below the melting temperature T of the pre-coating. melt The first heating stage, in which the temperature of the blank is higher than or equal to the melting temperature T of the pre-coating. melt Until heating temperature T heat The second heating stage.

[0296] The heating method is unrestricted and can be based on radiation, conduction, induction, or resistance.

[0297] The time spent on the billet during each of these two stages can be regulated by controlling the furnace, particularly by using a furnace with different sections, each with independent settings, for example, in terms of power and temperature, so that the heating rate can be adjusted independently of each of these sections. For example, if a high heating rate is desired in the first heating stage, the first section of the furnace can be set to high temperature and high power to ensure this rapid heating. If a low heating rate is then desired until the final heating temperature T is reached... heat Then several sections can be used, wherein the furnace temperature is slightly increased from one section to another to ensure that the temperature is above the melting temperature T of the pre-coating. meltSometimes it takes a relatively long time. Or conversely, if it is desired to obtain the pre-coating from its melting temperature T... melt and heating temperature T heat The initial short heating time allows for setting a high temperature in a section of the furnace, and the billet can be transferred to the target heating temperature T before reaching that temperature. heat The section.

[0298] In addition, technicians know how to determine the melting temperature of a pre-coating whose composition is known, for example, using a ternary phase diagram.

[0299] The heated blank is then held at the heating temperature T heat This process is carried out to obtain a fully austenitic structure in the steel.

[0300] Preferably, the total residence time in the furnace, including heating and holding, is between 1.5 min and 15 min.

[0301] The heated blank is then transferred to a die (or hot stamping press). The transfer time is preferably at most 15 s, still preferably at most 10 s or at most 8 s.

[0302] The die is then closed to stamp the blank into a part; the temperature of the blank at the time the die is closed is marked as T. close .

[0303] Preferably, the time elapsed between transferring the heated blank into the mold and closing the mold is less than 8 seconds.

[0304] Heating and holding result in interdiffusion between the pre-coating and the steel substrate. Specifically, during heating and holding, iron diffuses from the steel substrate toward the coating, and aluminum diffuses from the coating toward the steel substrate; this interdiffusion leads to the formation of an interdiffusion layer of the coating.

[0305] In addition, depending on the composition of the pre-coating, one or more intermetallic phases can be generated by interdiffusion in the form of a solid solution over the interdiffusion layer (and thus in the outer layer of the component).

[0306] According to the present invention, the thickness e of the flat portion of the blank that is transformed into an undeformed portion during hot stamping is determined... Bflat To control the heating, holding, transfer, closing, and cooling of the mold, so as to achieve a coating greater than or equal to dC in the undeformed portion of the hot-stamped steel part. min The crack line density dC.

[0307] In particular, the inventors have discovered that achieving a desired crack line density, i.e., a crack line density dC greater than or equal to dC in the coating of the undeformed portion, is possible. min The relevant factor is not the time spent in the furnace, but the temperature T of the workpiece when the mold is closed.close The blanks involved in the hot stamping process are above the melting temperature T of the pre-coating. melt The time t spent M .

[0308] The time t M Including heating in the second stage mentioned above (above T) melt ), at heating temperature T heat The holding time t heat The transfer time and the period during which the blank remains in the die (before and after stamping) until the formed blank reaches temperature T during cooling. melt The time spent.

[0309] The time t M During the hot stamping process, the temperature of the blank during heating can be monitored and adjusted as detailed above to be above T. melt The time spent, by controlling the heating temperature T heat The holding time t heat Furthermore, by controlling the transfer of the blank to the mold and its cooling to T within the mold... melt The time spent during this period is determined and controlled.

[0310] The inventors have discovered that in order to obtain at least dC min Crack line density, temperature T close It must be between 720°C and 820°C, and above the melting temperature T of the pre-coating. melt The time t spent M It must be included in the minimum time t Mmin With maximum time t Mmax Between, this depends on the thickness e of the flat portion of the blank that is designated to be formed into the undeformed part. Bflat Heating temperature T heat and the melting temperature T of the pre-coating melt .

[0311] Minimum time t Mmin and maximum time t Mmax The following limitations apply:

[0312]

[0313] and

[0314] In these expressions, t Mmin and t Mmax T is expressed in seconds. heat T represents the heating temperature of the workpiece in °C. melt This indicates the melting temperature of the pre-coating, expressed in °C, and eBflat This indicates the thickness of the flat portion of the blank, measured in mm, that does not deform during hot stamping and thus specifies the undeformed portion that becomes the part.

[0315] In fact, the inventors have discovered that meeting these conditions allows for hot-stamped coated steel parts to be achieved after hot stamping and cooling, such that the crack line density in the coating of the undeformed portion is higher than or equal to dC. min .

[0316] In contrast, when time t M The above relationship is not satisfied and / or temperature T close At temperatures above 820°C or below 720°C, the crack line density is insufficient, making it impossible to simultaneously achieve excellent spot weldability and excellent spray coating adhesion.

[0317] Specifically, below the minimum time t Mmin The crack line density is insufficient, and the coating adhesion and spot welding properties are unsatisfactory.

[0318] On the other hand, higher than the maximum time t Mmax The crack line density is insufficient, and even if the coating adhesion can be improved by taking longer at temperatures above the melting temperature, the spot weldability will become too low.

[0319] Of course, assuming t Mmin and t Mmax All depend on the heating temperature T heat Then not only time t M Moreover, there is also a heating temperature T. heat All can be adjusted to make time t M Included in t Mmin With t Mmax between.

[0320] In the implementation method, the blank has a uniform thickness e B The thickness e B Then the thickness e of the portion of the blank that is designated as the undeformed part of the component is equal to the thickness of the blank. Bflat , t Mmin and t Mmax Therefore, it becomes:

[0321]

[0322] and

[0323] If the blank has a variable thickness, then preferably, and depending on the desired characteristics, time t M It can be adjusted so that time t M Included in time t Mmin (e Bi) and t Mmax (e Bi Between ) for use in blanks having a thickness e Bi Additional sections / areas.

[0324] In this case, the thickness e of that part of the blank can be used as a reference. Bi Calculate t for each part of the considered part. Mmin and t Mmax The value of . Then, in the minimum time t calculated. Mmin The maximum value and the maximum calculation time t Mmax Selecting time t between the minimum values M :

[0325]

[0326] t Mmin and t Mmax The value of is an increasing function of thickness, which can be simplified as follows:

[0327]

[0328] In other words, if the expectation is to ensure that the crack line density is higher than the minimum crack line density dC of the part produced from two or more portions of the blank with different thicknesses, then... min Then time t M Then, it includes the minimum time t defined for the thickest part. Mmin The maximum time t defined for the thinnest part Mmax between.

[0329] Preferably, if hot stamping is performed to hot stamp the blank to make it have a thickness e Bflat (i) (where i = 1...n) are undeformed to produce a hot-stamped coated steel part comprising two or more undeformed parts, then time t M Including the minimum time t required for the thickest part Mmin The maximum time t required for the thinnest part Mmax between.

[0330]

[0331] In this case, ensure that time t is used in all considered flat portions of the blank corresponding to the undeformed portion of the component. M Production, at time t M Includes the actual thickness e corresponding to all the flat portions considered. Bflat(i) Minimum time t Mmin With maximum time t Mmax between.

[0332] Preferably, the blank has a minimum thickness e Bmin and maximum thickness e Bmax Time t M Included in the maximum thickness e Bmax Minimum required time t Mmin With minimum thickness e Bmin Maximum required time t Mmax between:

[0333]

[0334] In this case, ensure that all areas of the blank are within time t. M Hot stamping is performed for a time t. M Includes the minimum time t corresponding to the actual thickness of all regions. Mmin With maximum time t Mmax between.

[0335] Preferably, the temperature T of the blank when the mold is closed is... close The temperature must be at least 740°C.

[0336] In the implementation method, temperature T close The maximum temperature is 800°C.

[0337] Specifically, temperatures up to 800°C T close Allows implementation of up to 4*dC min The crack line density is reduced, thereby decreasing the risk of powdering.

[0338] The heated blank is hot-stamped and quenched in a mold.

[0339] In hot stamping, as detailed above, one or more parts of the blank are not deformed, and at least one part is deformed by hot stamping.

[0340] The undeformed portion of the blank becomes the undeformed portion of the hot-stamped coated steel component obtained through this process.

[0341] The deformed portion of the blank forms the deformed portion of the hot-stamped coated steel component.

[0342] Due to the geometry of the final part and the forming tool, the pattern and amount of deformation in the deformed section differ from one place to another. For example, some areas may be in a state of expansion, while other areas may be deformed under constraints. Regardless of the deformation pattern, as outlined above, the equivalent deformation ε b It can be defined at each location in the pressed hardened component as , where ε1 and ε2 are the principal deformations. Therefore, ε bThis represents the strain introduced by the hot stamping process in each zone of the deformed part.

[0343] In the undeformed portion, the equivalent deformation is at most 0.01.

[0344] The hot-stamped blank is then held in the mold to ensure an appropriate cooling rate and to avoid part deformation due to shrinkage and phase transformation.

[0345] Hot-stamped blanks are primarily cooled by heat conduction through the die. The die may include coolant circulation to increase the cooling rate or a heating element to decrease the cooling rate. Therefore, the cooling rate can be adjusted by achieving this.

[0346] The hot-stamped blank is cooled to a temperature below 400°C.

[0347] The cooling rate applied depends on the composition of the steel and the desired microstructure and mechanical properties.

[0348] From temperature T heat The average cooling rate up to 400°C (including cooling during transfer and cooling in the mold) is typically at least 27°C / s, preferably at least 50°C / s, and typically less than 200°C / s.

[0349] Preferably, the blank is cooled within the mold from its temperature when the mold is closed to 400°C at an average cooling rate of at least 30°C / s, still preferably at least 50°C / s.

[0350] For example, if the steel has a composition according to the first preferred composition described above, the hot-stamped blank is preferably first cooled in a temperature range between 750°C and 450°C at a first average cooling rate including between 40°C / s and 360°C / s. Within this range, austenite transforms to martensite and possibly bainite. In another step, the hot-stamped blank is cooled in a temperature range including 450°C and 250°C at an average cooling rate between 15°C / s and 150°C / s, which is slower than the first cooling rate.

[0351] If the steel has a composition according to the second preferred composition, the hot-stamped blank is preferably cooled in the die from the furnace outlet to a temperature below 400°C at an average cooling rate of at least 30°C / s to achieve a microstructure consisting essentially of martensite.

[0352] If the steel has a composition according to the third preferred composition, the hot-stamped blank is preferably cooled in a die to obtain a microstructure consisting of martensite or a microstructure consisting of martensite and bainite.

[0353] In any case, the hot-stamped blank is cooled in the die to a temperature below 400°C and then cooled to room temperature to obtain a hot-stamped coated steel part.

[0354] Example

[0355] Steel having compositions A, B, C, and D, expressed as weight percentages according to Table 1, has been cut from pre-coated steel sheets to a uniform thickness e. B It is provided in the form of a blank.

[0356] The blank is pre-coated on both sides with an Al-Si pre-coating. This Al-Si pre-coating has a composition comprising 8% to 11% Si, 2% to 4% Fe, with the remainder being Al and impurities from the smelting process. All pre-coatings have a melting temperature T of 577°C. melt .

[0357] The pre-coating thickness is adjusted to a range of 19.91 μm to 40 μm in each case, and the pre-coating thickness is related to the plate thickness e. B The reports are in Table 2 below.

[0358]

[0359] Table 1

[0360] The remainder of the composition is iron and unavoidable impurities (unavoidable impurities include Cu, the Cu content of which is reported above).

[0361] These pre-coated steel sheets have been cut into blanks.

[0362] The blank is then placed in the furnace at temperature T heat Heat and maintain at T heat It is then placed down and then transferred to the mold within 8 seconds.

[0363] The blank is then hot-stamped in a die to form a part with a deformed portion and a flat, undeformed portion. In each case, the die is used at a temperature T on the blank. close It closes at 750°C.

[0364] The hot-stamped blank is then cooled in the mold to a temperature below 400°C, and then removed from the mold and cooled to room temperature to obtain a hot-stamped coated steel part.

[0365] For each steel composition, the heating, holding, transferring, and cooling in the mold are regulated to be above the melting temperature T of the pre-coating. melt When reaching different times t M .

[0366] For each example, the blank is at a melting temperature T higher than that of the pre-coating. melt The time t spent M The report is in Table 2, and is related to the minimum time t. Mmin and maximum time t Mmax Compare the minimum time t. Mmin and maximum time t Mmax According to the thickness e of the blank B Using the above equation, the thickness e is calculated. B It is equal to the thickness of the undeformed portion of the blank.

[0367]

[0368] Table 2

[0369] The hot-stamped blank is then cooled in a die to a temperature below 400°C to achieve a hot-stamped coated steel part with a martensitic structure.

[0370] Hot-stamped coated steel parts have a uniform thickness e P Thickness e P The thickness e of the blank used to produce these components is equal to the thickness of the blank. B (and make e) P = e Pflat ).

[0371] Samples were taken from the undeformed portion of each component and prepared as described above.

[0372] The total coating thickness e is determined using the method disclosed above. coating and the thickness e of the interdiffusion layer IDL And the crack line density in the coating.

[0373] The crack line density in the coating of the undeformed portion was determined in each case by observing ten fields of view for each of the two samples, with a total observation length of 5.29 mm for the two samples.

[0374] The total thickness e of the measured coating is calculated according to the above formula. coating and the thickness e of the interdiffusion layer IDL Determined E pc The values ​​are reported in Table 3 below.

[0375] Crack linear density dC and minimum crack linear density dC min Also reported in Table 3.

[0376] The coating adhesion assessment for each component is as follows.

[0377] Samples were taken from the undeformed portion of each component.

[0378] The sample was first degreased, then washed with Gardoclean® 5176 and a surfactant at 55°C for 6 min. Refining was then performed using Gardolene® ZL6.

[0379] The phosphating step is achieved by immersing the sample in a bath containing Gardobond® R24 TA and additives at 50°C for 3 minutes.

[0380] A 20 μm electrophoretic coating was then deposited as follows: the sample was immersed in a bath containing PPG Industries' Pigment Paste® W9712-N6 and Resin Blend® W7911-N6, and a nominal voltage with a voltage rise time of 30 s was applied at 30°C for a total duration of 180 seconds. The sample was then wiped in an oven at 175°C and cured for 30 min.

[0381] Then, a dry spray adhesion test was performed on the first group of samples by drawing crosshairs (1 mm intervals) on the electrophoretic coating with a cutter, tearing the electrophoretic coating with adhesive tape (at 460 N / m), and visually assessing the amount of electrophoretic coating removed according to ISO 2409:2013: 0 indicates excellent, in other words, little or no paint was removed, and 5 indicates very poor, in other words, a large amount of paint (> 65%) was removed.

[0382] Then, a wet spray adhesion test was performed on the second group of samples. The samples were immersed in a sealed chamber containing demineralized water at 50°C for 10 days.

[0383] After immersion, the surface of the electrophoretic coating was marked with crosshairs (scratches spaced 1 mm) using a cutter. The electrophoretic coating was then torn with adhesive tape (at 460 N / m), and the amount of electrophoretic coating removed was assessed visually according to ISO 2409:2013. Again, 0 indicates excellent, meaning little or no coating was removed, and 5 indicates very poor, meaning a large amount of coating (>65%) was removed.

[0384] In addition, spot weldability was evaluated by determining the weld range for each sample in the sample according to standard SEP 1220-2 (2011).

[0385] For Examples 7 to 12, which have a thickness of 1.2 mm, the following parameters are used:

[0386] - Electrode: F1-16-20-5.5

[0387] - Welding force: 4 kN

[0388] - Welding current: Medium frequency DC

[0389] - Soldering time: 320 ms per pulse

[0390] - Hold time: 200 ms

[0391] For Examples 1 to 6 with a thickness of 1.8 mm, the following parameters are used:

[0392] - Electrode: F1-20-20-8

[0393] - Welding force: 5 kN

[0394] - Welding current: Medium frequency DC

[0395] - Soldering time: 3 pulses of 200 ms (pause time of 40 ms)

[0396] - Hold time: 300 ms

[0397] The dry coating adhesion and wet coating adhesion, as well as the welding range, as assessed are reported in Table 3.

[0398]

[0399] Table 3

[0400] Referring to Tables 2 and 3, Examples 2, 5, 8, and 11 were produced using the method according to the invention and have a crack line density greater than or equal to the minimum crack line density dC. min Crack line density.

[0401] Figure 2 The coating of Example 2 is illustrated in cross-section, as seen in one field of view used to assess crack line density. In this image, which is used for comparison only for illustrative purposes, several cracks extending from the surface of the coating toward the steel substrate are observed.

[0402] Of course, as mentioned above, the crack line density in Example 2 was not determined based on that single field of view, but rather by observing 20 fields of view to ensure that the crack line density over a sufficient observation length is dC. min (i.e., 15.5 cracks / mm) or more.

[0403] In the examples of the present invention, it is observed that the cracks are generally uniformly distributed in the coating on the flat, undeformed portion, which is not the case for Comparative Examples 1, 3, 4, 6, 7, 9, 10 and 12 discussed below.

[0404] These examples 2, 5, 8, and 11 therefore exhibit excellent dry spray adhesion (less than 1 in each case, and in fact 0, meaning little or no paint removal during testing), excellent wet spray adhesion (at most 1), and excellent spot weldability with a weld range greater than 1 kA.

[0405] Therefore, Examples 2, 5, 8, and 11 show that, according to the invention, dC is greater than or equal to dC. min The hot-stamped coated steel parts with high crack line density achieve excellent spray coating adhesion and spot weldability.

[0406] In contrast, Examples 1, 4, 7, and 10 involve melting at a time t above the pre-coating temperature. M These were produced under conditions that were too short. Therefore, these examples have a value below dC. min Crack line density. Figure 3 The above figure illustrates an example of the field of view in a cross-section of the coating in a flat, undeformed portion of Example 1. In this field of view, no cracks are present in the coating. Therefore, Examples 1, 4, 7, and 10 exhibit low spray adhesion, whether it is a dry spray adhesion of always 1 or a wet spray adhesion higher than 1 in each case.

[0407] Examples 3, 6, 9, and 12 also have values ​​below dC. min The crack line density. Even though dry spray adhesion is excellent and wet spray adhesion is good for Examples 3 and 6, spot weldability is greatly reduced compared to the parts according to the invention due to the longer time spent at temperatures above the melting temperature of the coating. Therefore, these examples do achieve the purpose of the invention.

[0408] Furthermore, Example 12 shows the minimum crack line density dC required to achieve this purpose. min It does indeed depend on the thickness. In particular, this example, by comparison with Examples 2 and 5, shows that if the part has a thickness of 1.2 mm, a crack line density of, for example, 15.5 / mm is insufficient, as in the case of Example 12, but by contrast, a higher thickness of 1.8 mm, as in the cases of Examples 2 and 5, allows the target characteristics to be achieved.

[0409] The example thus confirms that if the undeformed portion of the component is such that its coating has a value greater than or equal to dC... min The high crack line density ensures excellent spot weldability and excellent spray coating adhesion, at least in this part.

[0410] Additionally, examples show that by adjusting the process to make temperature T closeBy properly controlling the time spent on the blank at a temperature higher than the melting temperature of the coating, sufficient crack line density and therefore desired properties can be achieved.

[0411] Therefore, the steel components manufactured according to the present invention can be advantageously used to manufacture chassis or body-in-white components or suspension arms for motor vehicles.

[0412] According to embodiments of the present invention, the following notes are also disclosed:

[0413] Appendix 1. A hot-stamped coated steel component, the hot-stamped coated steel component comprising a steel substrate and an aluminum alloy coating on at least one surface of the steel substrate, the coating comprising, from the steel substrate outwards, a mutually diffused layer and an outer layer, wherein the total thickness e of the coating is... coating and the thickness e of the interdiffusion layer IDL The following conditions must be met:

[0414] 40 ≤ E pc ≤ 80

[0415] in

[0416]

[0417] e IDL The thickness of the interdiffusion layer is expressed in µm, and e coating The total thickness of the coating is expressed in µm.

[0418] Furthermore, the hot-stamped coated steel component includes a thickness e of 0.6 mm to 3.5 mm. Pflat The undeformed portion and at least one deformed portion, wherein the crack line density dC in the coating of the undeformed portion is greater than or equal to the minimum crack line density dC. min (e Pflat The minimum crack linear density dC min (e Pflat Limited to:

[0419]

[0420] Among them, dC and dC min (e Pflat ) is expressed as the number of cracks per mm, and e pflat This indicates the thickness of the undeformed portion, expressed in mm.

[0421] Note 2. The hot-stamped coated steel component according to Note 1, wherein the crack line density dC in the coating of the undeformed portion is less than or equal to 4*dC. min (e Pflat ).

[0422] Note 3. A hot-stamped coated steel component according to any one of Notes 1 or 2, wherein the hot-stamped coated steel component has a uniform thickness e comprising between 0.6 mm and 3.5 mm. P .

[0423] Appendix 4. A hot-stamped coated steel component according to any one of Appendices 1 or 2, wherein the hot-stamped coated steel component has a variable thickness, the hot-stamped coated steel component comprising different thicknesses e, each including between 0.6 mm and 3.5 mm. Pi The hot-stamped coated steel component has two or more regions having an average thickness e between 0.6 mm and 3.5 mm. P .

[0424] Note 5. A hot-stamped coated steel component according to any one of Notes 1 to 4, wherein the hot-stamped coated steel component comprises components each having a thickness e of 0.6 mm to 3.5 mm. pflat (i) two or more undeformed portions, wherein the crack linear density dC(i) in the coating of each undeformed portion is greater than or equal to dC min (e pflat (i)), where:

[0425]

[0426] Among them, e pflat (i) represents the thickness of the undeformed portion under consideration, expressed in mm, where i = 1…n, n ≥ 2, and dC(i) and dC min (e pflat (i) Expressed as the number of cracks per mm and respectively representing the thickness e pflat (i) the crack line density and the minimum crack line density in the coating in the undeformed portion under consideration.

[0427] Note 6. A hot-stamped coated steel component according to any one of Notes 1 to 5, wherein, in cross-section, cracks in the coating of the undeformed portion extend from the uppermost surface of the coating toward the steel substrate in a direction substantially orthogonal to the surface of the steel substrate to a depth of at least 5 µm, and each crack has a width of less than 2 µm in a direction substantially parallel to the surface of the steel substrate.

[0428] Note 7. A hot-stamped coated steel component according to any one of Notes 1 to 6, wherein the crack line density is determined as the ratio between the total number of cracks observed by a bright-field optical microscope in several cross sections of the undeformed portion over a total observation length of at least 5 mm in a direction parallel to the surface of the steel substrate and the total observation length.

[0429] Note 8. A hot-stamped coated steel component according to any one of Notes 1 to 7, wherein the hot-stamped coated steel component is an integral component or a hot-stamped welded component, the hot-stamped welded component comprising at least two hot-stamped coated sub-components and at least one hot-stamped welded portion joining the hot-stamped coated sub-components together.

[0430] Note 9. The hot-stamped coated steel component according to Note 8, wherein the hot-stamped coated steel component or each hot-stamped coated sub-component has a microstructure comprising at least 60% martensite, up to 20% bainite, up to 5% ferrite and up to 15% austenite by volume.

[0431] Note 10. A hot-stamped coated steel component according to any one of Notes 8 or 9, wherein the steel in the hot-stamped coated steel component or each hot-stamped coated sub-component has a chemical composition comprising, by weight percent, the following:

[0432] 0.062% ≤ C ≤ 0.4%

[0433] 0.4% ≤ Mn ≤ 3.9%

[0434] 0.10% ≤ Si ≤ 1.5%

[0435] 0.005% ≤ Al ≤ 1.0%

[0436] 0.001% ≤ Cr ≤ 2.0%

[0437] 0.001% ≤ Ti ≤ 0.2%

[0438] 0.0005% ≤ B ≤ 0.010%

[0439] Ni ≤ 2%

[0440] Nb ≤ 0.1%

[0441] Mo ≤ 0.65%

[0442] W ≤ 0.30%

[0443] N ≤ 0.010%

[0444] 0.0001% ≤ S ≤ 0.05%

[0445] 0.0001% ≤ P ≤ 0.1%

[0446] Ca ≤ 0.005%

[0447] The composition includes iron and unavoidable impurities produced by refining.

[0448] Note 11. The hot-stamped coated steel component according to Note 10, wherein the steel in the hot-stamped coated steel component or at least one hot-stamped coated sub-component has a chemical composition comprising, by weight percent, the following:

[0449] 0.062% ≤ C ≤ 0.095%

[0450] 1.4% ≤ Mn ≤ 1.9%

[0451] 0.2% ≤ Si ≤ 0.5%

[0452] 0.020% ≤ Al ≤ 0.070%

[0453] 0.02% ≤ Cr ≤ 0.1%

[0454] Of which, 1.5% ≤ (C + Mn + Si + Cr) ≤ 2.7%

[0455] 0.0035% ≤ Ti ≤ 0.072%

[0456] 0.0002% ≤ B ≤ 0.004%

[0457] 0.04% ≤ Nb ≤ 0.06%

[0458] Among them, 0.044% ≤ (Nb+Ti) ≤ 0.09%

[0459] 0.001% ≤ N ≤ 0.009%

[0460] 0.0005% ≤ S ≤ 0.003%

[0461] 0.0001% ≤ P ≤ 0.020%

[0462] Ca ≤ 0.005%,

[0463] The composition includes iron and unavoidable impurities produced by refining.

[0464] Note 12. The hot-stamped coated steel component according to Note 10, wherein the steel in the hot-stamped coated steel component or at least one hot-stamped coated sub-component has a chemical composition comprising, by weight percent, the following:

[0465] 0.15% ≤ C ≤ 0.30%

[0466] 0.5% ≤ Mn ≤ 3.0%

[0467] 0.10% ≤ Si ≤ 0.50%

[0468] 0.005% ≤ Al ≤ 0.1%

[0469] 0.01% ≤ Cr ≤ 1.0%

[0470] 0.001% ≤ Ti ≤ 0.2%

[0471] 0.0002% ≤ B ≤ 0.010%

[0472] 0.0005% ≤ N ≤ 0.010%

[0473] 0.0001% ≤ S ≤ 0.05%

[0474] 0.0001% ≤ P ≤ 0.1%

[0475] Ca ≤ 0.005%

[0476] The remainder consists of Fe and unavoidable impurities produced during refining.

[0477] Note 13. The hot-stamped coated steel component according to Note 10, wherein the steel in the hot-stamped coated steel component or at least one hot-stamped coated sub-component has a chemical composition comprising, by weight percent, the following:

[0478] 0.3% ≤ C ≤ 0.4%

[0479] 0.5% ≤ Mn ≤ 1.0%

[0480] 0.40% ≤ Si ≤ 0.80%

[0481] 0.01% ≤ Al ≤ 0.1%

[0482] 0.1% ≤ Cr ≤ 1.0%

[0483] 0.008% ≤ Ti ≤ 0.03%

[0484] 0.0005% ≤ B ≤ 0.003%

[0485] Ni ≤ 0.5%

[0486] 0.01% ≤ Nb ≤ 0.1%

[0487] 0.1% ≤ Mo ≤ 0.5%

[0488] N ≤ 0.005%

[0489] 0.0001% ≤ S ≤ 0.004%

[0490] 0.0001% ≤ P ≤ 0.02%

[0491] Ca ≤ 0.0010%

[0492] The composition includes iron and unavoidable impurities produced by refining.

[0493] Appendix 14. A method for producing hot-stamped coated steel parts, the method comprising the following sequential steps:

[0494] - Provides an average thickness e B The steel billet is between 0.6 mm and 3.5 mm thick, and the billet includes an aluminum or aluminum alloy pre-coating on at least one surface, the pre-coating having an average thickness between 19.91 μm and 40 μm.

[0495] - The steel billet is heated in a furnace to a heating temperature T between 850°C and 970°C. heat And the steel billet is held at the heating temperature T. heat This is done so that a fully austenitic structure can be obtained in the steel of the billet.

[0496] - Transfer the heated blank to the mold, and then close the mold.

[0497] - The blank is hot-stamped in the mold to achieve a thickness e Bflat The flat portion of the blank, ranging from 0.6 mm to 3.5 mm in diameter, is not deformed, and at least a portion of the blank is deformed by hot stamping, thereby obtaining a hot-stamped blank comprising an undeformed portion and at least one deformed portion.

[0498] - Cool the hot-stamped blank to a temperature below 400°C to obtain a hot-stamped coated steel part.

[0499] Wherein, when the mold is closed, the temperature T of the blank is closeIncluding between 720°C and 820°C, and during heating, holding, transferring, and hot stamping, the blank is at a temperature higher than the melting temperature T of the pre-coating. melt The time t spent M Included in minimum time t Mmin With maximum time t Mmax Between, among which:

[0500]

[0501] and

[0502]

[0503] Among them, t Mmin and t Mmax T is expressed in seconds. heat T represents the heating temperature of the blank, expressed in °C. melt The melting temperature of the pre-coating is expressed in °C, and e Bflat This indicates the thickness of the undeformed portion of the blank, expressed in mm.

[0504] Note 15. The method according to Note 14, wherein the pre-coating is an aluminum alloy pre-coating comprising, by weight, 7% to 15% silicon, 2% to 4% iron and optionally 0.0015% to 0.0030% calcium, with the remainder being aluminum and unavoidable impurities.

[0505] Note 16. The method according to Note 15, wherein the pre-coating is an aluminum alloy pre-coating comprising, by weight, the following: 8% to 11% silicon, 2% to 4% iron, optionally 0.0015% to 0.0030% calcium, with the remainder being aluminum and unavoidable impurities.

[0506] Note 17. The method according to any one of Notes 14 to 16, wherein, when the blanks are hot-stamped in the mold, each blank has a thickness e of 0.6 mm to 3.5 mm. Bflat (i) Two or more flat portions are not subjected to deformation, and the blank is at a melting temperature T higher than that of the pre-coating. melt The time t spent M Including the minimum time t required for the flat portion with the maximum thickness Mmin (Max(e Bflat(i) The maximum time t required for the flat portion with minimum thickness Mmax (Min(e Bflat(i) )between.

[0507] Note 18. The method according to any one of Notes 14 to 17, wherein, when the blank is hot-stamped in the mold, the blank has a minimum thickness e Bmin up to maximum thickness e Bmax The blank has a variable thickness within a range, and the blank is at a melting temperature T higher than that of the pre-coating. melt The time t spent M Including the maximum thickness e Bmax Minimum required time t Mmin (e Bmax ) and the minimum thickness e Bmin Maximum required time t Mmax (e Bmin )between.

[0508] Note 19. The method according to any one of Notes 14 to 18, wherein the steel billet is a custom welded billet produced by an integral billet, a custom rolled billet, or by welding at least two sub-billets together.

[0509] Note 20. The method according to Note 19, wherein the blank or each sub-blank is produced by cutting a pre-coated steel sheet having a chemical composition comprising, by weight percent, the following:

[0510] 0.062% ≤ C ≤ 0.4%

[0511] 0.4% ≤ Mn ≤ 3.9%

[0512] 0.10% ≤ Si ≤ 1.5%

[0513] 0.005% ≤ Al ≤ 1.0%

[0514] 0.001% ≤ Cr ≤ 2.0%

[0515] 0.001% ≤ Ti ≤ 0.2%

[0516] 0.0005% ≤ B ≤ 0.010%

[0517] Ni ≤ 2%

[0518] Nb ≤ 0.1%

[0519] Mo ≤ 0.65%

[0520] W ≤ 0.30%

[0521] N ≤ 0.010%

[0522] 0.0001% ≤ S ≤ 0.05%

[0523] 0.0001% ≤ P ≤ 0.1%

[0524] Ca ≤ 0.005%

[0525] The composition includes iron and unavoidable impurities produced by refining.

[0526] Note 21. The method according to Note 20, wherein the chemical composition of the blank or at least one sub-blank comprises, in weight percent, the following:

[0527] 0.062% ≤ C ≤ 0.095%

[0528] 1.4% ≤ Mn ≤ 1.9%

[0529] 0.2% ≤ Si ≤ 0.5%

[0530] 0.020% ≤ Al ≤ 0.070%

[0531] 0.02% ≤ Cr ≤ 0.1%

[0532] Of which, 1.5% ≤ (C + Mn + Si + Cr) ≤ 2.7%

[0533] 0.0035% ≤ Ti ≤ 0.072%

[0534] 0.0002% ≤ B ≤ 0.004%

[0535] 0.04% ≤ Nb ≤ 0.06%

[0536] Among them, 0.044% ≤ (Nb+Ti) ≤ 0.09%

[0537] 0.001% ≤ N ≤ 0.009%

[0538] 0.0005% ≤ S ≤ 0.003%

[0539] 0.0001% ≤ P ≤ 0.020%

[0540] Ca ≤ 0.005%,

[0541] The composition includes iron and unavoidable impurities produced by refining.

[0542] Note 22. The method according to Note 20, wherein the chemical composition of the blank or at least one sub-blank comprises, in weight percent, the following:

[0543] 0.15% ≤ C ≤ 0.30%

[0544] 0.5% ≤ Mn ≤ 3.0%

[0545] 0.10% ≤ Si ≤ 0.50%

[0546] 0.005% ≤ Al ≤ 0.1%

[0547] 0.01% ≤ Cr ≤ 1.0%

[0548] 0.001% ≤ Ti ≤ 0.2%

[0549] 0.0002% ≤ B ≤ 0.010%

[0550] 0.0005% ≤ N ≤ 0.010%

[0551] 0.0001% ≤ S ≤ 0.05%

[0552] 0.0001% ≤ P ≤ 0.1%

[0553] Ca ≤ 0.005%

[0554] The remainder consists of Fe and unavoidable impurities produced during refining.

[0555] Note 23. The method according to Note 20, wherein the chemical composition of the blank or at least one sub-blank comprises, in weight percent, the following:

[0556] 0.3% ≤ C ≤ 0.4%

[0557] 0.5% ≤ Mn ≤ 1.0%

[0558] 0.40% ≤ Si ≤ 0.80%

[0559] 0.01% ≤ Al ≤ 0.1%

[0560] 0.1% ≤ Cr ≤ 1.0%

[0561] 0.008% ≤ Ti ≤ 0.03%

[0562] 0.0005% ≤ B ≤ 0.003%

[0563] Ni ≤ 0.5%

[0564] 0.01% ≤ Nb ≤ 0.1%

[0565] 0.1% ≤ Mo ≤ 0.5%

[0566] N ≤ 0.005%

[0567] 0.0001% ≤ S ≤ 0.004%

[0568] 0.0001% ≤ P ≤ 0.02%

[0569] Ca ≤ 0.0010%

[0570] The composition includes iron and unavoidable impurities produced by refining.

[0571] Note 24. The method according to any one of Notes 19 to 23, wherein providing the blank or each sub-blank comprises the following sequential steps:

[0572] - Provide steel semi-finished products,

[0573] - Optionally, the semi-finished product is reheated to a temperature between 1100°C and 1300°C.

[0574] - The semi-finished product is hot-rolled to obtain hot-rolled steel sheet.

[0575] - The hot-rolled steel sheet is wound at a winding temperature of 750°C or lower.

[0576] - Optionally, the hot-rolled steel sheet is pickled.

[0577] - Optionally, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.

[0578] - Heat the hot-rolled steel sheet or the cold-rolled steel sheet to an annealing temperature between Ac1 and Ac3.

[0579] - The steel sheet is hot-dip coated in an aluminum or aluminum alloy bath at a temperature of 670°C to 680°C.

[0580] - Cool the pre-coated steel sheet to room temperature.

[0581] - The pre-coated steel sheet is cut to obtain the blank or the sub-blank.

[0582] - Optionally, the sub-blanks can be welded together to produce custom welded blanks.

[0583] Note 25. Use of a hot-stamped coated steel component according to any one of Notes 1 to 13, or a hot-stamped coated steel component produced by the method according to any one of Notes 14 to 24, for manufacturing chassis or body-in-white components or suspension arms for motor vehicles.

Claims

1. A hot-stamped coated steel part comprising a steel substrate and an aluminum alloy coating on at least one face of the steel substrate, the coating comprising, in order outward from the steel substrate, an interdiffusion layer and an outer layer, wherein, the total thickness e of the coating coating and the thickness e of the interdiffusion layer IDL satisfies the following condition: 40 ≤ E pc ≤ 80 wherein e IDL denotes the thickness of the interdiffusion layer in µm, and e coating denotes the total thickness of the coating in µm, Furthermore, the hot-stamped coated steel component includes a thickness e of 0.6 mm to 3.5 mm. Pflat The component comprises an undeformed portion and at least one deformed portion, wherein the undeformed portion or each undeformed portion is a flange of the component or a flat portion located between two deformed portions of the component, and the crack line density dC in the coating of the undeformed portion is greater than or equal to the minimum crack line density dC. min (e Pflat The minimum crack linear density dC min (e Pflat Limited to: wherein dC and dC min (e Pflat ) is expressed in number of cracks per mm, and e pflat denotes the thickness of the undeformed portion in mm.

2. The hot-stamped coated steel part according to claim 1, wherein, the crack line density dC in the coating in the undeformed portion is lower than or equal to 4*dC min (e Pflat ).

3. Hot-stamped coated steel part according to either one of claims 1 or 2, wherein, The hot-stamped coated steel part has a uniform thickness e comprised between 0.6 mm and 3.5 mm P .

4. Hot-stamped coated steel part according to either of Claims 1 or 2, wherein, The hot-stamped coated steel part has a variable thickness, the hot-stamped coated steel part comprising two or more zones each comprising a different thickness e comprised between 0.6 mm and 3.5 mm Pi The hot-stamped coated steel part has an average thickness e comprised between 0.6 mm and 3.5 mm P .

5. Hot-stamped coated steel part according to either of Claims 1 or 2, wherein, The hot-stamped coated steel part comprises each a thickness e comprised between 0.6 mm and 3.5 mm pflat a crack line density dC(i) in the coating of each undeformed portion higher than or equal to dC min (e pflat (i)) wherein: wherein e pflat (i) represents the thickness of the considered undeformed portion in mm, with i = 1... n, n > 2, and dC(i) and dC min (e pflat (i)) in number of cracks per mm and respectively represents the crack line density in the coating and the minimum crack line density in the considered undeformed portion of thickness e pflat (i).

6. The hot-stamped coated steel part according to either one of claims 1 or 2, wherein, In a cross-section, the cracks in the coating of the undeformed portion extend in a direction substantially normal to the surface of the steel substrate from the uppermost surface of the coating towards the steel substrate over a depth of at least 5 pm, each of the cracks having a width of less than 2 pm in a direction substantially parallel to the surface of the steel substrate.

7. Hot-stamped coated steel part according to either one of claims 1 or 2, wherein, The crack line density is determined as the ratio between the total number of cracks observed in several cross-sections of the undeformed portion with light field optical microscopy over a total observation length of at least 5 mm in a direction parallel to the surface of the steel substrate.

8. The hot-stamped coated steel part according to either one of claims 1 or 2, wherein, The hot-stamped coated steel part is a monolithic part, or a hot-stamped welded part comprising at least two hot-stamped coated sub-parts and at least one hot-stamped weld joining the hot-stamped coated sub-parts together.

9. The hot-stamped coated steel part according to claim 8, wherein, The hot-stamped coated steel part or each hot-stamped coated sub-part has a microstructure comprising at least 60% martensite, at most 20% bainite, at most 5% ferrite and at most 15% austenite by volume.

10. The hot-stamped coated steel part according to claim 8, wherein, The steel in the hot-stamped coated steel part or each hot-stamped coated sub-part has a chemical composition comprising in weight %: 0.062% ≤ C ≤ 0.4% 0.4% < Mn < 3.9% 0.10% < Si < 1.5% 0.005% ≤ Al ≤ 1.0% 0.001% ≤ Cr ≤ 2.0% 0.001% < Ti < 0.2% 0.0005% ≤ B ≤ 0.010% Ni < 2% Nb < 0.1% Mo < 0.65% W ≤ 0.30% N ≤ 0.010% 0.0001% ≤ S ≤ 0.05% 0.0001% ≤ P ≤ 0.1% Ca < 0.005% the balance of the composition comprising iron and unavoidable impurities resulting from the refining.

11. The hot-stamped coated steel part according to claim 10, wherein, The steel in the hot-stamped coated steel part or at least one hot-stamped coated sub-part has a chemical composition comprising in weight %: 0.062% ≤ C ≤ 0.095% 1.4% < Mn < 1.9% 0.2% < Si < 0.5% 0.020% ≤ Al ≤ 0.070% 0.02% ≤ Cr ≤ 0.1% wherein 1.5% < (C + Mn + Si + Cr) < 2.7% 0.0035% < Ti < 0.072% 0.0002% ≤ B ≤ 0.004% 0.04% < Nb < 0.06% wherein, 0.044% < (Nb + Ti) < 0.09% 0.001% ≤ N ≤ 0.009% 0.0005% ≤ S ≤ 0.003% 0.0001% ≤ P ≤ 0.020% Ca < 0.005%, the balance of the composition comprising iron and unavoidable impurities resulting from the refining.

12. The hot-stamped coated steel part according to claim 10, wherein, The steel in the hot-stamped coated steel part or at least one hot-stamped coated sub-part has a chemical composition comprising in weight %: 0.15% ≤ C ≤ 0.30% 0.5% < Mn < 3.0% 0.10% < Si < 0.50% 0.005% ≤ Al ≤ 0.1% 0.01% ≤ Cr ≤ 1.0% 0.001% < Ti < 0.2% 0.0002% ≤ B ≤ 0.010% 0.0005% ≤ N ≤ 0.010% 0.0001% ≤ S ≤ 0.05% 0.0001% ≤ P ≤ 0.1% Ca < 0.005% the balance being Fe and unavoidable impurities resulting from the refining.

13. The hot-stamped coated steel part according to claim 10, wherein the steel in the hot-stamped coated steel part or at least one hot-stamped coated sub-part has a chemical composition comprising in weight %: 0.3% ≤ C ≤ 0.4% 0.5% < Mn < 1.0% 0.40% < Si < 0.80% 0.01% ≤ Al ≤ 0.1% 0.1% ≤ Cr ≤ 1.0% 0.008% < Ti < 0.03% 0.0005% ≤ B ≤ 0.003% Ni < 0.5% 0.01 % < Nb < 0.1% 0.1% < Mo < 0.5% N ≤ 0.005% 0.0001% ≤ S ≤ 0.004% 0.0001% ≤ P ≤ 0.02% Ca < 0.0010% the remainder including iron and unavoidable impurities resulting from the refining.

14. A method for producing a hot-stamped coated steel part, the method comprising the following successive steps: - providing a steel blank having an average thickness e B of 0.6 mm to 3.5 mm, the steel blank comprising on at least one face an aluminum or aluminum alloy pre-coating having an average thickness comprised between 19.91 pm and 40 pm, - heating said billets in a furnace to a heating temperature T comprised between 850°C and 970°C heat , and maintaining said billets at said heating temperature T heat , so as to obtain a fully austenitic structure in the steel of the billets, said heating comprising a first heating phase in which the temperature of the billets remains lower than the melting temperature T melt of said pre-coating; and a second heating phase in which the temperature of the billets is higher than or equal to the melting temperature T melt of said pre-coating, up to said heating temperature T heat . - transferring the heated blank to a die, then closing the die, - hot stamping the blank in the mould such that the portion of the blank having a thickness e Bflat the flat portion of the blank having a thickness of 0.6 mm to 3.5 mm is not subjected to deformation and at least a portion of the blank is deformed by hot stamping, thereby obtaining a hot stamped blank comprising an undeformed portion and at least one deformed portion, the or each undeformed portion being a flange of the component or a flat portion located between two deformed portions of the component, - cooling the hot-stamped blank to a temperature of less than 400°C to obtain a hot- stamped coated steel part, wherein, when the mould is closed, the temperature T of the blank close between 720°C and 820°C, and during the second heating phase, heating, holding, transferring and hot stamping, the blank is at a temperature higher than the melting temperature T melt of the pre-coat, for a time t M comprised between a minimum time t Mmin and a maximum time t Mmax wherein: and wherein t Mmin and t Mmax is expressed in seconds, T heat represents the heating temperature of the blank in °C, T melt represents the melting temperature of the pre-coat in °C, and e Bflat represents the thickness of the portion of the blank not subjected to deformation in mm.

15. The method according to claim 14, wherein the pre-coating is an aluminum alloy pre- coating comprising by weight: 7 to 15% of silicon, 2 to 4% of iron and optionally 0.0015 to 0.0030% of calcium, the remainder being aluminum and unavoidable impurities.

16. The method of claim 15, wherein, the pre-coating is an aluminum alloy pre-coating comprising by weight: 8 to 11% of silicon, 2 to 4% of iron, optionally 0.0015 to 0.0030% of calcium, the remainder being aluminum and unavoidable impurities.

17. The method of any one of claims 14 or 15, wherein, when the blank is hot-stamped in the die, each of the flat portions of the blank has a thickness e comprised between 0.6 mm and 3.5 mm Bflat (i) is not subjected to deformation and the blank spends a time t melt above the melting temperature T M of the pre-coating Mmin comprised between a minimum time t Bflat(i) (Max(e Mmax ) and a maximum time t Bflat(i) (Min(e Bflat(i) ) required by the flat portion having the minimum thickness.

18. The method of any one of claims 14 or 15, wherein, when the blank is hot-stamped in the die, the blank has a variable thickness ranging from a minimum thickness e Bmin to a maximum thickness e Bmax , the time t melt spent by the blank above the melting temperature T M of the pre-coating includes a minimum time t Bmax required at the maximum thickness e Mmin (e Bmax ) and a maximum time t Bmin (e Mmax ) required at the minimum thickness e Bmin .

19. The method of any one of claims 14 or 15, wherein, the steel blank is a tailor-welded blank produced by a monolithic blank, a tailor-rolled blank or by welding at least two sub-blanks together.

20. The method of claim 19, wherein, the blank or each sub-blank is produced by cutting a pre-coated steel sheet having a chemical composition comprising by weight %: 0.062% ≤ C ≤ 0.4% 0.4% < Mn < 3.9% 0.10% < Si < 1.5% 0.005% ≤ Al ≤ 1.0% 0.001% ≤ Cr ≤ 2.0% 0.001% < Ti < 0.2% 0.0005% ≤ B ≤ 0.010% Ni < 2% Nb < 0.1% Mo < 0.65% W ≤ 0.30% N ≤ 0.010% 0.0001% ≤ S ≤ 0.05% 0.0001% ≤ P ≤ 0.1% Ca < 0.005% the remainder including iron and unavoidable impurities resulting from the refining.

21. The method of claim 20, wherein, the chemical composition of the blank or of at least one sub-blank comprises by weight %: 0.062% ≤ C ≤ 0.095% 1.4% < Mn < 1.9% 0.2% < Si < 0.5% 0.020% ≤ Al ≤ 0.070% 0.02% ≤ Cr ≤ 0.1% wherein 1.5% < (C + Mn + Si + Cr) < 2.7% 0.0035% < Ti < 0.072% 0.0002% ≤ B ≤ 0.004% 0.04 % < Nb < 0.06% wherein 0.044% < (Nb + Ti) < 0.09% 0.001% ≤ N ≤ 0.009% 0.0005% ≤ S ≤ 0.003% 0.0001% ≤ P ≤ 0.020% Ca < 0.005%, the remainder including iron and unavoidable impurities resulting from the refining.

22. The method of claim 20, wherein, the chemical composition of the blank or of at least one sub-blank comprises by weight %: 0.15% ≤ C ≤ 0.30% 0.5% < Mn < 3.0% 0.10% < Si < 0.50% 0.005% ≤ Al ≤ 0.1% 0.01% ≤ Cr ≤ 1.0% 0.001% < Ti < 0.2% 0.0002% ≤ B ≤ 0.010% 0.0005% ≤ N ≤ 0.010% 0.0001% ≤ S ≤ 0.05% 0.0001% ≤ P ≤ 0.1% Ca < 0.005% the remainder being Fe and unavoidable impurities resulting from the refining.

23. The method of claim 20, wherein, the chemical composition of the blank or of at least one sub-blank comprises by weight %: 0.3% ≤ C ≤ 0.4% 0.5% < Mn < 1.0% 0.40% < Si < 0.80% 0.01% ≤ Al ≤ 0.1% 0.1% ≤ Cr ≤ 1.0% 0.008% < Ti < 0.03% 0.0005% ≤ B ≤ 0.003% Ni < 0.5% 0.01 % < Nb < 0.1% 0.1% < Mo < 0.5% N ≤ 0.005% 0.0001% ≤ S ≤ 0.004% 0.0001% ≤ P ≤ 0.02% Ca < 0.0010% The balance of the composition includes iron and unavoidable impurities resulting from the refining.

24. The method of claim 19, wherein, providing said blank or each sub-blank comprises the following successive steps: - providing a steel semi-product, - optionally, reheating said semi-product to a temperature between 1100°C and 1300°C, - hot rolling said semi-product to obtain a hot-rolled steel sheet, - coiling said hot-rolled steel sheet at a coiling temperature lower than or equal to 750°C, - optionally, pickling said hot-rolled steel sheet, - optionally, cold rolling said hot-rolled steel sheet to obtain a cold-rolled steel sheet, - heating said hot-rolled steel sheet or said cold-rolled steel sheet to an annealing temperature between Ac1 and Ac3, - hot dip coating said steel sheet in an aluminum or aluminum alloy bath at a temperature between 670°C and 680°C, - cooling the pre-coated steel sheet to room temperature, - cutting said pre-coated steel sheet to obtain said blank or said sub-blanks, - optionally, welding said sub-blanks together to produce a tailor-welded blank.

25. Use of the hot-stamped coated steel part according to claim 1 or produced by the method according to claim 14 for manufacturing a chassis or a body-in-white part or a suspension arm for a motor vehicle.

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