Steel sheet with excellent bending anisotropy and high-strength press-hardened steel component and method of manufacturing the same

Abstract

A steel plate having a chemical composition comprising, in weight %, the following: 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 plate comprising, from the main body to the surface of the coated steel plate, a main body and a surface layer occupying the outermost 10% of the thickness on both sides of the main body, the main body containing an inclusion group, 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 This allows the production of hot-pressed parts with a tensile strength equal to or greater than 1300 MPa and a bending anisotropy equal to or lower than 7°.

Description

Technical Field

[0001] The present invention relates to steel sheets and to high strength press hardened steel components. Background Art

[0002] High-strength press-hardened components can be used as structural elements in motor vehicles for anti-intrusion or energy absorption functions.

[0003] In this type of application, it is desirable to produce steel components that combine high mechanical strength with high impact resistance. Furthermore, in view of global environmental protection, one of the main challenges in the automotive industry is to reduce the weight of vehicles to improve their fuel efficiency without neglecting safety requirements.

[0004] This weight reduction can be achieved in particular due to the use of steel components having a predominantly martensite microstructure.

[0005] Producing very high-strength steels that also exhibit good and uniform resistance to crack formation under bending is challenging. Indeed, very high-strength steels tend to crack at an early age when subjected to bending loads. This is detrimental to the crashworthiness of components produced from such high-strength steels because, although the material is able to withstand very high loads due to its high tensile strength, once cracks initiate in a component, they will rapidly propagate under continued load, and the component will fail prematurely.

[0006] In particular, producing very high-strength press-hardened steels 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 directions. This anisotropic behavior introduces constraints when designing steel components and when developing hot stamping processes. Therefore, there is great interest in providing steel sheets and methods for producing such steels that allow very similar bending behavior to be achieved in all directions. Summary of the Invention

[0007] The object of the present invention is to solve the above challenges and to provide a press-hardened steel component having a combination of high mechanical properties: a tensile strength after hot stamping higher than or equal to 1300 MPa and a very small bending angle anisotropy between rolling direction and transverse direction.

[0008] Another object of the present invention is to provide a steel sheet which can be transformed into such a press-hardened steel component by hot forming, and to provide a method for manufacturing such a steel sheet.

[0009] The object of the present invention is achieved by providing a steel sheet according to claim 1, optionally with 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. The steel component may also include the features of claims 6 to 7. Another object of the present invention is a method for manufacturing said hot stamped component according to claim 8, optionally with the features of claim 9. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will now be described in detail and by way of non-limiting examples and with reference to Figure 1 To illustrate the present invention, Figure 1 is a schematic cross section of a steel plate according to the present invention. DETAILED DESCRIPTION

[0011] A steel blank is a flat sheet of steel that has been cut into any shape suitable for its intended use. The blank has a top and bottom surface, also referred to as the top and bottom sides or as the top and bottom surfaces. The distance between these surfaces is designated as the blank's thickness. Thickness can be measured, for example, using a micrometer whose spindle and anvil are placed on the top and bottom surfaces. Similarly, the thickness of formed parts can also be measured.

[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, forming the blank at high temperature by stamping it, and quenching the formed part to obtain a microstructure with very high strength. Hot stamping allows very high strength parts with complex shapes to be obtained and presents many technical advantages. It should be understood that the heat treatment to which the part is subjected includes not only the thermal cycles of the hot stamping process itself described above, but may also include other subsequent heat treatment cycles, such as a paint baking step performed after the part has been painted in order to cure the paint. The following mechanical properties of hot stamped parts are mechanical properties measured after all thermal cycles (optionally including, for example, a paint baking step, if paint baking has indeed been performed).

[0013] The ultimate tensile strength is measured according to ISO standard ISO 6892-1 published in October 2009. Tensile test specimens are cut from a flat area of ​​the hot stamped part. If necessary, undersized tensile test specimens are used to accommodate the full available flat area on the part.

[0014] Bend angles are measured according to the VDA-238-100 bending standard, as of the June 2017 edition. Bend angles are measured using a laser measuring device. When performing bend tests on hot-stamped parts, samples are cut from a flat area of ​​the part. If necessary, use undersized samples to accommodate the full available flat area on the part.

[0015] The bend angle of a component represents the component's ability to resist deformation without crack formation.

[0016] The bend 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° to the rolling direction. The bend anisotropy is defined as the absolute value of the difference between the bend angles measured in the rolling direction and in the transverse direction for a given sample.

[0017] If the rolling direction on a hot stamped part is not known, it can be determined using electron backscatter diffraction (EBSD) analysis across a cross section of the sample in a scanning electron microscope (SEM). The intensity of the orientation density function (ODF) of the main fibers under 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 For the definition of , see Figures 2.2 and 2.3).

[0018] The composition of the steel according to the present invention will now be described, with the contents expressed in weight percentages. 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 present invention. Where preferred ranges for a given element are disclosed, all possible combinations of these preferred ranges for each individual element are also disclosed.

[0019] 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 be reduced. If the carbon content is below 0.2%, the tensile strength will not reach the target value. In a specific embodiment, the carbon content ranges from 0.2% to 0.3% to ensure sufficient strength while further controlling excellent weldability and bendability. In a specific embodiment, the carbon content ranges from 0.2% to 0.25% to ensure sufficient strength while even further controlling excellent weldability and bendability.

[0020] The manganese content ranges from 0.8% to 2.0%. Additions above 2.0% increase the risk of MnS formation, impairing bendability. Below 0.8%, the hardenability of the steel sheet during the hot stamping process decreases. In a specific embodiment, the manganese content ranges from 1.0% to 1.4% to further improve the hardenability of the steel and further limit the formation of MnS, thereby improving bendability.

[0021] The silicon content ranges from 0.1% to 0.5%. Silicon is an element that participates in solid solution hardening. It is added to limit carbide formation. Above 0.5%, silicon oxides form on the surface, which impairs the paintability of the steel. Furthermore, the weldability of components produced from the steel sheet may be reduced. In one specific embodiment, the silicon content ranges from 0.1% to 0.4% to further improve paintability and weldability. In another specific embodiment, the silicon content ranges from 0.15% to 0.35% to further harden the steel and further improve paintability and weldability.

[0022] According to the present invention, the aluminum content ranges from 0.01% to 0.1% because it is a very effective element for deoxidizing the steel in the liquid phase during processing. If the titanium content is insufficient, aluminum can protect the boron. 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 steel in the liquid phase, while further avoiding oxidation problems and ferrite formation during press hardening.

[0023] According to the present invention, the titanium content ranges from 0.01% to 0.1% to protect the boron that would otherwise be trapped in BN precipitates. The titanium content is limited to 0.1% to avoid excessive TiN formation. In a specific embodiment, the Ti content ranges from 0.02% to 0.06% to further protect the boron while further avoiding excessive TiN formation.

[0024] According to the present invention, the boron content ranges from 0.0005% to 0.005%. Boron improves the hardenability of the steel. Boron content of no more than 0.005% prevents slab breakage during continuous casting. In a specific embodiment, the boron content ranges from 0.002% to 0.004% to further ensure the hardenability of the steel and further prevent slab breakage.

[0025] Phosphorus is controlled to be less than or equal to 0.040% because it causes brittleness and weldability problems. In a specific embodiment, the P content is controlled to be less than or equal to 0.020% to further avoid brittleness and weldability problems.

[0026] Calcium is controlled to be less than or equal to 0.01% because the presence of calcium in the molten steel may cause the formation of coarse inclusions that are detrimental to bendability. In a specific embodiment, the Ca content is controlled to be less than or equal to 0.005% to further avoid the coarse inclusion problem.

[0027] Sulfur is controlled to be less than or equal to 0.006% because the presence of sulfur in the molten steel may lead to the formation of MnS precipitates that are detrimental to bendability. In a specific embodiment, the S content is controlled to be less than or equal to 0.005% to further avoid the formation of MnS precipitates.

[0028] Nitrogen is controlled to be lower than or equal to 0.01%, preferably lower than or equal to 0.008%, even more preferably lower 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 bendability.

[0029] Chromium is optionally added up to 0.4%. Chromium can be used to provide strength through solid solution hardening and improve the hardenability of the steel sheet during hot stamping. Chromium is limited to 0.4% to limit costs and avoid processing problems.

[0030] Molybdenum is optionally added up to 0.3%. Molybdenum improves the hardenability of the steel. Molybdenum is limited to 0.3% to limit costs and avoid processing problems.

[0031] Niobium is optionally added up to 0.1%. Niobium improves the ductility of the steel. Niobium is limited to 0.1% to limit costs and avoid processing problems.

[0032] Vanadium is optionally added up to 0.3%. Vanadium improves the hardenability of the steel. Vanadium is limited to 0.3% to limit costs and avoid processing problems.

[0033] In the case of adding one or more of the above elements, the following formula is further verified: Cr+Mo+Nb+V≤0.5% in order to limit costs and avoid processing problems.

[0034] In a specific embodiment, the chemical composition is further controlled so that the following conditions are verified:

[0035] (S-Ca*32 / 40)+(30*Ti*N)≤0.0045

[0036] The present inventors have found that this allows further control of the inclusion population of the steel sheet and thus further improvement in bendability and bending anisotropy.

[0037] The remainder of the steel's composition is iron and impurities resulting from the processing. The level of impurities resulting from the processing will depend on the production route used and the level of scrap used in the steel solution. For example, when a basic oxygen furnace route with low levels of scrap (recycled steel) is used, the level of impurities will remain very low. However, large amounts of scrap can also be added to the pig iron produced in the basic oxygen furnace in the converter, which will increase the level of impurities. In addition, when steel is processed using an electric furnace, for example with a very high ratio of recycled scrap, the level of impurities will increase significantly. When high levels of scrap are used, the level 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%.

[0038] The microstructure of the steel plate according to the present invention will now be described.

[0039] The steel plate has, on any analyzed section, a microstructure consisting, as a surface fraction, of:

[0040] -75% to 90% ferrite,

[0041] - The remainder consists of carbides Fe3C and hard phases such as martensite and bainite.

[0042] Reference Figure 1 The steel sheet 1 according to the present invention comprises a main body portion 3 and top and bottom skin layers 2. The total thickness of the steel sheet 1 is t0, and the thickness ts of the skin layer 2 is such that ts = t0 * 10%. In other words, the skin layer 2 occupies the outermost 10% of the thickness on both sides of the main body, and the main body of the steel sheet occupies 80% of the thickness of the steel sheet.

[0043] The present inventors have found that there is a correlation between the bending anisotropy of the steel plate and the inclusion group in the main portion. In particular, when the sum of the clustering indexes of MnS and TiN / Ti(C,N) inclusions in the main portion of the steel is controlled to be less than or equal to 300 μm / mm 2 When , the bending anisotropy can be limited to less than or equal to 7°.

[0044] 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 approach and that others may also be implemented.

[0045] The cross section of the steel plate on which the inclusions are observed is cut in the rolling direction of the steel. In other words, the plane of the observed cross section has the transverse direction as its normal direction.

[0046] The inclusions present in the steel plate were characterized using a scanning electron microscope (SEM) with a field effect gun (FEG). A Tescan Mira 3 SEM was used at a 14 kV power setting. This allows the detection of particles as small as 0.5 μm. Using the FEG SEM setup allows stable images with excellent resolution to be obtained over long periods of time, which may be necessary to perform image analysis over large areas - using the FEG SEM setup image fields can be obtained for periods of up to 48 hours, which may be necessary for multi-sample analysis. In addition, the inclusions were analyzed using energy dispersive spectroscopy (EDS). A 120 mm SEM with a large effective surface was used. 2 Bruker EDS probe to detect light elements (O, N) and achieve high count rates, thus allowing accurate quantification. method was used to obtain accurate quantification.

[0047] The Automated Steel Cleanliness Analysis Tool (ASCAT) from the RJ Lee group, based on computer-controlled scanning electron microscopy technology, was used to test the SEM and the associated EDS. Six individual samples could be analyzed in the same batch. The sample surface was divided into three regions (top surface, bottom surface, bulk as previously described). Each region was divided into fields. In each field, inclusions were detected. In order to detect fine particles, the scanning pixel size was set to a very low value of 0.11 μm. This is to reduce the matrix noise of the SEM image. As will be seen, only objects with a diameter exceeding 0.5 μm were actually considered. The first selection of objects to be referred to as particles was performed by selecting objects that formed solids and had a gray level below 150 or above 220 (extreme values ​​were excluded) on a scale of 0 to 255.

[0048] 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 and taking into account the chemical and morphological characteristics of all analyzed particles for all acquired images.

[0049] Of all the analyzed particle groups, only particles with a size greater than 0.5 μm and an iron content less than 80% were retained for subsequent analysis and were referred to as inclusions—other particles were considered part of the matrix and were not relevant for subsequent analysis.

[0050] 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.

[0051] Table 1 - Classification criteria for inclusions based on weight % of Ti, M, S, O, Nb

[0052]

[0053] Then for each inclusion type the following properties are calculated:

[0054] - average diameter, in micrometers,

[0055] -Density, as the number of inclusions / mm 2 count,

[0056] The clustering index calculation method is based on the DBSCAN (Density-Based Spatial Clustering of Applications with Noise) algorithm, as described in the article "Adensity-based algorithm for discovering clusters in large spatial databases with noise" in the Proceedings of the Second International Conference on Knowledge Discovery and Data Mining (KDD-96). AAAI Press., Ester, Martin; Kriegel, Hans-Peter; Sander, Details are given in Xu, Xiaowei (1996), pp. 226-231.

[0057] The clustering index is determined by taking two parameters: maximum_distance and minimum_number_of_points. Clustering is characterized by the following features:

[0058] - it contains only particles of the same type,

[0059] - in a given cluster, the distances of an inclusion to at least one other inclusion are all less than max_distance,

[0060] - It contains a number of individual inclusions equal to or greater than min_points.

[0061] For the present invention, the inventors have found that with a maximum_distance of 30 μm maximum distance and a minimum_number of inclusions / minimum_number of clusters of 4, good detection of clusters is obtained.

[0062] The length L of a given cluster is calculated as follows:

[0063] The convex hull of the cluster is first determined using known algorithms (see, for example, the chapter “Convex Hulls: Basic Algorithms” in Computational Geometry, Preparata, FP, Shamos, MI, 1985, Texts and Monographs in Computer Science, Springer, New York, NY).

[0064] - The maximum Feret diameter of the convex hull is then determined, which will be called Dmax, and also the Feret diameter taken in a direction perpendicular to Dmax, which will be called Dperpendicular. Information on Feret diameter measurements can be found, for example, in "Particle Size Measurements: Fundamentals, Practice, Quality" Springer. Henk G. Merkus (January 1, 2009).

[0065] - Calculate the length L of the cluster as

[0066]

[0067] For each type of inclusion, the average length L_mean of all clusters is calculated.

[0068] The cluster density of inclusions of a given type C_density is the number of clusters / mm 2 .

[0069] 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, C_index = L_average * C_density. Clustering index in μm / mm 2 The inventors have found that the clustering index allows samples with different properties to be compared using a unique number and that it correlates well with the bending behavior of the samples.

[0070] The steel sheet according to the present invention can be produced by any appropriate manufacturing method, and a method can be defined by those skilled in the art. However, it is preferred to use the method according to the present invention comprising the following steps.

[0071] In the following description, the term ladle refers to a container used to hold molten steel during the refining process, which is the step of adjusting the final chemical composition and temperature of the melt before casting the steel into its first solidified form, such as before casting it into slabs that will subsequently be hot rolled.

[0072] To successfully control the inclusion population of steel, for example, the following process can be implemented:

[0073] -Tapping molten steel from a previous steelmaking process step into a 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.

[0074] - measuring the sulfur content of the molten steel before the refining step, for example by removing a sample of the molten steel and analyzing it using a spark spectrometer. The sulfur content is measured, for example, by sampling the molten steel directly in a ladle or by sampling the molten steel as it is tapped into the ladle. The sulfur content before the refining step, measured in wt. %, will be referred to as S_start in the following description.

[0075] Aluminum is added to the ladle at the start of the refining process to deoxidize the molten steel. This Al addition is performed, for example, while the steel is being tapped into the ladle. Advantageously, this allows for time savings and thus increases productivity, while ensuring that the molten steel remains sufficiently hot. The amount of Al added to the molten steel at the start of the refining process (expressed in kg aluminum per ton of molten steel (kg / ton)) will be referred to as Al_addition in the following description.

[0076] - In a subsequent optional step, for example, if the temperature of the molten steel is too low, or if it is foreseeable that the waiting time between the end of the refining step and the subsequent process (e.g., continuous casting) requires it, the molten steel is reheated by aluminothermic heating. This is done by adding a defined amount of aluminum and co-blowing a defined amount of oxygen into the molten steel, the defined amount of oxygen corresponding to the stoichiometric ratio required to form Al2O3 with the added aluminum. The strongly exothermic reaction between Al and O2 allows the temperature of the molten steel to be increased. The amount of O2 injected during this optional step will be referred to as O2_injection and is expressed in standard cubic meters of O2 per ton of molten steel (Nm3). 3 The amount of Al injected for thermite reheating is not considered separately in the present description because there is a direct stoichiometric relationship between the O2 injection and the associated Al injection for thermite reheating. It should be noted that the Al injected for thermite reheating is different from the Al addition mentioned previously.

[0077] - the composition of the slag above the melt is adjusted by adding appropriate amounts of minerals to ensure that the %CaO / %Al2O3 ratio of the slag is above 1, the amount of slag per ton of liquid steel is at least 10 kg / ton of liquid steel, and that the slag is kept liquid to promote chemical exchange with the steel, to enable the steel to enter under the slag and to enable the steel and / or the slag to be tapped separately (the liquid state of the slag is verified visually and / or using thermodynamic rules based on its composition and temperature).

[0078] - In a subsequent step, the molten steel is stirred by blowing an inert gas into it, for example Ar. This is done to promote the exchange between the molten steel and the slag, which will allow the sulfur content of the molten steel to be reduced.

[0079] In a separate step, Ca is added to the ladle to spheroidize inclusions present in the molten steel. For example, Ca is added in the form of silicon calcium (SiCa), iron calcium (FeCa), or 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 the amount of Ca added to be easily controlled by controlling the length of the core wire inserted into the melt and the injection speed. The amount of Ca added to the molten steel (measured in wt. % in the molten steel) will be referred to as Ca_addition in the following description.

[0080] Taking into account the above process, the present inventors have found that satisfactory inclusion levels can be obtained to achieve the desired bending anisotropy properties after hot stamping by controlling the above-mentioned levels of sulfur measured at the start of the refining process (S_start, measured in wt. %), Al addition at the start of the refining process (Al_addition, measured in kg / ton), 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 (which will be referred to as C1 in the rest of the description) remains below a given cut-off value:

[0081] Al_Add +0.1953 * (S_Start * 1000+O2_injection)-9.367 * Ca_addition (C1)

[0082] In practice, the specific cut-off value below which the combination C1 needs to be controlled will depend on the specific industrial setting used to produce steel. It will depend on the production route in the steelmaking plant, the geometry of the ladle used to handle the molten steel, the equipment used to add the different additives, the oxygen blowing configuration, etc.

[0083] In order to determine the relationship between these parameters for a given industrial plant and production route, it is recommended to apply the following approach:

[0084] -Several heats were performed using the chemical composition ranges described previously.

[0085] The smelts were processed using different refining process parameters, in particular different levels of measured sulfur at the start of the refining process, Al additions at the start of the refining process, Ca additions during the refining process, and the volume of O2 gas sparging. The range of refining process parameters tested was selected to represent the industrial variation in these parameters. For example, a group of 6 different smelts with 6 different sets of refining process parameters was selected. For example, a group of 8 different smelts with 8 different sets of refining process parameters was selected.

[0086] - The heat was processed according to the industrial route described below and the inclusion population of the steel was characterized using the method described above.

[0087] The sum of the clustering indices for MnS and TiN / Ti(C,N) inclusions in the bulk of the steel plate and the associated refining process parameters is then recorded. The combination C1 of these refining process parameters is calculated. As a general trend, the higher the combination C1, the higher the sum of the clustering indices.

[0088] - using the above data set relating clustering indices and refining process parameters, determining a cutoff value below which the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk of the steel sheet is less than or equal to 300 μm / mm 2 The cut-off value of the combined C1 will determine how to control the refining process for the specific industrial facility under consideration. By controlling C1 below the cut-off value, it will be possible to produce a bulk fraction having a sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions below or equal to 300 μm / mm. 2 The steel plate can therefore achieve an excellent bending anisotropy of less than or equal to 7°.

[0089] For example, in the case of the specific industrial facility where the inventors conducted their experiments, the cut-off value was equal to 1.80.

[0090] After the molten steel refining step, the method for manufacturing the steel plate according to the present invention preferably comprises the following steps:

[0091] Continuously casting the molten steel into a semi-finished product suitable for hot rolling. During the casting step, special care should be taken to avoid oxygen absorption and, therefore, high inclusion levels in the semi-finished product. For example, in the case of a continuous casting process, where the semi-finished product is a slab produced in a continuous sequence by casting multiple heats in a mold that are poured into a tundish, specific refractory materials and linings can be used in the tundish, and specific distribution rules can be used for the first slab in the sequence and for the transition slab between the two different heats, etc.

[0092] The semi-finished product is then optionally reheated at a temperature of 1150°C to 1300°C.

[0093] -The steel sheet is then hot rolled at a finishing hot rolling temperature of 800°C to 950°C.

[0094] -The hot rolled steel is then cooled and heated to a temperature below 670°C 卷取 The steel is coiled and optionally pickled to remove oxidation.

[0095] The coiled steel sheet is then optionally cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction ratio preferably ranges from 20% to 80%. A reduction ratio below 20% hinders recrystallization during subsequent heat treatment, potentially impairing the ductility of the steel sheet. A reduction ratio above 80% creates a risk of edge cracking during cold rolling.

[0096] In one embodiment of the present invention, the steel sheet is heated in an annealing furnace to a soaking temperature of 700° C. to 850° C. and maintained at the soaking temperature for a soaking time of 10 seconds to 20 minutes.

[0097] In one embodiment of the invention, the thus annealed steel sheet is cooled to a temperature in the range of 400° C. to 700° C. and further coated with a metallic coating. The metallic coating is, for example, an aluminum-based metallic coating comprising at least 50% by weight of aluminum. The metallic coating is, for example, a zinc-based metallic coating comprising at least 50% by weight of zinc.

[0098] - In one embodiment of the invention, the steel sheet is then cooled to room temperature.

[0099] In summary, the above process preferably includes the following consecutive steps:

[0100] - producing a molten steel having 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 the O2 gas purge are controlled to verify the combined Al_addition+0.1953 * (S_Start * 1000+O2_injection)-9.367 * The Ca addition (C1) is kept below a predetermined cut-off value. The cut-off value is determined for the specific industrial equipment being used, so that when C1 is below the cut-off value, the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the main part of the steel is less than or equal to 300 μm / mm 2 .

[0101] - casting said molten steel to obtain semi-finished products capable of being hot rolled,

[0102] - optionally heating the semi-finished product at a temperature T of 1100°C to 1300°C 再加热 Reheat,

[0103] - hot rolling the semi-finished product at a finishing hot rolling temperature of 800°C to 950°C,

[0104] -The hot rolled steel sheet is coiled at a temperature T lower than 670°C 卷取 Coil down to obtain coiled steel plate,

[0105] - optionally pickling the coiled steel sheet,

[0106] - optionally, cold rolling the coiled steel sheet at a reduction ratio ranging from 20% to 80% to obtain a cold-rolled steel sheet,

[0107] - optionally heating the hot-rolled or cold-rolled steel sheet up to a soaking temperature of 700° C. to 850° C. and keeping the steel sheet at said temperature for a soaking time of 10 seconds to 20 minutes to obtain an annealed steel sheet,

[0108] - optionally cooling the annealed steel sheet to a temperature in the range of 400° C. to 700° C.,

[0109] - optionally coating the annealed steel sheet with a metallic coating,

[0110] - Optionally cooling the coated steel sheet to room temperature.

[0111] The pressed component manufacturing process and subsequent characteristics of the pressed components will now be described in detail.

[0112] Steel blanks are cut from the steel sheets according to the present invention and heated in an austenitizing furnace. Preferably, the steel blanks are heated to a temperature of 880°C to 950°C for 10 seconds to 15 minutes to obtain heated steel blanks. The heated blanks are then transferred to a press, where they are hot-formed and press-hardened to obtain pressed parts.

[0113] Optionally, the hot stamped component is further subjected to a paint bake step in which the component is heated to a temperature of 150° C. to 250° C. for a duration of 10 minutes to 2 hours.

[0114] The microstructure of the pressed part comprises, as a surface fraction, more than 95% martensite and less than 5% bainite + ferrite on any analyzed cross section. Furthermore, the pressed part according to the present invention comprises a main body portion and a top surface layer and a bottom surface layer, wherein the surface layer occupies 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 of the main body portion is less than or equal to 300 μm / mm. 2 .

[0115] The pressed component according to the present invention has a tensile strength greater than 1300 MPa, preferably greater than 1350 MPa, and preferably greater than 1400 MPa, and a bending angle anisotropy less than or equal to 7°. Such high tensile strength and low bending anisotropy provide the component with excellent mechanical resistance, especially in the event of a crash, and furthermore allow for very predictable and uniform behavior in all directions. These properties provide excellent energy absorption and anti-intrusion capabilities in all directions, thereby enhancing vehicle safety.

[0116] The invention will now be illustrated by the following examples which are in no way limiting.

[0117] Eight different samples of steel produced using the industrial production route from eight different heats A, B, C, D, E, F, G and H were tested. Samples I1, I2, I3 and I4 were according to the invention, samples R1, R2, R3 and R4 were reference samples.

[0118] All samples were produced following the same industrial production process in the steelworks. After annealing, all samples were coated with an AlSi-based coating containing 8 to 12 wt% Si, 2 to 4 wt% Fe, and the balance Al.

[0119] Table 2 - Sample composition

[0120] The compositions tested are summarized in the table below, where the element contents are expressed in weight percentages, the remainder of the composition being iron and unavoidable impurities resulting from the processing:

[0121]

[0122] Table 3 - Steelmaking plant process parameters and clustering index of MnS and TiN / Ti(C,N) inclusions in the bulk of the steel sum of numbers

[0123] The following process parameters were applied in a steelmaking plant and the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk of the following steels were observed - underlined values ​​are not according to the invention:

[0124]

[0125]

[0126] * C1=Al_addition+0.1953 * (S_Start * 1000+O2_injection)-9.367 * Ca_added

[0127] As can be seen, under the industrial conditions tested, 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 less than or equal to 300 μm / mm. 2 As previously explained, this cut-off value of 1.80 is specific to the industrial setting in which the experiments are conducted, and the appropriate cut-off factor for a given industrial setting will need to be determined, for example by following the method described above.

[0128] Table 4 - Additional process conditions

[0129] The following process parameters are applied along the production route:

[0130]

[0131] Table 5 - Microstructure, bending angle and tensile strength

[0132] The following microstructure (based on surface fraction), bend angle, bend angle anisotropy and tensile strength were measured on the samples, the underlined values ​​are not according to the invention:

[0133]

[0134]

[0135] Table 5 shows that the samples according to the present invention have a tensile strength (in both RD and TD) higher than 1300 MPa while having a bending anisotropy lower than 7°. On the other hand, the reference sample, despite having a comparable tensile strength level higher than 1300 MPa, has a bending anisotropy higher than 7°.

[0136] The inventors have found that this very good bending anisotropy is related to the sum of the clustering indices of the MnS and TiN / Ti(C,N) inclusions in the bulk of the steel, as can be seen in Table 3. A higher clustering index of the MnS and TiN / Ti(C,N) inclusions in the bulk of the steel results in a higher bending anisotropy. 300 μm / mm 2 A cutoff value of allows controlling the flexural anisotropy to less than or equal to 7°. Due to this very low level of flexural anisotropy, the hot stamped component behaves very uniformly when subjected to loads from any direction. This very stable behavior of hot stamped components under load allows for simplified component design, such as in the case of automotive components, and ensures very good, robust, and stable crashworthiness of the hot stamped component.

Claims

1. A steel plate made of steel having the following composition, wherein the composition comprises, by weight percentage: C: 0.2% to 0.25% Mn: 0.8% to 2.0% Si: 0.15% to 0.35% 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 contains: Cr≤0.4% Mo≤0.3% Nb≤0.1% V≤0.3% Cr+Mo+Nb+V≤0.5% The remainder of the composition is iron and unavoidable impurities resulting from the processing. The steel plate has a microstructure comprising, by surface fraction, 75% to 90% ferrite, the remainder being a microstructure consisting of Fe3C and hard phases including martensite and bainite, The steel plate includes, from the main body to the surface of the steel plate: - a body representing 80% of the thickness of the steel plate, - the top of such a body is the top and bottom surface layers occupying the outermost 10% of the thickness on both sides of the body, the body comprising 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: Mn: 1.0% to 1.4%, 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%。 3. The steel plate according to claim 2, wherein: N≤0.005%。 4. The steel sheet according to claim 1 or 2, wherein the steel sheet is coated with a metal coating comprising at least 50% by weight of Al.

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

6. The steel sheet according to claim 1 or 2, wherein the chemical composition further complies with the following conditions, with all elements expressed in weight %: 。 7. A press-hardened steel component, the steel component having the following composition, the composition comprising, by weight percentage: C: 0.2% to 0.25% Mn: 0.8% to 2.0% Si: 0.15% to 0.35% 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 contains: Cr≤0.4% Mo≤0.3% Nb≤0.1% V≤0.3% Cr+Mo+Nb+V≤0.5% The remainder of the composition is iron and unavoidable impurities resulting from the processing. The steel component has a microstructure comprising, by surface fraction, more than 95% of martensite and up to 5% of bainite or ferrite, The steel component comprises, from the main body to the surface of the steel component: - Subject, - the top of such a body is the top and bottom skin layers occupying the outermost 10% of the thickness on both sides of said body, The body comprises 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 .

8. The press-hardened steel component according to claim 7, wherein: Mn: 1.0% to 1.4%, 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%。 9. The press-hardened steel component according to claim 8, wherein: N≤0.005%。 10. The press-hardened steel component according to claim 7 or 8, wherein the chemical composition further complies with the following conditions, all elements expressed in weight %: (S-Ca * 32 / 40)+(30 * Ti * N)≤0.0045.

11. The press-hardened steel component according to claim 7 or 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°.

12. A method for manufacturing a steel plate according to claim 1 or 2, comprising the following consecutive steps: - providing molten steel having the following chemical composition, wherein the chemical composition comprises, by weight percentage: C: 0.2% to 0.25% Mn: 0.8% to 2.0% Si: 0.15% to 0.35% 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 contains: Cr≤0.4% Mo≤0.3% Nb≤0.1% V≤0.3% Cr+Mo+Nb+V≤0.5% The remainder of the composition is iron and unavoidable impurities, - casting said molten steel to obtain semi-finished products capable of being hot rolled, - hot rolling the semi-finished product at a finishing 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 steel plate is then coiled to obtain a coiled steel plate.

13. The method according to claim 12, wherein: Mn: 1.0% to 1.4%, 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%。 14. The method according to claim 13, wherein: N≤0.005%。 15. The method according to claim 12, wherein the step of providing the molten steel comprises a stage of refining the molten steel, 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 the O2 gas purge are controlled to verify the combination C1 = Al_addition + 0.1953 * (S_Start * 1000+O2_injection)-9.367 * Ca addition is kept below a predetermined cut-off value, Al_added is the Al added at the beginning of the refining, in kg aluminum / ton of molten steel, S_start is the sulfur content before said refining, in % by weight, O2_injection is the amount of O2 injected during the optional thermite heating step, expressed in Nm3 O2 / ton of molten steel, Ca_addition is the amount of Ca added to the molten steel, measured as wt% in the molten steel.

16. A method for manufacturing a press-hardened steel component according to claim 7 or 8, comprising the following consecutive steps: - providing a steel sheet according to claim 1 or 2, - cutting the steel sheet into predetermined shapes in order to obtain steel blanks, - heating the steel blank to a temperature of 880° C. to 950° C. during 10 seconds to 15 minutes to obtain a heated steel blank, - transferring the heated blank to a forming press, - thermoforming said heated blank in said forming press to obtain a formed part, - press hardening the formed part.

17. The method according to claim 16, further comprising a paint baking 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.

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