Steel sheet and high-strength press-hardened steel component having excellent bending anisotropy, and method for manufacturing same

Abstract

A steel sheet having a chemical composition comprising, in% by weight, C: 0.2% to 0.4%, Mn: 0.8% to 2.0%, Si: 0.1% to 0.5%, Al: 0.01% to 0.1%, Ti: 0.01% to 0.1%, B: 0.0005% to 0.005%, P < = 0.040%, Ca < = 0.01%, S < = 0.006%, N < = 0.01%, the steel sheet comprising, from the body to the surface of the coated steel sheet, a body comprising a population of inclusions, where MnS and TiN / Ti (C, Si) (C, Mn), S < = 0.006%, N < = 0.01%, and a skin layer occupying the outermost 10% thickness on both sides of the body. N) the sum of clustering indexes of inclusions is less than or equal to 300 [mu] m / mm < 2 >. This allows the manufacture of a hot-pressed part having a tensile strength equal to or greater than 1300 MPa and a bending anisotropy equal to or less than 7 DEG.

Description

[0001] This invention patent application is a divisional application of the invention patent application filed on April 26, 2024, with application number 202480001638.9 and invention title "Steel plate with excellent bending anisotropy and high-strength press-hardened steel component and method of manufacturing the same". Technical Field

[0002] This invention relates to steel plates and high-strength press-hardened steel components. 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 and uniform resistance to the formation of bending cracks is also challenging. In fact, very high-strength steels tend to crack early when subjected to bending loads. This is detrimental to the impact resistance of components made from such high-strength steels, because although 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.

[0007] In particular, producing very high-strength press-hardened steel with isotropic bending behavior is challenging. Indeed, it is well known that, in general, the bending behavior of steel sheets is better in the rolling direction than in the transverse direction. This anisotropic behavior introduces constraints when designing steel components and conceiving hot stamping processes. Therefore, it is of great interest to provide steel sheets and methods for producing such steel that allow for very similar bending behavior in all directions. Summary of the Invention

[0008] 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 1300 MPa or higher after hot stamping and very small anisotropy of bending angle between the rolling direction and the transverse direction.

[0009] Another object of the present invention is to provide a steel sheet that can be thermoformed into such press-hardened steel parts, and to provide a method for manufacturing such a steel sheet.

[0010] The object of the present invention is achieved by providing a steel plate according to claim 1 (optionally having the features of claims 2 to 4). Another object of the present invention is achieved by providing a press-hardened steel component according to claim 5. This steel component may also include the features of claims 6 to 7. Another object of the present invention is a method for manufacturing the hot-stamped component according to claim 8 (optionally including the features of claim 9). Attached Figure Description

[0011] The invention will now be described in detail, and by way of examples without limitation and reference. 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

[0012] 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 simply top surface and bottom surface. The distance between these surfaces is specified as the thickness of the billet. The thickness can be measured, for example, using a micrometer, with its spindle and anvil placed on the top and bottom surfaces. Similarly, the thickness of shaped parts can also be measured.

[0013] Hot stamping is a forming technique that involves heating a blank to a temperature at which the microstructure of steel is at least partially transformed into austenite, 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 offers numerous 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 to cure the paint 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, where baking has indeed been performed).

[0014] 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 used to accommodate all available flat areas on the part.

[0015] The bending angle is measured according to the VDA-238-100 bending standard, based on the version published in June 2017. A laser measuring device is used to measure the bending angle. 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 used to accommodate the entire available flat area on the part.

[0016] The bending angle of a component represents its ability to resist deformation without forming cracks.

[0017] The bending angles are measured in the rolling direction (RD) (i.e., the direction in which the steel sheet travels during the hot rolling step) and in the transverse direction (TD) (i.e., oriented at 90° relative to the rolling direction). Bending anisotropy is defined as the absolute value of the difference between the bending angles measured in the rolling direction and the transverse direction on a given sample.

[0018] If the rolling direction on a hot-stamped part is not known, it can be determined in scanning electron microscopy (SEM) using electron backscatter diffraction (EBSD) analysis across the sample cross section. The rolling direction is determined based on the representative... The strength of the orientation density function (ODF) of the main fibers is used to determine this, where... Euler angles as defined in "H.-J. Bunge: Texture Analysis in Materials Science - Mathematical Methods. Butterworth Co., first English edition (publication) 1982" (for...) For the definition, see Figures 2.2 and 2.3).

[0019] The composition of the steel according to the invention will now be described, with the content expressed as a weight percentage. The chemical composition is given with lower and upper limits of the composition range, which are included within the possible composition range according to the invention. Where preferred ranges for a given element are disclosed, the invention also discloses all possible combinations of these preferred ranges for each individual element.

[0020] According to the present invention, the carbon content ranges from 0.2% to 0.4% to ensure satisfactory strength. Above 0.4% carbon, the weldability and bendability of the steel sheet may decrease. If the carbon content is below 0.2%, the tensile strength will not reach the target value. In one specific embodiment, the carbon content ranges from 0.2% to 0.3% to ensure sufficient strength while further controlling excellent weldability and bendability. In another specific embodiment, the carbon content ranges from 0.2% to 0.25% to ensure sufficient strength while even further controlling excellent weldability and bendability.

[0021] The manganese content ranges from 0.8% to 2.0%. Adding more than 2.0% increases the risk of MnS formation, thus impairing flexibility. Below 0.8%, the hardenability of the steel sheet decreases during the hot stamping process. In one specific embodiment, the manganese content ranges from 1.0% to 1.4% to further improve the hardenability of the steel and further limit MnS formation, thereby improving flexibility.

[0022] The silicon content ranges from 0.1% to 0.5%. Silicon is an element involved in the hardening of solid solutions. Adding silicon limits carbide formation. Above 0.5%, silicon oxides form on the surface, which impairs the coatability of the steel. Furthermore, the weldability of parts produced from said steel sheet may be reduced. In one specific embodiment, the silicon content ranges from 0.1% to 0.4% to further improve coatability and weldability. In one specific embodiment, the silicon content ranges from 0.15% to 0.35% to further harden the steel and further improve coatability and weldability.

[0023] According to the invention, the aluminum content ranges from 0.01% to 0.1%, as it is a very effective element for deoxidizing liquid-phase steel during processing. 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 0.02% to 0.06% to further ensure good deoxidation of the liquid-phase steel, while further avoiding oxidation problems and ferrite formation during press hardening.

[0024] According to the present invention, the titanium content ranges from 0.01% to 0.1% to protect boron that would otherwise be trapped in the BN precipitate. The titanium content is limited to 0.1% to avoid excessive TiN formation. In one specific embodiment, the Ti content ranges from 0.02% to 0.06% to further protect boron while further avoiding excessive TiN formation.

[0025] According to the present invention, the boron content ranges from 0.0005% to 0.005%. Boron improves the hardenability of steel. The boron content is not higher than 0.005% to avoid slab breakage problems during continuous casting. In one specific embodiment, the boron content ranges from 0.002% to 0.004% to further ensure the hardenability of the steel and further avoid slab breakage problems.

[0026] Phosphorus is controlled to be less than or equal to 0.040% because it causes brittleness and solderability problems. In one specific embodiment, the P content is controlled to be less than or equal to 0.020% to further avoid brittleness and solderability problems.

[0027] Calcium is controlled to be less than or equal to 0.01%, as the presence of calcium in molten steel can lead to the formation of coarse inclusions that are detrimental to bending properties. In one specific embodiment, the Ca content is controlled to be less than or equal to 0.005% to further avoid the problem of coarse inclusions.

[0028] Sulfur content is controlled to be less than or equal to 0.006%, as the presence of sulfur in molten steel can lead to the formation of MnS precipitates that are detrimental to flexural properties. In one specific embodiment, the S content is controlled to be less than or equal to 0.005% to further prevent the formation of MnS precipitates.

[0029] Nitrogen is controlled to be less than or equal to 0.01%, preferably less than or equal to 0.008%, and even more preferably less than or equal to 0.005%. The presence of nitrogen may lead to the formation of precipitates such as TiN or TiNbCN that are detrimental to flexibility.

[0030] Chromium may be added at a maximum of 0.4%. Chromium can be used to provide strength through solution hardening and to improve the hardenability of steel sheets during hot stamping. Chromium is limited to 0.4% to limit costs and avoid processing problems.

[0031] Molybdenum may be added up to 0.3%. Molybdenum improves the hardenability of steel. The amount of molybdenum is limited to 0.3% to limit costs and avoid processing problems.

[0032] Niobium may be added optionally up to 0.1%. Niobium improves the ductility of steel. Niobium is limited to 0.1% to limit costs and avoid processing problems.

[0033] Vanadium may be added up to 0.3%. Vanadium improves the hardenability of steel. Vanadium is limited to 0.3% to limit costs and avoid processing problems.

[0034] With the addition of one or more of the above elements, further verify the following formula: Cr+Mo+Nb+V≤0.5% to limit costs and avoid processing problems.

[0035] In one specific implementation, further control of the chemical composition allows for verification of the following conditions:

[0036] (S-Ca*32 / 40)+(30*Ti*N)≤0.0045

[0037] The inventors have discovered that this allows for further control over the inclusion group in the steel plate, and thus further improves bending and bending anisotropy.

[0038] The remaining components of steel are iron and impurities generated during processing. The level of impurities generated during processing will depend on the production route used and the level of scrap steel used in the molten steel. For example, when using a basic oxygen converter route with a low level of scrap steel (recycled steel), the level of impurities will remain very low. However, a large amount of scrap steel can be added to the pig iron produced in a basic oxygen converter, which will increase the level of impurities. Furthermore, when steel is processed using an electric furnace, for example with a very high ratio of recycled scrap steel, the level of impurities will increase significantly. When using high levels of scrap steel, the levels of Cu can rise to 0.25%, Ni to 0.25%, Sn to 0.05%, As to 0.03%, Sb to 0.03%, and Pb to 0.03%.

[0039] The microstructure of the steel plate according to the present invention will now be described.

[0040] The steel plate has a microstructure, in terms of surface fraction, comprising the following components at any analytical section:

[0041] -75% to 90% ferrite,

[0042] The remainder consists of carbides Fe3C and hard phases such as martensite and bainite.

[0043] Reference Figure 1 According to the present invention, the steel plate 1 includes a main body portion 3 and a top surface layer and a bottom surface layer 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 outermost 10% of the thickness of the surface layer 2 occupies both sides of the main body, and the main body of the steel plate accounts for 80% of the thickness of the steel plate.

[0044] The inventors have discovered a correlation between the bending anisotropy of steel plates and the inclusion groups in the bulk portion. Specifically, this correlation is observed when the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk portion of the steel is controlled to be less than or equal to 300 μm / mm. 2 When this is the case, the bending anisotropy can be limited to less than or equal to 7°.

[0045] 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.

[0046] The section of the steel plate on which inclusions are observed is cut in the rolling direction of the steel. In other words, the plane of the observed section has a transverse direction as its normal direction.

[0047] 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. This allowed for the detection of particles as small as 0.5 μm. The use of the FEG SEM setting allowed for the acquisition of stable images with excellent resolution over long periods, which can be necessary for image analysis over large areas—the FEG SEM setting can acquire image fields over periods up to 48 hours, which can be necessary for multi-sample analysis. Furthermore, inclusions were analyzed using energy-dispersive spectroscopy (EDS). A 120 mm microscope with a large effective surface area was used. 2 Bruker EDS probes are used to detect light elements (O, N) and achieve high count rates, thus allowing for precise quantification. The -ρ-Z method is used to obtain accurate quantification.

[0048] Based on computer-controlled scanning electron microscopy (SEM) technology, the RJ Lee group's Automated Steel Cleanliness Analysis Tool (ASCAT) is used for testing SEM and related EDS. Six individual samples can be analyzed in the same batch. The sample surface is divided into three regions (as previously described: top surface, bottom surface, and bulk). Each region is further divided into fields. Inclusions are detected within each field. To detect fine particles, the scanning pixel size is set to a very low value of 0.11 μm. This is to reduce matrix noise in the SEM image. As will be seen, only objects with a diameter greater than 0.5 μm are actually considered. The initial selection of objects referred to as particles is made by selecting objects that form solids and have a grayscale level below 150 or above 220 (extreme values ​​are excluded) on a scale of 0 to 255.

[0049] Each individual particle was then magnified to capture its morphological features and subjected to EDS analysis. A database of all particles was created using ASCAT, taking into account the chemical and morphological characteristics of all analyzed particles across all acquired images.

[0050] Of all the particle groups analyzed, only particles larger than 0.5 μm and with an iron content of less than 80% were retained for subsequent analysis and referred to as inclusions—other particles were considered part of the matrix and were not relevant to subsequent analysis.

[0051] Using information from the EDS probe, various inclusions are then classified into one of the following categories: TiN, alumina, complex oxides, oxysulfide particles, MnS, etc. For example, Table 1 details the precise rules used by the inventors to classify MnS and TiN / Ti(C,N) inclusions. A high-performance EDS detector enables oxygen quantification. Oxygen levels are checked to separate TiN from TiO2 and MnS from complex oxysulfide inclusions.

[0052] Table 1 - Criteria for classifying inclusions by weight percent of Ti, M, S, O, and Nb

[0053]

[0054] Then, for various types of inclusions, the following properties are calculated:

[0055] -Average diameter, in micrometers

[0056] -Density, expressed as the number of inclusions / mm 2 count,

[0057] The clustering index calculation method is based on the DBSCAN (Density-Based Spatial Clustering of Applications with Noise) algorithm, as seen in the article "A density-based algorithm for discovering clusters in large spatial databases with noise" in Proceedings of the Second International Conference on Knowledge Discovery and Data Mining (KDD-96). AAAI Press., by Ester, Martin; Kriegel, Hans-Peter; Sander. Xu, Xiaowei (1996), pp. 226-231.

[0058] The clustering index is determined by two parameters: maximum distance and minimum number of points. Clustering is characterized by the following features:

[0059] -It contains only the same type of particles.

[0060] - In a given cluster, the distance between an inclusion and at least one other inclusion is always less than the maximum distance.

[0061] - It contains many individual inclusions that are equal to or greater than the minimum number of points.

[0062] Regarding this invention, the inventors have discovered that good cluster detection can be obtained by using the maximum distance of the maximum distance of 30μm and the number of minimum inclusions / minimum number of clusters.

[0063] The length L of a given cluster is calculated as follows:

[0064] - First, the convex hull of the cluster is determined using a known algorithm (for example, see the chapter "Convex Hulls: Basic Algorithms" in Computer Science, Springer, New York, NY, Computational Geometry, Preparata, FP, Shamos, MI, 1985, Texts and Monographs).

[0065] Then, the maximum Feret diameter, which will be referred to as D-maximum, is determined, and the Feret diameter, which will be referred to as D-perpendicular, is also determined in the direction perpendicular to D-maximum. Information on Feret diameter measurement can be found, for example, in "Particle Size Measurements: Fundamentals, Practice, Quality" Springer, Henk G. Merkus (January 1, 2009).

[0066] -Calculate the length L of the cluster as

[0067]

[0068] For each type of inclusion, calculate the average length L_average of all clusters.

[0069] The cluster density C_ of a given type of inclusion is the number of clusters / mm. 2 .

[0070] The clustering index C_index of a given type of inclusion is defined as the product of the average length of the cluster and its density, where C_index = L_average. * C-density. Clustering index in μm / mm 2 The inventors have discovered that the clustering index allows for the use of a unique number to compare samples with different properties, and that it is sufficiently correlated with the bending behavior of the samples.

[0071] The steel plate according to the invention can be produced by any suitable manufacturing method, and those skilled in the art can define one method. However, it is preferred to use the method according to the invention, which includes the following steps.

[0072] In the following description, the term ladle refers to a container used to hold molten steel during the refining process. The refining process is referred to as the steps of conditioning the final chemical composition and temperature of the melt before casting the steel into its first solidified form (e.g., before casting it into a slab that will subsequently be hot-rolled).

[0073] To successfully control inclusion groups in steel, the following processes can be implemented, for example:

[0074] - The molten steel is tapped from the previous steelmaking process step into the ladle. For example, in the case of an electric arc furnace production route, the previous process step 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 step is the converter process.

[0075] - The sulfur content of molten steel prior to the refining step is measured, for example, by taking a sample of the molten steel and analyzing it using a spark spectrometer. The sulfur content is measured, for example, by directly sampling the molten steel in the ladle or by sampling the molten steel as it is being poured into the ladle. The sulfur content prior to the refining step, measured in wt%, will be referred to as S_start in the following description.

[0076] - Aluminum is added to the ladle at the beginning of the refining process to deoxidize the molten steel. This Al addition, for example, occurs simultaneously with the steel being tapped into the ladle – advantageously, this allows for time savings and thus increased productivity, and ensures that the molten steel remains sufficiently hot. The amount of Al added to the molten steel at the beginning of the refining process (expressed in kg aluminum / ton of steel (kg / ton)) will be referred to as Al_addition in the following description.

[0077] - In subsequent optional steps, for example, if the temperature of the molten steel is too low, or if a waiting time is anticipated between the end of the refining step and subsequent processes (e.g., continuous casting), the molten steel is reheated by aluminothermic heating. This is done by adding a predetermined amount of aluminum and co-blowing a predetermined amount of oxygen into the molten steel, the predetermined amount of oxygen corresponding to the stoichiometric ratio required to form Al₂O₃ with the added aluminum. The strong exothermic reaction between Al and O₂ allows for an increase in the temperature of the molten steel. The amount of O₂ injected during this optional step will be referred to as O₂ injection and expressed in standard cubic meters of O₂ / ton of molten steel (Nm³). 3 The amount is expressed in tons ( / ton). Because there is a direct stoichiometric relationship between the O2 injection used for aluminothermic reheating and the associated Al injection, the amount of Al used for the aluminothermic injection will not be considered separately in this description. It should be noted that the Al used for the aluminothermic injection is different from the previously mentioned Al addition.

[0078] - Adjust the slag composition above the melt by adding appropriate amounts of minerals to ensure that the %CaO / %Al2O3 ratio of the slag is greater than 1, the amount of slag per ton of molten steel is at least 10 kg / ton of molten steel, and the slag is kept in a liquid state to promote chemical exchange with the steel, so that the steel can enter below the slag and the steel can be tapped separately and / or the slag can be tapped separately (the liquid state of the slag is visually verified and / or verified using thermodynamic rules based on its composition and temperature).

[0079] In subsequent steps, the molten steel is agitated by blowing an inert gas, such as Ar, into it. This is done to promote exchange between the molten steel and the slag, which will allow for a reduction in the sulfur content of the steel.

[0080] - In a further step, Ca is added to the ladle to spheroidize inclusions present in the molten steel. For example, Ca is added in the form of calcium silicate (SiCa), iron-calcium (FeCa), or as pure calcium. This addition is performed, for example, by adding SiCa or FeCa to the ladle in the form of a core wire—advantageously, this allows for easy control of the amount of Ca added by controlling the length of the core wire inserted into the melt and the injection rate. The amount of Ca added to the molten steel (measured as a percentage by weight in the molten steel) will be referred to as Ca_addition in the following description.

[0081] Considering the above process, the inventors have discovered that by controlling the sulfur level (S_start, measured in wt%) at the start of the refining process, the Al addition at the start of the refining process (Al_addition, measured in kg / ton), the Ca addition during the refining process (Ca_addition, measured in kg / ton), and the volume of O2 blowing (O2_injection, measured in Nm3 / ton) to verify that the following combination (hereinafter referred to as Cl in the remaining description) remains below a given cutoff value, a satisfactory inclusion level can be obtained to achieve the desired flexural anisotropy properties after hot stamping:

[0082] Al_Add +0.1953 * (S_start) * 1000+O2 (injection) -9.367 * Ca_Add(C1)

[0083] In practice, the specific threshold required to control the combined C1 below its value will depend on the particular industrial setup used for steel production. This will depend on the production route in the steelmaking plant, the geometry of the ladle used to process the molten steel, the equipment used to add different additives, the oxygen blowing setup, and so on.

[0084] To determine the relationship between these parameters for a given industrial equipment and production route, the following method is recommended:

[0085] - Several smeltings were performed using the previously described range of chemical compositions.

[0086] - The smelting was treated using different refining process parameters, specifically different levels of measured sulfur at the start of the refining process, Al addition at the start of the refining process, Ca addition during the refining process, and the volume of O2 blowing. A range of refining process parameters was selected to represent industrial variations of these parameters. For example, a set of six different smeltings with six different sets of refining process parameters was selected. Alternatively, a set of eight different smeltings with eight different sets of refining process parameters was selected.

[0087] - The smelting is processed according to the industrial route described below, and the inclusion groups of the steel are characterized using the methods described above.

[0088] Then, record the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the main body of the steel plate and the associated refining process parameters. Calculate the combination C1 of these refining process parameters. As a general trend, it will be seen that the higher the combination C1, the higher the sum of the clustering indices.

[0089] Using the aforementioned data set of clustering indices and refining process parameters, a cutoff value was determined below which the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk of the steel plate is less than or equal to 300 μm / mm. 2 The cutoff value for C1 will determine how to control the refining process of the specific industrial facility under consideration. By controlling C1 below the cutoff value, the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the main body can be produced to be less than or equal to 300 μm / mm. 2 The steel plate thus achieves excellent bending anisotropy of less than or equal to 7°.

[0090] For example, in the case of a specific industrial facility where the inventors conducted their experiments, the cutoff value was equal to 1.80.

[0091] After the steel refining step, the method for manufacturing the steel plate according to the invention preferably includes the following steps:

[0092] - 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 high inclusion levels in the semi-finished products. For example, in the case of continuous casting processes, where the semi-finished products are slabs produced sequentially by casting products poured into a tundish through multiple melting processes in a mold, specific refractory materials and linings can be used in the tundish, specific distribution rules can be applied to the first slab in the sequence and the transition slab between two different melting processes, etc.

[0093] -Then optionally, the semi-finished product is heated at a temperature of 1150°C to 1300°C.

[0094] -Then the steel plate is hot rolled at a fine hot rolling temperature of 800°C to 950°C.

[0095] -Then the hot-rolled steel is cooled and heated to a temperature below 670°C. 卷取 Take the roll up and optionally acid wash it to remove oxidation.

[0096] - The coiled steel sheet is then optionally cold-rolled to obtain cold-rolled steel sheet. The cold rolling reduction rate is preferably in the range of 20% to 80%. Below 20%, recrystallization is unfavorable during subsequent heat treatment, which may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling.

[0097] - In one embodiment of the invention, the steel plate is heated in an annealing furnace to a homogenization temperature of 700°C to 850°C and held at the homogenization temperature for 10 seconds to 20 minutes.

[0098] - In one embodiment of the invention, the annealed steel sheet is cooled to a temperature range of 400°C to 700°C and further coated with a metallic coating. The metallic coating is, for example, an aluminum-based coating containing at least 50% aluminum by weight. The metallic coating is, for example, a zinc-based coating containing at least 50% zinc by weight.

[0099] - In one embodiment of the invention, the steel plate is then cooled to room temperature.

[0100] In summary, the above process preferably includes the following sequential steps:

[0101] - Producing molten steel with the above chemical composition, wherein during the steel refining stage, the sulfur level measured at the start of the refining process, the Al addition at the start of the refining process, the Ca addition during the refining process, and the volume of O2 blowing are controlled to verify the combined Al_addition +0.1953 * (S_start) * 1000+O2 (injection) -9.367 * Ca addition (C1) is maintained below a predetermined cutoff value. This cutoff value is determined for the specific industrial equipment being used, such that when C1 is below the cutoff value, the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk of the steel is less than or equal to 300 μm / mm. 2 .

[0102] - Cast the molten steel to obtain a semi-finished product that can be hot-rolled.

[0103] -Optionally, the semi-finished product is subjected to a temperature T between 1100°C and 1300°C. 再加热 Reheating

[0104] - The semi-finished product is hot-rolled at a fine hot rolling temperature of 800℃ to 950℃.

[0105] - The hot-rolled steel sheet is coiled at a temperature T below 670°C. 卷取 The lower winding process yields the wound steel sheet.

[0106] -Optional pickling of the coiled steel sheet

[0107] -Optionally, the coiled steel sheet is cold-rolled at a reduction rate ranging from 20% to 80% to obtain cold-rolled steel sheet.

[0108] - Optionally, hot-rolled or cold-rolled steel sheet is heated to a homogenization temperature of 700°C to 850°C, and held at said temperature for a homogenization time of 10 seconds to 20 minutes to obtain annealed steel sheet.

[0109] - Optionally, the annealed steel sheet may be cooled to a temperature range of 400°C to 700°C.

[0110] -Optionally, the annealed steel sheet is coated with a metallic coating.

[0111] -Optionally, the coated steel sheet is cooled to room temperature.

[0112] The manufacturing process of pressed parts and the subsequent characteristics of pressed parts will now be described in detail.

[0113] Steel billets are cut from the steel sheet according to the invention and heated in an austenitizing 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.

[0114] Optionally, the hot-stamped parts are further subjected to a baking paint step, in which the parts are heated to a temperature of 150°C to 250°C for a duration of 10 minutes to 2 hours.

[0115] The microstructure of the pressed component, in any analytical section, contains more than 95% martensite and less than 5% bainite + ferrite by surface fraction. Furthermore, the pressed component according to the invention comprises a main body portion and top and bottom surface layers, wherein the surface layers occupy the outermost 10% of the thickness on both sides of the main body. The sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the main body portion is less than or equal to 300 μm / mm. 2 .

[0116] The pressed component according to the invention has a tensile strength greater than 1300 MPa, preferably greater than 1350 MPa, and more preferably greater than 1400 MPa, and a bending angle anisotropy of less than or equal to 7°. Such high tensile strength and low bending anisotropy endow the component with excellent mechanical resistance, especially in the event of a collision, and further allow for highly predictable and uniform behavior in all directions. These characteristics provide excellent energy absorption and intrusion resistance in all directions, thereby improving vehicle safety.

[0117] The invention will now be illustrated by way of the following examples, which are by no means limiting.

[0118] Eight different samples of steel produced using an industrial production route from eight different smelting processes A, B, C, D, E, F, G, and H were tested. Samples I1, I2, I3, and I4 are based on the present invention, while samples R1, R2, R3, and R4 are reference samples.

[0119] All produced samples followed the same industrial production process in the steelmaking workshop. After annealing, all samples were coated with an AlSi-based coating comprising 8 to 12 wt% Si, 2 to 4 wt% Fe, and the balance Al.

[0120] Table 2 – Sample Composition

[0121] The tested compositions are summarized in the table below, where elemental content is expressed as a weight percentage. The remaining components consist of iron and unavoidable impurities from the processing:

[0122]

[0123] Table 3 – Clustering Indices of MnS and TiN / Ti(C,N) Inclusions in the Main Body of Steelmaking Plant (Process Parameters) sum of numbers

[0124] Apply the following process parameters in a steelmaking workshop and observe the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk of the following steel—the underlined values ​​are not based on the present invention:

[0125]

[0126]

[0127] * C1 = Al_Add + 0.1953 * (S_start) * 1000+O2 (injection) -9.367 * Ca_add

[0128] As can be seen, under the tested industrial conditions, by ensuring that the refining process parameters are properly controlled to keep C1 equal to or below 1.80, the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk of the steel can be controlled to be below or equal to 300 μm / mm. 2 As previously explained, the cutoff value of 1.80 is specific to the industrial setting where the test is being conducted, and it will be necessary to determine the appropriate cutoff factor for a given industrial setting, for example, by following the methods described above.

[0129] Table 4 - Additional Process Conditions

[0130] Apply the following process parameters along the production route:

[0131]

[0132] Table 5 – Microstructure, Bending Angle and Tensile Strength

[0133] The following microstructures (in terms of surface fraction), bending angle, bending angle anisotropy, and tensile strength were measured on the sample. The underlined values ​​are not based on the present invention:

[0134]

[0135]

[0136] Table 5 shows that the sample according to the invention has a tensile strength greater than 1300 MPa (on both RD and TD) and a flexural anisotropy less than 7°. On the other hand, the reference sample, although having a comparable tensile strength level of greater than 1300 MPa, has a flexural anisotropy greater than 7°.

[0137] The inventors have discovered that this excellent bending anisotropy is related to the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk of the steel, as can be seen in Table 3. Higher clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk of the steel result in higher bending anisotropy. (300 μm / mm) 2 The cutoff value allows for controlling bending anisotropy to be less than or equal to 7°. Due to this very low level of bending anisotropy, hot-stamped parts behave very uniformly when subjected to loads from any direction. This highly stable behavior of hot-stamped parts under load allows for simplified part design, for example, in automotive components, and ensures very good, robust, and stable crashworthiness.

[0138] According to embodiments of the present invention, the following notes are also disclosed:

[0139] Note 1. A steel plate made of steel having the following composition, said composition comprising, by weight percentage:

[0140] C: 0.2% to 0.3%

[0141] Mn: 0.8% to 2.0%

[0142] Si: 0.1% to 0.5%

[0143] Al: 0.01% to 0.1%

[0144] Ti: 0.01% to 0.1%

[0145] B: 0.0005% to 0.005%

[0146] P≤0.040%

[0147] Ca ≤ 0.01%

[0148] S≤0.006%

[0149] N≤0.01%

[0150] And optionally include:

[0151] Cr≤0.4%

[0152] Mo≤0.3%

[0153] Nb≤0.1%

[0154] V≤0.3%

[0155] Where Cr+Mo+Nb+V≤0.5%

[0156] The remaining component consists of iron and unavoidable impurities produced during the processing.

[0157] The steel plate has a microstructure comprising, by surface fraction, 75% to 90% ferrite, with the remainder consisting of Fe3C and hard phases such as martensite and bainite.

[0158] The steel plate includes, from its main body to its surface:

[0159] -The main body, comprising 80% of the thickness of the steel plate,

[0160] - The top of such a body consists of a top surface layer and a bottom surface layer occupying the outermost 10% thickness on both sides of the body. The body contains a cluster of inclusions, wherein the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions is less than or equal to 300 μm / mm. 2 .

[0161] Note 2. The steel plate according to Note 1, wherein:

[0162] C: 0.2% to 0.25%, and / or

[0163] Mn: 1.0% to 1.4%, and / or

[0164] Si: 0.1% to 0.4%, preferably 0.15% to 0.35%, and / or

[0165] Al: 0.02% to 0.06%, and / or

[0166] Ti: 0.02% to 0.06%, and / or

[0167] B: 0.002% to 0.004%, and / or

[0168] P≤0.020%, and / or

[0169] Ca ≤ 0.005%, and / or

[0170] S≤0.005%, and / or

[0171] N ≤ 0.008%, preferably N ≤ 0.005%.

[0172] Note 3. The steel plate according to Note 1 or 2, wherein the steel plate is coated with a metallic coating comprising at least 50% Al by weight.

[0173] Note 4. The steel plate according to Note 1 or 2, wherein the steel plate is coated with a metal coating containing at least 50% Zn by weight.

[0174] Note 5. The steel plate according to any one of Notes 1 to 4, wherein the chemical composition also conforms to the following conditions, and all elements are expressed in weight percent:

[0175] (S-Ca*32 / 40)+(30*Ti*N)≤0.0045.

[0176] Appendix 6. A press-hardened steel component, the steel component having the following composition, the composition comprising, by weight percentage:

[0177] C: 0.2% to 0.3%

[0178] Mn: 0.8% to 2.0%

[0179] Si: 0.1% to 0.5%

[0180] Al: 0.01% to 0.1%

[0181] Ti: 0.01% to 0.1%

[0182] B: 0.0005% to 0.005%

[0183] P≤0.040%

[0184] Ca ≤ 0.01%

[0185] S≤0.006%

[0186] N≤0.01%

[0187] And optionally include:

[0188] Cr≤0.4%

[0189] Mo≤0.3%

[0190] Nb≤0.1%

[0191] V≤0.3%

[0192] Where Cr+Mo+Nb+V≤0.5%

[0193] The remaining component consists of iron and unavoidable impurities produced during the processing.

[0194] The steel component has a microstructure comprising more than 95% martensite and up to 5% bainite or ferrite by surface fraction.

[0195] The steel component includes, from its body to its surface:

[0196] -main body,

[0197] - The top of such a main body consists of a top surface layer and a bottom surface layer that occupy the outermost 10% of the thickness on both sides of the main body.

[0198] The main body comprises a group of inclusions, wherein the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions is less than or equal to 300 μm / mm. 2 .

[0199] Note 7. The press-hardened steel component according to Note 6, wherein:

[0200] C: 0.2% to 0.25%, and / or

[0201] Mn: 1.0% to 1.4%, and / or

[0202] Si: 0.1% to 0.4%, preferably 0.15% to 0.35%, and / or

[0203] Al: 0.02% to 0.06%, and / or

[0204] Ti: 0.02% to 0.06%, and / or

[0205] B: 0.002% to 0.004%, and / or

[0206] P≤0.020%, and / or

[0207] Ca ≤ 0.005%, and / or

[0208] S≤0.005%, and / or

[0209] N ≤ 0.008%, preferably N ≤ 0.005%.

[0210] Note 8. For the press-hardened steel components according to Note 6 or 7, wherein the chemical composition also conforms to the following conditions, all elements are expressed in weight %: (S-Ca) * 32 / 40)+(30 * Ti * N)≤0.0045.

[0211] Note 9. The press-hardened steel component according to any one of Notes 6 to 8, wherein the press-hardened steel component has a tensile strength TS of at least 1300 MPa and a bending angle anisotropy of less than or equal to 7°.

[0212] Note 10. A method for manufacturing a steel plate according to any one of Notes 1 to 5, comprising the following sequential steps:

[0213] - Provide molten steel having the following chemical composition, wherein the chemical composition comprises, by weight percentage:

[0214] C: 0.2% to 0.3%

[0215] Mn: 0.8% to 2.0%

[0216] Si: 0.1% to 0.5%

[0217] Al: 0.01% to 0.1%

[0218] Ti: 0.01% to 0.1%

[0219] B: 0.0005% to 0.005%

[0220] P≤0.040%

[0221] Ca ≤ 0.01%

[0222] S≤0.006%

[0223] N≤0.01%

[0224] And optionally include:

[0225] Cr≤0.4%

[0226] Mo≤0.3%

[0227] Nb≤0.1%

[0228] V≤0.3%

[0229] Where Cr+Mo+Nb+V≤0.5%

[0230] The remaining portion of the composition consists of iron and unavoidable impurities.

[0231] - Cast the molten steel to obtain a semi-finished product that can be hot-rolled.

[0232] - The semi-finished product is hot-rolled at a fine hot rolling temperature of 800°C to 950°C.

[0233] - The hot-rolled steel sheet is coiled at a temperature T below 670°C. 卷取 The coil is then wound up to obtain the coiled steel sheet.

[0234] Note 11. According to the method described in Note 10, wherein:

[0235] C: 0.2% to 0.25%, and / or

[0236] Mn: 1.0% to 1.4%, and / or

[0237] Si: 0.1% to 0.4%, preferably 0.15% to 0.35%, and / or

[0238] Al: 0.02% to 0.06%, and / or

[0239] Ti: 0.02% to 0.06%, and / or

[0240] B: 0.002% to 0.004%, and / or

[0241] P≤0.020%, and / or

[0242] Ca ≤ 0.005%, and / or

[0243] S≤0.005%, and / or

[0244] N ≤ 0.008%, preferably N ≤ 0.005%.

[0245] Note 12. According to the method described in Note 10 or 11, the step of providing the molten steel in the method includes a refining stage, during which the sulfur level measured at the start of the refining, the Al addition at the start of the refining, the Ca addition during the refining, and the volume of O2 blowing are controlled to verify the combination C1 = Al_addition + 0.1953 * (S_start * 1000 + O2_injection) - 9.367 * Ca_ is added to keep it below the predetermined cutoff value.

[0246] Al_ added refers to the amount of Al added at the start of the refining process, expressed in kg aluminum per ton of molten steel.

[0247] S_ begins as the sulfur content prior to the refining process, expressed as a percentage by weight.

[0248] O2 injection refers to the amount of O2 injected during the optional aluminothermic heating step, expressed in standard cubic meters of O2 per ton of molten steel.

[0249] Ca_addition refers to the amount of Ca added to the molten steel, measured as a percentage by weight in the molten steel.

[0250] Appendix 13. A method for manufacturing a press-hardened steel component according to any one of Appendices 6 to 9, comprising the following sequential steps:

[0251] - Provide steel plates according to any one of Annexes 1 to 5,

[0252] - The steel plate is cut into a predetermined shape to obtain a steel billet.

[0253] - 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.

[0254] - Transfer the heated blank to the forming machine.

[0255] - The heated blank is thermoformed in the forming machine to obtain the formed part.

[0256] - The molded part is subjected to compression molding and quenching.

[0257] Note 14. The method according to Note 13 further includes a baking paint step, in which the molded part is heated to a temperature of 150°C to 250°C for a duration of 10 minutes to 2 hours.

Claims

1. A steel plate made of steel having the following composition, said composition comprising, by weight percentage: C: 0.2% to 0.4% Mn: 0.8% to 2.0% Si: 0.1% to 0.5% Al: 0.01% to 0.1% Ti: 0.01% to 0.1% B: 0.0005% to 0.005% P≤0.040％ Ca ≤ 0.01% S≤0.006％ N≤0.01％ And optionally include: Cr≤0.4％ Mo≤0.3% Nb≤0.1% V≤0.3％ Where Cr+Mo+Nb+V≤0.5% The remaining component consists of iron and unavoidable impurities produced during the processing. The steel plate has a microstructure comprising, by surface fraction, 75% to 90% ferrite, with the remainder consisting of Fe3C and hard phases including martensite and bainite. The steel plate has a thickness between 1.2 mm and 2.1 mm. The steel plate includes, from its main body to its surface: -The main body, comprising 80% of the thickness of the steel plate, - The top of such a body consists of a top surface layer and a bottom surface layer occupying the outermost 10% thickness on both sides of the body. The body contains a cluster of inclusions, wherein the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions is less than or equal to 300 μm / mm. 2 .

2. The steel plate according to claim 1, wherein: C: 0.2% to 0.25%, and / or Mn: 1.0% to 1.4%, and / or Si: 0.1% to 0.4%, preferably 0.15% to 0.35%, and / or Al: 0.02% to 0.06%, and / or Ti: 0.02% to 0.06%, and / or B: 0.002% to 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. The steel plate according to claim 1 or 2, wherein the steel plate is coated with a metal coating comprising at least 50% Al by weight.

4. The steel plate according to claim 1 or 2, wherein the steel plate is coated with a metal coating comprising at least 50% Zn by weight.

5. The steel plate according to claim 1 or 2, wherein the chemical composition further conforms to the following conditions, and all elements are expressed in weight percent: (S-Ca*32 / 40)+(30*Ti*N)≤0.0045.

6. A press-hardened steel component, the steel component having the following composition, the composition comprising, by weight percentage: C: 0.2% to 0.4% Mn: 0.8% to 2.0% Si: 0.1% to 0.5% Al: 0.01% to 0.1% Ti: 0.01% to 0.1% B: 0.0005% to 0.005% P≤0.040％ Ca ≤ 0.01% S≤0.006％ N≤0.01％ And optionally include: Cr≤0.4％ Mo≤0.3% Nb≤0.1% V≤0.3％ Where Cr+Mo+Nb+V≤0.5% The remaining component consists of iron and unavoidable impurities produced during the processing. 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 main body consists of a top surface layer and a bottom surface layer that occupy the outermost 10% of the thickness on both sides of the main body. The main body comprises a group of inclusions, wherein the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions is less than or equal to 300 μm / mm. 2 .

7. The press-hardened steel component according to claim 6, wherein: C: 0.2% to 0.25%, and / or Mn: 1.0% to 1.4%, and / or Si: 0.1% to 0.4%, preferably 0.15% to 0.35%, and / or Al: 0.02% to 0.06%, and / or Ti: 0.02% to 0.06%, and / or B: 0.002% to 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%.

8. The press-hardened steel component according to claim 6 or 7, wherein the chemical composition further conforms to the following conditions, with all elements expressed in weight %: (S-Ca) * 32 / 40)+(30 * Ti * N)≤0.0045.

9. The press-hardened steel component according to claim 6 or 7, wherein the press-hardened steel component has a tensile strength TS of at least 1300 MPa and a bending angle anisotropy of less than or equal to 7°.

10. A method for manufacturing a steel plate according to any one of claims 1 to 5, comprising the following sequential steps: - Provide molten steel having the following chemical composition, wherein the chemical composition comprises, by weight percentage: C: 0.2% to 0.4% Mn: 0.8% to 2.0% Si: 0.1% to 0.5% Al: 0.01% to 0.1% Ti: 0.01% to 0.1% B: 0.0005% to 0.005% P≤0.040％ Ca ≤ 0.01% S≤0.006％ N≤0.01％ And optionally include: Cr≤0.4％ Mo≤0.3% Nb≤0.1% V≤0.3％ Where Cr+Mo+Nb+V≤0.5% The remaining portion of the composition consists of iron and unavoidable impurities. - Cast the molten steel to obtain a semi-finished product that can be hot-rolled. - The semi-finished product is hot-rolled at a fine hot rolling temperature of 800°C to 950°C. - The hot-rolled steel sheet is coiled at a temperature T below 670°C. 卷取 The coil is then wound up to obtain the coiled steel sheet.

11. The method of claim 10, wherein: C: 0.2% to 0.25%, and / or Mn: 1.0% to 1.4%, and / or Si: 0.1% to 0.4%, preferably 0.15% to 0.35%, and / or Al: 0.02% to 0.06%, and / or Ti: 0.02% to 0.06%, and / or B: 0.002% to 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%.

12. The method of claim 10 or 11, wherein the step of providing the molten steel includes a refining stage during which the sulfur level measured at the start of refining, the Al addition at the start of refining, the Ca addition during refining, and the volume of O2 blowing are controlled to verify the combination Cl = Al - addition + 0.1953 * (S_start * 1000 + O2_injection) - 9.367 * Ca_ is added to keep it below the predetermined cutoff value. Al_ added refers to the amount of Al added at the start of the refining process, expressed in kg aluminum per ton of molten steel. S_ begins as the sulfur content prior to the refining process, expressed as a percentage by weight. O2 injection refers to the amount of O2 injected during the optional aluminothermic heating step, expressed in standard cubic meters of O2 per ton of molten steel. Ca_addition refers to the amount of Ca added to the molten steel, measured as a percentage by weight in the molten steel.

13. A method for manufacturing a press-hardened steel component according to claim 6 or 7, 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 the formed part. - The molded part is subjected to compression molding and quenching.

14. The method of claim 13, further comprising a baking paint step, wherein the molded part is heated to a temperature of 150°C to 250°C for a duration of 10 minutes to 2 hours.

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