Hot-stamped coated steel component with crack having excellent spot weldability and excellent coating adhesion

Abstract

To provide a hot-stamped steel component having improved spot-weldability and improved coating adhesiveness, and a manufacturing method thereof.SOLUTION: In a hot-stamped coated steel component including a steel substrate and an aluminum alloy coating having an interdiffusion layer and an outer layer outward from the steel substrate, the total thickness ecoating of the coating and the thickness eIDL of the interdiffusion layer satisfy a following condition. When 40≤Epc≤80, the hot-stamped coated steel component has a non-deformation portion having a thickness ePflat of 0.6 mm to 3.5 mm, and at least one deformation portion. Linear density dC of cracks on the coating at the non-deformation portion is not less than minimum linear density dCmin(ePflat) of crack defined as follows.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a hot-stamped coated steel part comprising a steel substrate and an aluminum alloy coating on at least one surface of the steel substrate, the coating having an optimized crack density and the part having excellent paint adhesion and excellent spot weldability. The present invention also relates to a method for manufacturing a hot-stamped coated steel part.

Background Art

[0002] Such parts can also be used, for example, in the automotive industry for manufacturing structural elements for intrusion prevention or energy absorption functions.

[0003] In this type of application, it is desirable to have steel parts with high mechanical strength, high impact resistance, good corrosion resistance and dimensional accuracy. Automotive parts such as front or rear rails, roof rails, B-pillars, and chassis parts such as lower control arms and engine cradles more specifically require these properties.

[0004] To meet these requirements, such parts are currently generally manufactured by a hot stamping process (also called press hardening). In the hot stamping process, as disclosed in particular in French Patent No. 2780984 and French Patent No. 2807447, a blank cut from a steel sheet 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 hot stamped in a mold. During stamping, the part is held in the mold to achieve 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-coating alloy alloys with the steel substrate to form a compound that provides protection of the steel surface against decarburization and scale formation.

[0005] Recently, focus has been placed on the coating of parts after hot stamping and how it affects the properties of the parts during use.

[0006] In International Publication No. 2008 / 053273, in order to achieve the preferred continuity and morphology of the continuous layer during the coating of parts and bring about improved weldability, the pre-coating thickness is limited to between 20 and 33 μm at all locations, and it is proposed to control the hot stamping process, particularly the heating rate and austenitizing parameters.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

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

[0009] In particular, the inventors have found that even when paint adhesion can be improved, such improvement is achieved by sacrificing spot weldability, and thus it is still desirable to provide parts that have both excellent paint adhesion and excellent spot weldability.

[0010] Accordingly, an object of the present invention is to provide a hot-stamped coated steel part including a steel substrate and an aluminum alloy coating on at least one surface of the steel substrate, which has excellent paint adhesion and excellent spot weldability, and a method for manufacturing the same.

[0011] In particular, it is desirable to provide a hot-stamped coated steel part having at least a non-deformed portion with a welding range exceeding 1 kA as measured according to standard SEP 1220-2 (2011), together with excellent paint adhesion. The paint adhesion is evaluated by performing a dry paint test and a wet paint adhesion test according to standard ISO 2409:2013. If the result of the dry paint test is strictly less than 1 and the result of the wet paint adhesion test is 1 or less, the paint adhesion is considered excellent.

Means for Solving the Problems

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

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

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

[0015] This 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 part according to the present invention or a hot-stamped coated steel part manufactured by the method according to the present invention for the manufacture of a chassis or a white body part or a suspension arm for a motor vehicle.

[0017] The present invention will be described in detail and illustrated by way of examples without limitation in consideration of the following drawings.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

[0019] The present invention relates to a hot-stamped coated steel part.

[0020]

[0021] The hot-stamped coated steel part is a non-flat part manufactured by hot-stamping a blank.

[0022] The steel sheet refers to a flat steel sheet. Here, the steel sheet refers to a coiled hot-rolled steel sheet or cold-rolled steel sheet, or a sheet cut from such a coil.

[0023] The steel blank is a flat sheet of steel cut into any shape suitable for its use, or two or more steel sheet materials having different thicknesses or different compositions are cut into the required shape, assembled together, and in particular, a blank welded together.

[0024] ​ The average thickness of a part or a part of a part means the overall average thickness of the material that makes up the part after it has been formed from a flat sheet into a three-dimensional part.

[0025] Uniform thickness means that the thickness of the blank, part, sheet, or region or part thereof under consideration is constant, and the maximum variation in the thickness of the blank, part, sheet, or region or part thereof above or below the average thickness of the blank, part, sheet, or region or part thereof is at most 0.1 mm. In particular, uniform thickness means that no change in thickness was spontaneously brought about during manufacturing, in particular during the manufacture of the sheet in hot and / or cold rolling, and during the forming operations in the manufacture of the part.

[0026] Hereinafter, the thickness of a blank, part, sheet, or region or part thereof having a uniform thickness is defined as the average thickness of this blank, part, sheet, or region or part thereof.

[0027] Furthermore, the term "thickness" is used to refer to the thickness of a blank, part, sheet, or region or part thereof having a uniform thickness, while "average thickness" is more generally used to refer to the average thickness of a blank, part, sheet, or region or part thereof, whether the thickness is uniform or variable.

[0028] Tailor-welded blanks are manufactured by assembling, for example by laser welding, several sheets of steel or cut-out blanks known as sub-blanks together in order to optimize the performance of the part in its different regions, to reduce the overall part weight, to reduce the overall part cost, and to reduce material scrap. The sub-blanks forming the tailor-welded blank can be assembled with or without overlap, for example they can be laser butt-welded (without overlap) or spot-welded to each other (with overlap).

[0029] A flexible blank is a type of tailor-welded blank that includes regions where at least a portion of the connection between different sub-blanks is not rigid, allowing the sub-blanks to move in different directions during forming operations in the corresponding regions.

[0030] In contrast to a tailor-welded blank, a monolithic blank refers to a blank consisting of a single sub-blank without several sub-blanks being combined together.

[0031] A tailor-rolled blank is a blank having a variable thickness, i.e., a change in thickness along the blank, obtained by differential rolling during the steel sheet manufacturing process.

[0032] Hot stamping is a forming technique that includes heating the blank to a temperature at which the fine structure of the steel is at least partially transformed into austenite, forming the blank at a high temperature by stamping the blank, and quenching the formed part to obtain a fine structure with high strength. Hot stamping enables the production of very high-strength parts with complex shapes and presents many technical advantages.

[0033] A monolithic part is a hot-stamped part manufactured from a monolithic blank.

[0034] Monolithic parts are manufactured, for example, from a monolithic blank having a uniform thickness or from a monolithic tailor-rolled blank.

[0035] Hot-stamped welded steel parts or hot-stamped laser-welded steel parts are hot-stamped parts manufactured from a tailor-welded blank, such as a flexible blank. Thus, a hot-stamped welded steel part comprises two or more hot-stamped sub-parts and one or more hot-stamped welds joining the hot-stamped sub-parts together.

[0036] In one embodiment, the hot-stamped coated steel part of the present invention is a monolithic part.

[0037] In another embodiment, the hot-stamped coated steel part is a hot-stamped welded steel part.

[0038] The hot-stamped coated steel part of the present invention preferably has an average thickness e included between 0.6 mm and 3.5 mm. P It has.

[0039] The range of 0.6 mm to 3.5 mm is the normal thickness used for the manufacture of structural or reinforcement parts for the automotive industry. This thickness range is also suitable for industrial press-hardening tools, particularly hot-stamping presses or dies.

[0040] Furthermore, as detailed below, the thermal process applied to a given steel sheet thickness affects the formation of cracks in the coating, particularly in the coating of the flat non-deformed parts desired in the present invention.

[0041] Preferably, the average thickness e of the hot-stamped coated steel part P is within the range of 0.7 mm to 3.0 mm.

[0042] In one embodiment, the hot-stamped coated steel part has a uniform thickness e of 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm. P It has.

[0043] In another embodiment, the hot-stamped coated steel part has a variable thickness (and thus is non-uniform). In that case, the hot-stamped coated steel part preferably consists of two or more regions each having a different uniform thickness e of 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm. Pi It consists of.

[0044] As an example, a hot-stamped coated steel part having a variable thickness can be manufactured from a tailor-rolled blank having a variable thickness obtained by differential rolling during the steel sheet manufacturing process.

[0045] As another example, a hot-stamped coated steel part having a variable thickness can be a hot-stamped welded steel part manufactured from a tailor-welded blank made of blanks having different thicknesses.

[0046] The hot-stamped coated steel part comprises a steel substrate (also called a steel base material) having two main surfaces.

[0047] The steel in the substrate is steel for hot stamping, i.e., steel that can be hardened after austenitization and quenched by quenching.

[0048] In one embodiment, the part is manufactured from a monolithic blank and the steel substrate is made of a single steel.

[0049] In another embodiment, the part is a hot-stamped welded steel part and the steel substrate consists of two or more regions (or sub-parts) that can be made of the same steel or different steels and can have the same or different microstructures.

[0050] Hereinafter, the microstructure and composition of the steel of the steel substrate means the microstructure and composition of the steel substrate (when the substrate is made of a single steel), or the microstructure or composition of one or more regions or sub-parts of the steel substrate.

[0051] The composition of the steel depends on the desired mechanical properties of the part. However, preferably, in the steel substrate, or in each region of the steel substrate, the steel is, 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% has a composition comprising the following, the balance of the composition consisting of iron and unavoidable impurities resulting from processing:

[0052] The levels of impurities resulting from the processing route depend on the manufacturing route used. For example, when using a blast furnace route with low levels of steel scrap (recycled steel), the impurity levels remain very low. On the other hand, when refining steel using an electric arc furnace with a very high proportion of recycled scrap steel, the impurity levels increase significantly. In the latter case, for example, the level of Cu can increase up to 0.25%, Ni can increase up to 0.25%, Sn can increase up to 0.05%, As can increase up to 0.03%, Sb can increase up to 0.03%, and Pb can increase up to 0.03%.

[0053] Thus, in one embodiment, the steel contains, as unavoidable impurities, a maximum of 0.25% Cu, a maximum of 0.05% Sn, a maximum of 0.03% As, a maximum of 0.03% Sb and / or a maximum of 0.03% Pb.

[0054] The above composition is advantageous for achieving high mechanical properties, in particular a tensile strength TS in the range of 950 MPa to 2100 MPa.

[0055] The tensile strength is measured in accordance with the ISO standard NF EN ISO 6892-1 issued in October 2009. The tensile test specimens are cut from the flat part of the hot-stamped parts.

[0056] Unless otherwise specified, the content in the elements is expressed in weight percent below.

[0057] The carbon content depends on the desired tensile strength TS of the hot-stamped coated steel part.

[0058] If the C content is less than 0.062%, it is difficult to obtain a tensile strength of at least 950 MPa after hot stamping under any cooling conditions. If it exceeds 0.4%, in combination with other elements of the composition, the adhesion of the coating after hot stamping may not be satisfactory, and the resistance and toughness of the steel to delayed cracking decrease. In one embodiment, the C content is at most 0.38%.

[0059] The C content depends on the desired tensile strength TS of the hot-stamped part produced by hot stamping the steel sheet. In one embodiment, the C content is 0.062% - 0.095%. If a higher tensile strength is desired, at about 1500 MPa, the C content can be increased to the range of 0.15% - 0.30%. If it is necessary to further increase the tensile strength to at least 1800 MPa, the C content can be added up to a content of 0.4%.

[0060] Apart from the role of deoxidation, manganese has an important effect on hardenability, especially when its content is at least 0.4%. If it exceeds 3.9%, the stabilization of austenite by Mn may be too important, which can lead to the formation of an overly prominent banded structure. Preferably, the Mn content is at most 3.0%.

[0061] Silicon is added in a content of at least 0.10% to assist in the deoxidation of molten steel and contribute to the hardening of the steel by precipitation in the solid solution. However, its content is generally limited to avoid the excessive formation of silicon oxide that impairs the coating property of the steel. Therefore, the silicon content is generally 1.5% or less, for example, 0.80% or less.

[0062] Aluminum may be added as a deoxidizer with a content of at least 0.005%. Further, when the titanium content is insufficient, Al can protect boron by binding with N. The Al content is preferably at least 0.01%. To avoid oxidation problems and avoid the formation of ferrite during hot stamping, the Al content is generally 1.0% or less. Preferably, the Al content is at most 0.1%.

[0063] Cr may be added to increase the hardenability of the steel and contribute to achieving the desired tensile strength after hot stamping. When Cr is added, its content is 0.01% or more, preferably 0.1% or more, and at most 2.0%. If no optional addition of Cr is made, the Cr content may be present at a low content of about 0.001% as an impurity.

[0064] When titanium is added, its content is preferably at least 0.008% and at most 0.2%. When the Ti content is included between 0.008% and 0.2%, precipitation at very high temperatures occurs in the form of TiN, and then at lower temperatures in austenite in the form of fine TiC, resulting in hardening. Further, when titanium is added in addition to boron, titanium hinders the binding of boron and nitrogen, and nitrogen binds with titanium. Therefore, the titanium content is preferably higher than N, where N is the N content by weight percentage in the composition. However, the Ti content should preferably remain at 0.2% or less, preferably 0.1% or less, and more preferably at most 0.05% to avoid the precipitation of coarse TiN precipitates. If no optional addition of Ti is made, Ti is present as an impurity with a content of at least 0.001%. * Higher than N, where N is the N content by weight percentage in the composition. However, the Ti content should preferably remain at 0.2% or less, preferably 0.1% or less, and more preferably at most 0.05% to avoid the precipitation of coarse TiN precipitates. If no optional addition of Ti is made, Ti is present as an impurity with a content of at least 0.001%.

[0065] Boron is added with 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%.

[0066] In one embodiment, Ni can be added in a content of up to 2%, generally at least 0.25%, preferably up to 0.5%, in order to reduce the susceptibility to delayed fracture by concentrating on the surface of the parts. In the case of not adding, Ni may be present in a low content of about 0.001% as an impurity. Depending on the manufacturing route used, the Ni content as an impurity can be as high as 0.25% (for example, when manufacturing steel with a high ratio of recycled scrap steel), or as high as 0.1% (for example, when using a lower level of steel scrap).

[0067] In order to obtain fine structures such as precipitation hardening and austenite grain size, niobium of up to 0.1% is optionally added. Nb further improves the ductility of the steel. When adding Nb, its content is preferably at least 0.01%. The Nb content is preferably up to 0.06% in order to avoid the formation of coarse (Ti,Nb)(C,N) precipitates.

[0068] Molybdenum can be added in a content of up to 0.65%. When Mo is added, its content is preferably at least 0.05%. Mo is preferably added together with Nb and Ti to form co-precipitates that are very stable at high temperatures. Mo may also be added to increase the toughness of the steel that acts as a grain boundary strengthener in the solid solution state. Optimal effects are obtained when the Mo content is included between 0.15% and 0.25%.

[0069] W may be added to increase the hardenability and hardening property of the steel by forming tungsten carbide. When adding W, its content is from 0.001% or more to 0.30% or less.

[0070] Sulfur, phosphorus and nitrogen generally exist in the steel composition as impurities.

[0071] The nitrogen content is generally at least 0.0005%. In order to prevent the precipitation of coarse TiN precipitates, the N content is generally at most 0.010%, preferably at most 0.005%.

[0072] In the case of excessive amounts, sulfur and phosphorus reduce ductility. Therefore, their contents are limited to 0.05% and 0.1% respectively.

[0073] In particular, the presence of S in the molten steel can lead to the formation of MnS precipitates that are harmful to the properties. Preferably, the S content is at most 0.01%, more preferably at most 0.005%. Achieving a very low S content, i.e., less than 0.0001%, is very costly and unprofitable. Therefore, generally the S content is 0.0001% or more.

[0074] Preferably, the phosphorus content is at most 0.05%, more preferably at most 0.02%. Achieving a very low P content, i.e., less than 0.0001%, is very costly. Therefore, generally the P content is 0.0001% or more.

[0075] The steel may be subjected to globularization treatment of sulfides with calcium, and the globularization of MnS has the effect of improving the bending angle. Therefore, the steel composition may contain at least 0.0001% and at most 0.005% of Ca.

[0076] The remainder of the steel composition is iron and impurities resulting from the processing process. As detailed above, the impurities resulting from the processing process may contain 0.25% or less of Cu, 0.05% or less of Sn, 0.03% or less of As, 0.03% or less of Sb and / or 0.03% or less of Pb.

[0077] The steel composition can be selected according to the desired mechanical properties, particularly with respect to strength and ductility.

[0078] In particular, when a tensile strength in the range of 950 - 1200 MPa is desired together with a bending angle higher than 75° (measured according to the VDA 238 - 100 bending standard of July 2020), in the whole part or at least one area of the part, the steel of the steel substrate or at least one area of the steel substrate is preferably, 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% 1.5% ≤ (C + Mn + Si + Cr) ≤ 2.7% 0.0035% ≤ Ti ≤ 0.072% 0.0002% ≤ B ≤ 0.004% 0.04% ≤ Nb ≤ 0.06% 0.044% ≤ (Nb + Ti) ≤ 0.09% 0.001% ≤ N ≤ 0.009% 0.0005% ≤ S ≤ 0.003% 0.0001% ≤ P ≤ 0.020% Ca ≤ 0.005%, having a composition according to a first preferred composition, comprising the balance consisting of iron and unavoidable impurities due to processing.

[0079] On the other hand, when a tensile strength of at least 1400 MPa is required, the steel of the steel substrate or at least one region of the steel substrate preferably has, 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% having a composition according to a second preferred composition, comprising the balance consisting of Fe and unavoidable impurities due to processing.

[0080] When a higher tensile strength of 1800 MPa or more is required, the composition of the steel substrate or at least one region of the steel substrate preferably contains, 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% According to a third preferred composition including the above, the balance of the composition consists of iron and unavoidable impurities resulting from processing.

[0081] The steel substrate of the hot-stamped coated steel part generally has a microstructure consisting of at least 60% martensite, at most 20% bainite, at most 5% ferrite, and at most 15% austenite by volume fraction.

[0082] The martensite fraction can be as high as 100%, and the bainite, ferrite, and austenite fractions can each be as low as 0%.

[0083] This description of the microstructure applies to most of the steel substrate, which means that this microstructure is present in at least 95% of the volume of the steel substrate, preferably the entire volume of the steel substrate.

[0084] The microstructure is determined by the following method: The test piece is cut from the hot-stamped coated steel part, polished as detailed below, and etched with Nital 2% (for 10 seconds) to reveal the microstructure. Thereafter, the cross-section is inspected with an optical microscope at a magnification of 500 times, and if it is necessary to distinguish martensite and bainite, it is inspected using a scanning electron microscope (SEM) (backscattered electron mode, magnification 500 times, EHT (electron high voltage) = 15.00 kV, scale 10 micrometers). The measurement of the volume fraction of each component (martensite, bainite, ferrite, austenite) is performed by image analysis by a method known per se.

[0085] In one embodiment, the austenite fraction is at most 5% by volume and / or the bainite fraction is at most 10% by volume.

[0086] In one embodiment, the microstructure consists of, by volume, at least 80% martensite, at most 10% bainite, at most 5% austenite, and at most 5% ferrite.

[0087] In a preferred embodiment, the microstructure is essentially martensite, i.e., it consists of, by volume, at least 95% martensite and at most 5% bainite and / or ferrite.

[0088] More preferably, the microstructure is completely martensite.

[0089] The hot-stamped coated steel part comprises an undeformed part and at least one deformed part.

[0090] In one embodiment, the hot-stamped coated steel part includes two or more undeformed parts.

[0091] In fact, during stamping, especially hot stamping, for manufacturing parts, one or more parts of the blank are not deformed, while other parts are deformed to reach the final non-flat geometry of the part. The non-deformed part or each non-deformed part has not been deformed during hot stamping and may not have been deformed even during previous cold pre-deformation of the blank, if any.

[0092] By non-deformed, it is to be understood that during the stamping process, the non-deformed part has undergone an equivalent deformation of at most 0.01.

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[0093] For example, the main deformations

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[0094] For example, when the above method cannot be applied because only a part of the fully formed part is available, or for very local deformation evaluation in specific regions such as edges, electron backscatter diffraction (EBSD) measurements combined with scanning electron microscope (SEM) observations can be performed. This depends on the correlation existing between the deformation and the local crystal orientation deviation. For example, the following reference gives an example of such measurements: "Kamaya M. Assessment of local deformation using EBSD: quantification of accuracy of measurement and definition of local gradient. Ultramicroscopy. 2011 Jul;111(8):1189-99.doi:10.1016 / j.ultramic.2011.02.004.Epub 2011 Feb 21.PMID:21763236".

[0095] Another methodology applicable to determine the principal deformation of the formed part is to measure the thickness of the deformed material within the forming area and compare it with the thickness of the non-deformed area.

[0096] The non-deformed part or each non-deformed part is the flat part of the part.

[0097] The non-deformed part or each non-deformed part is, for example, the flange of the part or the flat part located between two deformed parts of the part. As an example, a hat-shaped part can further include the flat top of the hat-shaped stamped part as the flat part.

[0098] Examples of such parts are schematically shown in FIG. 1.

[0099] FIG. 1 shows a hot-stamped coated steel part 1 welded to a flat part 2 by a plurality of spot welds 3. The spot welds 3 are located, in this example, on the flange 8 of the hot-stamped coated steel part 1.

[0100] The exemplary hot-stamped coated steel part 1 comprises a flat top 4, two first curved portions 5 (or radii) extending from two opposite longitudinal edges of the flat top 4, two side walls 6 each extending from a longitudinal edge of the first curved portion 6, two second curved portions 7 each extending outwardly from a longitudinal edge of the side wall 6, and two flat flanges 8 each extending from a longitudinal edge of the second curved portion 7 to the outer edge of the part 1, and is a hat-shaped part (or an "omega" shaped part). Thus, the flange 8 forms the edge of the part 1.

[0101] In this example, the flange 8 and the flat top 4 are flat non-deformed portions of the hot-stamped coated steel part 1.

[0102] The non-deformed portions can be distinguished from the deformed portions by observing cracks in their shape and / or in the coating of these portions.

[0103] In fact, as will be detailed below, the coating of the deformed portions contains wide cracks that are not present or are hardly present in the coating of the non-deformed portions.

[0104] Each non-deformed portion has not undergone deformation but has undergone the same thermal cycle as the deformed portions.

[0105] The hot-stamped coated steel part of the present invention not only includes at least one deformed part resulting from hot stamping, but also the entire hot-stamped coated steel part is different from the flat blank manufactured by austenitization and quenching without deformation in that it undergoes a thermal cycle by hot stamping, which is different from the thermal cycle received by the non-deformed steel sheet.

[0106] In particular, each non-deformed part has not undergone deformation, but has experienced the same thermal cycle as the deformed part in hot stamping during heating, transfer to the mold, and retention in the mold.

[0107] Therefore, in the hot-stamped coated steel part, the deformed part or each deformed part is deformed during hot stamping, and the non-deformed part or each non-deformed part has not been deformed but has received the same thermal cycle as the deformed part during hot stamping.

[0108] The hot-stamped coated steel part of the present invention obtained by the in-mold hot stamping process is also different from a sample manufactured by heating in a laboratory furnace or any technique without surface contact, such as a Gleeble machine. In fact, such a sample has no non-deformed part and has undergone uniaxial deformation (tensile deformation), but in hot stamping, the deformation is not uniform through the part and is three-dimensional. Furthermore, the thermal cycle received by the hot-stamped coated steel part is different from the thermal cycle received in a laboratory furnace without transfer or stamping in the mold.

[0109] The flat non-deformed part or each flat non-deformed part of the part is a part having a uniform thickness e of 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm pflat and having such thickness.

[0110] For example, if the flange consists of two regions having different thicknesses e pflat(1) and e pflat(2) each of these regions is a separate non-deformed part.

[0111] When the hot-stamped coated steel part has a uniform thickness, the thickness of the flat non-deformed part (or each non-deformed part) is the thickness e of the hot-stamped coated steel part P is equal to.

[0112] When the hot-stamped coated steel part has a variable thickness and includes two or more non-deformed parts, the thickness of the non-deformed parts is e Pflat(i) as shown, where i = 1...n are the indices associated with each non-deformed part, n is the number of non-deformed parts, and the thicknesses of the non-deformed parts may be different from each other or the same.

[0113] The thickness e pflat can be measured, for example, using a micrometer.

[0114] The hot-stamped coated steel part comprises a coating on at least the surface of the steel substrate.

[0115] The coating is an aluminum alloy coating.

[0116] This coating contains aluminum and iron, and preferably further contains silicon.

[0117] The coating preferably contains more than 50% aluminum.

[0118] For example, the coating contains 8 - 12 wt% Si and 3 - 5 wt% Fe, and the balance is Al and unavoidable impurities.

[0119] The coating results from the interdiffusion between the aluminum alloy precoating and the steel during hot stamping.

[0120] The coating comprises an interdiffusion layer located at the innermost of the coating, i.e., in contact with the steel substrate.

[0121] The interdiffusion layer generally has an Fe content of at least 80% and at most 95%, and an Al content of 4% to 20% with a maximum of 2% Si.

[0122] In one embodiment, the interdiffusion layer has a composition consisting of 86 - 95% Fe, 4 - 12% Al, and 0 - 2% Si.

[0123] The coating further comprises an outer layer extending from the diffusion layer to the surface of the coating.

[0124] The outer layer generally contains or consists of an intermetallic compound of Fe, Al, and optionally Si.

[0125] The outer layer may be composed of a single layer or itself may be composed of sub - layers of different intermetallic compounds.

[0126] For example, the outer layer consists of 1 - 4 sub - layers.

[0127] However, in the present invention, the inventors have found that the object of the present invention is achieved if, regardless of what the layers in the coating are, especially regardless of their composition and number, the linear density of cracks follows the conditions defined below.

[0128] In the hot - stamped coated steel part according to the present invention, the total thickness e of the coating coating and the thickness e of the interdiffusion layer IDL satisfy the following conditions. 40 ≦ E pc ≦ 80 wherein

Number

[0129] E pc If the value of E is less than 40, the coating may not sufficiently protect the part for its intended use, and excellent paint adhesion and spot weldability may not be guaranteed. Epc When the value of pc exceeds 80, the coating becomes brittle and dust may be generated.

[0130] In the hot-stamped coated steel part according to the present invention, the coating has 40 ≦ E pc ≦ 80 in at least one non-deformed part of the hot-stamped coated steel part, provided that the coating has a minimum linear density dC of cracks pflat depending on the thickness e min (e pflat ) or more, and is defined as follows.

Equation

[0131] In this equation,

Equation

[0132] Here, the "linear density of cracks" indicating the number of cracks per millimeter is, of course, not a volume density, but a linear density in the sense of measuring the number of cracks present in the cross-section of the coating over a given length in a direction parallel to the surface of the steel substrate.

[0133] Actually, the inventors conducted intensive research to solve the problem of improving spot weldability and paint adhesion. Surprisingly, contrary to the established idea that cracks are harmful to the properties of hot-stamped coated steel parts, it was discovered that when the linear density of cracks exceeds a threshold value, both spot weldability and paint adhesion increase dramatically.

[0134] The inventors have also found that, with respect to the linear density of cracks, the relevant part of the component, even though it has undergone the same thermal cycle as the deformed part, has not been deformed because it is located on the flange of the hot-stamped component or in the flat area between two deformed parts of the component (for example, the flat top of the hat-shaped component shown in FIG. 1).

[0135] In fact, in the deformed part, the coating is deformed during stamping, which, due to the difference in expansion between the substrate and the coating and depending on the temperature of the deformation and the applied deformation rate at each location, causes cracks in the coating. However, the inventors have found that having a large number of cracks in some deformed parts is not sufficient to guarantee such properties in at least other parts of the hot-stamped coated steel component, in terms of ensuring excellent spot weldability and excellent paint adhesion.

[0136] Preferably, the non-deformed part is located within the flange of the component, which is the area most likely to be spot welded, so it is particularly desirable to have excellent spot weldability (and excellent paint adhesion) in this part.

[0137] The inventors have further found that the threshold value of the linear density of cracks is not an absolute value but depends on the thickness of the non-deformed part, and that a higher linear density of cracks is required as the thickness of the non-deformed part decreases in order to achieve the desired spot weldability and paint adhesion.

[0138] As a result of these considerations, the inventors have found that when 40 ≦ E pc ≦ 80 for the coating of the component, excellent spot weldability and paint adhesion are achieved when the linear density of cracks in the coating in the non-deformed part is dC min or more.

[0139] Preferably, the linear density of cracks in the coating is at most 4 * dC min and preferably at most 3 * dCmin remains as it is.

[0140] Preferably, the hot-stamped coated steel part includes two or more non-deformed parts, and the linear crack density dC(i) in each non-deformed part is related to the thickness e pflat (i) of this non-deformed part.

Number

[0141] Therefore, each undeformed part has a thickness e pflat (i) of 0.6 mm to 3.5 mm, where i = 1...n is an index related to one of the non-deformed parts, n ≧ 2 is the number of non-deformed parts, and the thickness e pflat The linear crack density dC(i) of the cracks in the coating in each non-deformed part of (i) is

Number

Number

Number

[0142] This formula is valid whether the non-deformed parts are all of the same thickness or of different thicknesses.

[0143] The cracks in the coating extend at least 5 μm deep in a direction substantially perpendicular to the surface of the steel substrate (i.e., the interface between the steel substrate and the coating). The cracks have a width of less than 2 μm (in the direction parallel to the surface of the steel substrate).

[0144] Therefore, the cracks are different from possible gaps in the coating having a width exceeding 2 μm, cracks in deformed parts which may also have a width exceeding 2 μm, and voids or coating defects affecting the coating over a depth of less than 5 μm.

[0145] Furthermore, as described above, in the non-deformed part, the cracks generally have a maximum width of 1 μm. In particular, the average width of the maximum cracks, which are cracks having a width greater than 90% of the width of the cracks in the coating, is less than 1 μm. In other words, the average width of 10% of the cracks having the maximum width among the observed cracks is less than 1 μm. Here, the average indicates the average calculated from the widths of all the maximum cracks.

[0146] In contrast, in the deformed part, the coating has more cracks having a width greater than 1 μm, and the average width of the cracks can be as large as 2 μm or even 3 μm so that the average width of the cracks generally becomes greater than 1 μm. In any case, in the deformed part or each deformed part, the average width of the cracks having a width greater than 90% of the width of the cracks is greater than 1 μm.

[0147] Preferably, the coating of the non-deformed part does not have gaps or cracks having a width of 2 μm or more.

[0148] The linear density of the cracks is determined as the ratio of the number of cracks observed over the entire length of observation of at least 5 mm (in the direction parallel to the surface of the steel substrate) in the cross-section of the non-deformed part with a bright-field optical microscope to this entire length (i.e., the number of cracks divided by the entire length).

[0149] In particular, the linear density of the cracks in the coating is determined by observing the cross-sections of the coatings of two samples taken from the non-deformed part of a hot-stamped coated steel part with a 500-fold bright-field microscope over a plurality of fields of view such that the entire length of the field of view (in the direction parallel to the surface of the steel substrate) is at least 5 mm. Then, the linear density of the cracks is determined as the ratio of the number of observed cracks to the entire length of the field of view.

[0150] In fact, in order to ensure the targeted excellent spot weldability and excellent paint adhesion, it is not sufficient to have cracks with a line density of at least dC over only a very small part of the coating. min They further found that in order to achieve these properties, it is necessary to have a crack line density of at least dC measured over a length of at least 5 mm. min

[0151] The average width of the maximum cracks is determined by measuring the widths of all the cracks observed over the length of the observation, identifying the maximum cracks from these measurements, and calculating the average value of the widths of these maximum cracks.

[0152] Specifically, the above-mentioned coating characteristics are determined as follows.

[0153] First, samples are taken from the parts and prepared.

[0154] For that purpose, in the non-deformed part of the hot-stamped coated steel part, the part is cut at the center of its length to produce a sample of a suitable size of 20×30 mm.

[0155] The cutting process is carefully carried out using a hard iron cutting wheel (e.g., Struers 60A25) or, preferably, a micro-cutting device in order to avoid excessive stress that may damage the sample.

[0156] Then, the samples are washed (preferably in an ultrasonic bath) and dried with ethanol and compressed air.

[0157] Next, the sample is cold-mounted with resin. Cold mounting is selected instead of hot mounting because the gap between the resin and the sample is very small or non-existent. This is important because the gap can cause several preparation problems such as etching problems, scratches, or coating damage. The resin is preferably a liquid EpoFix(R) resin mixed with Epofix Hardener(R). The polymerization lasts for 10 hours. To protect the coating from surface damage during polishing, two protective metal sheets (guard plates) are provided on both sides of the sample. Each time, two samples are attached together to ensure that these two samples are subjected to the same polishing.

[0158] Next, the sample is carefully polished. Polishing is an important step in evaluating the coating characteristics because incorrect polishing can cause defects in the coating. These defects include the attachment of diamond particles, damage to the coating such as coating peeling, and more importantly, cracks in the coating, especially longitudinal cracks. Longitudinal cracks extending in a direction substantially parallel to the surface of the steel substrate may reflect preparation problems and must be avoided during polishing to ensure reliable results when counting the number of cracks.

[0159] Polishing is divided into three main stages: disk polishing, die diamond polishing, and oxide polishing. To remove polishing contamination, the sample must be washed and dried after each polishing step and checked with an optical microscope.

[0160] First, using a sample holder rotation speed of 150 RPM (revolutions per minute) and a disk rotation speed of 100 RPM, both rotating in the same direction, the sample is polished with a polishing disk of SiC paper P320 for 120 seconds. A force of 15 N is applied, and the sample is washed with water.

[0161] Next, 9μm diamond polishing is used to remove material from the surface without causing scratches or deformation. This step is important to ensure the flatness of the polished surface. The rotational speed is the same as in the previous step, but the sample rotation is opposite to the disk rotation. This stage is set for 300 seconds and a force of 20N is used. The polishing disk is damped by the lubricant in an optimal amount (0.5mL / 30 seconds) to ensure better material removal. The sample is washed with water.

[0162] The final step is oxide polishing using a colloidal silica solution (1mL / 5 seconds). This step continues for 90 seconds with the rotational speed of the sample holder being 150RPM and the rotational speed of the disk being 60RPM, both rotating in the same direction. The applied force is 20N. The sample is first washed with water and then with ethanol.

[0163] Next, the sample is etched with nital (2%, 5 - 10 seconds) to clarify the microstructure of the steel and the steel / coating interface.

[0164] The total thickness e of the coating coating and the thickness e of the interdiffusion layer IDL To determine these, the sample is imaged using a scanning electron microscope (SEM) (backscattered electron mode, magnification 500x, preferably WD (working distance) of 10mm, EHT (electron high voltage) = 15.00kV, scale 10 micrometers) to show at least a partial cross-section of the coating and the base steel.

[0165] From the image, the interdiffusion layer can be identified as the lightest layer closest to the steel substrate.

[0166] The total thickness of the coating (including the interdiffusion layer) and the thickness of the interdiffusion layer are measured at 5 spots spaced 15μm apart horizontally.

[0167] Next, the total thickness of the coating and the thickness of the interdiffusion layer are calculated as the average of the obtained values. Thus, the total thickness e of the coating coating and the thickness e of the interdiffusion layerIDL is the average thickness.

[0168] In addition, the composition of the interdiffusion layer can be determined as follows.

[0169] Observe the sample with a scanning electron microscope (SEM) to display the cross-section of the coating and at least a part of the base steel.

[0170] By energy dispersive spectroscopy (EDS), at five different horizontal locations, two spots spaced vertically within the interdiffusion layer are considered, and the composition is determined at ten spots.

[0171] Next, the composition of the interdiffusion layer is calculated as the average of the obtained values.

[0172] If necessary, the composition of the entire coating can be measured by EDS.

[0173] The determination of the line density of cracks in the non-deformed part of the coating is carried out as follows.

[0174] Two samples attached to the same resin are imaged with a bright-field optical microscope (magnification 500x), showing the cross-section of the coating and at least a part of the steel substrate at multiple locations of the samples.

[0175] Next, for each sample, ten separate non-overlapping fields of view are randomly selected and observed. Each field of view has a length of at least 250 μm (in the direction parallel to the surface of the steel substrate).

[0176] Therefore, the total length of the observation, which is the total length of the fields of view of the two samples (in the direction parallel to the surface of the steel substrate), is 5 mm or more (> 250 μm * 2 * × 10).

[0177] In each field of view, first, it is confirmed that there are no longitudinal cracks extending in a direction substantially parallel to the surface of the steel substrate. If there are longitudinal cracks, the presence of such cracks means that the sample preparation (cutting, polishing) was erroneously performed at this location, so the field of view is ignored and replaced.

[0178] Next, in each field of view, cracks extending in the direction of the steel substrate are identified from the top surface of the coating, substantially perpendicular to the surface of the steel substrate.

[0179] To distinguish between possible gaps and cracks 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 a magnification of 500 times, but if necessary, the width of the cracks can be measured as detailed below. Further, to distinguish cracks from voids or coating defects, the length of the cracks is measured, and only cracks with a depth of at least 5 μm (in a direction substantially perpendicular to the surface of the steel substrate) are considered cracks.

[0180] The number of cracks is counted in each field of view, and the total number of cracks in the 20 observed fields of view is calculated.

[0181] Next, the linear density of the cracks in the coating is determined as the ratio of the total number of cracks to the total length of the fields of view of the two samples.

[0182] The determination of the average width of the largest crack, or more generally, the width of any crack, can be performed as follows.

[0183] The cross-section used to evaluate the linear density of the cracks is observed with a bright-field microscope at a magnification of 500 times using a camera with a resolution of 3072 × 2048 pixels, and the image is displayed such that 1 pixel of the camera is displayed as 1 pixel on the image. The magnification during imaging is 3020 times.

[0184] The width of each crack in all fields of view is determined.

[0185] Therefore, for each crack, the width is measured at three locations located at the center of the crack and at a distance of 2 μm on both sides of the center. The average value of the three widths is determined as the width of the crack.

[0186] Next, the maximum width, i.e., cracks larger than 90% of the width of the cracks in the coating, are identified. Thus, these cracks are 10% of the cracks having the maximum width. Then, the average width of these cracks is calculated as the sum of their individual widths divided by their number.

[0187] If the number of cracks is such that 90% of the number of cracks is not an integer, the 90% result based on the number of cracks is rounded to the nearest integer (e.g., 90% of 108 cracks is considered 97 cracks, and 90% of 135 cracks is considered 122 cracks).

[0188] The hot-stamped coated steel part according to the present invention has a welding range exceeding 1 kA when measured according to standard SEP 1220-2 (2011), at least in the non-deformed part.

[0189] The hot-stamped coated steel part further has excellent paint adhesion in that, when subjected to dry and wet paint adhesion tests according to standard ISO 2409:2013, at least in the non-deformed part, the dry paint adhesion is strictly less than 1 and the wet paint adhesion is 1 or less.

[0190] Next, a method for manufacturing a hot-stamped coated steel part according to the present invention is disclosed.

[0191] This method generally includes providing a steel blank having an average thickness of 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm. This thickness generally corresponds to the average thickness e of the hot-stamped coated steel part that is to be manufactured from the blank. P is the same.

[0192] The blank has, on at least one surface, an aluminum or aluminum alloy pre - coating, and the pre - coating has an average thickness in the range of 19.91 μm to 40 μm.

[0193] Preferably, the blank has, on each of its two main surfaces, an aluminum or aluminum alloy pre - coating, and the pre - coating has an average thickness in the range of 19.91 μm to 40 μm.

[0194] This pre - coating can be aluminum or an aluminum alloy (containing more than 50% aluminum).

[0195] Advantageously, the pre - coating is an aluminum - silicon alloy containing, by weight, 7% - 15% silicon, 2% - 4% iron, and optionally 0.0015% - 0.0030% calcium, with the balance being aluminum and unavoidable impurities resulting from processing.

[0196] Preferably, the pre - coating contains, by weight, 8% - 11% Si, 2% - 4% Fe, optionally 0.0015% - 0.0030% Ca, and the balance is Al and impurities resulting from smelting.

[0197] The pre - coating is generally obtained by hot - dip plating in an Al or Al alloy bath.

[0198] In one embodiment, the blank is a monolithic blank, i.e., a blank consisting of a single sub - blank obtained by cutting a pre - coated steel sheet.

[0199] A monolithic blank is, for example, a tailor - rolled blank, i.e., a blank having a variable thickness obtained by differential rolling during the steel sheet manufacturing process.

[0200] In another embodiment, the blank is a tailor-welded blank produced by assembling a plurality of blanks cut from different pre-coated steel sheets known as sub-blanks, in particular by welding them together, for example by laser welding.

[0201] In one embodiment, the blank has a uniform thickness of 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm.

[0202] In another embodiment, the blank is a tailor-rolled blank or a tailor-welded blank and the blank has a variable thickness.

[0203] In this case, the blank consists of two or more regions each having a different thickness e of 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm. Bi For a given thickness e Bi each region of the blank having that thickness e Pi corresponds to the region of the final hot-stamped coated steel part having that thickness e.

[0204] In any case, the blank comprises a flat portion having a uniform thickness e defined to be the non-deformed portion of the hot-stamped coated steel part after the hot stamping process. The thickness e Bflat is also 0.6 mm to 3.5 mm, preferably 0.7 mm to 3.0 mm. Bflat That is to say, the thickness e

[0205] is the thickness of the portion of the blank that is converted into the corresponding non-deformed portion of the hot-stamped coated steel part during hot stamping. Bflat The thickness of the portion of the blank

[0206] is equal to the thickness e Bflat of the corresponding non-deformed portion of the part. pflat That is, the thickness e of the portion of the blank is equal to the thickness e of the corresponding non-deformed portion of the part.

[0207] In one embodiment, the blank is defined to be two or more non-deformed portions of the hot-stamped coated steel part after the hot stamping process, each having a uniform thickness e Bflat(i) (where i = 1...n, and n is the number of such portions).

[0208] Each thickness e Bflat(i) is in the range of 0.6 mm to 3.5 mm, preferably in the range of 0.7 mm to 3.0 mm. Further, each thickness e Bflat(i) is equal to the thickness e Pflat(i) of the corresponding non-deformed portion of the part.

[0209] When the thickness of the blank is uniform, the thickness e Bflat , or each thickness e Bflat(i) if applicable, is equal to the thickness e B of the blank.

[0210] The blank, or if the blank is a tailor-welded blank, each sub-blank is preferably made of steel having the composition disclosed above, particularly according to the first, second or third preferred composition. The sub-blanks may have the same steel composition or different steel compositions.

[0211] The blank or each sub-blank is manufactured, for example, as follows.

[0212] Semifinished products in the form of slabs, thin slabs or / and ingots that can be further hot-rolled are preferably provided with the steel composition described above. The thickness of this semifinished product is typically in the range of 50 to 250 mm.

[0213] If necessary, this semifinished product is generally heated to a temperature in the range of 1100 °C to 1300 °C and then hot-rolled at a finishing rolling temperature preferably in the range of 880 to 950 °C to obtain a hot-rolled steel sheet. The hot-rolled steel sheet is coiled at a temperature Tc below 750 °C, generally above the Ms temperature of the steel.

[0214] At this stage, the thickness of the hot-rolled steel sheet may be in a typical range of 1.5 to 4 mm. Depending on the thickness of the hot-rolled steel sheet and the desired thickness of the sheet, the steel sheet may be pickled under normal conditions, further cold-rolled, or directly annealed in the process described below.

[0215] If the blank to be manufactured is a tailor-rolled blank, the steel sheet can be manufactured by differential rolling (or continuous flexible rolling), i.e., a process in which the sheet thickness obtained after rolling is variable in the rolling direction in relation to the load applied to the sheet through the rollers during the rolling process.

[0216] After hot rolling or cold rolling, in preparation for coating, the steel sheet is annealed at a temperature typically included between Ac1 and Ac3, generally in the range of 700 to 850 °C, and if cold rolling has been performed, the crystal grains are recrystallized. The sheet is then generally hot dip galvanized in an Al or Al alloy bath at a temperature of about 670 to 680 °C (the exact temperature depends on the composition of the bath).

[0217] A preferred precoating is Al-Si, which is obtained by hot dip galvanizing the sheet in a bath containing 7% to 15% Si, 2% to 4% Fe, and optionally 0.0015% to 0.0030% Ca by weight, the balance being Al and impurities resulting from steelmaking.

[0218] Preferably, the bath contains 8% to 11% Si, 2% to 4% Fe, and optionally 0.0015% to 0.0030% Ca by weight, the balance being Al and impurities resulting from steelmaking.

[0219] Thereafter, the precoated steel sheet is cooled to room temperature.

[0220] The precoated steel sheet is cut to obtain a blank (or sub-blank), and the geometry of the blank is related to the final geometry of the hot-stamped coated steel part.

[0221] If the blank from which the part is manufactured is a tailor-welded blank, the sub-blanks manufactured as described above are welded to each other.

[0222] In one embodiment, the sub-blanks have the same composition. In another embodiment, the compositions of the sub-blanks are different. This applies especially when different mechanical properties are required at different locations of the final part. As an example, the composition of the steel in the first sub-blank is selected from among the three preferred compositions described above, and the composition of the second sub-blank is selected from among the two other preferred compositions or has a different specific composition within the same preferred composition as the first sub-blank.

[0223] In one embodiment, the sub-blanks have the same uniform thickness. In another embodiment, the sub-blanks have different thicknesses.

[0224] Optionally, before the heating and hot stamping process in the mold, the blank can be cold formed to obtain a pre-deformed blank. This cold pre-deformation can reduce the amount of deformation in the subsequent hot stamping process. In any case, after the hot stamping process, the part of the blank that is to become the non-deformed part of the hot-stamped coated steel part is not deformed during such cold pre-deformation. Thus, this part of the blank remains flat. On the other hand, the part of the blank that is then deformed by hot stamping is only partially deformed during cold pre-deformation. As a result, it is ensured that the cold pre-deformation does not affect the target coating properties.

[0225] Next, the blank (flat or cold pre-deformed) is heated in a furnace to a temperature T in the range of 850 to 970 °C. heat is heated.

[0226] The heating is carried out in a first heating stage in which the temperature of the blank is maintained below the melting temperature T of the pre-coating, and in a second heating stage in which the temperature of the blank is at least the melting temperature T of the pre-coating and the hot temperature T. melt of the pre-coating and in a second heating stage in which the temperature of the blank is at least the melting temperature T melt of the pre-coating and is above, and the hot temperature T heatIt includes a second heating stage that rises up to

[0227] The heating means is not limited and can be based on radiation, conduction, induction, or resistance.

[0228] The time consumed by the blank between each of these two stages can be adjusted by controlling the furnace, in particular, by using a furnace having different sections with independent settings respectively regarding, for example, power and temperature. As a result, the heating rates in each of these sections can be adjusted independently of each other. For example, if a high heating rate is desired in the first heating stage, the first section of the furnace can be set to a high temperature and high power to ensure such rapid heating. Then, when a low heating rate up to the final heating temperature T heat is desired, a plurality of sections can be used to ensure a longer time exceeding the melting temperature T melt of the precoating, and the temperature of the furnace can be slightly increased between the sections. Conversely, when a short heating time is desired from the melting temperature T melt of the precoating and the heating temperature T heat , the sections of the furnace can be set to a high temperature, and the blank can be transferred to the section of the target heating temperature T heat before reaching this temperature.

[0229] Furthermore, those skilled in the art know how to determine the melting temperature of a precoating whose composition is known, for example, by using a ternary phase diagram.

[0230] Then, the heated blank is held at the heating temperature T heat to obtain a fully austenite structure in the steel.

[0231] Preferably, the total residence time in the furnace including heating and holding is in the range of 1.5 minutes to 15 minutes.

[0232] Subsequently, the heated blank is transferred to the mold (or hot stamping press). The transfer time is preferably at most 15 seconds, more preferably at most 10 seconds or at most 8 seconds.

[0233] Subsequently, the mold is closed to punch the blank into a part, and the temperature of the blank when the mold is closed is recorded as T close and is recorded as such.

[0234] Preferably, the elapsed time between the transfer of the heated blank to the mold and the closing of the mold is less than 8 seconds.

[0235] Heating and holding cause interdiffusion between the pre - coating and the steel substrate. In particular, during heating and holding, iron diffuses from the steel substrate towards the coating, and aluminum diffuses from the coating towards the steel substrate, and this interdiffusion results in the formation of an interdiffusion layer of the coating.

[0236] Furthermore, depending on the composition of the pre - coating, one or more intermetallic phases can be formed by interdiffusion on top of the interdiffusion layer (and thus within the outer layer of the part) in the form of a solid solution.

[0237] According to the present invention, heating, holding, transfer, closing of the mold, and cooling within the mold are controlled as a function of the thickness e min of the flat portion of the blank that is converted into the non - deformed portion during hot stamping in order to achieve a crack linear density dC Bflat in the coating of the non - deformed portion of the hot - stamped steel part of dC or more.

[0238] In particular, the inventors have found that the relevant factors for achieving the desired crack linear density in the coating of the non - deformed portion, i.e., dC min or more, are not the time spent in the furnace, but rather the temperature T close of the blank when the mold is closed, in combination with the time t melt spent by the blank above the melting temperature T Mfound that it is as follows.

[0239] This time t M is the heating in the above-mentioned second stage (the above T melt ), the holding time T heat at the heating temperature T heat , the transfer time, and the time (before and after stamping) consumed by the blank in the mold until the temperature of the formed blank reaches T melt during cooling.

[0240] This time t M is determined and controlled during the hot stamping process by monitoring the temperature of the blank during heating and adjusting the time spent exceeding T melt as detailed above, controlling the holding time t heat at the heating temperature T heat , and controlling the time consumed by the blank during transfer to the mold and during cooling in the mold to T melt .

[0241] The inventors have found that in order to obtain a crack line density of at least dC min , the temperature T close must be in the range of 720 to 820 °C, and the time t melt spent exceeding the melting temperature T M of the precoating must be in the range between the minimum time t Bflat and the maximum time t heat which depends on the thickness e melt of the flat part of the blank defined to be the non-deformed part, the heating temperature T Mmin , and the melting temperature T Mmax of the precoating.

[0242] The minimum time t Mmin and the maximum time t Mmax are defined as follows.

Equation

[0243] In these equations, t Mmin and t Mmax are expressed in seconds, T heat represents the heating temperature of the blank in °C, T melt represents the melting temperature of the pre - coating in °C, and e Bflat represents the thickness of the flat part of the blank, determined so as not to be deformed during hot stamping and thus to be the non - deformed part of the component, in mm.

[0244] In fact, the inventors have found that by satisfying these conditions, after hot stamping and cooling, a hot - stamped coated steel component can be achieved in which the linear density of cracks in the coating of the non - deformed part is dC min or more.

[0245] On the other hand, when the time t M does not satisfy the above - mentioned relationship, and / or when the temperature T close is above 820 °C and below 720 °C, the linear density of cracks becomes insufficient, and it is impossible to achieve both excellent spot weldability and excellent paint adhesion.

[0246] In particular, when it is less than the minimum time t Mmin , the linear density of cracks is insufficient, and neither the paint adhesion nor the spot weldability is satisfactory.

[0247] On the other hand, when it exceeds the maximum time t Mmax , the linear density of cracks becomes insufficient, and even if the paint adhesion can be improved by increasing the time exceeding the melting temperature, the spot weldability becomes too low.

[0248] Naturally, [Number] and [Number] Considering that both depend on the heating temperature T heat and the time t M not only, but also the heating temperature T heat and the time t M are

Number

Number

[0249] In one embodiment, a blank having a uniform thickness e B is equal to the thickness e of the portion of the blank defined to be the non-deformed portion of the part, and thus, t Bflat and t Mmin and t Mmax are as follows.

Number

Number

[0250] If the blank has a variable thickness, preferably, depending on the required characteristics, the time t M is between the time t Bi for an additional portion / region of the blank having a thickness e Mmin (e Bi ) and the time t Mmax (e Bi ) and may be adjusted to be included.

[0251] In that case, the values of t Mmin and t Mmax can be calculated for each of the portions considered as a function of the thickness of this portion of the blank e Bi . Next, the time t M is the maximum value of the calculated minimum time t Mmin and the calculated maximum time t Mmaxis selected between the minimum value. [Mathematics]

[0252] t Mmin and t Mmax The values of are functions of the increasing thickness, which can be simplified as follows. [Mathematics]

[0253] In other words, if it is desired to ensure a crack linear density exceeding the minimum crack linear density dC in two or more parts of the component, manufactured from two or more parts of the blank having different thicknesses min the time t M is in the range between the minimum time t Mmin defined for the thickest part and the maximum time t Mmax defined for the thinnest part.

[0254] Preferably, when hot stamping is performed to form a hot-stamped coated steel component including two or more non-deformed parts by hot stamping the blank so that two or more parts having a thickness e Bflat (i) for i = 1...n are not deformed M the time t Mmin is in the range between the minimum time t Mmax required for the thickest part and the maximum time t [Mathematics]

[0255] In that case, it is guaranteed that all considered flat parts of the blank corresponding to the non-deformed parts of the component are manufactured with a time t Bflat(i) in the range between the minimum time t Mmin corresponding to their actual thickness e Mmax and the maximum time t M .

[0256] More preferably, the blank has a minimum thickness e Bmin and a maximum thickness e Bmax and the time t M is in the range between the minimum time t Bmax required for the maximum thickness e Mmin and the maximum time t Bmin required for the minimum thickness e Mmax and. [Number]

[0257] In that case, all regions of the blank are guaranteed to be hot stamped at a time t Mmin in the range between the minimum time t Mmax corresponding to their actual thickness and the maximum time t M .

[0258] Preferably, the temperature T close of the blank when the mold is closed is at least 740 °C.

[0259] In one embodiment, the temperature T close is at most 800 °C.

[0260] In particular, a temperature T close below 800 °C makes it possible to achieve a crack linear density of at most 4 * dC min and reduces the risk of powdering.

[0261] The heated blank is hot stamped in the mold and mold quenched.

[0262] 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.

[0263] The non-deformed part of the blank becomes the non-deformed part of the hot-stamped coated steel part obtained by the process.

[0264] The deformed portion of the blank forms the deformed portion of the hot-stamped coated steel part.

[0265] The mode and amount of deformation in the deformed portion vary from place to place due to the geometry of the final part and the forming tool. For example, some areas may be expanding, while other areas are constrained and deformed. Whatever the deformation mode outlined above, the equivalent deformation

Number

Number

Number

Number

Number

[0266] In the undeformed portion, the equivalent deformation is at most 0.01.

[0267] Next, the hot-stamped blank is held in the mold to ensure an appropriate cooling rate and avoid part distortion due to shrinkage and phase transformation.

[0268] The hot-stamped blank is cooled mainly by conduction due to heat transfer with the mold. The mold may include a coolant circulation to increase the cooling rate or a heating cartridge to decrease the cooling rate. Thus, the cooling rate can be adjusted by the implementation of such means.

[0269] The hot-stamped blank is cooled to a temperature below 400 °C.

[0270] The applicable cooling rate depends on the composition of the steel as well as the desired structure and mechanical properties.

[0271] Temperature T heat The average cooling rate from the temperature T to 400 °C (including cooling during transfer and cooling in the mold) is generally at least 27 °C / second, preferably at least 50 °C / second, and usually less than 200 °C / second.

[0272] Preferably, the blank is cooled in the mold at an average cooling rate of at least 30 °C / second, more preferably at least 50 °C / second, from the temperature of the blank when the mold is closed to 400 °C.

[0273] For example, when the steel has a composition according to the above first preferred composition, the hot-stamped blank is preferably first cooled at a first average cooling rate in the range of 40 - 360 °C / second in a temperature range between 750 - 450 °C. In this range, transformation from austenite to martensite, and in some cases bainite, occurs. In a further step, the hot-stamped blank is cooled at an average cooling rate of 15 - 150 °C / second, slower than the first cooling rate, in a temperature range including 450 °C to 250 °C.

[0274] When the steel has a composition according to the second preferred composition, the hot-stamped blank is preferably cooled in the mold at an average cooling rate of at least 30 °C / second from the outlet of the furnace to a temperature below 400 °C in order to achieve a structure consisting essentially of martensite.

[0275] When the steel has a composition according to a third preferred composition, the hot-stamped blank is preferably cooled in a mold to obtain a structure consisting of martensite or consisting of martensite and bainite.

[0276] In any case, the hot-stamped blank is cooled in a mold to a temperature below 400 °C and then to room temperature to obtain a hot-stamped coated steel part.

[0277] Examples Steels having compositions A, B, C and D according to Table 1, expressed in weight percent, are provided in the form of blanks cut from a pre-coated steel sheet having a uniform thickness e B The blanks were coated on both sides with an Al-Si precoating having a composition containing 8% - 11% Si, 2% - 4% Fe, the balance being Al and impurities resulting from the smelting. All the precoatings had a melting temperature T

[0278] of 577 °C. melt The precoating thickness was adjusted in the range of 19.91 - 40 μm in each case and the precoating thickness is reported in Table 2 below together with the sheet thickness e

[0279] B

[0280] [Table 1]

[0281]

[0282] The balance of the composition is iron and unavoidable impurities (unavoidable impurities including Cu whose content is reported above).

[0282] These pre-coated steel sheets were cut into blanks.

[0283] Then, the blank is heated in a furnace at a temperature T heat and T heatIt was maintained at [temperature] and then transferred into the mold within 8 seconds.

[0284] Next, the blank was hot stamped in the mold to produce a part having a deformed portion and a flat non-deformed portion. In each case, the temperature T of the blank close was 750 °C when the mold was closed.

[0285] Next, the hot-stamped blank was cooled in the mold to a temperature below 400 °C, then removed from the mold and cooled to room temperature to obtain a hot-stamped coated steel part.

[0286] Heating, holding, transferring, and cooling in the mold were adjusted for each steel composition to reach various times t melt exceeding the melting temperature of the precoating T M was reached.

[0287] For each example, the time t melt consumed by the blank exceeding the melting temperature T of the precoating M was reported in Table 2 and compared with the minimum time t Mmin and the maximum time t Mmax The minimum time t Mmin and the maximum time t Mmax were calculated using the above formula from the thickness e B of the blank, which is equal to the thickness of the portion of the blank where the non-deformed portion occurs.

[0288]

Table 2

[0289] Next, the hot-stamped blank was cooled in the mold to a temperature below 400 °C to obtain a hot-stamped coated steel part having a martensite structure.

[0290] The hot-stamped coated steel parts have a uniform thickness e B equal to the thickness e P of the blank from which these parts were manufactured (and eP = e Pflat so as to become).

[0291] Samples were taken from the non-deformed parts of each component and prepared as described above.

[0292] The total thickness e of the coating coating and the thickness e of the interdiffusion layer IDL were determined using the protocol disclosed above and the linear density of cracks in the coating.

[0293] The linear density of cracks in the coating of the non-deformed parts was determined in each case by observing 10 fields of view for each of the two samples, and the total length of the observations for the two samples was 5.29 mm.

[0294] The measured total thickness e of the coating coating and the thickness e of the interdiffusion layer IDL From, E determined according to the above formula pc The values of are reported in Table 3 below.

[0295] The linear density of cracks dC is the minimum linear density of cracks dC min Also reported in Table 3 together with.

[0296] The paint adhesion was evaluated as follows for each part.

[0297] Samples were taken from the non-deformed parts of each component.

[0298] The samples were first degreased and then washed at 55 °C for 6 minutes using Gardoclean(R) 5176 and a surfactant. Refining was carried out using Gardolene(R) ZL6.

[0299] The phosphate treatment step was achieved by immersing the samples in a bath containing a solution of Gardobond(R) R24 TA and an additive at 50 °C for 3 minutes.

[0300] Next, the sample was immersed in a bath containing an aqueous solution comprising Pigment Paste(R)W9712-N6 and Resin blend(R)W7911-N6 of PPG Industries, and a 20-μm e-coating layer was deposited by applying a nominal voltage with a voltage rise time of 30 seconds for a total of 180 seconds at 30°C. Subsequently, the sample was wiped and cured in an oven at 175°C for 30 minutes.

[0301] Next, a dry paint adhesion test was performed on the first set of samples by cross-hatching the e-coating layer with a cutter (scratches at 1-mm intervals), peeling off the e-coating layer with an adhesive tape (460 N / m), and evaluating the amount of the removed e-coating layer visually according to ISO 2409:2013. Here, 0 means excellent, that is, in other words, little or no paint was removed, and 5 means very poor, that is, in other words, a large amount of paint (>65%) was removed.

[0302] Next, a wet paint adhesion test was performed on the second set of samples. The samples were immersed in a sealed box containing pure water at a temperature of 50°C for 10 days.

[0303] After immersion, the surface of the e-coating was cross-hatched with a cutter (scratches at 1-mm intervals), the e-coating was peeled off with an adhesive tape (460 N / m), and the amount of the removed e-coating was evaluated visually according to ISO 2409:2013. Here too, 0 means excellent, that is, in other words, little or no paint was removed, and 5 means very poor, that is, in other words, a large amount of paint (>65%) was removed.

[0304] Furthermore, the spot weldability was evaluated by determining the welding range of each sample according to the standard SEP 1220-2 (2011).

[0305] For Examples 7 to 12 having a thickness of 1.2 mm, the following parameters were used. - Electrode: F1-16-20-5.5 - Welding strength: 4 kN - Welding current: Medium-frequency direct current - Welding time: 1 pulse, 320 ms - Holding time: 200 ms

[0306] For Examples 1 to 6 having a thickness of 1.8 mm, the following parameters were used. - Electrode: F1-20-20-8 - Welding strength: 5 kN - Welding current: Medium-frequency direct current - Welding time: 3 pulses of 200 ms (pause time 40 ms) - Holding time: 300 ms

[0307] The dry and wet paint adhesion and welding range evaluated in this way are reported in Table 3.

[0308]

Table 3

[0309] Referring to Tables 2 and 3, Examples 2, 5, 8, and 11 were manufactured using the method according to the present invention, and the linear density of cracks is the minimum linear density dC of cracks min or more.

[0310] Figure 2 shows a cross-section of the coating of Example 2 as seen in one of the fields of view observed to evaluate the linear density of cracks. In this image, which is contrasted for illustrative purposes only, a plurality of cracks extending from the surface of the coating towards the steel substrate are observed.

[0311] Of course, as described above, the linear density of cracks in Example 2 was not determined based on this single field of view, but rather by observing 20 fields of view to confirm that the linear density of cracks over a sufficient observation length is dC min (i.e., 15.5 cracks / mm) or more.

[0312] In the examples of the present invention, it was observed that the cracks were almost uniformly distributed in the coating of the flat non-deformed part, which was not observed in the case of Comparative Examples 1, 3, 4, 6, 7, 9, 10, and 12 described below.

[0313] As a result, Examples 2, 5, 8, and 11 all have a dry paint adhesion of less than 1, actually 0, meaning that the paint was not removed at all or hardly removed during the test, excellent dry paint adhesion, excellent wet paint adhesion of up to 1, and excellent spot weldability, with a welding range exceeding 1 kA.

[0314] Therefore, Examples 2, 5, 8, and 11 min demonstrate that the hot-stamped coated steel parts according to the present invention having a crack linear density of dC or more achieve excellent paint adhesion and spot weldability.

[0315] In contrast, Examples 1, 4, 7, and 10 were produced at a time t that was too short and exceeded the melting temperature of the pre-coating. M As a result, these examples have a crack linear density lower than dC. min An example of the field of view of the cross-section of the coating of the flat non-deformed part of Example 1 is shown in FIG. 3. In this field of view, there are no cracks in the coating. As a result, Examples 1, 4, 7, and 10 have low paint adhesion because the dry paint adhesion is always 1 and the wet paint adhesion is higher than 1 in all cases.

[0316] Examples 3, 6, 9, and 12 also have a crack linear density lower than dC. min For Examples 3 and 6, even though the dry paint adhesion was excellent and the wet paint adhesion was good, the spot weldability was significantly reduced compared to the parts according to the present invention because a long time was spent exceeding the melting temperature of the coating. Therefore, these examples do not achieve the object of the present invention.

[0317] Furthermore, Example 12 has the minimum crack linear density dC to achieve this object.min demonstrate that it actually depends on the thickness. In particular, this example shows that having a crack linear density of, for example, 15.5 / mm as in Example 12, as compared to Examples 2 and 5, is not sufficient when the part has a thickness of 1.2 mm, whereas in contrast, it enables achieving the targeted properties for a higher thickness of 1.8 mm as in Examples 2 and 5.

[0318] Thus, the example confirms that excellent spot weldability and excellent paint adhesion are achieved at least in this part if the non-deformed part of the part has a crack linear density such that its coating has min dC or more.

[0319] Furthermore, the example demonstrates that by adjusting the process so that the temperature T exceeds the melting temperature of the coating and the time spent by the blank is appropriately controlled, a sufficient desired crack linear density, and thus the desired properties, can be achieved. close

[0320] Therefore, the steel parts manufactured according to the present invention can be advantageously used for manufacturing chassis or white body parts or suspension arms for motor vehicle vehicles.

Claims

1. A hot stamped coated steel part comprising a steel substrate and an aluminum alloy coating on at least one surface of the steel substrate, the coating comprising an interdiffusion layer and an outer layer proceeding outwardly from the steel substrate, the coating having a total thickness e ｃｏａｔｉｎｇ and the thickness of the interdiffusion layer e ＩＤＬ is the following condition, 40≦E ｐｃ ≦80 In [0010] Fulfilling e ＩＤＬ is the thickness of the interdiffusion layer in μm, e ｃｏａｔｉｎｇ denotes the total coating thickness in μm, The hot stamped coated steel parts have a thickness of 0.6 mm to 3.5 mm e Ｐｆｌａｔ and at least one deformed portion, wherein the linear density dC of cracks in the coating in the non-deformed portion is a minimum linear density dC of cracks defined as follows: ｍｉｎ (e Ｐｆｌａｔ ) and [0025] Wherein dC and dC ｍｉｎ (e Ｐｆｌａｔ ) is expressed in number of cracks per mm, e ｐｆｌａｔ indicates the thickness of the non-deformed part in mm, Hot stamped coated steel parts.

2. The linear density of cracks in the coating in the non-deformed part, dC, is 4 ＊ dC ｍｉｎ (e Ｐｆｌａｔ 2. The hot stamped coated steel part of claim 1, wherein the thickness of the coated steel part is equal to or less than 1 mm.

3. Hot stamped coated steel parts have a uniform thickness e ranging from 0.6 mm to 3.5 mm. Ｐ 3. The hot stamped coated steel part according to claim 1 or 2, having

4. The hot stamped coated steel parts have a variable thickness, and the hot stamped coated steel parts have a discrete thickness e ranging from 0.6 to 3.5 mm. Ｐｉ The hot stamped coated steel part comprises two or more regions having an average thickness e in the range of 0.6 mm to 3.5 mm. Ｐ 3. The hot stamped coated steel part according to claim 1 or 2, having

5. The hot stamped coated steel parts each have a thickness of 0.6 mm to 3.5 mm. ｐｆｌａｔ (i), and the linear density of cracks in the coating in each of the non-deformed portions, dC(i), is ｍｉｎ (e ｐｆｌａｔ (i)) or more, [0030] In the formula, e ｐｆｌａｔ (i) denotes the thickness in mm of the non-deformed part considered, i = 1...n, n >= 2, and dC(i) and dC ｍｉｎ (e ｐｆｌａｔ (i)) are expressed in number of cracks per mm, respectively, for thickness e Ｐｆｌａｔ 5. The hot stamped coated steel part according to any one of claims 1 to 4, exhibiting a linear density of cracks in the coating of the considered non-deformed part of (i) and a minimum linear density of cracks.

6. 6. The hot stamped coated steel part according to any one of claims 1 to 5, wherein in cross section, the cracks in the coating of the non-deformed portions extend from a top surface of the coating towards the steel substrate in a direction generally perpendicular to the surface of the steel substrate for a depth of at least 5 μm, and each crack has a width in a direction generally parallel to the surface of the steel substrate of less than 2 μm.

7. 7. The hot stamped coated steel part according to any one of claims 1 to 6, wherein the linear density of cracks is determined as the ratio of the total number of cracks observed in several cross sections of the non-deformed part with 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 to the total observation length.

8. 8. The hot stamped coated steel part according to any one of claims 1 to 7, wherein the hot stamped coated steel part is a monolithic part or a hot stamped welded part consisting of at least two hot stamped coated sub-parts and at least one hot stamped weld joining the hot stamped coated sub-parts together.

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

10. The steel in the hot stamped coated steel part or in each hot stamped coated subpart comprises, in weight percent: 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% and having a chemical composition comprising 10. The hot stamped coated steel part according to claim 8 or 9, wherein the balance of the composition consists of iron and inevitable impurities resulting from processing.

11. The steel in the hot stamped coated steel part or in at least one hot stamped coated sub-part comprises, in weight percent: 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% 1.5%≦(C+Mn+Si+Cr)≦2.7% 0.0035%≦Ti≦0.072% 0.0002%≦B≦0.004% 0.04%≦Nb≦0.06% 0.044%≦(Nb+Ti)≦0.09% 0.001%≦N≦0.009% 0.0005%≦S≦0.003% 0.0001%≦P≦0.020% Ca≦0.005%, and having a chemical composition comprising 11. The hot stamped coated steel part of claim 10, wherein the balance of the composition is iron and unavoidable impurities resulting from processing.

12. The steel in the hot stamped coated steel part or in at least one hot stamped coated sub-part comprises, in weight percent: 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% and having a chemical composition comprising 11. The hot stamped coated steel part of claim 10, wherein the balance consists of Fe and unavoidable impurities resulting from processing.

13. The steel in the hot stamped coated steel part or in at least one hot stamped coated sub-part comprises, in weight percent: 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% and having a chemical composition comprising 11. The hot stamped coated steel part of claim 10, wherein the balance of the composition is iron and unavoidable impurities resulting from processing.

14. 1. A method for producing a hot stamped coated steel part, comprising the following successive steps: Average thickness e of 0.6 mm to 3.5 mm Ｂ providing a steel blank having a thickness of 19.91 μm to 40 μm, the steel blank being provided on at least one side with an aluminum or aluminum alloy pre-coating, the pre-coating having an average thickness comprised between 19.91 μm and 40 μm; In order to obtain a fully austenitic structure in the steel of the blank, the steel blank is heated in a furnace to a heating temperature T in the range of 850 ° C. to 970 ° C. ｈｅａｔ The steel blank is heated to a heating temperature T ｈｅａｔ and holding the - transferring the heated blank into a mould and then closing the mould; - Thickness e of 0.6mm to 3.5mm Ｂｆｌａｔ hot stamping the blank in a die such that a flat portion of the blank having the blank is not deformed and at least a portion of the blank is deformed by the hot stamping, thereby obtaining a hot stamped blank comprising a non-deformed portion and at least one deformed portion; - cooling the hot stamped blank to a temperature below 400°C to obtain a hot stamped coated steel part; Including, The temperature of the blank when the mold is closed, T ｃｌｏｓｅ is in the range of 720°C to 820°C, and the melting temperature T ｍｅｌｔ Time spent by the blank beyond t Ｍ is the minimum time t Ｍｍｉｎ and maximum time t Ｍｍａｘ and [0045] and [0050] In the formula, t Ｍｍｉｎ and t Ｍｍａｘ is expressed in seconds, T ｈｅａｔ indicates the heating temperature of the blank in ° C., T ｍｅｌｔ indicates the melting temperature of the precoating in ° C., e Ｂｆｌａｔ denotes the thickness of the undeformed part of the blank in mm, method.

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

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

17. In the hot stamping of the blank in the mold, each of the blanks is cut to a thickness e of 0.6 mm to 3.5 mm. Ｂｆｌａｔ (i) Two or more flat portions of the blank are not deformed and the melting temperature T ｍｅｌｔ Time spent by blanks exceeding t Ｍ is the minimum time t required for the flat part with the highest thickness Ｍｍｉｎ (Max (e Ｂｆｌａｔ（ｉ） ) and the maximum time t required for the flat part with the lowest thickness Ｍｍａｘ (Min(e Ｂｆｌａｔ（ｉ） 17. The method according to claim 14, wherein the method is included between

18. The blank is formed to a minimum thickness e during hot stamping of the blank in the die. Ｂｍｉｎ From maximum thickness e Ｂｍａｘ and the melting temperature T ｍｅｌｔ The time the blank spends beyond t Ｍ is the maximum thickness e Ｂｍａｘ The minimum time t required for Ｍｍｉｎ (e Ｂｍａｘ ) and minimum thickness e Ｂｍｉｎ The maximum time t required for Ｍｍａｘ (e Ｂｍｉｎ 18. The method according to any one of claims 14 to 17, wherein the ratio of

19. The method according to any one of claims 14 to 18, wherein the steel blank is a monolithic blank, a tailor rolled blank or a tailor welded blank produced by welding together at least two sub-blanks.

20. The blank or each subblank is, in weight percent, 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% and having a chemical composition comprising 20. The method of claim 19, produced by cutting pre-coated steel sheet, the remainder of the composition being iron and unavoidable impurities resulting from processing.

21. The chemical composition of the blank or at least one sub-blank is, in weight percent: 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% 1.5%≦(C+Mn+Si+Cr)≦2.7% 0.0035%≦Ti≦0.072% 0.0002%≦B≦0.004% 0.04%≦Nb≦0.06% 0.044%≦(Nb+Ti)≦0.09% 0.001%≦N≦0.009% 0.0005%≦S≦0.003% 0.0001%≦P≦0.020% Ca≦0.005%, Contains 21. The method of claim 20, wherein the balance of the composition consists of iron and inevitable impurities resulting from processing.

22. The chemical composition of the blank or at least one sub-blank is, in weight percent: 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% Contains 21. The method of claim 20, wherein the balance is Fe and unavoidable impurities resulting from processing.

23. The chemical composition of the blank or at least one sub-blank is, in weight percent: 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% Contains 21. The method of claim 20, wherein the balance of the composition consists of iron and inevitable impurities resulting from processing.

24. The steps of providing the blank or each sub-blank may comprise the following successive steps: - providing a steel semi-finished product, - optionally reheating the semi-finished product to a temperature between 1100°C and 1300°C; - hot rolling the semi-finished product to obtain a hot rolled steel sheet; Coiling the hot-rolled steel sheet at a coiling temperature of −750° C. or less; - optionally pickling the hot-rolled steel sheet; Optionally, cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; - heating a hot-rolled or cold-rolled steel sheet to an annealing temperature between Ac1 and Ac3, hot-dip galvanizing the steel sheet in an Al or Al alloy bath at a temperature of -670°C to 680°C; - cooling the precoated steel sheet to room temperature; - cutting the pre-coated steel sheet to obtain blanks or sub-blanks; - optionally welding the sub-blanks together to produce a tailor-welded blank; The method according to any one of claims 19 to 23, comprising:

25. Use of a hot stamped coated steel part according to any one of claims 1 to 13 or produced by the method according to any one of claims 14 to 24 for the manufacture of a chassis or body-in-white part or a suspension arm for an automotive vehicle.

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