Steel sheet produced from steel

Description

1 / 25 “STEEL SHEET PRODUCED FROM A DIVIDED STEEL” FROM BR 11 2024 017233-3, FILED ON 04 / 25 / 2024 Field of the Invention

[001] The present invention relates to steel sheets and high-strength hardened steel parts. Background of the Invention

[002] High-strength pressed parts can be used as structural elements in automotive vehicles for anti-intrusion or energy absorption functions.

[003] In such types of applications, it is desirable to produce steel parts that combine high mechanical strength and high impact resistance. Furthermore, one of the main challenges in the automotive industry is to reduce the weight of vehicles in order to improve their fuel efficiency in view of global environmental conservation, without neglecting safety requirements.

[004] This weight reduction can be achieved in particular thanks to the use of steel parts with a predominantly martensitic microstructure.

[005] It is a challenge to produce ultra-high-strength steels that also have good resistance to cracking under bending. In fact, high-strength steels tend to crack prematurely when subjected to a bending load. This is detrimental to the crack value of a part produced with high-strength steel, because even though the material has the capacity to withstand very high loads thanks to its high tensile strength, once cracks begin to appear in the part, these cracks will propagate rapidly under continuous loading and the part will fail prematurely. Description of the Invention

[006] The objective of the present invention is to address the challenge mentioned above and provide a press-hardened steel part with Petition 870260055143, dated 08 / 06 / 2026, page 13 / 75 2 / 25 a combination of high mechanical properties, with a tensile strength after hot stamping greater than or equal to 1300 MPa and a high bending angle measured in the transverse direction greater than 48° when normalized to a thickness of 1.5 mm.

[007] Another object of the invention is to provide a steel sheet that can be transformed by hot forming into a press-hardened steel part and to provide a process for manufacturing this steel sheet.

[008] The objective of the present invention is achieved by providing a steel sheet according to claim 1, optionally having the characteristics of claims 2 to 4. Another objective of the present invention is achieved by providing a pressed hardened steel part according to claim 5. The steel part may also comprise characteristics of claims 6 to 7. A further objective of the present invention is a method of manufacturing said hot-stamped part according to claim 8, optionally comprising the characteristics of claim 9. Description of Embodiments of the Invention

[009] The invention will now be described in detail and illustrated by examples without introducing limitations, and making reference to Figure 1, which is a schematic cross-section of a steel sheet according to the invention.

[010] A piece of raw steel refers to a flat sheet of steel that has been cut into any shape suitable for its use. A steel sheet has an upper and a lower face, which are also called upper and lower sides or as upper and lower surfaces. The distance between these faces is designated as the thickness of the sheet. The thickness can be measured, for example, using a micrometer, whose axis and anvil are placed on the upper and lower faces. Similarly, the thickness Petition 870260055143, dated 08 / 06 / 2026, p. 14 / 75 3 / 25 can also be measured in a formed part.

[011] Hot stamping is a forming technology that involves heating a blank to a temperature at which the steel microstructure has been at least partially transformed into austenite, forming the blank at high temperature by stamping it and then rapidly cooling the formed part to obtain a microstructure with very high strength. Hot stamping allows for the production of parts with very high strength and complex shapes, and offers many technical advantages. It should be understood that the heat treatment to which a part is subjected includes not only the thermal cycle described above for the hot stamping process itself, but also possibly other subsequent heat treatment cycles, such as, for example, the paint curing stage, performed after the part has been painted in order to cure the paint.The mechanical properties of the hot-stamped parts below are those measured after the complete thermal cycle, optionally including, for example, an ink baking step, if ink baking was actually performed.

[012] Tensile strength is measured according to the ISO 6892-1 standard, published in October 2009. Tensile test specimens are cut from flat portions of the hot-stamped part. If necessary, small-sized tensile test samples are collected to accommodate the total flat area available on the part.

[013] The bending angle is measured according to the VDA-238 bending standard. For the same material, the bending angle depends on the thickness. For simplicity, the bending angle values ​​of the present invention refer to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle value needs to be normalized to 1.5 mm by the following calculation, where α 1.5 is the normalized bending angle. Petition 870260055143, dated 08 / 06 / 2026, page 15 / 75 4 / 25 in 1.5 mm, t is the thickness and at is the bending angle for thickness t: a1.5 = (at x Vt) / Vi ,5.

[014] In the present invention, the bending angle was measured in the transverse direction, i.e., in the direction transverse to the rolling direction along which the steel sheet traveled during the hot rolling step. The bending angle was measured using a laser measuring device. The reported values ​​are those after the spring effect. When performing bending tests on hot-stamped parts, samples are cut from flat areas of the part. If necessary, small-sized samples are collected to accommodate the total flat area available on the part. If the rolling direction on the hot-stamped part is not known, it can be determined using electron backscatter diffraction (EBSD) analysis across the entire sample section in a scanning electron microscope (SEM).The lamination direction is determined according to the intensity of the Orientation Density Function (ODF) representative of the principal fibers at φ2 = 45°, where φ2 is the Euler angle as defined in H.-J. Bunge: Texture Analysis in Materials Science-Mathematical Methods. 1st English edition by Butterworth Co (Publ.) 1982 (see Figures 2.2 and 2.3 for the definition of φ2).

[015] The bending angle of a part is representative of the part's ability to resist deformation without cracking.

[016] The composition of the steel according to the invention will now be described, the content being expressed as a percentage by weight. The chemical compositions are given in terms of a lower and upper limit of the composition range, these limits being within the possible composition range according to the invention. In the case where preferred ranges for a given element are disclosed, the present invention also discloses all possible combinations of these preferred ranges for each individual element. Petition 870260055143, dated 08 / 06 / 2026, page 16 / 75 5 / 25

[017] According to the invention, the carbon content varies from 0.2% to 0.4% to ensure satisfactory strength. Above 0.4% carbon, the weldability and bendability of the steel sheet may be reduced. If the carbon content is less than 0.2%, the tensile strength will not reach the desired value. In a specific embodiment, the carbon content varies from 0.2% to 0.3% in order to ensure sufficient strength while further controlling excellent weldability and bendability. In a specific embodiment, the carbon content varies from 0.2% to 0.25% in order to ensure sufficient strength while further controlling excellent weldability and bendability.

[018] The manganese content varies from 0.8% to 2.0%. Above 2.0% addition, the risk of MnS formation is increased at the expense of foldability. Below 0.8%, the hardenability of the steel sheet during the hot stamping process is reduced.

[019] In one particular embodiment, the manganese content varies from 1.0% to 1.4% in order to further improve the hardenability of the steel and further limit the formation of MnS, thereby improving the foldability.

[020] The silicon content varies from 0.1% to 0.5%. Silicon is an element that participates in hardening in solid solution. Silicon is added to limit carbide formation. Above 0.5%, silicon oxides form on the surface, which impairs the coating capacity of the steel. In addition, the weldability of the part produced with said steel sheet may be reduced. In a specific embodiment, the silicon content varies from 0.1% to 0.4% in order to further improve coating capacity and weldability. In a specific embodiment, the silicon content varies from 0.15% to 0.35% in order to further harden the steel and further improve coating capacity and weldability.

[021] According to the invention, the aluminum content varies from 0.01 Petition 870260055143, dated 08 / 06 / 2026, page 17 / 75 6 / 25% to 0.1%, as it is a very effective element for deoxidizing steel in the liquid phase during fabrication. Aluminum can protect boron if the titanium content is insufficient. The aluminum content is less than 0.1% to avoid oxidation problems and ferrite formation during press hardening. Preferably, the aluminum content ranges from 0.02% to 0.06% to ensure good deoxidation of the steel in the liquid phase, while avoiding oxidation problems and ferrite formation during press hardening.

[022] According to the invention, the titanium content varies from 0.01% to 0.1% in order to protect the boron, which would otherwise be trapped within BN precipitates. The titanium content is limited to 0.1% to avoid excessive TiN formation. In a specific embodiment, the Ti content varies from 0.02% to 0.06% in order to further protect the boron and additionally avoid excessive TiN formation.

[023] According to the invention, the boron content varies from 0.0005% to 0.005%. Boron improves the hardenability of steel. The boron content is not higher than 0.005% to avoid plate breakage problems during continuous casting. In a specific embodiment, the boron content varies from 0.002% to 0.004% in order to further ensure the hardenability of the steel and additionally avoid plate breakage problems.

[024] Phosphorus is controlled to be below or equal to 0.040% because it leads to brittleness and solderability problems. In a specific embodiment, the P content is controlled to be below or equal to 0.020% to further avoid brittleness and solderability problems.

[025] Calcium is controlled to below or equal to 0.01% because the presence of calcium in liquid steel can lead to the formation of coarse inclusions that are detrimental to bendability. In a specific embodiment, the Ca content is controlled to below or equal to 0.005% to further prevent Petition 870260055143, dated 08 / 06 / 2026, page 18 / 75 7 / 25 problems with gross inclusions.

[026] Sulfur is controlled to below or equal to 0.006% because the presence of sulfur in liquid steel can lead to the formation of MnS precipitates, which are detrimental to bendability. In a specific embodiment, the S content is controlled to below or equal to 0.005% to further prevent the formation of MnS precipitates.

[027] Nitrogen is controlled below 0.01%, preferably below or equal to 0.008%, even more preferably below 0.005%. The presence of nitrogen can lead to the formation of precipitates such as TiN or TiNbCN, which are detrimental to foldability.

[028] Chromium is optionally added up to 0.4%. Chromium can be used to provide strength through solid solution hardening and to improve the hardenability of steel sheet during hot stamping. Chromium is limited to 0.4% to limit costs and avoid processing problems.

[029] Molybdenum is optionally added up to 0.3%. Molybdenum improves the hardenability of steel. Molybdenum is limited to 0.3% to limit costs and avoid processing problems.

[030] Niobium is optionally added up to 0.1%. Niobium improves the ductility of steel. Niobium is limited to 0.1% to limit costs and avoid processing problems.

[031] Vanadium is optionally added up to 0.3%. Vanadium improves the hardenability of steel. Vanadium is limited to 0.3% to limit costs and avoid processing problems.

[032] If one or more of the above elements are added, the following formula is additionally checked: Cr + Mo + Nb + V < 0.5% in order to limit costs and avoid processing problems.

[033] In a specific embodiment, the chemical composition is Petition 870260055143, dated 08 / 06 / 2026, page 19 / 75 8 / 25 additionally controlled so that the following condition is verified: 5.22*(S-Ca*32 / 40)*104+ 11.4*(Ti2*N)*106+ 136.5 < 300.

[034] The inventors found that this allowed them to further control the population of inclusions in the steel sheet and thus improve the foldability even further.

[035] The remainder of the steel composition is iron and impurities resulting from the manufacturing process. The level of impurities resulting from the manufacturing process will depend on the production route used and the level of scrap used in the steel smelting. For example, when using a basic oxygen furnace route with a low level of steel residue (recycled steel), the level of impurities will remain low. However, it is also possible to add a large amount of scrap in the converter to the pig iron produced in the basic oxygen furnace, which will increase the level of impurities. On the other hand, when manufacturing steel using an electric furnace, with a very high ratio of recycled residual steel, the level of impurities will be significantly increased. In the latter case, for example, the level of Cu can increase by up to 0.25%, Ni can increase by up to 0.25%, Sn can increase by up to 0.05%, As can increase by up to 0.03%, Sb can increase by up to 0.03%, and Pb can increase by up to 0.03%.

[036] The microstructure of the steel sheet according to the invention will now be described.

[037] The steel sheet has a microstructure comprising a surface fraction in any cross-sectional analysis: - 75% to 90% ferrite, - the remainder being comprised of FesC carbides and hard phases such as martensite and bainite.

[038] With reference to Figure 1, the steel plate 1 according to the invention comprises a main portion 3 and an upper and lower coating layer 2. The total thickness of the steel plate 1 is t0 and the thickness ts Petition 870260055143, dated 08 / 06 / 2026, page 20 / 75 9 / 25 of the skin layers 2 is such that ts = tO*1O %. In other words, the skin layers 2 occupy the outermost 10% of the thickness on both sides of the volume, and the volume of the steel plate represents 80% of the thickness of the steel plate.

[039] The inventors found that there is a correlation between the bending angle and the population of inclusions in the skin portion of the steel sheet. In particular, it is possible to ensure that the normalized bending angle α1.5 in the transverse direction is strictly greater than 48° by controlling that both the density of TiN / Ti(C,N) inclusions on the surface is less than 240 particles / mm2 and the agglomeration index of MnS inclusions on the surface portion of the steel is less than 110 pm / mm2

[040] The following is a description of the methodology that was used to characterize the inclusions in the steel sheet and steel parts. It should be understood that this is only one possible methodology and that other protocols may also be implemented.

[041] The cross-sections of the steel plate in which the inclusions are observed are taken in the direction of the steel's rotation. In other words, the plane of the observed cross-section has the transverse direction as its normal direction.

[042] The inclusions present in the steel plate were characterized using a Scanning Electron Microscope (SEM) with Field Effect Gun (FEG). A Tescan Mira 3 SEM was used in a 14 kV power setting. This allows the detection of particles as small as 0.5 µm. The use of a FEG SEM configuration allows obtaining stable images with excellent resolution over a long period of time, which may be necessary to complete image analysis in large areas – using a FEG SEM configuration, it is possible to acquire image fields for up to 48 hours, which may be necessary for the analysis of multiple samples. In addition Petition 870260055143, dated 08 / 06 / 2026, page 21 / 75 10 / 25 of this, the inclusions were analyzed using Energy Dispersive Spectrometry (EDS). A 120 mm² Bruker EDS probe with a large active surface was used to detect light elements (O, N) and obtain a high counting rate, thus allowing precise quantification. The phi-rho-Z method was used to obtain precise quantification.

[043] The RJ Lee group's Automated Steel Cleaning Analysis Tool (ASCAT) is used to pilot the SEM and associated EDS, based on Computer Controlled Scanning Electron Microscopy technologies. Six individual samples can be analyzed in the same batch. The sample surface is divided into three areas (top skin, bottom skin, volume, as described previously). Each area is divided into fields. In each field, inclusions are detected. In order to detect fine particles, the scan pixel size is set to the very low value of 0.11 µm. This is intended to reduce matrix noise in the SEM images. As will be seen, only objects with diameters above 0.5 µm are actually taken into consideration.A first selection of objects, which will be called particles, is made by selecting objects that form a solid and have a gray level, on a scale of 0 to 255, below 150 or above 220 (extreme values ​​are excluded).

[044] An enlarged image is then taken of each individual particle to capture its morphological characteristics and perform an EDS analysis. A database of all particles is created using ASCAT and taking into account, for all acquired images, the chemical and morphological characteristics of all particles analyzed.

[045] From the set of all particles analyzed, only those with a size greater than 0.5 pm and an iron content below 80% are retained for subsequent analysis and will be called inclusions - the other particles are considered part of the matrix and are not relevant for subsequent analysis. Petition 870260055143, dated 08 / 06 / 2026, page 22 / 75 11 / 25

[046] Using the information from the EDS probe, each inclusion is then classified into one of the following families: TiN, alumina, complex oxides, oxysulfide particles, MnS, and others. For example, Table 1 details the precise rules that were used by the inventors to classify MnS and TiN / Ti(C,N) inclusions. Oxygen quantification is possible thanks to high-performance EDS detectors. The oxygen level is checked in order to separate TiN from TiO2 and MnS from complex oxysulfide inclusions. Table 1 - Criteria in weight % of Ti, M, S, O, Nb for classifying inclusions Ti (%) S > 2% O < 5%

[047] For each inclusion family, the following characteristics are then calculated: - average diameter in microns, - density in number of inclusions per mm2.

[048] The method for calculating the aggregation index is based on the DBSCAN algorithm (for Density-Based Spatial Aggregation of Applications with Noise), as detailed in the article “A density-based algorithm for discovering clusters in large spatial databases with noise”, Ester, Martin; Kriegel, Hans-Peter; Sander, Jorg; Xu, Xiaowei (1996), in Proceedings of the Second International Conference on Knowledge Discovery and Data Mining (KDD-96). AAAI Press, pp. 226-231.

[049] Determining the aggregation index uses 2 parameters: Max_distance and Min_points. An aggregation is characterized by the following features: - it comprises only particles of the same type, - In a given aggregation, the inclusions are all within a... Petition 870260055143, dated 08 / 06 / 2026, page 23 / 75 12 / 25 distance less than Max_distance from at least one other inclusion, - it comprises a number of individual inclusions equal to or greater than Min_points.

[050] For the present invention, the inventors found that good aggregation detection is obtained with a maximum distance Max_distance of 30 pm and a minimum number of inclusions per aggregation Min_points of 4.

[051] The length L of a given aggregation is calculated as follows: - The convex hull of the aggregation is first determined using a known algorithm (see, for example, the chapter Convex Hulls: Basic Algorithms in: Computational Geometry, Preparata, FP, Shamos, M.I., 1985, Texts and Monographs in Computer Science. Springer, New York, NY). The maximum Feret diameter of said convex hull, which will be denoted Dmax, is then determined, and the Feret diameter taken in the direction perpendicular to Dmax, which will be denoted Dperp, is also determined. Information on measuring the Feret diameter is available, for example, in "Particle Size Measurements: Fundamentals, Practice, Quality" Springer, by Henk G. Merkus (January 1, 2009). The length L of said aggregation is calculated as: L = Jomax2 + Dperp2

[052] For each type of inclusion, an average length of all aggregations L_average is calculated.

[053] The density of aggregations C_density of a given type of inclusion is the number of aggregations per mm2.

[054] The C_index aggregation index for a given type Petition 870260055143, dated 08 / 06 / 2026, page 24 / 75 The inclusion index (C_index) is defined as the product of the average length of the aggregates by their density. C_index = L_average * C_density. The aggregation index is expressed in pm / mm². The inventors found that said aggregation index allows comparing samples with different characteristics using a single number and that it is well correlated with the bending behavior of said samples.

[055] The steel sheet according to the invention can be produced by any suitable manufacturing method and a person skilled in the art can define one. It is, however, preferable to use the method according to the invention comprising the steps described below.

[056] In the following description, the term shell refers to the container used to hold the liquid steel during the refining process. The refining process is defined as the stage in which the final chemical composition and temperature of the liquid metal are adjusted before melting the steel into its first solidified form (for example, before melting it into plates, which will later be hot-rolled).

[057] In order to successfully control the inclusion population in steel, the following process can, for example, be implemented: - Liquid steel is poured into the ladle of the previous stage of the steelmaking process. For example, in the case of an electric arc furnace production route, the previous process stage is the electric arc furnace process itself. For example, in the case of a blast furnace and converter process (or in the case of a direct reduced iron and converter process), the previous process stage is the converter. The sulfur content of liquid steel before the refining stage is measured, for example, by taking a sample of said liquid steel and analyzing it using a spark spectrometer. Said sulfur content is measured, for example, by directly sampling the liquid steel in the ladle or by taking a... Petition 870260055143, dated 08 / 06 / 2026, page 25 / 75 14 / 25 sample when the liquid steel is being poured into said ladle. Said sulfur content before the refining stage, measured in % by weight, will be referred to as S_initial in the following description. Aluminum is added to the ladle at the beginning of the refining process to deoxidize the liquid steel. This addition of Al, for example, is carried out at the same time as the steel is being poured into the ladle—which saves time and therefore increases productivity and ensures that the liquid steel remains sufficiently hot. The amount of Al added to the liquid steel at the beginning of the refining process, expressed in kg of aluminum per ton of liquid steel (kg / ton), will be referred to as Al_added in the following description. - In an optional subsequent step, for example, if the temperature of the liquid steel is too low or if the waiting time between the end of the refining stage and the subsequent process (e.g., continuous casting) is justified, the liquid steel is reheated by aluminothermic heating. This is achieved by adding a specified amount of aluminum and simultaneously blowing a specified amount of oxygen into the liquid steel, corresponding to the stoichiometric ratio required to form Al2O3 with the added aluminum. The strongly exothermic reaction between Al and O2 allows the temperature of the liquid steel to increase. The amount of O2 injected during this optional step will be denoted O2_inj and is expressed in standard cubic meters of O2 per ton of liquid steel (Nm3 / ton).Since there is a direct stoichiometric relationship between O2_inj and the associated Al injection for aluminothermic reheating, the said quantity of Al injected for aluminothermic reheating will not be considered separately in the present description. It should be noted that the said Al injected for aluminothermic reheating is distinct from the Al_added mentioned previously. The composition of the slag above the melt is adjusted. Petition 870260055143, dated 08 / 06 / 2026, page 26 / 75 15 / 25 adding the appropriate amount of minerals in order to ensure that the % CaO / % Al2O3 ratio of the slag is greater than 1, that the amount of slag per ton of liquid steel is at least 10 kg / ton of liquid steel and that the slag remains liquid in order to promote chemical exchanges with the steel, to be able to access the steel below the slag and to be able to dump the steel and / or the slag separately (the liquid state of the slag is verified visually and / or using thermodynamic rules based on its composition and temperature), - In a subsequent step, the liquid steel is agitated by blowing an inert gas, for example, air, into the liquid steel. This is done in order to promote exchanges between the liquid steel and the slag, which will reduce the sulfur content of the liquid steel. - In a further step, Ca is added to the ladle in order to globularize the inclusions present in the liquid steel. For example, Ca is added in the form of silicon-calcium (SiCa) or in the form of iron-calcium (FeCa), or as pure calcium. For example, said addition is carried out by adding SiCa or FeCa to the ladle in the form of a tubular wire – advantageously, this allows easy control of the amount of Ca added by controlling the length and injection speed of the tubular wire inserted into the melt. The amount of Ca added to the liquid steel, measured as a weight % within the liquid steel, will be referred to as Ca_added in the following description.

[058] Considering the process described above, the inventors found that a satisfactory level of inclusions to achieve the desired level of bending after hot stamping can be obtained by controlling the levels described above of Sulfur measured at the beginning of the refining process (S_initial measured in % by weight), addition of Al at the beginning of the refining process (Al_added measured in kg / ton), addition of Ca during the refining process (Ca_added measured in kg / ton) and volume of O2 blowing (O2_injected measured in Petition 870260055143, dated 08 / 06 / 2026, page 27 / 75 16 / 25 Nm3 / ton) in order to verify that the combination below (which will be referred to as C1 in the rest of the description) remains below a given limit value: 217.8 - 315.1*Ca_added + 41.5*O2_injected + 18700*S_start-40*AI_added (C1).

[059] In fact, the specific cutoff value below which the C1 combination needs to be controlled will depend on the specific industrial arrangement that is used to produce the steel. It will depend on the production route in the steel mill, the geometric configuration of the ladles that are used to process the liquid steel, the equipment used to add the different additions, the oxygen blowing configuration, etc.

[060] In order to determine the relationship between these parameters for a given industrial equipment and production route, it is recommended to apply the following method: - Several heating processes are carried out using the chemical composition ranges described previously, - The aforementioned heating processes are carried out using different refining process parameters, in particular different levels of sulfur measured at the beginning of the refining process, addition of Al at the beginning of the refining process, addition of Ca during the refining process, and O2 blow volume. The ranges of refining process parameters tested are chosen to be representative of the industrial variation of these parameters. For example, a set of 6 different heating processes with 6 different sets of refining process parameters is chosen. For example, a set of 8 different heating processes with 8 different sets of refining process parameters is chosen. The aforementioned heating processes are carried out according to the industrial route described below, and the inclusion population of the steels is characterized using the method described above. Petition 870260055143, dated 08 / 06 / 2026, page 28 / 75 17 / 25 - The density of TiN / Ti(C,N) inclusions on the surface and the agglomeration index of MnS inclusions in the surface portion of the steel, as well as the associated refining process parameters, are then recorded. The C1 combination of refining process parameters is calculated. As a general trend, it will be seen that the larger the said combined C1 portion, the greater the density of TiN / Ti(C,N) inclusions in the skin and the agglomeration index of MnS inclusions in the skin portion of the steel. Using the dataset described above, which associates the characteristics of surface inclusions and refining process parameters, a cutoff value is determined below which both the density of TiN / Ti(C,N) inclusions on the surface is less than 240 particles / mm² and the agglomeration index of MnS inclusions on the surface portion of the steel is less than 110 pm / mm². This cutoff value of the C1 combination will determine how to control the refining process for the specific industrial plant under consideration. By controlling C1 below this cutoff value, it will be possible to produce steel sheets with a density of TiN / Ti(C,N) inclusions on the surface less than 240 particles / mm² and an agglomeration index of MnS inclusions on the surface portion of the steel less than 110 pm / mm². In this way, it will be possible to achieve the excellent associated bending level of more than 48° normalized bending angle at 1.5 in the transverse direction.

[061] For example, in the case of the specific industrial facilities in which the inventors carried out experiments, the cutoff value said is equal to 270.

[062] After the liquid steel refining stage, the method for manufacturing the steel sheet according to the present invention preferably comprises the following steps: - Continuous casting of liquid steel into a suitable semi-finished product Petition 870260055143, dated 08 / 06 / 2026, page 29 / 75 18 / 25 for hot rolling. During the casting stage, particular care must be taken to avoid oxygen absorption and therefore a higher level of inclusions in the semi-finished product. For example, in the case of a continuous casting process where the semi-finished products are slabs produced in a continuous sequence by casting in a mold from multiple heatings poured into a crucible, specific refractories and linings may be used in the crucible, specific allocation rules may be used for the first slabs in the sequence and transitional slabs between two different heatings, etc. The semi-finished product is then optionally reheated to a temperature between 1150 °C and 1300 °C. The steel sheet is then hot-rolled to a final hot-rolling temperature between 800 °C and 950 °C. The hot-rolled steel is then cooled and coiled at a temperature Tcoil below 670 °C, and optionally pickled to remove oxidation. - The coiled steel sheet is then optionally cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling preferably ranges from 20% to 80%. Below 20%, recrystallization during subsequent heat treatment is not favored, which can impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling. - In one embodiment of the invention, the steel sheet is heated in an annealing furnace to an immersion temperature between 700 °C and 850 °C and said immersion temperature is maintained for an immersion time between 10 seconds and 20 minutes. - In one embodiment of the invention, the annealed steel sheet is cooled to a temperature range of 400 °C to 700 °C and additionally Petition 870260055143, dated 08 / 06 / 2026, p. 30 / 75 19 / 25 coated with a metallic coating. Said metallic coating is, for example, an aluminum-based metallic coating comprising at least 50% aluminum by weight. Said metallic coating is, for example, a zinc-based metallic coating comprising at least 50% zinc by weight. In one embodiment of the invention, the steel sheet is then cooled to room temperature.

[063] In summary, the process described above preferably comprises the following successive steps: - To produce a liquid steel with the chemical composition described above, wherein during the refining phase of the liquid steel the levels of Sulfur measured at the beginning of the refining process, Al addition at the beginning of the refining process, Ca addition during the refining process, and O2 blow volume are controlled to verify that the combination 217.8 - 315.1*Ca_added + 41.5*O2_injected + 18700*S_initial - 40*Al_added (C1) remains below a predetermined cutoff value. Said cutoff value having been determined for the specific industrial equipment being used, such that when C1 is below the cutoff value, the density of TiN / Ti(C,N) inclusions in the skin is less than 240 particles / mm2 and the agglomeration index of MnS inclusions in the skin portion of the steel is less than 110 pm / mm2, - to melt liquid steel to obtain a semi-finished product capable of being hot-rolled, - optionally reheating the semi-finished product to a reheating temperature between 1100 °C and 1300 °C, - to roll the semi-finished product at a final rolling temperature between 800 °C and 950 °C, - To coil hot-rolled steel sheet at a cooling temperature Tcoil lower than 670 °C to obtain a coiled steel sheet, Petition 870260055143, dated 08 / 06 / 2026, page 31 / 75 20 / 25 - optionally pickle the coiled steel sheet, - optionally rolling the cold-rolled steel sheet with a reduction ratio ranging from 20% to 80% to obtain a cold-rolled steel sheet, - optionally heating the hot-rolled steel sheet or the cold-rolled steel sheet to an immersion temperature between 700 °C and 850 °C and maintaining the steel sheet at said temperature for an immersion time between 10 seconds and 20 minutes, to obtain an annealed steel sheet, - optionally resin-coating the annealed steel sheet for a temperature range of 400 °C to 700 °C, - optionally coating the annealed steel sheet with a metallic coating, - optionally cooling the coated steel sheet to ambient temperature.

[064] The manufacturing process of the pressed part and the subsequent characteristics of the pressed part will now be detailed.

[065] A blank piece of steel is cut from the steel sheet according to the invention and heated in an austenitizing furnace. Preferably, the steel sheet is heated to a temperature between 880 °C and 950 °C for 10 seconds to 15 minutes to obtain a heated steel sheet. The heated blank piece is then transferred to a forming press before being hot-formed and rapidly cooled to obtain a pressed piece.

[066] Optionally, the hot-stamped part is additionally subjected to an ink curing step, in which the part is heated to a temperature between 150 °C and 250 °C for a duration of 10 minutes to 2 hours. Petition 870260055143, dated 08 / 06 / 2026, page 32 / 75 21 / 25

[067] The microstructure of the pressed part comprises, in surface fraction in any cross-section analyzed, more than 95% martensite and less than 5% bainite + ferrite. Furthermore, the pressed part according to the invention comprises a bulk portion and an upper and lower skin layer, wherein the skin layers occupy the outermost 10% of the thickness on each side of the bulk. Said skin layer has a TiN / Ti(C,N) inclusion density of less than 240 particles / mm2 and a MnS inclusion aggregation index in the steel skin portion of less than 110 pm / mm2.

[068] The pressed part according to the invention has a tensile strength above 1300 MPa, preferably above 1350 MPa, and a normalized bending angle of α1.5 in the transverse direction strictly greater than 48°. Such high tensile strength and high bending angle give this part very good mechanical strength, especially in the event of an accident. These characteristics provide very good energy absorption capacity and anti-intrusion capability, thus increasing vehicle safety.

[069] The invention will now be illustrated by the following examples, which are by no means limiting.

[070] Eleven different samples from eleven different batches A, B, C, D, E, F, G, H, I, J, and K of steel produced using an industrial production route were tested. Samples I1, I2, I3, I4, I5, I6, and I7 are according to the invention, samples R1, R2, R3, and R4 are reference samples.

[071] All samples produced followed the same industrial production process in the steel mill. All samples were coated after annealing using an AISi-based coating containing 8-12 wt% Si, 2-4 wt% Fe, with the remainder being Al. Petition 870260055143, dated 08 / 06 / 2026, page 33 / 75 22 / 25 Table 2 - Sample composition

[072] The compositions tested are compiled in the following table where the element contents are expressed as a percentage by weight, with the remainder of the composition being iron and unavoidable impurities resulting from the manufacturing process: Reference heat % C % Mn % Si % Al % Ti % B % P % Ca s % N % Cr 5.22* (S- Ca *32 / 40) *104 + 11.4* (Ti2 *N) *106 + 136.5 A 0.2 1.2 0.2 0.03 0.04 0.003 0.013 0.002 0.002 0.005 0.2 248 B 0.2 1.2 0.3 0.03 0.04 0.003 0.014 0.002 0.001 0.004 0.2 178 C 0.2 1.2 0.3 0.03 0.04 0.002 0.013 0.002 0.002 0.005 0.2 248 D 0.2 1.2 0.2 0.03 0.04 0.003 0.011 0.002 0.002 0.002 0.2 193 E 0.2 1.2 0.3 0.03 0.04 0.003 0.015 0.001 0.001 0.005 0.2 238 F 0.2 1.2 0.3 0.04 0.04 0.003 0.016 0.001 0.001 0.004 0.2 219 G 0.2 1.2 0.2 0.05 0.03 0.003 0.013 0.003 0.002 0.004 0.2 156 H 0.2 1.2 0.3 0.04 0.04 0.003 0.013 0.001 0.002 0.007 0.2 326 1 0.2 1.2 0.2 0.05 0.04 0.003 0.010 0.000 0.002 0.004 0.2 313 J 0.2 1.2 0.2 0.04 0.03 0.002 0.011 0.001 0.004 0.006 0.2 365 K 0.2 1.2 0.2 0.04 0.03 0.002 0.011 0.001 0.004 0.006 0.2 365 Table 3 - Steelmaking process parameters, inclusion density of TlN / Tl(C,N) in the skin and aggregation index of MnS inclusions in the skin.

[073] The following process parameters were applied in the steel mill and the following density of TiN / Ti(C,N) inclusions in the skin and aggregation index of MnS inclusions in the skin portion of the steel were observed; the underlined values ​​are not in accordance with the invention: Reference sample Reference heat SJníci 0 (% by weight) Added Al (kg / ton) Added Ca (kg / ton) Injected O2 (Nm3 / ton) C1* MnS aggregation index of the skin layer (pm / mm2) TiN / Ti(C,N) density of the skin layer (nb of particles / mm2) 11 A 0.005 2.500 0.138 1.439 227 12 207 I2 B 0.007 1.939 0.153 0.000 223 8 165 I3 C 0.007 2.438 0.137 1.171 257 12 187 I4 D 0.006 1.768 0.153 0.000 211 5 146 I5 E 0.005 1.349 0.154 0.000 209 21 214 I6 F 0.005 1.742 0.153 0.000 195 2 197 I7 G 0.006 2.387 0.236 0.000 163 11 173 Petition 870260055143, dated 08 / 06 / 2026, p. 34 / 75 23 / 25 Reference sample Reference heat SJníci 0 (% by weight) Added Al (kg / ton) Added Ca (kg / ton) Injected O2 (Nm3 / ton) C1* MnS aggregation index of the skin layer (pm / mm2) TiN / Ti(C,N) density of the skin layer (nb of particles / mm2) R1 H 0.010 2.027 0.139 0.000 285 8 246 R2 I 0.008 2.439 0.000 0.993 305 146 193 R3 J 0.010 2.458 0.122 1.407 332 144 202 R4 K 0.010 2.458 0.122 1,407 332 173 215 *C1 = 217.8 - 315.1 *Ca_added + 41.5O2_injected + 18700*S_start 40*AI_added.

[074] As can be seen, under the tested industrial conditions, the density of TiN / Ti(C,N) inclusions on the surface can be controlled to less than 240 particles / mm2 and the aggregation index of MnS inclusions on the surface portion of the steel to less than 110 pm / mm2, ensuring that the refining process parameters are adequately controlled to keep C1 below 270. As explained earlier, this cutoff value of 270 is specific to the industrial setting in which the tests were performed and the appropriate cutoff factor for a given industrial setting will need to be determined, for example, by following the methodology described above. Table 4 - Additional Process Conditions

[075] The following process parameters were applied along the production route: Ref. of Sample Plate Reheating Temperature (°C) Lamination Temperature (°C) Winding Temperature (°C) Cold Lamination Reduction Rate (%) Immersion Temperature (°C) Immersion Time (s) Hot Forming Austenite T(°C) Hot Forming Austenite Time (s) Ink Baking T (°C) Ink Baking Time (min) 11 1235 889 537 60 756 92 900 360 170 20 I2 1241 891 532 59 760 90 900 360 170 20 I3 1238 887 524 58 762 96 900 360 170 20 I4 1198 867 562 57 755 94 900 360 170 20 I5 1211 862 535 57 757 90 900 360 170 20 Petition 870260055143, dated 08 / 06 / 2026, p. 35 / 75 24 / 25 Ref. of Sample Plate Reheating Temperature (°C) Lamination Temperature (°C) Winding Temperature (°C) Cold Lamination Reduction Rate (%) Immersion Temperature (°C) Immersion Time (s) Hot Forming Austenite T(°C) Hot Forming Austenite Time (s) Ink Baking T (°C) Ink Baking Time (min) 16 1257 841 537 52 762 74 900 380 170 20 17 1220 875 551 54 751 43 900 400 170 20 R1 1220 888 577 54 752 42 900 405 170 20 R2 1248 877 554 60 751 38 900 380 170 20 R3 1209 896 580 52 750 34 900 410 170 20 R4 1209 857 548 52 751 34 900 410 170 20 Table 5 - Microstructure, Bending Angles and Tensile Strength

[076] The following microstructures (as a surface fraction), bending angles, bending angle anisotropies, and tensile strength were measured on the samples; underlined values ​​do not conform to the invention: Steel sheet Hot stamped part Sample Ref. Thickness (mm) Steel sheet structure Yield Limit (MPa) TD Tensile Strength (MPa) TD Bending angle α1.5 normalized to 1.5 mm (TD) Hot stamped part structure 11 1.3 75%-90% ferrite. the remainder being FesC. martensite and bainite 1210 1520 51 100% martensite I2 1.3 1190 1485 53 I3 1.2 1186 1485 55 I4 1.2 1204 1506 50 I5 1.3 1153 1462 50 I6 1.4 1216 1525 53 I7 1.6 1192 1509 49 R1 1.7 1150 1474 46 R2 1.4 1185 1490 47 R3 1.7 1142 1463 48 R4 1.7 1144 1474 44

[077] Table 5 shows that the samples according to the invention have a tensile strength above 1300 MPa in the transverse direction, while possessing a normalized bending angle α1.5 measured in the transverse direction strictly greater than 48°. On the other hand, the samples Petition 870260055143, dated 08 / 06 / 2026, page 36 / 75 Reference 25 / 25, although they have comparable tensile strength levels above 1300 MPa, all have a normalized bending angle of α1.5 measured in the transverse direction equal to or less than 48°.

[078] The inventors found that this very good level of foldability is correlated with the density of TiN / Ti(C,N) inclusions in the skin and the aggregation index of MnS inclusions in the skin portion of the steel.

[079] When the aggregation index of MnS inclusions on the surface is equal to or greater than 110 pm / mm2, the bending angle is reduced, as is the case with reference samples R1, R2 and R3 which all have an aggregation index of MnS inclusions on the surface equal to or greater than 110 pm / mm2 and a normalized bending angle of 1.5 mm in the transverse direction equal to or less than 48°.

[080] When the density of TiN / Ti(C,N) inclusions on the surface is equal to or greater than 240 pm / mm2, the bending angle is reduced, as is the case with reference sample R1 which has a density of TiN / Ti(C,N) inclusions on the surface of 246 pm / mm2 and exhibits a bending angle normalized to 1.5 mm in the transverse direction of 46° or less.

[081] By controlling the population of inclusions in the skin within the ranges described above, the resulting steel sheet makes it possible to produce hot-stamped parts with very good impact resistance, robust and stable for use, for example, in the automotive industry. Petition 870260055143, dated 08 / 06 / 2026, page 37 / 75

Claims

1 / 2 Claims 1. STEEL SHEET (1) PRODUCED FROM A STEEL characterized by comprising a composition consisting of, in % by weight: C: 0.2-0.3%; Mn: 0.8-2.0%; Si: 0.1-0.5%; Al: 0.01-0.1%; Ti: 0.01-0.1%; B: 0.0005-0.005%; P < 0.040%; Ca < 0.01%; S < 0.006%; N < 0.01%; and optionally comprising: Cr < 0.4%; Mo < 0.3%; Nb < 0.1%; V < 0.3%; wherein Cr + Mo + Nb + V < 0.5%; the remainder of the composition being iron and unavoidable impurities resulting from the manufacturing process, wherein the chemical composition additionally respects the following condition, all elements being expressed in % by weight: 5.22*(SCa*32 / 40)*104+11.4*(Ti2*N)*106+136.5 < 280, wherein the steel sheet (1) has a microstructure in surface fraction comprising 75% to 90% ferrite, the remainder being comprised of FeaC and hard phases made of martensite and / or bainite, Petition 870260055143, dated 08 / 06 / 2026, page.38 / 75 2 / 2 wherein the steel plate (1) comprises from the volume to the surface of the steel plate (1): - the volume (3) representing 80% of the thickness of the steel plate (1), - wherein the volume (3) is covered by an upper skin layer (2) and a lower skin layer (2) occupying the outermost 10% of the thickness on both sides of the volume (3), the density of TiN / Ti(C,N) inclusions in the skin being less than 240 particles / mm2 and the agglomeration index of MnS inclusions in the skin being less than 110 pm / mm2.

2. STEEL SHEET (1), according to claim 1, characterized in that it consists of, in % by weight: C: 0.2 - 0.25 % and / or Mn: 1.0 - 1.4 % and / or Si: 0.1 - 0.4 %, preferably 0.15 - 0.35 %, and / or Al: 0.02 - 0.06 % and / or Ti: 0.02 - 0.06 % and / or B: 0.002 - 0.004 % and / or P < 0.020 % and / or Ca < 0.005 % and / or S < 0.005 % and / or N < 0.008 %, preferably N < 0.005 %.

3. STEEL SHEET (1), according to any one of claims 1 to 2, characterized in that the steel sheet (1) is coated with a metallic coating comprising at least 50% Al by weight.

4. STEEL SHEET (1), according to any one of claims 1 to 2, characterized in that the steel sheet (1) is coated with a metallic coating comprising at least 50% Zn by weight. Petition 870260055143, dated 08 / 06 / 2026, p. 39 / 75

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