Steel plate with excellent bending anisotropy, high-strength press-hardened steel part, and manufacturing method thereof

Abstract

Steel plates and high-strength press-hardened steel parts and their manufacturing methods. The steel plate has a chemical composition including, by weight percent, C: 0.2-0.4%, Mn: 0.8-2.0%, Si: 0.1-0.5%, Al: 0.01-0.1%, Ti: 0.01-0.1%, B: 0.0005-0.005%, P≦0.040%, Ca≦0.01%, S≦0.006%, and N≦0.01%, The steel sheet has a bulk and a skin layer occupying the outermost 10% of the thickness on both sides of the bulk from the bulk to the surface of the coated steel sheet, and the bulk has a clustering index of MnS and TiN / Ti(C,N) inclusions of 300 μm / mm 2 This makes it possible to manufacture hot pressed parts having a tensile strength of 1300 MPa or more and a bending anisotropy of 7° or less.

Description

[Technical field]

[0001] The present invention relates to steel plates and high strength press hardened steel parts. [Background technology]

[0002] High strength press hardened parts can be used as structural elements in automotive vehicles for anti-intrusion or energy absorbing functions.

[0003] For such types of applications, it is desirable to produce steel parts that combine high mechanical strength with high impact resistance. Furthermore, one of the main challenges in the automotive industry is to reduce the weight of vehicles to improve their fuel efficiency in an environmentally friendly way, without neglecting safety requirements.

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

[0005] It is difficult to produce very high strength steels that also have a good and uniform resistance to the formation of cracks under bending. Indeed, very high strength steels tend to crack prematurely when subjected to bending loads. This is detrimental to the crash resistance of parts produced with such high strength steels, since, although the material can withstand very high loads thanks to its high tensile strength, once cracks begin to appear in the part, these cracks propagate rapidly under continued loads and the part fails prematurely.

[0006] In particular, it is difficult to produce very high strength press hardened steels with isotropic bending behavior. In fact, generally speaking, it is well known that the bending behavior of steel sheets is better in the rolling direction than in the transverse direction. This anisotropic behavior poses constraints when designing steel parts and when considering hot stamping processes. It is therefore very interesting to provide steel sheets and methods for producing such steels that make it possible to reach very similar bending behavior in all directions. Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to address the above-mentioned problems and to provide a press-hardened steel part having a combination of high mechanical properties, a tensile strength after hot stamping of more than 1300 MPa and a very small anisotropy of the bending angle between the rolling direction and the transverse direction.

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

[0009] The object of the present invention is achieved by providing a steel sheet 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 part according to claim 5. The steel part may also include the characteristics of claims 6 to 7. A further object of the present invention is a manufacturing method for said hot stamped part according to claim 8, optionally including the characteristics of claim 9.

[0010] The invention will now be explained in more detail and illustrated by means of examples, without introducing any limitations, with reference to FIG. 1, which is a schematic cross-sectional view of a steel sheet according to the invention. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 shows a steel sheet 1 according to the invention comprising a bulk portion 3 and an upper and lower skin layer 2 . DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] A steel blank refers to a flat sheet of steel cut into any shape suitable for its use. The blank has an upper and lower surface, also referred to as the upper and lower surfaces or the upper and lower surfaces. The distance between said surfaces is designated as the thickness of the blank. The thickness can be measured, for example, using a micrometer, whose spindle and anvil are placed on the upper and lower surfaces. Similarly, the thickness can also be measured on a formed part.

[0013] 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 the blank, and quenching the formed part to obtain a microstructure with very high strength. Hot stamping makes it possible to obtain very high strength parts with complex shapes, and presents many technical advantages. It should be understood that the heat treatment that the part undergoes includes not only the thermal cycle of the hot stamping process itself described above, but also other subsequent heat treatment cycles, which may possibly be carried out after the part has been painted, such as for example a paint baking step, in order to harden the paint. The mechanical properties of the hot stamped parts below are measured after the complete thermal cycle, optionally including for example a paint baking step if a paint baking has actually been carried out.

[0014] Ultimate tensile strength is measured according to ISO standard ISO 6892-1, published October 2009. Tensile specimens are cut from the flat areas of the hot stamped parts. If necessary, smaller sizes of tensile test samples are taken to fit the entire available flat area on the part.

[0015] The bend angle is measured according to the VDA-238-100 bending standard according to the version published in June 2017. The bend angle was measured using a laser measuring device. When performing bend tests on hot stamped parts, samples are cut from the flat area of ​​the part. If necessary, smaller size samples are taken to fit the entire available flat area on the part.

[0016] The bend angle of a part represents the ability of the part to withstand deformation without forming cracks.

[0017] The bend angles are measured in the rolling direction (RD), i.e. the direction in which the steel sheet traveled during the hot rolling step, and in the transverse direction (TD), i.e. an orientation of 90° compared to the rolling direction. The bending anisotropy is defined as the absolute value of the difference between the bend angles measured in the rolling and transverse directions for a given sample.

[0018] If the rolling direction on a hot stamped part is unknown, it can be determined using electron backscatter diffraction (EBSD) analysis over the cross section of the sample in a scanning electron microscope (SEM). The rolling direction is determined according to the intensity of the orientation density function (ODF) representing the main fibers at φ2=45°, where φ2 is the Euler angle defined in "H.-J. Bunge: Texture Analysis in Materials Science-Mathematical Methods. 1st English Edition by Butterworth Co (Publ.) 1982" (see Figure 2.2 and Figure 2.3 for the definition of φ2).

[0019] The composition of the steel according to the invention is now described, the contents being expressed in weight percent. The chemical composition is given in terms of lower and upper compositional range limits, said limits being included within the possible compositional 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, carbon is in the range of 0.2%-0.4% to ensure satisfactory strength. If carbon exceeds 0.4%, the weldability and bendability of the steel plate may be reduced. If the carbon content is less than 0.2%, the tensile strength will not reach the target value. In a specific embodiment, carbon is in the range of 0.2%-0.3% to ensure sufficient strength while still controlling excellent weldability and bendability. In a specific embodiment, carbon is in the range of 0.2%-0.25% to ensure sufficient strength while still controlling excellent weldability and bendability.

[0021] The manganese content ranges from 0.8% to 2.0%. If the amount added exceeds 2.0%, the risk of MnS formation increases and the bendability is impaired. If it is less than 0.8%, the hardenability of the steel sheet during the hot stamping process is reduced.

[0022] In a particular embodiment, the manganese content is in the range of 1.0% to 1.4% to further improve the hardenability of the steel and further limit the formation of MnS and thus improve the bendability.

[0023] The silicon content ranges from 0.1% to 0.5%. Silicon is an element involved in hardening in solid solution. Silicon is added to limit the formation of carbides. Above 0.5%, silicon oxide forms on the surface, which impairs the coatability of the steel. Furthermore, the weldability of parts produced with said steel plate may be reduced. In a particular embodiment, the silicon content ranges from 0.1% to 0.4% to further improve the coatability and weldability. In a particular embodiment, the silicon content ranges from 0.15% to 0.35% to further harden the steel and further improve the coatability and weldability.

[0024] According to the present invention, the aluminum content is in the range of 0.01%-0.1% because aluminum is a very effective element for deoxidizing steel in the liquid phase during smelting. Aluminum can protect boron when the titanium content is not sufficient. The aluminum content is less than 0.1% to avoid oxidation problems and ferrite formation during press hardening. Preferably, the aluminum content is in the range of 0.02%-0.06% to further ensure good deoxidation of steel in the liquid phase while further avoiding oxidation problems and ferrite formation during press hardening.

[0025] According to the present invention, the titanium content ranges from 0.01% to 0.1% to protect boron that would otherwise be trapped in BN precipitates. The titanium content is limited to 0.1% to avoid excessive TiN formation. In a particular embodiment, the Ti content ranges from 0.02% to 0.06% to further protect boron while still avoiding excessive TiN formation.

[0026] According to the present invention, the boron content is in the range of 0.0005% to 0.005%. Boron improves the hardenability of the steel. The boron content is less than or equal to 0.005% to avoid the problem of slab breakage during continuous casting. In a particular embodiment, the boron content is in the range of 0.002% to 0.004% to further ensure the hardenability of the steel and further avoid the problem of slab breakage.

[0027] Phosphorus leads to brittleness and weldability issues and is therefore controlled to 0.040% or less. In a particular embodiment, the P content is controlled to 0.020% or less to further avoid brittleness and weldability issues.

[0028] Since the presence of calcium in the molten steel can lead to the formation of coarse inclusions that are detrimental to bendability, calcium is controlled to 0.01% or less. In certain embodiments, the Ca content is controlled to 0.005% or less to further avoid the problem of coarse inclusions.

[0029] Since the presence of sulfur in the molten steel can result in the formation of MnS precipitates that are detrimental to bendability, sulfur is controlled to 0.006% or less. In a particular embodiment, the S content is controlled to 0.005% or less to further avoid the formation of MnS precipitates.

[0030] Nitrogen is controlled to less than 0.01%, preferentially less than 0.008%, and even more preferentially less than 0.005%. The presence of nitrogen can lead to the formation of precipitates such as TiN or TiNbCN which are detrimental to bendability.

[0031] Chromium is optionally added up to a maximum of 0.4%. Chromium provides strength through solid solution hardening and can be used to improve the hardenability of the steel during hot stamping. Chromium is limited to 0.4% to limit cost and avoid processing problems.

[0032] Molybdenum is optionally added up to a maximum of 0.3%. Molybdenum improves the hardenability of the steel. Molybdenum is limited to 0.3% to limit cost and to avoid processing problems.

[0033] Niobium is optionally added up to a maximum of 0.1%. Niobium improves the ductility of the steel. Niobium is limited to 0.1% to limit cost and to avoid processing problems.

[0034] Vanadium is optionally added up to a maximum of 0.3%. Vanadium improves the hardenability of the steel. Vanadium is limited to 0.3% to limit cost and to avoid processing problems.

[0035] When one or several of the above elements are added, in order to limit the cost and avoid processing problems, the following formula is further verified: Cr+Mo+Nb+V≦0.5%.

[0036] In certain embodiments, the chemical composition is further controlled such that the following conditions are verified:

[0037] (S-Ca*32 / 40)+(30*Ti*N)≦0.0045

[0038] The inventors have found that this allows for further control of the inclusion population in the steel sheet and therefore further improves bendability and bending anisotropy.

[0039] The remainder of the steel composition is iron and impurities resulting from the smelting process. The level of impurities resulting from the smelting process depends on the production route used and the level of scrap used in the steel melt. For example, when using the basic oxygen furnace route with low levels of steel scrap (recycled steel), the level of impurities remains very low. However, it is also possible to add large amounts of scrap in the converter to the pig iron produced in the basic oxygen furnace, which increases the level of impurities. Furthermore, when smelting steel using, for example, an electric furnace with a very high proportion of recycled scrap steel, the level of impurities increases significantly. When using high levels of scrap, the level of Cu can rise to 0.25%, Ni can rise to 0.25%, Sn can rise to 0.05%, As can rise to 0.03%, Sb can rise to 0.03%, and Pb can rise to 0.03%.

[0040] Next, the microstructure of the steel sheet according to the present invention will be described.

[0041] The steel plate has a microstructure in any analytical cross section that includes the following surface fractions: -75%~90% ferrite, - The remainder is composed of carbides Fe3C and hard phases such as martensite and bainite.

[0042] 1, a steel plate 1 according to the invention comprises a bulk portion 3 and upper and lower skin layers 2. The total thickness of the steel plate 1 is t0 and the thickness ts of the skin layers 2 is such that ts=t0*10%. In other words, the skin layers 2 occupy the outermost 10% of the thickness on both sides of the bulk, and the bulk of the steel plate corresponds to 80% of the thickness of the steel plate.

[0043] The inventors have found that there is a correlation between bending anisotropy and the inclusion population in the bulk part of the steel sheet. In particular, when the sum of the clustering indexes of MnS and TiN / Ti(C,N) inclusions in the bulk part of the steel is less than 300 μm / mm 2 If controlled below this value, it is possible to limit the bending anisotropy to 7° or less.

[0044] Below is a description of the methodology used to characterize inclusions in steel plates and steel parts, it should be understood that this is only one possible methodology and that other protocols may also be implemented.

[0045] The cross-section of the steel sheet in which the inclusions are observed is taken 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 sheets have been characterized using a scanning electron microscope (SEM) equipped with a field effect gun (FEG). A Tescan Mira 3 SEM was used with a power setting of 14 kV. This makes it possible to detect particles as small as 0.5 μm. With the FEGSEM setting it is possible to obtain stable images with excellent resolution over long periods of time, which may be necessary to complete an extensive image analysis, and with the FEGSEM setting it is possible to acquire image fields for periods of up to 48 hours, which may be necessary for multiple sample analyses. Furthermore, the inclusions were analyzed using energy dispersive spectroscopy (EDS). A 120 mm SEM with a large active surface was used to detect light elements (O, N) and to obtain a high count rate and therefore allow for rigorous quantification. 2A Bruker EDS probe was used. Accurate quantification was obtained using the phi-rho-Z method.

[0047] The SEM and associated EDS are piloted using the Automated Steel Cleanliness Analysis Tool (ASCAT) of the RJ Lee Group, based on computer-controlled scanning electron microscopy technology. Six individual samples can be analyzed in the same batch. The sample surface is divided into three regions (top skin, bottom skin, bulk, as mentioned before). Each region is divided into fields. In each field, inclusions are detected. To detect fine particles, the scanning pixel size is set to a very low value of 0.11 μm. This is to reduce the matrix noise of the SEM image. As will be understood, only objects with a diameter greater than 0.5 μm are actually considered. The initial selection of objects, called particles, is done by selecting objects that form solids and have a gray level that is either less than 150 or greater than 220 on a scale of 0 to 255 (extreme values ​​are excluded).

[0048] Then, zoom in on each individual particle, capture their morphological features and perform EDS analysis. A database of all particles is created using ASCAT, taking into account the chemical and morphological features of all analyzed particles for all acquired images.

[0049] Of the total set of particles analyzed, only those with a size greater than 0.5 μm and an iron content less than 80% were retained for subsequent analysis and called inclusions, the other particles were considered to be part of the matrix and not relevant for subsequent analysis.

[0050] Using the information of the EDS probe, each inclusion is then classified into one of the following families: TiN, alumina, complex oxides, oxysulfide particles, MnS, etc. For example, Table 1 details the strict rules we used to classify MnS and TiN / Ti(C,N) inclusions. Oxygen quantification is made possible by the high performance EDS detector. Oxygen levels are checked to separate TiN from TiO2 and MnS from complex oxysulfide inclusions.

[0051] Table 1 - Criteria for weight percent of Ti, M, SO, and Nb to classify inclusions

[0052] [Table 1]

[0053] The following properties are then calculated for each inclusion family: - average diameter in microns, -mm 2 The density of the number of inclusions per unit area.

[0054] The method for calculating the clustering index is based on the DBSCAN (Density-based Spatial Clustering for Applications with Noise) algorithm, as detailed in the paper "A density-based algorithm for discovering clusters in large spatial databases with noise", Ester, Martin; Kriegel, Hans-Peter; Sander, Jorg; Xu, Xiaowei (1996), Proceedings of the Second International Conference on Knowledge Discovery and Data Mining (KDD-96). AAAI Press. pp. 226-231.

[0055] The determination of the clustering index takes two parameters, Max_distance and Min_points. Clusters are characterized by the following features: -It contains only particles of the same type, - in a given cluster, all inclusions are at a distance less than Max_distance from at least one other inclusion, -It contains a number of individual inclusions equal to or greater than Min_point.

[0056] In the context of the present invention, the inventors have found that a maximum distance, Max_distance, of 30 μm and a minimum number of inclusions per cluster, Min_points, of four, results in good detection of the clusters.

[0057] The length L of a given cluster is calculated as follows: - 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) Then, determine the maximum ferret diameter of said convex hull, called Dmax, and also determine the ferret diameter taken in a direction perpendicular to Dmax, called Dperp. Information on ferret diameter measurements is available, for example, in "Particle Size Measurements: Fundamentals, Practice, Quality" Springer. Henk G. Merkus (1 January 2009).

[0058] Calculate the length L of the cluster as follows:

[0059]

number

[0060] For each inclusion type, the average length L_average of all clusters is calculated.

[0061] The cluster density C_density of a given type of inclusion is given in mm 2 is the number of clusters per

[0062] The cluster index C_index of a given type of inclusion is defined as the product of the average length of the clusters and their density, C_index=L_average*C_density. The cluster index is expressed in μm / mm 2 The inventors have found that the cluster index allows samples with different properties to be compared using a unique number that correlates well with the bending behavior of the samples.

[0063] The steel sheet according to the invention can be produced by any suitable manufacturing method, which the skilled person can define, however it is preferable to use the method according to the invention, which comprises the steps described below.

[0064] In the following description, the term ladle refers to the vessel used to contain molten steel during the refining process, which refers to the adjustment of the final chemical composition and temperature of the melt before casting the steel into its first solidification form (e.g., before casting into a slab that is subsequently hot rolled).

[0065] To successfully control the inclusion population of steel, for example, the following steps can be implemented: - Liquid steel is tapped into the ladle from a previous steelmaking process step. For example, in the case of an electric arc furnace production route, said 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), said previous process step is the converter.

[0066] The sulfur content of the molten steel before the smelting step is measured, for example, by taking a sample of the molten steel and analysing it using a spark spectrometer. The sulfur content is measured, for example, by directly sampling the molten steel in the ladle or by taking a sample when the molten steel is being tapped into the ladle. The sulfur content before the refining step is measured in wt. % and is referred to as S_start in the following description.

[0067] - Aluminium is added to the ladle at the start of the refining process in order to deoxidise the molten steel. Said Al addition is carried out, for example, at the same time that the steel is tapped into the ladle, advantageously making it possible to ensure that the molten steel remains hot enough, saving time and thus increasing productivity. The amount of Al added to the molten steel at the start of the refining process is expressed in kg of aluminium per tonne of molten steel (kg / tonne) and is referred to as Al_added in the following description.

[0068] - In a subsequent optional step, for example if the molten steel temperature is too low or if the waiting time between the end of the refining step and the subsequent process (for example continuous casting) is expected to warrant it, the molten steel is reheated by aluminothermic heating. This is carried out by adding a determined amount of aluminum and co-injecting into the molten steel a determined amount of oxygen, corresponding to the stoichiometric ratio required to form Al2O3 with the added aluminum. The strong exothermic reaction between Al and O2 makes it possible to increase the temperature of the molten steel. The amount of O2 injected during this optional step is called O2_inj and is expressed in standard cubic meters of O2 per tonne of molten steel (Nm 3 The amount of Al injected for aluminothermy is not considered separately in this description since there is a direct stoichiometric relationship between O2_inj and the associated Al injection for aluminothermic reheat. Note that the injected Al for aluminothermy is different from Al_added above.

[0069] - the slag ratio %CaO / %Al2O3 is greater than 1, the amount of slag per tonne of liquid steel is at least 10 kg / tonne of liquid steel, and the slag composition above the melt is adjusted by adding appropriate amounts of minerals to ensure that the slag remains liquid in order to facilitate chemical exchange with the steel, the steel below the slag can be accessed, and the steel and / or slag can be tapped separately (the liquid state of the slag is confirmed visually and / or using thermodynamic rules based on its composition and temperature); - In a subsequent step, the molten steel is stirred by injecting an inert gas, for example Ar, into the molten steel in order to promote the exchange between the molten steel and the slag, which makes it possible to lower the sulphur content of the molten steel.

[0070] - In a further step, Ca is added to the ladle in order to spheroidize the inclusions present in the molten steel. For example, Ca is added in the form of silicon calcium (SiCa) or in the form of ferrocalcium (FeCa) or as pure calcium. For example, said addition is carried out by adding SiCa or FeCa to the ladle in the form of a cored wire, advantageously this makes it possible to easily control the amount of Ca added by controlling the length of the cored wire inserted in the melt and the injection speed. The amount of Ca added in the molten steel is measured in weight % in the molten steel and is referred to as Ca_added in the following description.

[0071] Taking the above steps into consideration, the inventors have found that by controlling the above levels of sulphur measured at the start of the refining process (S_start measured in wt%), Al added at the start of the refining process (Al_added measured in kg / ton), Ca added during the refining process (Ca-added measured in kg / ton) and O2 injection (O2_inj measured in Nm3 / ton) to verify that the following combination (referred to as C1 in the remainder of this specification) remains below a given cut-off value, it is possible to obtain a satisfactory level of inclusions to reach the desired bending anisotropy properties after hot stamping.

[0072] Al_added+0.1953*(S_start*1000+O2_inj)-9.367*Ca_added(C1) In practice, the particular cut-off value at which combination C1 must be controlled depends on the particular industrial equipment used to produce the steel: it depends on the production route in the steelworks, the geometric configuration of the ladles used to process the molten steel, the equipment used to add the different additives, the oxygen blowing configuration, etc.

[0073] To determine the relationship between these parameters for a given industrial device and production path, it is recommended to apply the following method: - Several heatings are carried out using the aforementioned chemical composition ranges.

[0074] -The heats are processed using different smelting process parameters, in particular different levels of measured sulfur at the start of the smelting process, Al addition at the start of the smelting process, Ca addition during the smelting process, and O2 injection. The range of smelting process parameters tested is selected to represent the industrial variation of these parameters. For example, six different sets of heats are selected with six different sets of smelting process parameters. For example, eight different sets of heats are selected with eight different sets of smelting process parameters.

[0075] said heat is treated according to the industrial route described below and the inclusion population of the steel is characterized using the method described above.

[0076] Then, the sum of the clustering indexes of MnS and TiN / Ti(C,N) inclusions in the bulk part of the steel plate and the associated refining process parameters are recorded. A combination C1 of the refining process parameters is calculated. It can be seen that, as a general trend, the higher the combination C1, the higher the sum of the clustering index.

[0077] Using the above-mentioned data set correlating the clustering index with the refining process parameters, the sum of the clustering indexes of MnS and TiN / Ti(C,N) inclusions in the bulk part of the steel plate is 300 μm / mm 2 A cut-off value is determined that is equal to or less than 300 μm / mm. The cut-off value of combination C1 determines how to control the refining process for the particular industrial facility under consideration. By controlling C1 below the cut-off value, the refining process can be controlled to a value below 300 μm / mm. 2 It becomes possible to produce steel sheets having a sum of clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk part below 7° and thus reach an associated excellent bending anisotropy below 7°.

[0078] For example, for the particular industrial installation in which the inventors carried out their experiments, said cut-off value is equal to 1.80.

[0079] After the steel refining step, the method for producing a steel plate according to the invention preferably comprises the following steps:

[0080] -Continuous casting of molten steel into semi-finished products suitable for hot rolling. During the casting step, particular care should be taken to avoid oxygen pick-up and therefore higher inclusion levels in the semi-finished products. For example, in the case of continuous casting processes where the semi-finished products are slabs produced in successive sequence by casting in a mold the products of several heats poured into a tundish, specific refractories and linings can be used in the tundish, specific allocation rules can be used for the first of the sequence slabs and transitional slabs between two different heats, etc.

[0081] The semi-finished product is then optionally reheated at a temperature comprised between 1150°C and 1300°C.

[0082] The steel sheet is then hot rolled at a finish hot rolling temperature comprised between 800°C and 950°C.

[0083] -The hot rolled steel is then cooled and coiled at a temperature T below 670 °C and optionally pickled to remove oxidation.

[0084] The coiled steel sheet is then optionally cold rolled to obtain a cold rolled steel sheet. The cold rolling reduction is preferably in the range of 20% to 80%. Below 20%, recrystallization during the subsequent heat treatment is unfavorable and may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracks during cold rolling.

[0085] In an embodiment of the present invention, the steel sheet is heated in an annealing furnace to a soaking temperature comprised between 700° C. and 850° C. and maintained at said soaking temperature for a soaking time comprised between 10 seconds and 20 minutes.

[0086] In an embodiment of the invention, the steel sheet thus annealed is cooled to a temperature range between 400°C and 700°C and further coated with a metal coating, for example an aluminium-based metal coating comprising at least 50% aluminium by weight, for example a zinc-based metal coating comprising at least 50% zinc by weight.

[0087] In an embodiment of the invention, the steel plate is then cooled to room temperature.

[0088] In summary, the above-mentioned method preferably comprises the following successive steps:

[0089] - Producing molten steel having the above mentioned chemical composition, during the molten steel refining stage, the level of sulphur measured at the start of the refining process, the amount of Al added at the start of the refining process, the amount of Ca added during the refining process and the amount of O2 injection are controlled to verify that the combination Al_added+0.1953*(S_start*1000+O2_inj)-9.367*Ca_added(C1) remains below a predefined cut-off value, which is determined if C1 is below the cut-off value, such that the sum of the clustering indexes of MnS and TiN / Ti(C,N) inclusions in the bulk part of the steel is less than 300 μm / mm2 It has been determined for the particular industrial equipment being used as follows:

[0090] - casting said molten steel to obtain a semi-finished product which can be hot rolled, Optionally, the semi-finished product is heated to a temperature T comprised between 1100 ° C and 1300 ° C. reheat reheating with - hot rolling the semi-finished product at a finishing hot rolling temperature comprised between 800°C and 950°C - Hot-rolled steel sheets are coiled at a temperature T of less than 670°C. coil and winding the steel sheet in a coil shape. - optionally pickling the coiled steel sheet; - optionally cold rolling the coiled steel sheet at a reduction in the range of 20% to 80% to obtain a cold rolled steel sheet. - optionally, heating the hot-rolled or cold-rolled steel sheet to a soaking temperature comprised between 700 ° C and 850 ° C and maintaining the steel sheet at said temperature for a soaking time comprised between 10 seconds and 20 minutes to obtain an annealed steel sheet; - optionally cooling the annealed steel sheet to a temperature range of 400°C to 700°C; - optionally coating the annealed steel sheet with a metallic coating - Optionally, cooling the coated steel sheet to room temperature.

[0091] Next, the manufacturing process of the pressed parts and the subsequent securing of the properties of the pressed parts will be described in detail.

[0092] A steel blank is cut from the steel sheet according to the invention and heated in an austenitizing furnace. Preferably, the steel blank is heated to a temperature comprised between 880°C and 950°C for 10 seconds to 15 minutes to obtain a heated steel blank. The heated blank is then transferred to a forming press, after which it is hot formed and die quenched to obtain a pressed part.

[0093] Optionally, the hot stamped part is further subjected to a paint baking step in which the part is heated to a temperature of 150°C to 250°C for a duration of 10 minutes to 2 hours.

[0094] The microstructure of the pressed part comprises, at the surface fraction on any analyzed cross section, more than 95% martensite and less than 5% bainite + ferrite. Furthermore, the pressed part according to the present invention comprises a bulk portion and upper and lower skin layers, the skin layers occupying the outermost 10% of the thickness on both sides of the bulk. The bulk portion has a surface fraction of 300 μm / mm 2 The sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions is as follows:

[0095] The pressed parts according to the invention have a tensile strength of more than 1300 MPa, preferably more than 1350 MPa, preferably more than 1400 MPa, and a bending angle anisotropy of less than or equal to 7°. Such high tensile strength and low bending anisotropy give the parts very good mechanical resistance, especially in the case of a crash, and furthermore make it possible to have a very predictable and uniform behavior in all directions. These properties give them very good energy absorption and intrusion resistance capabilities in all directions, thereby increasing the safety of the vehicle. EXAMPLES

[0096] The present invention is illustrated by the following examples which are not intended to be limiting in any way.

[0097] Eight different samples were tested, originating from eight different heats A, B, C, D, E, F, G and H of steel produced using the industrial production route. Samples I1, I2, I3 and I4 are according to the invention and samples R1, R2, R3 and R4 are reference samples.

[0098] All produced samples followed the same industrial production process in the steel mill. All samples were coated after annealing using an AlSi-based coating containing 8-12 wt.% Si, 2-4 wt.% Fe, and the balance Al.

[0099] Table 2 - Sample composition The compositions tested are summarized in the table below, with elemental contents expressed as weight percent, the remainder of the composition being iron and unavoidable impurities resulting from the smelting process.

[0100] [Table 2]

[0101] Table 3 - Steelworks process parameters and sum of clustering indexes of MnS and TiN / Ti(C,N) inclusions in the bulk part of steel The following process parameters were applied in the steel plant and the following sums of clustering indices of MnS and TiN / Ti(C,N) inclusions in the bulk part of the steel were observed - the underlined values ​​are not according to the invention:

[0102] [Table 3]

[0103] As can be seen, in the tested industrial conditions, the sum of the clustering indexes of MnS and TiN / Ti(C,N) inclusions in the bulk part of the steel can be reduced to 300 μm / mm by ensuring that the refining process parameters are properly controlled to keep C1 below 1.80. 2 As mentioned above, this cutoff value of 1.80 is specific to the industrial facility for which the trial was performed, and the appropriate cutoff factor for a given industrial facility must be determined, for example, by following the methodology described above.

[0104] Table 4 - Further process conditions The following process parameters were applied along the production route:

[0105] [Table 4]

[0106] Table 5 - Microstructure, bending angle and tensile strength The following microstructure (surface fraction), bend angle, bend angle anisotropy and tensile strength were measured on the samples, with the underlined values ​​not being in accordance with the present invention.

[0107] [Table 5]

[0108] Table 5 shows that the samples according to the invention have tensile strengths of over 1300 MPa (both RD and TD) while having bending anisotropy of less than 7°, whereas the reference samples have comparable tensile strength levels of over 1300 MPa but have bending anisotropy of more than 7°.

[0109] The inventors have found that this very good bending anisotropy correlates with the sum of the clustering indexes of MnS and TiN / Ti(C,N) inclusions in the bulk part of the steel, as seen in Table 3. The higher the sum of the clustering indexes of MnS and TiN / Ti(C,N) inclusions in the bulk part of the steel, the higher the bending anisotropy. The cutoff value is set at 300 μm / mm 2 By using the bending anisotropy, it is possible to control the bending anisotropy to below 7°. Due to this very low level of bending anisotropy, the behavior of the hot stamped part becomes very homogeneous when subjected to a load coming from any direction. This very stable behavior of the hot stamped part under load allows for a simplified design of the part, for example in the case of automotive parts, and ensures very good, robust and stable crash resistance of the hot stamped part.

Claims

1. A steel plate made of steel, the steel having, in weight percent: C: 0.2-0.3% Mn: 0.8-2.0% Si: 0.1-0.5% Al: 0.01~0.1% Ti: 0.01~0.1% B: 0.0005-0.005% P≦0.040% Ca≦0.01% S≦0.006% N≦0.01% and, optionally, 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 inevitable impurities resulting from the smelting process; The steel plate has a microstructure containing 75% to 90% ferrite on the surface, and the remaining portion is Fe. ３ C and a hard phase consisting of martensite and / or bainite, The steel plate has a bulk and a surface, - with a bulk equivalent to 80% of the thickness of the plate, the bulk is covered on either side by an upper and a lower skin layer occupying the outermost 10% of the thickness of the bulk, the sum of the clustering indices of MnS and TiN / Ti(C,N) inclusions being less than 300 μm / mm ２ 1. A steel sheet comprising an inclusion population comprising:

2. C: 0.2 to 0.25%, and / or Mn: 1.0 to 1.4%, and / or Si: 0.1 to 0.4% 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 The steel sheet according to claim 1, wherein N≦0.008%.

3. 3. The steel sheet according to claim 1 or 2, wherein the steel sheet is coated with a metallic coating comprising at least 50% Al by weight.

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

5. The chemical composition is as follows:

3. The steel sheet according to claim 1 or 2, further adhering to: (S-Ca*32 / 40)+(30*Ti*N)≦0.0045, where all elements are expressed in weight percent.

6. The steel plate according to claim 2, wherein the composition includes, by weight percent, Si: 0.15 to 0.35%.

7. The steel plate according to claim 2, wherein the composition includes, in weight percent, N≦0.005%.

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