Steel plate, press-hardened steel part and process for manufacturing a press-hardened steel part.

Abstract

steel sheet, press-hardened steel part and process for manufacturing a press-hardened steel part. A steel plate made of a steel having a composition comprising C: 0.3 - 0.4%, Mn: 0.5 - 1.0%, Si: 0.4 - 0.8%, Cr: 0.1 - 1.0%, Mo: 0.1 - 0.5%, Nb: 0.01 - 0.1%, Al: 0.01 - 0.1%, Ti: 0.008 - 0.03%, B: 0.0005 - 0.003%, P = 0.020%, Ca = 0.001%, S = 0.004%, N = 0.005% and optionally comprising Ni < 0.5%, having a microstructure comprising, in surface fraction, 60% to 95% ferrite, the remainder being islands of martensite-austenite, pearlite or bainite, and comprising a volume and a coating layer occupying the outermost 10% of the thickness on both sides of the volume, said coating layer having a coating layer inclusion population in which the surface fraction of oxides is equal to or less than 60*10-6.

Description

1 / 21 “STEEL PLATE, PRESS-HARDENED STEEL PART AND PROCESS FOR MANUFACTURING A PRESS-HARDENED STEEL PART” Field of Invention

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

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

[003] In this type of application, it is desirable to produce steel parts that combine high mechanical strength and high impact resistance. Furthermore, one of the major challenges in the automotive industry is to reduce vehicle weight in order to improve fuel efficiency with a view to global environmental conservation, without neglecting safety requirements.

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

[005] It is a challenge to produce very high-strength steels that also have good resistance to cracking under bending. In fact, very high-strength steels tend to crack prematurely when subjected to bending load. This is detrimental to the impact resistance of a part produced with high-strength steel, because even if the material is able 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 load and the part will fail prematurely. Description of the Invention

[006] The objective of the present invention is to meet the aforementioned challenge and provide a press-hardened steel part having a Petition 870260000890, dated 06 / 01 / 2026, page 11 / 38 2 / 21 combination of high mechanical properties with a tensile strength after hot stamping greater than or equal to 1800 MPa and a bending angle in the rolling direction, normalized to 1.5 mm equal to or greater than 50° as measured by the VDA-238 standard.

[007] Another objective of the invention is to obtain a steel sheet that can be transformed by hot forming into a press-hardened steel part.

[008] The objective of the present invention is achieved by providing a steel plate according to claim 1, optionally having the characteristics of claim 2. Another objective of the present invention is achieved by providing a press-hardened steel part according to claim 3. The steel part may also comprise the characteristics of claim 4. Another objective is achieved by providing the method according to claim 5. Description of Embodiments of the Invention

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

[010] A steel blank refers to a flat sheet of steel that has been cut into any shape suitable for its use. A blank has an upper and a lower face, which are also called the upper and lower sides or upper and lower surfaces. The distance between these faces is designated as the thickness of the blank. The thickness can be measured, for example, using a micrometer, whose spindle and anvil are placed on the upper and lower faces. Similarly, the thickness can also be measured on a formed part.

[011] Hot stamping is a forming technology that involves heating a raw workpiece to a temperature at which the Petition 870260000890, dated 06 / 01 / 2026, p. 12 / 38 3 / 21 The microstructure of the steel was at least partially transformed into austenite, shaping the raw part at high temperature by stamping and quenching the formed part to obtain a microstructure having very high resistance. Hot stamping allows obtaining parts with very high resistance and complex shapes and presents 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 of the hot stamping process itself, but also possibly other subsequent heat treatment cycles, such as the paint curing step, performed after the part has been painted 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, a paint curing step, if paint curing was actually performed.

[012] Maximum tensile strength is measured according to ISO 6892-1, published in October 2009. Tensile test specimens are cut from flat areas 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 α1.5 is the bending angle normalized to 1.5 mm, t is the thickness, and at is the bending angle for thickness t: a1,5 = ^tx Vt) / Vi ,5

[014] In the present invention, the bending angle was measured in the rolling direction, that is, the direction along which the steel sheet bends. Petition 870260000890, dated 06 / 01 / 2026, page 13 / 38 4 / 21 shifted during the hot rolling stage. The bending angle was measured using a laser measuring device. 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 unknown, it can be determined using Electron Backscatter Diffraction (EBSD) analysis through the 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 represents the ability of the part to resist deformation without the formation of cracks.

[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, said limits being within the possible composition range according to the invention.

[017] According to the invention, the carbon content varies from 0.3% 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 0.3%, the tensile strength will not reach the desired value.

[018] The manganese content varies from 0.5% to 1.0%. Above 1.0% addition, the risk of MnS formation increases at the expense of foldability. Below 0.5% the reversibility of the steel sheet is reduced.

[019] The silicon content varies from 0.4% to 0.8%. Silicon is a Petition 870260000890, dated 06 / 01 / 2026, page 14 / 38 5 / 21 element that participates in solid solution hardening. Silicon is added to limit carbide formation. Above 0.8%, silicon oxides form on the surface, which impairs the coating capacity of the steel. In addition, the weldability of the steel sheet may be reduced.

[020] The chromium content ranges from 0.1% to 1.0%. Chromium is an element involved in solid solution hardening and must be greater than 0.1% to ensure sufficient strength. The chromium content is preferably less than 0.4% to limit processability and cost issues. Preferably, the chromium content ranges from 0.1% to 0.4%.

[021] The molybdenum content varies from 0.1% to 0.5%. Molybdenum improves the reversibility of steel. Below 0.1% tensile strength is not achieved. Molybdenum is preferably not higher than 0.4% to limit costs.

[022] Niobium ranges from 0.01% to 0.1%. Niobium improves the ductility of steel. Above 0.1%, the risk of formation of NbC or Nb(C,N) carbides increases, to the detriment of foldability. Preferably, the niobium content ranges from 0.03% to 0.06%.

[023] According to the invention, the aluminum content varies from 0.01% to 0.1% because it is a very effective element for deoxidizing the 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 varies from 0.03% to 0.05%.

[024] According to the invention, the titanium content varies from 0.008% to 0.03% to protect the boron, which would be trapped in the BN precipitates. The titanium content is limited to 0.03% to avoid excessive formation of TiN. As will be explained in more detail, it is possible to add the appropriate amount of Ti to capture the residual N content by measuring the N level of the liquid steel before adding Ti. Petition 870260000890, dated 06 / 01 / 2026, page 15 / 38 6 / 21

[025] According to the invention, the boron content varies between 0.0005% and 0.003%. Boron improves the revererability of steel. The boron content is not higher than 0.003% to avoid problems of slab breakage during continuous casting.

[026] Phosphorus is controlled below 0.020%, as it causes brittleness and solderability problems.

[027] Calcium is controlled below 0.001% because the presence of calcium in liquid steel can lead to the formation of coarse precipitates that are detrimental to bendability.

[028] Sulfur is controlled below 0.004% because the presence of sulfur in liquid steel can lead to the formation of MnS precipitates that are detrimental to bendability.

[029] Nitrogen is controlled below 0.005%, preferably below 0.004%, even more preferably below 0.003%. The presence of nitrogen can lead to the formation of precipitates such as TiN or TiNbCN, which are detrimental to foldability.

[030] Nickel is optionally added, up to a level of 0.5%. Nickel can be used to protect steel against delayed cracking.

[031] The remainder of the steel composition is iron and impurities resulting from smelting.

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

[033] The steel sheet has a microstructure comprising, in the surface fraction, 60% to 95% ferrite, with the remainder being islands of martensite-austenite, pearlite or bainite.

[034] Ferrite is formed during the intercritical annealing of cold-rolled steel sheet. The remainder of the microstructure is austenite at the end of immersion, which transforms into islands of martensite-austenite, pearlite or bainite. Petition 870260000890, dated 06 / 01 / 2026, page 16 / 38 7 / 21 during the cooling of the steel plate.

[035] The total amount of ferrite in the microstructure of the steel sheet is a function of the chemical composition, the annealing temperature Ta, and the immersion time tA. The higher the annealing temperature Ta, in the range of 700 °C to 850 °C, and the longer the time tA, in the range of 10 seconds to 20 minutes, the more austenite will be formed during annealing. After annealing, the transformation of the austenite formed into martensite, bainite, or ferrite will depend mainly on the cooling rate. Preferably, the cooling rate is less than 10 °C / s to form as many soft phases (ferrite, bainite) as possible. This allows for good processability of the steel sheet before hot stamping.

[036] With reference to Figure 1, the steel sheet (1) according to the invention comprises a volume portion (3) and an upper and lower coating layer (2). The total thickness of the steel sheet (1) is t0 and the thickness ts of the coating layers (2) is such that ts = t0 * 10%. In other words, the coating layers (2) occupy the outermost 10% of the thickness on both sides of the volume.

[037] The aforementioned coating layers (2) have a coating layer inclusion population in which the surface fraction of oxides is equal to or less than 60*10-6. The method used to measure the said inclusion population will be detailed below.

[038] The inventors discovered that there is a correlation between the bending angle and the inclusion population of the coating layer, in particular the oxide population. By controlling said coating inclusion population it is possible to improve the bending angle without adversely affecting other product properties, such as, for example, tensile strength.

[039] The methodology used to characterize inclusions in steel plates and steel parts is described below. It should be understood that this is Petition 870260000890, dated 06 / 01 / 2026, page 17 / 38 8 / 21 is just one possible methodology, and other protocols can also be implemented.

[040] The inclusions present in the steel plate were characterized using Scanning Electron Microscopy (SEM) with Field Effect Gun (FEG). A Tescan Mira 3 SEM was used with a power setting of 14kV. In addition, the inclusions were analyzed by Energy Dispersive Spectrometry (EDS). A 120mm2 Bruker EDS probe was used.

[041] The sample is divided into 3 areas (top coating, bottom coating, volume, as described previously). Each area is divided into fields. Inclusions are detected in each field. A zoom is made on each inclusion to capture morphological features and perform EDS analyses. A dual gray-level threshold is defined to capture particles (on a scale ranging from 0 to 255, where 0 is black and 255 is white): - Classic dark particles, such as oxides, with a gray level <150; and - Bright particles, such as NbC particles, with a gray level > 220.

[042] Using the information from the EDS probe, the shape and brightness level, each particle is then classified into one of the following categories: TiN, NbC, TiNbCN, alumina, Complex oxides, Oxysulfides, MnS.

[043] The next step is to calculate the following characteristics for the entire set of inclusions and for each particle family: -average diameter in microns; and -density in number of inclusions / mm2; - Surface fraction of inclusions, defined as the total sum of all analyzed fields of the surface area occupied by a given inclusion family divided by the total surface area of ​​all analyzed areas. The surface fraction of inclusions can be calculated using the following formula. Petition 870260000890, dated 06 / 01 / 2026, page 18 / 38 9 / 21 (here for a type of particle called “X”): Surface area fraction of particle X Σ surface area of ​​all particles X surface area analyzed

[044] The surface fraction of inclusions combines in a single parameter information about both the density level of the particles and their average size. The inventors found that the surface fraction of inclusions is a good indicator of cleanliness and correlates well, in the case of specific inclusions, with some key properties in use, such as the bending angle.

[045] The coated steel sheet according to the invention can be produced by any suitable manufacturing method and the person skilled in the art can define one. However, it is preferred to use the method according to the invention comprising the steps described below.

[046] In the following description, the term ladle refers to the container used to hold liquid steel during the refining process. The term ladle refers to the container into which liquid steel is poured before being molded into the molds - the ladle is used in continuous casting: it allows a buffer of liquid steel to be available for casting between the finishing of the casting of one ladle and the opening of the next ladle.

[047] A semi-finished product capable of being subsequently hot-rolled is supplied with the steel composition described above. Special care must be taken in refining said semi-finished product, particularly in the liquid phase and during casting, in order to manage the inclusion population.

[048] In a first embodiment, the liquid steel refining process comprises the following steps: After the pig iron has been decarburized in the converter by oxygen blasting, the liquid steel is tapped in a ladle without the addition of Al or any other Petition 870260000890, dated 06 / 01 / 2026, page 19 / 38 10 / 21 deoxidizing element, such as Si or Mn, which would deoxidize the crude steel at this stage. This allows minimizing the subsequent uptake of nitrogen from the liquid steel; - The main alloying elements, in particular Mn, Si, Cr, Mo, Nb and B, but not Ti, are incorporated into the liquid steel under vacuum, for example, using a Ruhrstahl Heraeus (RH) vacuum degassing system or a vacuum tank degasser (VTD). This allows, among other advantages, ensuring a low nitrogen content; - A desulfurization step is carried out after the aforementioned vacuum degassing step in order to achieve the desired very low sulfur levels. The desulfurization step involves exchanges between the liquid steel and a slag formed by the addition of heat-sensitive fluxes, such as CaO-based fluxes. These fluxes can be added before the desulfurization step, for example, during the bypass after the converter; - Ti is added after the desulfurization step. Ti is added, for example, using the measured nitrogen composition to add just the right amount of Ti to precipitate the N in the form of TiN in the semi-finished product. For example, the amount of Ti added, as a percentage by weight, is equal to or slightly greater than 3.42 times the amount of Nitrogen measured after desulfurization; - To minimize the amount of calcium aluminates, which can be detrimental to the bending performance of the press-hardened part, additions containing Ca (such as SiCa, FeCa or pure Ca) are not made. Thanks to the very low S levels achieved using the target composition and process, the steel produced will contain a low population of MnS; - A minimum time is provided to promote the flotation of inclusions. Inclusion flotation refers to the phenomenon whereby inclusions in liquid steel, due to their lower density than steel, float to the slag covering the liquid steel. Once the inclusions become trapped in the Petition 870260000890, dated 06 / 01 / 2026, page 20 / 38 11 / 21 slag, they are removed from the liquid steel and will not be melted into the semi-finished product, thus decreasing the inclusion population. The inventors discovered that the aforementioned inclusion flotation time is correlated with the surface fraction of oxides in the surface layers of the steel sheet. The determination of the aforementioned inclusion flotation time depends on the specific process route and the equipment used to manufacture the steel. For example, in the case described above, where the additions of Mn, Si, Cr, Mo, Nb, and B are made using a vacuum degasser and the liquid steel is further desulfurized after the vacuum degasser, the inclusion flotation time is the sum of: - the time spent in the vacuum degasser after the addition of Mn, Si, Cr, Mo, Nb and B (the time is measured after the addition of the alloying elements because the addition of said elements can, by itself, nucleate inclusion particles that will need to float to the slag); - the time spent on the desulfurization stage; and - the waiting time between the desulfurization stage and the continuous casting operation itself. This waiting time may include gentle agitation using controlled injection of inert gas after desulfurization, ladle transport steps between the desulfurization station and the continuous casting operation, waiting time in the continuous casting stage, etc. The continuous casting stage begins when the ladle is opened to start pouring into the casting ladle. [ 049] In a second embodiment, the liquid steel refining process comprises the following steps: - After the pig iron has been decarburized in the converter by oxygen blowing, the liquid steel is tapped in a ladle. Optionally, at this stage, some of the alloying elements can be added, such as, for example, at least part of the Mo, Cr, and Mn content of the steel can be added; A desulfurization step is then carried out to reach the required levels. Petition 870260000890, dated 06 / 01 / 2026, page 21 / 38 12 / 21 very low levels of Sulfur desired. The desulfurization step involves exchanges between the liquid steel and a slag formed by the addition of heat fluxes, such as CaO-based fluxes. These fluxes can be added before the desulfurization step, for example, during the bypass after the converter; - The main alloying elements, in particular Mn, Si, Cr, Mo, Nb and B, but not Ti at this stage, are incorporated into the liquid steel under vacuum, for example using an RH vacuum degassing system or a VTD. After the addition of the main alloying elements, the steel is stirred under vacuum; this is known as the stirring step. For example, when using an RH vacuum degassing system, stirring is naturally induced in the system by the circulation of the liquid steel within the vacuum vessel snorkels. When using a VTD, stirring can be induced, for example, by bubbling argon into the liquid steel. This stirring step plays the role of both uniformly distributing the alloying elements within the liquid steel and promoting the flotation of inclusions. - Ti is added at the end of the vacuum degassing process. For example, the amount of Ti added is determined using the measured composition of Nitrogen to add just the right amount of Ti to precipitate N in the form of TiN in the semi-product. For example, the amount of Ti added, as a percentage by weight, is equal to or slightly greater than 3.42 times the amount of Nitrogen measured at the end of the stirring stage; - To minimize the amount of calcium aluminates, which can be detrimental to the bending performance of the press-hardened part, no additions containing Ca (such as SiCa, FeCa, or pure Ca) are made. Thanks to the very low S levels achieved using the target composition and process, the steel produced will contain a low population of MnS. The inventors found that with such a low MnS content, the bending performance was very good even without the addition of Ca to globularize the Petition 870260000890, dated 06 / 01 / 2026, page 22 / 38 13 / 21 population of MnS; - As in the first embodiment, a minimum time is provided to promote the flotation of inclusions. In this second embodiment, where the liquid steel is desulfurized before the addition of the main alloying elements under vacuum, the inclusion flotation time is the sum of: - the time spent in the vacuum degasser after the addition of Mn, Si, Cr, Mo, Nb, and B; and - The waiting time between the vacuum degasser and the continuous casting operation itself. This waiting time may include ladle transport steps between the vacuum degasser and the continuous casting operation, waiting time during the continuous casting stage, etc. The continuous casting stage begins when the ladle is opened to start pouring into the casting ladle.

[050] More generally, it is preferable to refine steel by making the main additions of Mn, Si, Cr, Mo, Nb and B under vacuum, using, for example, a vacuum degasser. This allows for a low nitrogen content in the steel and, in turn, allows for better control of nitrogen-containing inclusions in the steel.

[051] More generally, inclusion flotation time is defined as the total amount of time that liquid steel spends after the addition of Mn, Cr, Si, Mo, Nb and B and before the start of the casting stage.

[052] To control the inclusion surface fraction in the steel sheet coating, the inclusion flotation time must be controlled above a minimum inclusion flotation time tf. The value of tf will depend on the specific industrial setup used to produce the steel. It will depend on the production path in the steel mill and also on the geometric configuration of the ladles used to process the liquid steel. As the inclusion flotation time is related to fluid dynamics and the movement of small particles within the liquid steel, the minimum inclusion flotation time required to achieve the desired level of specific inclusions in Petition 870260000890, dated 06 / 01 / 2026, page 23 / 38 14 / 21 The steel coating will depend on the size of the shells, their diameter, height, volume, etc. For example, the minimum inclusion flotation time is 60 minutes. For example, the minimum inclusion flotation time is 53 minutes.

[053] To determine the minimum inclusion flotation time tf for a given steel composition and a given industrial equipment and production route, the following method is recommended: - Several heating tests are performed using the same chemical composition targets; - The aforementioned heating processes are produced using different inclusion flotation times. For example, a set of runs is carried out using inclusion flotation times ranging from a minimum inclusion flotation time that corresponds to the minimum feasible inclusion flotation time of the industrial route, then incrementally longer inclusion flotation times are applied, for example, using 10-minute time increments. For example, five different inclusion flotation times are applied to five different heating processes; 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; and - The surface oxide fraction of the coating layer and the respective inclusion flotation times are recorded. The inventors discovered that there is a correlation between said surface oxide fraction of the coating layer and said inclusion flotation time. The longer the inclusion flotation time, the lower the surface oxide fraction of the coating layer. The minimum inclusion flotation time tf is determined as the inclusion flotation time above which the surface oxide fraction of the coating layer is equal to or less than 60*10-6. For example, the inventors discovered that by using specific industrial equipment that was available to the inventors and applying the route of Petition 870260000890, dated 06 / 01 / 2026, page 24 / 38 15 / 21 processing of the first embodiment, the minimum inclusion flotation time was 60 minutes, preferably 53 minutes. This will be illustrated in the examples below.

[054] 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 semi-finished product suitable for hot rolling. During the casting stage, special care must be taken to avoid oxygen uptake and therefore higher levels of oxides 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 of the product from multiple heats poured into a ladle, specific refractories and linings may be used in the ladle, specific allocation rules may be used for first-sequence slabs and transitional slabs between two different runs, 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 at a finishing hot-rolling temperature between 800 °C and 950 °C; - The hot-rolled steel is then cooled and rolled at a Tenolar temperature 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 annealed steel sheet is heated to an annealing temperature Ta between Petition 870260000890, dated 06 / 01 / 2026, page 25 / 38 16 / 21 700 °C and 850 °C and maintained at said temperature Ta for a holding time tA between 10 seconds and 20 minutes; and - In one embodiment of the invention, the annealed steel sheet is cooled to a temperature range of 400 °C to 700 °C and then coated with a metallic coating. [ 055] 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 Mn, Si, Cr, Mo, Nb and B are added using a vacuum degasser and wherein a minimum inclusion flotation time tf is guaranteed, said inclusion flotation time being the total time that the liquid steel spends after the addition of Mn, Si, Cr, Mo, Nb and B and before the start of the casting stage, said minimum inclusion flotation time tf being defined as the minimum inclusion flotation time required to achieve a surface oxide fraction of the coating layer equal to or less than 60*10-6; - to melt the aforementioned liquid steel to obtain a semi-finished product capable of being hot-rolled; - optionally reheat the semi-finished product to a temperature ranging from 1100 °C to 1300 °C; - hot roll the semi-finished product at a finishing hot rolling temperature between 800 °C and 950 °C; - To roll the hot-rolled steel sheet at a Tenrolar rolling temperature below 670 °C to obtain a rolled steel sheet; - optionally, strip the rolled steel sheet; - optionally cold-roll the coiled steel sheet to obtain a cold-rolled steel sheet; - optionally heat the hot-rolled steel sheet or the cold-rolled steel sheet to an annealing temperature Ta of between 700 °C and 850 °C and maintain the steel sheet at said temperature. Petition 870260000890, dated 06 / 01 / 2026, page 26 / 38 17 / 21 temperature Ta during a holding time tA ranging from 10 seconds to 20 minutes, to obtain an annealed steel sheet; - optionally, cool the annealed steel sheet to a temperature range of 400 °C to 700 °C; - optionally, coat the annealed steel sheet with a metallic coating; and - Optionally, cool the coated steel sheet to room temperature.

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

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

[058] The microstructure of the pressed part comprises, in the surface fraction, more than 95% martensite and less than 5% bainite + ferrite. Furthermore, the pressed part according to the invention comprises a volume portion and an upper and lower coating layer, wherein the coating layers occupy the outermost 10% of the thickness on both sides of the volume. Said coating layers have a coating layer inclusion population in which the surface fraction of oxides is equal to or less than 60*10-6.

[059] The pressed part according to the invention has a bending angle in the rolling direction standardized to 1.5 mm of at least 50° and a tensile strength TS of at least 1800 MPa. This high tensile strength and bending angle give the part very good mechanical strength, especially in the event of a collision. They exhibit very good energy absorption capacity and anti-intrusion capability, thus increasing vehicle safety. Petition 870260000890, dated 06 / 01 / 2026, page 27 / 38 18 / 21

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

[061] Eight different samples from eight different heats A, B, C, D, E, F, G, H of steel produced by an industrial production route were tested. Samples I1, I2, I3, I4, I5 and I6 are in accordance with the invention, samples R1, R2 are reference samples. Table 1 - Sample composition

[062] The compositions tested are listed in the following table, where the element contents are expressed as a percentage by weight: Reference steel AB c DEFGHC 0.4 0.4 0.3 0.4 0.4 0.3 0.4 0.4 Mn 0.6 0.7 0.6 0.6 0.7 0.6 0.8 0.8 Si 0.5 0.5 0.5 0.6 0.5 0.5 0.6 0.5 Cr 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 Mo 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Nb 0.06 0.05 0.05 0.05 0.05 0.05 0.05 0.04 Al 0.03 0.03 0.05 0.04 0.03 0.04 0.05 0.03 Ti 0.013 0.009 0.010 0.011 0.009 0.012 0.013 0.014 B 0.0016 0.0018 0.0022 0.0020 0.0017 0.0016 0.0020 0.0018 P 0.012 0.012 0.011 0.011 0.012 0.011 0.009 0.012 Ca 0.0003 <0.0003 0.0004 0.0003 <0.0003 <0.0003 <0.0003 <0.0003 S 0.004 0.002 0.001 0.001 0.003 0.002 0.002 0.002 N 0.002 0.004 0.003 0.003 0.003 0.003 0.004 0.003 Ni 0.4 0.4 0.4 0.4 0.4 0.02 0.01 0.01 Table 2 - Parameters of the steelmaking process and inclusions in the coating layer.

[063] The following process parameters were applied in the steelmaking process and the following surface inclusion fraction of the coating layer Petition 870260000890, dated 06 / 01 / 2026, p. 28 / 38 19 / 21 was observed - the underlined values ​​do not correspond to the invention: Sample Reference I1 I2 I3 I4 I5 I6 R1 R2 Steel Reference ABCFGHDE Steelmaking Process Times (minutes) RH* 5 10 2 2 5 4 2 2 DS** 18 18 15 18 32 104 18 16 cc*** 46 36 41 33 18 22 24 32 Inclusion Float Time = RH + DS + CC 69 64 60 53 55 130 44 50 Surface Fraction of Inclusions in the Coating Layer (*10-6) NbC Particles 33 185 204 79 100 81 110 198 Oxide Particles 31 38 56 47 38 26 70 65 MnS Particles 27 18 14 12 24 9 11 15 TiNbCN Particles 1 0 0 0 0 0 0 0 Oxides + MnS + TiNbCN 59 56 70 59 62 35 81 80 *RH = RH Process time of the vacuum degasser after the addition of Mn, Si, and Cr; **DS = desulfurization process time; and ***CC = Time elapsed between the end of desulfurization and the beginning of continuous casting (=opening of the ladle to pour into the casting ladle). Table 3 - Other process conditions

[064] The following process parameters were applied along the production route: I1 I2 I3 I4 I5 I6 R1 R2 Plate reheating temperature (°C) 1220 1244 1244 1206 1238 1243 1253 1233 Rolling temperature (°C) 914 902 913 911 908 892 911 900 Tenolar winding temperature (°C) 537 532 525 527 537 535 526 526 Steel sheet annealing temperature (°C) 745 748 738 780 773 778 741 751 Steel sheet annealing time (minutes) 9 3 5 18 14 20 6 11 Hot forming annealing temperature (°C) 900 900 900 900 900 900 900 900 Hot forming annealing time (minutes) 6 6 6 7 7 7 6 6 Petition 870260000890, dated 06 / 01 / 2026, page 29 / 38 20 / 21 Table 4 - Microstructure, bending angles and tensile strength

[065] The following microstructures, bending angles and tensile strength were measured on the samples, the underlined values ​​do not conform to the invention: I1 I2 I3 I4 I5 I6 R1 R2 Steel plate Thickness (mm) 1.0 1.6 1.2 1.5 2.0 1.5 1.7 1.3 Ferrite content 88% 86% 93% 96% 89% 92% 91% 90% Martensite + austenite + pearlite + bainite 12% 14% 7% 5% 11% 8% 9% 10% Pressed part t Tensile strength (MPa) 1914 2025 1850 1801 1959 1954 1921 1998 Bending angle measured in the rolling direction (°) 63 49 57 59 49 60 46 48 Bending angle normalized to 1.5 mm in the rolling direction (°) 52 51 52 59 57 60 49 45 Martensite content 100% 100% 100% 100% 100% 100% 100% 100%

[066] Table 4 shows that the samples according to the invention (references I1, I2, I3, I4, I5 and I6) have a tensile strength above 1800MPa and a bending angle in the rolling direction normalized to 1.5mm above 50° thanks to their specific composition and inclusions in the coating layer.

[067] Referring to Table 2, there is a relationship between the inclusion flotation time and the surface oxide fraction of the coating layer. The inclusion flotation time represents the total time that the liquid steel spends after the addition of Mn, Cr, Si, Mo, Nb and B and before the start of the continuous casting stage.

[068] The inventors discovered that by using the specific composition of the invention and increasing the inclusion flotation time above a minimum inclusion flotation time tf, it is possible to control the oxide fraction of the coating layer below a critical level that ensures good Petition 870260000890, dated 06 / 01 / 2026, pp. 30 / 38 21 / 21 flexural strength. In the industrial setting used to produce the samples given in the current examples, the minimum inclusion flotation time tf is 53 minutes. The tf value will depend on the specific industrial setting used to produce the steel.

[069] When the inclusion flotation time is equal to or greater than tf = 60 minutes, the surface fraction of oxides in the coating layer is equal to or less than 60*10-6. When the inclusion flotation time is less than tf = 53 minutes, the surface fraction of oxides in the coating layer is greater than 60*10-6.

[070] The inventors discovered that when steel is subjected to a bending load, the surface inclusion fraction in the coating layers plays an important role in improving the material's resistance to cracking. Surprisingly, this is not the case for all types of inclusions. For example, NbC inclusions do not appear to have a significant impact on the bending properties of steel. On the other hand, the surface oxide fraction was found to play an important role in bending performance. Reducing the surface oxide fraction helps improve bending performance.

[071] Referring to Table 4, the samples according to the invention (I1, I2, I3, I4, I5 and I6), which all have a surface oxide fraction of coating layer equal to or less than 60*10-6, all have a bending angle in the normalized rolling direction for 1.5 mm of at least 50° and also a tensile strength of at least 1800 MPa. On the other hand, the reference samples (R1, R2), although maintaining a tensile strength above 1800 MPa, all have a bending angle in the normalized rolling direction for 1.5 mm below 50°. Therefore, the steel produced according to the invention will exhibit better resistance to crack formation when subjected to a load, while at the same time exhibiting a very high tensile strength, which will improve the impact resistance and safety of the part produced with said material. Petition 870260000890, dated 06 / 01 / 2026, pages 31 / 38

Claims

1 / 3 Claims 1. STEEL PLATE (1), characterized by being made of a steel having a composition comprising, in weight percentage: C: 0.3 - 0.4% Mn: 0.5 - 1.0% Si: 0.4 - 0.8% Cr: 0.1 - 0.4% Mo: 0.1 - 0.5% Nb: 0.01 - 0.1% Al: 0.01 - 0.1% Ti: 0.008 - 0.03% B: 0.0005 - 0.003% P < 0.020% Ca < 0.0010% S < 0.004% N < 0.005% and optionally comprising: Ni < 0.5% the remainder of the composition being iron and unavoidable impurities resulting from casting, the steel plate (1) having a microstructure comprising, in surface fraction, from 60% to 95% ferrite, the remainder being islands of martensite-austenite, pearlite or bainite, the steel plate (1) comprising from a volume (3) to the surface of the coated steel plate (1): - the volume (3), and - the volume (3) being covered by a coating layer (2) occupying the outermost 10% of the thickness in both Petition 870260000890, dated 06 / 01 / 2026, page.32 / 38 2 / 3 sides of volume (3), the coating layer (2) having a coating layer inclusion population (2) in which the surface fraction of oxides is equal to or less than 60*10-6.

2. STEEL SHEET (1), according to claim 1, characterized by further comprising a metallic coating on at least one side.

3. PRESS-HARDENED STEEL PART, characterized by the steel part having a composition comprising, in weight percentage: C: 0.3 - 0.4% Mn: 0.5 - 1.0% Si: 0.4 - 0.8% Cr: 0.1 - 0.4% Mo: 0.1 - 0.5% Nb: 0.01 - 0.1% Al: 0.01 - 0.1% Ti: 0.008 - 0.03% B: 0.0005 - 0.003% P < 0.020% Ca < 0.001% S < 0.004% N < 0.005% and optionally comprising: Ni < 0.5%, the remainder of the composition being iron and unavoidable impurities resulting from casting, the steel part having a microstructure comprising, in surface fraction, more than 95% martensite and up to 5% bainite or ferrite Petition 870260000890, dated 06 / 01 / 2026, page.33 / 38 3 / 3 the steel part comprises from a volume (3) to the surface of the steel part: - the volume (3), and - the volume (3) being covered by a coating layer (2) occupying the outermost 10% of the thickness on both sides of the volume (3), the coating layer (2) having a coating layer inclusion population (2) in which the surface fraction of oxides is equal to or less than 60*10'6.

4. PRESS-HARDENED STEEL PART, according to claim 3, characterized in that the press-hardened steel part has a tensile strength of at least 1800 MPa and a bending angle in the rolling direction normalized to 1.5 mm of at least 50°.

5. PROCESS FOR MANUFACTURING A PRESS-HARDENED STEEL PART, as defined in any one of claims 3 to 4, characterized by comprising the following successive steps: - providing the steel sheet (1), as defined in any one of claims 1 to 2; - cutting the steel sheet (1) into a predetermined shape, so as to obtain a blank steel part; - heating the blank steel part to a temperature of 880 °C to 950 °C for 10 seconds to 15 minutes to obtain a heated blank steel part; - transferring the heated blank to a forming press; - hot forming the heated blank in the forming press to obtain a formed part; and - tempering the formed part in a die. Petition 870260000890, dated 06 / 01 / 2026, pp. 34 / 38

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