Steel sheet and high-strength press-hardened steel part and method of manufacturing thereof

Abstract

A steel sheet made of a steel having a composition comprising C: 0.3 to 0.4%, Mn: 0.5 to 1.0%, Si: 0.4 to 0.8%, Cr: 0.1 to 1.0%, Mo: 0.1 to 0.5%, Nb: 0.01 to 0.1%, Al: 0.01 to 0.1%, Ti: 0.008 to 0.03%, B: 0.0005 to 0.003%, P ≤ 0.020%, Ca ≤ 0.001%, S ≤ 0.004%, N ≤ 0.005% and optionally comprising Ni < 0.5%, the steel sheet having a microstructure comprising 60 to 95% ferrite in terms of surface fraction, the rest being martensite-austenite islands, pearlite or bainite, and including a bulk and a surface layer occupying the outermost 10% thickness on both sides of the bulk, the surface layer having a population of surface layer inclusions wherein the surface fraction of oxides is equal to or lower than 60*10 ‑6 -2.

Description

[0001] This application is a divisional application of the invention patent application filed on April 29, 2022, with application number "202280031942.9" and invention title "Steel Plate and High-Strength Press-Hardened Steel Components and Manufacturing Method Thereof". Technical Field

[0002] This invention relates to steel plates, and more specifically to high-strength press-hardened steel components with good bending properties. Background Technology

[0003] High-strength press-hardened components can be used as structural elements in motor vehicles for intrusion prevention or energy absorption functions.

[0004] In this type of application, the goal is to produce steel components that combine high mechanical strength and high impact resistance. Furthermore, given global environmental protection efforts, one of the major challenges in the automotive industry is to reduce vehicle weight to improve fuel efficiency without neglecting safety requirements.

[0005] This weight reduction can be achieved, in particular, by using steel components with a martensitic microstructure.

[0006] Producing very high-strength steels that exhibit good resistance to the formation of bending cracks is also challenging. In fact, very high-strength steels tend to crack prematurely when subjected to bending loads. This is detrimental to the impact resistance of components made from such high-strength steels, because even if the material can withstand very high loads due to its high tensile strength, once cracks begin to appear in the component, these cracks will propagate rapidly under sustained loads, and the component will fail prematurely. Summary of the Invention

[0007] The object of the present invention is to address the above-mentioned challenges and to provide a press-hardened steel component having a combination of high mechanical properties: a tensile strength of 1800 MPa or higher after hot stamping and a bending angle in the rolling direction of 1.5 mm or higher than 50° as measured by VDA-238 standard.

[0008] Another object of the present invention is to obtain a steel sheet that can be thermoformed into such press-hardened steel parts.

[0009] The object of the present invention is achieved by providing a steel plate according to claim 1, optionally having the features of claim 2. Another object of the present invention is achieved by providing a press-hardened steel component according to claim 3. This steel component may further include the features of claim 4. Another object is achieved by providing the method according to claim 5. Attached Figure Description

[0010] The invention will now be described in detail, and through embodiments without limitation and with reference to Figure 1 The present invention will be illustrated by examples. Figure 1 This is a schematic cross-section of the steel plate according to the present invention. Detailed Implementation

[0011] A billet is a flat sheet of steel that has been cut into any shape suitable for its application. The billet has a top surface and a bottom surface, also referred to as top side and bottom side, or top surface and bottom surface. The distance between these surfaces is called the thickness of the billet. This thickness can be measured, for example, using a micrometer with its spindle and anvil placed on the top and bottom surfaces. Similarly, thickness can also be measured on shaped parts.

[0012] Hot stamping is a forming technique that involves heating a blank to a temperature at which the microstructure of the steel is at least partially transformed into austenite, thereby forming the blank at a high temperature by stamping, and quenching the formed part to obtain a microstructure with very high strength. Hot stamping allows for the production of very high-strength parts with complex shapes and presents many technical advantages. It should be understood that the heat treatment subjected to the part includes not only the thermal cycle of the hot stamping process itself, but may also include other subsequent heat treatment cycles, such as a baking process for repainting after the part has been painted. The mechanical properties of the following hot-stamped parts are measured after all thermal cycles (optionally including, for example, a baking process if repainting has indeed been performed).

[0013] Ultimate tensile strength was measured according to ISO 6892-1, published in October 2009. Tensile test specimens were cut from a flat area of ​​the hot-stamped part. If necessary, smaller tensile test specimens were taken to fit all available flat areas on the part.

[0014] 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 value of this invention refers 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 αt is the bending angle for thickness t:

[0015]

[0016] In this invention, the bending angle is measured along the rolling direction (i.e., the direction in which the steel sheet travels during the hot rolling step). The bending angle is measured using a laser measuring device. When performing a bending test on a hot-stamped part, a sample is cut from a flat area of ​​the part. If necessary, a small-sized sample is taken to fit the entire available flat area on the part. If the rolling direction on the hot-stamped part is not known, it can be determined using electron back-scattered diffraction (EBSD) analysis across the sample cross section in a scanning electron microscope (SEM). The rolling direction is determined based on the intensity of the orientation density function (ODF) representing the major fibers at φ2 = 45°, where φ2 is the Euler angle as defined in “H.-J. Bunge: Texture Analysis in Materials Science - Mathematical Methods. Butterworth Co., first English edition (publication) 1982” (for the definition of φ2, see Figures 2.2 and 2.3).

[0017] The bending angle of a component represents its ability to resist deformation without forming cracks.

[0018] The composition of the steel according to the invention will now be described, with contents expressed as a weight percentage. The chemical composition is given by the lower and upper limits of the composition range, which are included within the possible composition range according to the invention.

[0019] According to the present invention, the carbon content is in the range of 0.3% to 0.4% to ensure satisfactory strength. If the carbon content is greater than 0.4%, the weldability and bendability of the steel sheet may decrease. If the carbon content is less than 0.3%, the tensile strength will not reach the target value.

[0020] The manganese content ranges from 0.5% to 1.0%. Adding more than 1.0% increases the risk of MnS formation, thus impairing flexibility. Below 0.5%, the hardenability of the steel plate decreases.

[0021] The silicon content ranges from 0.4% to 0.8%. Silicon is an element involved in the hardening of solid solutions. Silicon is added to limit carbide formation. Above 0.8%, silicon oxides form on the surface, which impairs the coatability of the steel. Furthermore, the weldability of the steel sheet may be reduced.

[0022] The chromium content ranges from 0.1% to 1.0%. Chromium is an element involved in the hardening of solid solutions and must be above 0.1% to ensure sufficient strength. The chromium content is preferably below 0.4% to limit processability issues and costs. Preferably, the chromium content ranges from 0.1% to 0.4%.

[0023] The molybdenum content ranges from 0.1% to 0.5%. Molybdenum improves the hardenability of steel. Below 0.1%, tensile strength cannot be achieved. Preferably, the molybdenum content is no higher than 0.4% to limit costs.

[0024] The niobium content ranges from 0.01% to 0.1%. Niobium improves the ductility of steel. Above 0.1%, the risk of forming NbC or Nb(C,N) carbides increases, thereby impairing flexibility. Preferably, the niobium content ranges from 0.03% to 0.06%.

[0025] According to the invention, the aluminum content ranges from 0.01% to 0.1%, as it is a very effective element for deoxidizing liquid steel during refining. Aluminum can protect boron if the titanium content is insufficient. An aluminum content below 0.1% avoids oxidation problems and ferrite formation during press hardening. Preferably, the aluminum content ranges from 0.03% to 0.05%.

[0026] According to the present invention, the titanium content ranges from 0.008% to 0.03% to protect the boron to be trapped in the BN precipitates. Limiting the titanium content to 0.03% avoids the formation of excessive TiN. As will be explained in further detail, an appropriate amount of Ti can be added to capture residual N content by measuring the N level of the molten steel before adding Ni.

[0027] According to the present invention, the boron content ranges from 0.0005% to 0.003%. Boron improves the hardenability of steel. The boron content is not higher than 0.003% to avoid slab breakage during continuous casting.

[0028] Phosphorus should be kept below 0.020% because it causes brittleness and solderability problems.

[0029] Calcium content should be kept below 0.001%, as the presence of calcium in molten steel can lead to the formation of coarse precipitates that are detrimental to flexibility.

[0030] Sulfur content should be controlled below 0.004%, as the presence of sulfur in molten steel can lead to the formation of MnS precipitates that are detrimental to flexural properties.

[0031] Nitrogen should be controlled to below 0.005%, preferably below 0.004%, and even more preferably below 0.003%. The presence of nitrogen may lead to the formation of precipitates such as TiN or TiNbCN that are detrimental to flexibility.

[0032] Nickel may be added optionally up to a level of 0.5%. Nickel can be used to protect steel from delayed cracking.

[0033] The remaining components of steel are iron and impurities produced during smelting.

[0034] The microstructure of the coated steel plate according to the present invention will now be described.

[0035] The steel plate has a microstructure comprising 60% to 95% ferrite by surface fraction, with the remainder being martensite-austenite islands, pearlite, or bainite.

[0036] Ferrite forms during critical zone annealing of cold-rolled steel sheets. The remaining portion of the microstructure at the end of soaking is austenite, which transforms into martensite-austenite islands, pearlite, or bainite during the cooling of the steel sheet.

[0037] The total amount of ferrite in the microstructure of steel sheet is a function of chemical composition, annealing temperature (TA), and soaking time (tA). Higher annealing temperatures (TA) in the range of 700°C to 850°C and longer soaking times (tA) in the range of 10 seconds to 20 minutes result in the formation of more austenite during annealing. After annealing, the transformation of the formed austenite into martensite, bainite, or ferrite will depend primarily on the cooling rate. Preferably, the cooling rate is below 10°C / second to form as many soft phases (ferrite, bainite) as possible. This allows for good machinability of the steel sheet prior to hot stamping.

[0038] Reference Figure 1 According to the present invention, the steel plate 1 includes a main body portion 3 and top and bottom surface layers 2. The total thickness of the steel plate 1 is t0, and the thickness of the surface layer 2 is ts, such that ts = t0 * 10%. In other words, the surface layer 2 occupies the outermost 10% of the thickness on both sides of the main body.

[0039] The surface layer 2 has an oxide surface fraction equal to or less than 60*10. -6 The surface inclusions. The method for measuring the inclusions will be described in further detail below.

[0040] The inventors have discovered a correlation between the bending angle and the surface inclusion group, particularly the oxide group. By controlling the surface inclusion group, the bending angle can be improved without adversely affecting other product properties, such as tensile strength.

[0041] The following is a description of a method used to characterize inclusions in steel plates and steel components. It should be understood that this is only one possible method and other schemes may be implemented.

[0042] Inclusions present in the steel plate were characterized using scanning electron microscopy (SEM) with a field effect gun (FEG). A Tescan Mira 3 SEM was used at a power setting of 14 kV. Furthermore, the inclusions were analyzed using energy dispersive spectroscopy (EDS). A 120 mm² Bruker EDS probe was used.

[0043] The sample was divided into three regions (top surface, bottom surface, and bulk, as previously described). Each region was further divided into fields. Inclusions were detected within each field. Each inclusion was magnified to capture morphological features and subjected to EDS analysis. A dual grayscale threshold was set to capture particles (0 to 255, where 0 represents black and 255 represents white):

[0044] - Classical dark grain, such as oxide, with a grayscale value <150.

[0045] - Bright particles, such as NbC particles, with a grayscale value > 220.

[0046] Using information from EDS probes, shape, and brightness levels, the particles were then classified into one of the following categories: TiN, NbC, TiNbCN, alumina, composite oxides, oxysulfides, and MnS.

[0047] The next step is to calculate the following characteristics for the entire group of inclusions and each particle family:

[0048] - Average diameter in micrometers

[0049] - Density expressed as the number of inclusions per mm²

[0050] - The surface fraction of inclusions is defined as the sum of all analytical fields over the surface regions occupied by a given family of inclusions, divided by the total surface area of ​​all analytical regions. The surface fraction of inclusions can be calculated using the following formula (here, for the particle type referred to as "X"):

[0051]

[0052] The surface fraction of inclusions combines the particle density level and its average size in a single parameter. The inventors have found that the surface fraction of inclusions is a good indicator of cleanliness and, in the case of specific inclusions, is highly correlated with some key performance characteristics such as bending angle.

[0053] The coated steel sheet according to the invention can be produced by any suitable manufacturing method, and those skilled in the art can define the method. However, it is preferred to use the method according to the invention, which includes the following steps.

[0054] In the following description, the term ladle refers to a container used to hold molten steel during the refining process. The term tundish refers to a container for pouring molten steel before it is poured into a mold—the tundish is used in continuous casting: it allows for a buffer of molten steel available for casting between the completion of pouring one ladle and the opening of the next.

[0055] A semi-finished product having the above-described steel composition is provided, which can be further hot-rolled. Special care should be taken during the refining and casting of the semi-finished product, particularly the liquid-phase semi-finished product, to control inclusion groups.

[0056] In the first embodiment, the steel refining process includes the following steps:

[0057] - After decarburizing pig iron by oxygen blowing in the converter, the molten steel is tapped into the ladle. At this stage, no Al or any other deoxidizing element that would deoxidize the crude steel, such as Si or Mn, is added. This allows for minimizing subsequent nitrogen absorption by the molten steel.

[0058] - For example, the Ruhrstahl Heraeus (RH) vacuum degassing system or Vacuum Tank Degasser (VTD) incorporates the main alloying elements (particularly Mn, Si, Cr, Mo, Nb, and B, but not Ti) into the molten steel under vacuum. Among other advantages, this also allows for ensuring low nitrogen content.

[0059] - A desulfurization step is performed after the vacuum degassing step to achieve a desired very low sulfur level. The desulfurization step involves the exchange between molten steel and slag formed by adding flux (e.g., CaO-based flux) to the heat. These fluxes can be added before the desulfurization step, for example, during tapping after the converter.

[0060] - Add Ti after the desulfurization step. For example, Ti is added by utilizing the measured nitrogen composition to add just the right amount of Ti, thereby precipitating N in the semi-finished product as TiN. For example, the amount of Ti added, by weight percentage, is equal to or slightly more than 3.42 times the amount of nitrogen measured after desulfurization.

[0061] - To minimize the amount of calcium aluminate that could be detrimental to the bending properties of press-hardened parts, no Ca-containing additives (e.g., SiCa, FeCa, or pure Ca) are made. Because a very low S level is achieved using the targeted composition and process, the produced steel will contain low MnS groups.

[0062] - Provides a minimum amount of time to promote inclusion floating. Inclusion floating refers to the phenomenon that inclusions in molten steel float to the surface of the slag covering the molten steel due to their lower density than steel. Once the inclusions are captured in the slag, they are removed from the molten steel and will not be cast into the semi-finished product, thereby reducing the inclusion group. The inventors have found that the inclusion floating time is related to the surface fraction of oxides in the surface layer of the steel plate. The determination of the inclusion floating time depends on the specific process route and equipment used to manufacture the steel. For example, in the case of adding Mn, Si, Cr, Mo, Nb, and B using a vacuum degasser and further desulfurizing the molten steel after the vacuum degasser, the inclusion floating time is the sum of the following:

[0063] - The time spent in the vacuum degasser after the addition of Mn, Si, Cr, Mo, Nb and B (this time is measured after the addition of the alloying elements, as the addition of these elements may itself cause the inclusion particles that would otherwise need to float to the slag to nucleate).

[0064] - The time spent in the desulfurization process,

[0065] - The holding time between the desulfurization step and the continuous casting operation itself. This holding time may include soft stirring using controlled inert gas injection after desulfurization, ladle transport between the desulfurization station and the continuous casting operation, waiting time during the continuous casting step, etc. The continuous casting step begins when the ladle is opened to begin pouring into the casting tundish.

[0066] In the second embodiment, the steel refining process includes the following steps:

[0067] - After decarburizing the pig iron by oxygen blowing in the converter, the molten steel is tapped into a ladle. Optionally, a portion of the alloying elements may be added at this stage, such as at least a portion of the Mo, Cr, and Mn content in the steel.

[0068] - Then a desulfurization step is performed to achieve the desired very low sulfur level. The desulfurization step involves the exchange between molten steel and slag formed by adding flux (such as CaO-based flux) to the smelting process. These fluxes can be added before the desulfurization step, for example, during tapping after the converter.

[0069] - In this stage, major alloying elements (particularly Mn, Si, Cr, Mo, Nb, and B, but not Ti) are incorporated into the molten steel under vacuum, for example using an RH vacuum degassing system or a VTD. After the addition of the major alloying elements, the steel is stirred under vacuum; this is called the stirring step. For example, when using an RH vacuum degassing system, stirring is naturally initiated in the system by circulating the molten steel within the connecting pipes of the vacuum vessel. When using a VTD, stirring can be initiated, for example, by blowing argon gas into the molten steel. This stirring step serves both to ensure the alloying elements are evenly distributed within the molten steel and to promote the flotation of inclusions.

[0070] - Add Ti at the end of the vacuum degassing process. For example, the amount of Ti added is determined by the measured nitrogen composition so that just the right amount of Ti is added, thereby precipitating N in the semi-finished product as TiN. For example, the amount of Ti added, by weight percentage, is equal to or slightly more than 3.42 times the amount of nitrogen measured at the end of the stirring step.

[0071] - To minimize the amount of calcium aluminate, which could potentially harm the bending properties of press-hardened parts, no Ca-containing additives (e.g., SiCa, FeCa, or pure Ca) are made. Because a very low S level is achieved using the targeted composition and process, the produced steel will contain low MnS groups. The inventors have discovered that even with such low MnS content, bending properties are excellent, even without the addition of Ca to spheroidize the MnS groups.

[0072] - As in the first embodiment, a minimum amount of time is provided to promote the floating of inclusions. In this second embodiment, where the molten steel is desulfurized before the main alloying elements are added under vacuum, the inclusion floating time is the sum of the following:

[0073] - The time spent in the vacuum degasser after adding Mn, Si, Cr, Mo, Nb, and B.

[0074] - The holding time between the vacuum degasser and the continuous casting operation itself. This holding time may include the ladle transport step between the vacuum degasser and the continuous casting operation, the waiting time during the continuous casting step, etc. The continuous casting step begins when the ladle is opened to begin pouring into the casting tundish.

[0075] More generally, it is preferable to refine steel by using, for example, a vacuum degasser to perform the main addition of Mn, Si, Cr, Mo, Nb, and B under vacuum. This allows for low nitrogen content in the steel, and consequently, better control over nitrogen-containing inclusions in the steel.

[0076] More generally, inclusion floating time is defined as the total amount of time spent in molten steel after the addition of Mn, Cr, Si, Mo, Nb, and B and before the start of the casting step.

[0077] To control the surface fraction of inclusions in the steel plate, the inclusion floating time should be controlled to be higher than the minimum inclusion floating time tf. The value of tf will depend on the specific industrial setup used for steel production. It will depend on the production route in the steelmaking plant and the geometry of the ladle used to process the molten steel. Since inclusion floating time is related to hydrodynamics and the movement of small particles within the molten steel, the minimum inclusion floating time required to achieve a desired level of specific inclusions in the steel surface will depend on the size of the ladle, its diameter, height, volume, etc. For example, the minimum inclusion floating time is 60 minutes. Alternatively, the minimum inclusion floating time is 53 minutes.

[0078] To determine the minimum inclusion float time tf for a given steel composition and given industrial equipment and production route, the following method is recommended:

[0079] - Use the same chemical composition target material for multiple smeltings.

[0080] - The furnace runs are generated using different inclusion float times. For example, a set of smelting runs is performed using a range of inclusion float times starting from the minimum feasible inclusion float time corresponding to the industrial route, and then incrementally longer inclusion float times are applied, such as using a 10-minute time increment. For example, five different inclusion float times are applied to five different furnace runs.

[0081] - The furnace batches are processed according to the industrial route described below, and the inclusion groups in the steel are characterized using the methods described above.

[0082] - Record the surface oxide fraction and the corresponding inclusion floating time. The inventors discovered a correlation between the surface oxide fraction and the inclusion floating time. The longer the inclusion floating time, the lower the surface oxide fraction. A minimum inclusion floating time tf was determined to be an inclusion floating time above which the surface oxide fraction is equal to or less than 60*10. -6 For example, the inventors have discovered that when using specific industrial equipment available to them and applying the processing route of the first embodiment, the minimum inclusion floating time is 60 minutes, preferably 53 minutes. This will be illustrated in the following examples.

[0083] After the steel refining step, the method for manufacturing the steel plate according to the invention preferably includes the following steps:

[0084] - Molten steel is continuously cast into semi-finished products suitable for hot rolling. During the casting process, special care should be taken to avoid oxygen absorption and thus avoid high oxide levels in the semi-finished products. For example, in the case of a continuous casting process where the semi-finished products are slabs produced sequentially by casting products from multiple heats poured into the tundish into molds, specific refractory materials and linings can be used in the tundish, specific allocation rules can be applied to the first slab in the sequence and to the transition slabs between two different heats, and so on.

[0085] - The semi-finished product is then optionally reheated at a temperature of 1150°C to 1300°C.

[0086] - The steel plate is then hot-rolled at a fine hot rolling temperature of 800°C to 950°C.

[0087] - The hot-rolled steel is then cooled and coiled at a temperature below 670°C (T coiling), and optionally pickled to remove oxidation.

[0088] - The coiled steel sheet is then optionally cold-rolled to obtain cold-rolled steel sheet. The cold rolling reduction is preferably in the range of 20% to 80%. Below 20%, recrystallization during subsequent heat treatment is unfavorable, which may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling.

[0089] - In one embodiment of the invention, the annealed steel sheet is heated to an annealing temperature TA of 700°C to 850°C and held at said temperature TA for a holding time tA of 10 seconds to 20 minutes.

[0090] - In one embodiment of the invention, the annealed steel sheet is cooled to a temperature in the range of 400°C to 700°C and further coated with a metal coating.

[0091] In summary, the above method preferably includes the following sequential steps:

[0092] - Producing molten steel with the above chemical composition, wherein during the steel refining stage, Mn, Si, Cr, Mo, Nb, and B are added using a vacuum degasser, and wherein a minimum inclusion floating time tf is ensured, the inclusion floating time being the total time spent in the molten steel after the addition of Mn, Si, Cr, Mo, Nb, and B and before the start of the casting step, and the minimum inclusion floating time tf is defined as reaching or falling below 60*10 -6 The minimum inclusion floating time required for the surface oxide surface fraction.

[0093] - Cast the molten steel to obtain a semi-finished product that can be hot-rolled.

[0094] - Optionally, the semi-finished product may be reheated at a temperature T of 1100°C to 1300°C.

[0095] - The semi-finished product is hot-rolled at a fine hot rolling temperature of 800℃ to 950℃.

[0096] - Hot-rolled steel sheets are coiled at a coiling temperature T below 670°C to obtain coiled steel sheets.

[0097] - Optional pickling of the coiled steel sheet.

[0098] - Optionally, the coiled steel sheet is cold-rolled to obtain cold-rolled steel sheet.

[0099] - Optionally, hot-rolled or cold-rolled steel sheet is heated to an annealing temperature TA of 700°C to 850°C, and held at said temperature TA for a holding time tA of 10 seconds to 20 minutes to obtain an annealed steel sheet.

[0100] - Optionally, the annealed steel sheet may be cooled to a temperature in the range of 400°C to 700°C.

[0101] - Optionally, the annealed steel sheet is coated with a metallic coating.

[0102] - Optionally, the coated steel sheet can be cooled to room temperature.

[0103] The manufacturing process of the pressed parts and the characteristics of the subsequently pressed parts will now be described in detail.

[0104] Steel billets are cut from the steel sheet according to the invention and heated in an annealing furnace. Preferably, the steel billets are heated to a temperature of 880°C to 950°C over a period of 10 seconds to 15 minutes to obtain heated steel billets. The heated billets are then transferred to a forming machine, where they are thermoformed and die-hardened to obtain pressed parts.

[0105] The microstructure of the pressed component, by surface fraction, comprises more than 95% martensite and less than 5% bainite + ferrite. Furthermore, the pressed component according to the invention comprises a body portion and top and bottom surface layers, wherein the surface layers occupy the outermost 10% of the thickness on both sides of the body, and said surface layers have an oxide surface fraction equal to or less than 60 × 10⁻⁶. -6 Surface inclusions.

[0106] The pressed components according to the invention have a bending angle of at least 50° normalized to 1.5 mm in the rolling direction and a tensile strength TS of at least 1800 MPa. Such high tensile strength and bending angle endow the components with excellent mechanical resistance, especially in the event of a collision. They provide excellent energy absorption and intrusion resistance, thereby improving vehicle safety.

[0107] The invention will now be illustrated by way of examples, which are by no means limiting.

[0108] Eight different samples from eight different furnace batches A, B, C, D, E, F, G, and H, produced using an industrial production route, were tested. Samples I1, I2, I3, I4, I5, and I6 are based on the present invention, while samples R1 and R2 are reference samples.

[0109] Table 1 - Sample composition

[0110] The following table summarizes the tested compositions, with elemental content expressed as a weight percentage:

[0111]

[0112] Table 2 - Steelmaking shop process parameters and surface inclusions

[0113] Applying the following process parameters in a steelmaking workshop and observing the surface fraction of surface inclusions—the underlined values ​​are not based on the present invention:

[0114]

[0115] *RH = RH vacuum degasser process time after adding Mn, Si, and Cr

[0116] **DS = Desulfurization process time**

[0117] ***CC = Time spent between the end of desulfurization and the start of continuous casting (= ladle opened to pour into the casting tundish).

[0118] Table 3 - Additional process conditions

[0119] The following process parameters are applied along the production route:

[0120]

[0121] Table 4 - Microstructure, bend angle and tensile strength

[0122] The following microstructure, bending angle, and tensile strength of the sample were measured. The underlined values ​​are not based on the invention:

[0123]

[0124] Table 4 shows that the samples according to the present invention (references I1, I2, I3, I4, I5 and I6) have tensile strengths exceeding 1800 MPa and bending angles in the rolling direction exceeding 50° normalized to 1.5 mm due to their specific composition and surface inclusions.

[0125] Referring to Table 2, a relationship exists between inclusion floating time and surface oxide surface fraction. Inclusion floating time represents the total amount of time spent in molten steel after the addition of Mn, Cr, Si, Mo, Nb, and B and before the start of the continuous casting step.

[0126] The inventors have discovered that when using the specific composition of this invention and when increasing the inclusion floating time to above the minimum inclusion floating time tf, the surface oxide fraction can be controlled below a critical level, which ensures good bending resistance. In the industrial configuration used to produce the sample given in the current embodiment, the minimum inclusion floating time tf is 53 minutes. The value of tf will depend on the specific industrial settings used to produce the steel.

[0127] When the inclusion floating time is equal to or greater than tf=60 minutes, the surface oxide surface fraction is equal to or less than 60*10. -6 When the inclusion floating time is less than tf=53 minutes, the surface oxide surface fraction is higher than 60*10. -6 .

[0128] The inventors have discovered that the surface fraction of inclusions in the surface layer plays a significant role in improving the material's resistance to crack initiation when steel is subjected to bending loads. Surprisingly, this is not true for all types of inclusions. For example, NbC inclusions do not appear to have a significant effect on the bending properties of steel. On the other hand, the surface fraction of oxides has been found to play an important role in bending properties. Reducing the oxide surface fraction helps improve bending properties.

[0129] Referring to Table 4, all have a value equal to or less than 60*10 -6 The samples (I1, I2, I3, I4, I5, and I6) according to the invention, all having a surface oxide fraction, all possess a bending angle in the rolling direction of at least 50° (normalized to 1.5 mm) and a tensile strength of at least 1800 MPa. On the other hand, the reference samples (R1, R2), while maintaining a tensile strength higher than 1800 MPa, all possess a bending angle in the rolling direction of less than 50° (normalized to 1.5 mm). Therefore, the steel produced according to the invention exhibits very high tensile strength while also demonstrating better resistance to crack formation under load, which will improve the impact resistance and safety of components produced using said material.

Claims

1. A steel plate made of steel having the following composition, said composition comprising, by weight percentage: C: 0.3% to 0.4% Mn: 0.5% to 1.0% Si: 0.4% to 0.8% Cr: 0.1% to 1.0% Mo: 0.1% to 0.5% Nb: 0.01% to 0.1% Al: 0.01% to 0.1% Ti: 0.008% to 0.03% B: 0.0005% to 0.003% P≤0.020% Ca≤0.0010% S≤0.004% N≤0.005% And optionally include: Ni < 0.5% The remaining portion of the composition consists of iron and unavoidable impurities produced during smelting. The steel plate has a microstructure comprising 60% to 95% ferrite by surface fraction, with the remainder being martensite-austenite islands, pearlite, or bainite. The steel plate includes, from the body to the surface, the coated steel plate: - Main body, - The top of such a body is a surface layer occupying the outermost 10% thickness on both sides of the body, wherein the surface fraction of oxides therein is equal to or less than 60*10 -6 Surface inclusions.

2. The steel plate according to claim 1, further comprising a metal coating on at least one side.

3. A press-hardened steel component, the steel component having the following composition, the composition comprising, by weight percentage: C: 0.3% to 0.4% Mn: 0.5% to 1.0% Si: 0.4% to 0.8% Cr: 0.1% to 0.4% Mo: 0.1% to 0.5% Nb: 0.01% to 0.1% Al: 0.01% to 0.1% Ti: 0.008% to 0.03% B: 0.0005% to 0.003% P≤0.020% Ca≤0.001% S≤0.004% N≤0.005% And optionally include: Ni < 0.5% The remaining portion of the composition consists of iron and unavoidable impurities produced during smelting. The steel component has a microstructure comprising more than 95% martensite and up to 5% bainite or ferrite by surface fraction. The steel component includes, from its body to its surface: - Main body, - The top of such a body is a surface layer occupying the outermost 10% thickness on both sides of the body, wherein the surface fraction of oxides therein is equal to or less than 60*10 -6 Surface inclusions.

4. The press-hardened steel component according to claim 3, wherein the press-hardened steel component has a tensile strength TS of at least 1800 MPa and a bending angle of at least 50° normalized to 1.5 mm in the rolling direction.

5. A method for manufacturing a press-hardened steel component according to claim 3 or 4, comprising the following sequential steps: - Provide the steel plate according to claim 1 or 2, - The steel plate is cut into a predetermined shape to obtain a steel billet. - The steel billet is heated to a temperature of 880°C to 950°C over a period of 10 seconds to 15 minutes to obtain a heated steel billet. - Transfer the heated blank to the forming machine. - The heated blank is thermoformed in the forming machine to obtain a molded part. - The molded part is subjected to compression molding and quenching.

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