Steel sheet having excellent bendability and high strength press hardened

Abstract

A steel sheet having a chemical composition comprising, in 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 main body to the surface of the coated steel sheet, the main body and a surface layer occupying 10% of the outermost thickness on both sides of the main body, the top of such a body is a skin layer occupying 10% of the thickness of the outermost surface on both sides of the body, the density of TiN / Ti (C, N) inclusions in the skin layer being less than 240 particles / mm2, and the clustering index of MnS inclusions in the skin layer being less than 110 [mu] m / mm2. This allows the production of a hot-pressed part having a tensile strength equal to or greater than 1300 MPa and a bending angle strictly greater than 48 degrees standardized to 1.5 mm and measured in the transverse direction.

Description

[0001] The present patent application is a divisional application of the patent application with the application date of 25 April 2024, the application number 202480001634.0, and the invention title “Steel sheet with excellent bending properties and high strength press hardened steel part and method of manufacturing the same”. TECHNICAL FIELD

[0002] The present invention relates to a steel sheet and to a high strength press hardened steel part. BACKGROUND

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

[0004] In such type of applications, it is desirable to produce steel parts that combine high mechanical strength and high impact resistance. Moreover, in view of the 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 the safety requirements.

[0005] Such weight reduction can be achieved in particular thanks to the use of steel parts having a microstructure dominated by martensite.

[0006] It is challenging to produce very high strength steels that also have a good resistance to the formation of cracks under bending. Indeed, very high strength steels tend to crack early when subjected to bending loads. This is detrimental to the crashworthiness of parts produced with such high strength steels, because although the material is able to withstand very high loads thanks to its high tensile strength, once cracks start to appear in the part, these cracks will rapidly propagate under continued load and the part will fail prematurely. SUMMARY

[0007] It is an object of the present invention to solve the above challenges and to provide a press hardened steel part having such a combination of high mechanical properties: a tensile strength after hot stamping higher than or equal to 1300 MPa and a high bending angle measured in the transverse direction greater than 48° when normalized to a thickness of 1.5 mm.

[0008] It is another object of the present invention to provide a steel sheet that can be transformed into such a press hardened steel part by hot forming, and to provide a method of manufacturing such a steel sheet.

[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. This steel part can also comprise the features of claims 6 to 7. Another object of the present invention is a method of manufacturing said hot stamped part according to claim 8, optionally comprising the features of claim 9. Attached Figure Description

[0010] 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

[0011] A billet is a flat sheet of steel that has been cut into any shape suitable for its application. The billet has a top surface and a bottom surface, also referred to as top side and bottom side, or 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.

[0012] 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 presents many technical advantages. It should be understood that the heat treatment subjected to the part includes not only the thermal cycle of the hot stamping process itself, but may also include other subsequent heat treatment cycles, such as a baking process to cure the paint after the part has been painted. The mechanical properties of the following hot-stamped parts are those measured after all thermal cycles (optionally including, for example, a baking process, where baking has indeed been performed).

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

[0014] The bending angle is measured according to the VDA-238 bending standard. For the same material, the bending angle depends on the thickness. For simplicity, the bending angle value of this invention refers to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle value needs to be normalized to 1.5 mm using the following calculation, where α1.5 is the bending angle normalized to 1.5 mm, t is the thickness, and αt is the bending angle for thickness t:

[0015] α1.5=(αt×√t) / √1.5

[0016] In the present invention, the bend angle is measured in the transverse direction, i.e. the transverse direction to the rolling direction along which the steel sheet travels during the hot rolling step. The bend angle is measured using a laser measuring device. The value reported is after springback. When performing the bend test on the hot stamped part, a sample is cut from a flat area of the part. If necessary, a small size sample is taken to fit the whole available flat area on the part. If the rolling direction on the hot stamped part is not known, it can be determined in a Scanning Electron Microscope (SEM) with Electron Backscatter Diffraction (EBSD) analysis across the cross section of the sample. The rolling direction is determined from the intensity of the main fiber's Orientation Density Function (ODF) where is the Euler angle as defined in "H.-J. Bunge: Texture Analysis in Materials Science - Mathematical Methods. First English edition (publication) 1982" (for the definition of see Fig. 2.2 and Fig. 2.3).

[0017] The bend angle of the part represents the ability of the part to resist deformation without forming cracks.

[0018] The composition of the steel according to the present invention will now be described, the contents being expressed in weight percent. The chemical composition is given with a lower and an upper limit of the composition range, said limits being included in the possible composition range according to the present invention. In case a preferred range for a given element is disclosed, all possible combinations of these preferred ranges for the individual elements are also disclosed.

[0019] According to the present invention, the carbon ranges from 0.2% to 0.4% to ensure satisfactory strength. Above 0.4% of carbon, the weldability and bendability of the steel sheet can decrease. If the carbon content is below 0.2%, the tensile strength will not reach the targeted value. In a particular embodiment, the carbon ranges from 0.2% to 0.3% to ensure sufficient strength while further controlling excellent weldability and bendability. In a particular embodiment, the carbon 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%. Adding above 2.0%, the risk of MnS formation increases, impairing the bendability. Below 0.8%, the hardenability of the steel sheet decreases during the hot stamping process. In a particular 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, improving the bendability. ​

[0021] The silicon content ranges from 0.1% to 0.5%. Silicon is an element that participates in the hardening of the solid solution. Silicon is added to limit carbide formation. Above 0.5%, silicon oxide forms at the surface, which impairs the coatability of the steel. In addition, the weldability of the parts produced with the steel sheet can be reduced. In a particular embodiment, the silicon content ranges from 0.1% to 0.4% to further improve the coatability and the 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 the weldability.

[0022] According to the application, the aluminum content ranges from 0.01% to 0.1% as it is a very efficient element to deoxidize the steel in liquid phase during processing. Aluminum can protect boron if the titanium content is insufficient. The aluminum content is below 0.1% to avoid 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 liquid phase while further avoiding oxidation problems and ferrite formation during press hardening.

[0023] According to the application, 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 further avoiding excessive TiN formation.

[0024] According to the application, the boron content ranges from 0.0005% to 0.005%. Boron improves the hardenability of the steel. The boron content is not higher than 0.005% to avoid slab breakage problems during continuous casting. In a particular 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.

[0025] Phosphorus is controlled to be lower than or equal to 0.040% as it leads to brittleness and weldability problems. In a particular embodiment, the P content is controlled to be lower than or equal to 0.020% to further avoid brittleness and weldability problems.

[0026] Calcium is controlled to be lower than or equal to 0.01% as the presence of calcium in the liquid steel can lead to the formation of coarse inclusions that are detrimental to the bendability. In a particular embodiment, the Ca content is controlled to be lower than or equal to 0.005% to further avoid coarse inclusions problems.

[0027] Sulfur is controlled to be lower than or equal to 0.006% because the presence of sulfur in the liquid steel can lead to the formation of MnS precipitates which are detrimental to the bendability. In a particular embodiment, the S content is controlled to be lower than or equal to 0.005% to further avoid the formation of MnS precipitates.

[0028] Nitrogen is controlled to be lower than 0.01%, preferably lower than or equal to 0.008%, even more preferably lower than 0.005%. The presence of nitrogen can lead to the formation of precipitates such as TiN or TiNbCN which are detrimental to the bendability.

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

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

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

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

[0033] In case one or several of the above elements are added, it is further verified that Cr + Mo + Nb + V < 0.5% to limit the cost and to avoid processing issues.

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

[0035] 5.22 ＊ (S-Ca ＊ 32 / 40) ＊ 10 4 +11,4 ＊ (Ti 2＊ N) ＊ 10 6 +136.5<300

[0036] The inventors have found that this allows further controlling the inclusion population of the steel sheet and thus further improving the bendability.

[0037] The remainder of the composition of the steel 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 liquid. For example, when using a basic oxygen converter route with a low level of scrap (recycled steel), the level of impurities will remain very low. However, it is also possible to add a large amount of scrap to the pig iron produced in the basic oxygen converter, which will increase the level of impurities. In addition, when the steel is processed using an electric furnace, for example with a very high rate of recycled scrap, the level of impurities will increase significantly. When using a high level of scrap, 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 sheet according to the application will now be described.

[0039] The steel sheet has, on any analysis section, a microstructure comprising, in surface fraction:

[0040] - 75% to 90% of ferrite,

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

[0042] With reference to Figure 1 The steel sheet 1 according to the application comprises a bulk portion 3 as well as a top and a bottom surface layer 2. The total thickness of the steel sheet 1 is to and the thickness of the surface layer 2 is such that ts=to * 10%. In other words, the surface layer 2 occupies the outermost 10% of thickness on both sides of the bulk and the bulk of the steel sheet occupies 80% of the thickness of the steel sheet.

[0043] The inventors have found a correlation between the bending angle of the steel sheet and the population of inclusions in the surface layer portion. In particular, by controlling both: the density of TiN / Ti(C,N) inclusions in the surface layer to be less than 240 particles / mm 2 and the clustering index of MnS inclusions in the surface layer portion of the steel to be lower than 110 pm / mm 2 the normalized bending angle a1 5 in the transverse direction can be guaranteed to be strictly greater than 48°.

[0044] The following is a description of the method used in order to characterize the inclusions in the steel sheet and steel parts. It should be understood that this is only one possible method and other solutions can also be implemented.

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

[0046] Inclusions present in the steel sheet are characterized using a scanning electron microscope (SEM) with a field effect gun (FEG). A Tescan Mira 3 SEM is used at a 14 kV power setting. This allows detection of particles as small as 0.5 pm. The use of the FEG SEM setting allows stable images with excellent resolution to be obtained over long periods of time, which can be necessary to complete image analysis over large areas - up to 48 hours of image field can be obtained using the FEG SEM setting, which can be necessary for multi-sample analysis. Furthermore, inclusions are analyzed with energy dispersive spectroscopy (EDS). A Bruker EDS probe with a large active surface of 120 mm2is used to detect light elements (O, N) and obtain a high count rate, and thus allow accurate quantification. 2 Bruker EDS probe with a large active surface of 120 mm2is used to detect light elements (O, N) and obtain a high count rate, and thus allow accurate quantification. The method is used to obtain accurate quantification.

[0047] Based on computer-controlled scanning electron microscopy techniques, the Automated Steel Cleaniness Analysis Tool (ASCAT) of the RJ·Lee group is used to perform the test on the SEM and associated EDS. Six individual samples can be analyzed in the same batch. The sample surface is divided into three zones (top skin, bottom skin, bulk as previously described). Each zone 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 pm. This is to reduce the matrix noise of the SEM images. As will be seen, only objects with a diameter exceeding 0.5 pm are actually considered. The first selection of objects, which will be called particles, is made by selecting objects that form a solid and have a gray level below 150 or above 220 on a scale of 0 to 255 (extreme values are excluded).

[0048] Each individual particle is then zoomed in on to capture its morphological features and to perform EDS analysis. A database of all particles is created using ASCAT and considering the chemical and morphological features of all analyzed particles for all images obtained.

[0049] In the group of all analyzed particles, only particles with a size greater than 0.5 pm and an iron content lower than 80% are retained for subsequent analysis and are referred to as inclusions - other particles are considered as part of the matrix and are not relevant for subsequent analysis.

[0050] The information from the EDS probe is then used to classify the various inclusions in 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. High performance EDS detectors make it possible to quantify oxygen. The oxygen level is checked to separate TiN from TiO2 and MnS from complex oxysulfide inclusions.

[0051] Table 1 - criteria for classification of inclusions in terms of weight % of Ti, M, S, O, Nb

[0052]

[0053] Then for each inclusion category, the following characteristics are calculated:

[0054] - the average diameter, in microns,

[0055] - the density, in number of inclusions / mm2, 2

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

[0057] The determination of the clustering index takes 2 parameters: max_distance and min_points. A cluster is characterized by the following features:

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

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

[0060] - it contains as many individual inclusions as equal to or greater than min_points. ​

[0061] For the present invention, the inventors have found that a good detection of clusters is obtained with a max_distance of 30 pm maximum distance and a min_point_number of 4 minimum number of inclusions per cluster.

[0062] The length L of a given cluster is calculated in the following way:

[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, F. P., Shamos, M. I., 1985, Texts and Monographs in Computer Science, Springer, New York, NY).

[0064] - The largest Feret diameter of said convex hull, to be called Dmax, is then determined, and also the Feret diameter taken in a direction perpendicular to Dmax, to be called Dperp. 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] - The length L of said cluster is calculated as

[0066]

[0067] The average length L_average of all clusters is calculated for each type of inclusion.

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

[0069] The cluster index C_index of a given type of inclusion is defined as the product of the average length of clusters and their density, C_index = L_average * C_density. The cluster index is expressed in pm / mm 2 . The inventors have found that said cluster index allows to compare samples with different characteristics using a unique number, and that it is well correlated with the bending behavior of said samples.

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

[0071] In the following description, the term ladle refers to a vessel used to contain the molten steel during the refining process. The refining process is known as the step of adjusting the final chemical composition and temperature of the melt before the steel is cast in its first solidified form, for example before it is cast into slabs which will be subsequently hot rolled.

[0072] In order to successfully control the inclusion population of the steel, the following processes can be implemented for example:

[0073] - tapping the molten steel from a 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.

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

[0075] - adding aluminium to the ladle at the beginning of the refining process to deoxidise the molten steel. The Al addition is performed for example at the same time as the tapping of the steel into the ladle - advantageously, this allows time to be saved and therefore production rate to be increased, and ensures that the molten steel remains hot enough. The amount of Al added to the molten steel at the beginning of the refining process, expressed in kg of aluminium per ton of molten steel (kg / ton), will be referred to in the following description as Al_add.

[0076] - in a subsequent optional step, for example if the temperature of the molten steel is too low, or if a waiting time is foreseen between the end of the refining step and the subsequent process (for example, continuous casting), the molten steel is reheated by aluminothermic heating. This is performed by adding a determined amount of aluminium and co-blowing a determined amount of oxygen into the molten steel, the determined amount of oxygen corresponding to the stoichiometric ratio required to form AI2O3 with the aluminium added. The strong 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 normal cubic metres of O2 per ton of molten steel (Nm 3 / ton). Since there is a direct stoichiometric relationship between O2_injection for aluminothermic reheating and the associated Al injection, the amount of Al injected for aluminothermic will not be considered separately in the current description. It should be noted that the Al injected for aluminothermic is different from the Al_add mentioned previously.

[0077] - the slag composition above the melt is adjusted by adding the appropriate amount of mineral to ensure that the %CaO / %AI2O3 ratio of the slag is higher than 1, the amount of slag per ton of liquid steel is at least 10 kg / ton of liquid steel and the slag remains liquid to facilitate the chemical exchanges with the steel to enable the steel to go below the slag and to enable the steel to be tapped alone and / or the slag to be tapped (liquid state of the slag visually and / or using thermodynamic rules based on its composition and temperature to verify).

[0078] - in a subsequent step, the liquid steel is stirred by blowing an inert gas inside the liquid steel, for example by blowing Ar. This is done to facilitate the exchanges between the liquid steel and the slag which will allow to reduce the sulphur content of the liquid steel.

[0079] - in a further step, Ca is added to the ladle to spheroidize the inclusions present inside the liquid steel. For example, Ca is added in the form of calcium-silicon (SiCa) or in the form of iron-calcium (FeCa) or as pure calcium. The addition is made, for example, by adding SiCa or FeCa in the form of a cored wire to the ladle - advantageously, this allows to easily control the amount of Ca added by controlling the length and the injection speed of the cored wire inserted into the melt. The amount of Ca added to the liquid steel (measured in weight % inside the liquid steel) will be referred to in the rest of the description as Ca_add.

[0080] In view of the above process, the inventors have found that by controlling the above-mentioned level of sulphur measured at the beginning of the refining process (S_start, measured in weight %), the Al addition at the beginning of the refining process (Al_add, measured in kg / ton), the Ca addition during the refining process (Ca-add, measured in kg / ton) and the volume of O2 injection (O2_injection, measured in Nm3 / ton) to verify that the following combination (which will be referred to in the rest of the description as Cl) remains below a given cut-off value, it is possible to obtain a satisfactory level of inclusions to reach the desired level of bending after hot stamping:

[0081] 217.8 - 315.1 * Ca_add + 41.5 * O2_injection + 18700 * S_start - 40 * Al_add (Cl)

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

[0083] To determine the relationship between these parameters for a given industrial equipment and production route, it is proposed to apply the following method:

[0084] - several melts are treated using the chemical composition ranges described previously.

[0085] - the melts are treated using different refining process parameters, in particular different levels of measured sulphur at the beginning of the refining process, Al addition at the beginning of the refining process, Ca addition during the refining process and volume of O2 blowing. The ranges of refining process parameters tested are chosen so as to be representative of the industrial variations of these parameters. For example, a set of 6 different melts with 6 different refining process parameters is chosen. For example, a set of 8 different melts with 8 different refining process parameters is chosen.

[0086] - the melts are treated according to the industrial route described below and the inclusions population of the steels is characterized using the method described above.

[0087] - the density of TiN / Ti(C,N) inclusions in the surface layer and the clustering index of MnS inclusions in the surface layer portion of the steel and the associated refining process parameters are then recorded. The combination C1 of the refining process parameters is calculated. As a general trend, it will be seen that the higher the combination C1, the higher the density of TiN / Ti(C,N) inclusions in the surface layer and the clustering index of MnS inclusions in the surface layer portion of the steel.

[0088] - using the data set relating the surface layer inclusion characteristics and the refining process parameters described above, a cut-off value is determined below which not only the density of TiN / Ti(C,N) inclusions in the surface layer is less than 240 particles / mm 2 and the clustering index of MnS inclusions in the surface layer portion of the steel is lower than 110 pm / mm2. Said cut-off value of the combination C1 will determine how to control the refining process of the particular industrial facility under consideration. By controlling C1 below said cut-off value, it will be possible to produce a steel sheet with both a density of TiN / Ti(C,N) inclusions in the surface layer less than 240 particles / mm 2 and a clustering index of MnS inclusions in the surface layer portion of the steel lower than 110 pm / mm 2 . Thus, it will be possible to reach an associated excellent bending level of the normalized bending angle a1 5 in the transverse direction strictly greater than 48°.

[0089] For example, in the case of the particular industrial facility on which the inventors conducted the experiments, said cut-off value is equal to 270.

[0090] The method for manufacturing a steel sheet according to the application comprises, after the molten steel refining step, the following steps:

[0091] - the molten steel is continuously cast into a semi-finished product suitable for being hot-rolled. During the casting step, particular attention should be paid to avoiding the absorption of oxygen and thus to avoiding a high level of inclusions in the semi-finished product. For example, in the case of a continuous casting process, in which the semi-finished product is a slab produced in a continuous sequence by pouring a multiple melt in a mould into a tundish, specific refractory materials and linings can be used in the tundish, specific distribution rules can be used for the first of the sequential slabs and for the transition slabs between two different melts, 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 finish hot-rolling temperature of 800°C to 950°C.

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

[0095] - the coiled steel sheet is then optionally cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction rate preferably ranges from 20% to 80%. Below 20%, recrystallization is not favored during the subsequent heat treatment, which can impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold-rolling.

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

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

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

[0099] In summary, the process described above preferably comprises the following successive steps:

[0100] - producing a molten steel having the chemical composition described above, in which, during the molten steel refining phase, the level of sulphur measured at the beginning of the refining process, the Al addition at the beginning of the refining process, the Ca addition and the volume of O2 injection during the refining process are controlled to verify the combination 217.8-315.1 * Ca_ addition + 41.5 * O2_ injection + 18700 * S_ start - 40* The Al addition (C1) is kept below a predetermined cut-off value. The cut-off value is determined for the specific industrial equipment being used, such that when C1 is below the cut-off value, the density of TiN / Ti(C,N) inclusions in the surface layer is less than 240 particles / mm 2 , and the clustering index of MnS inclusions in the surface layer portion of the steel is below 110 pm / mm 2 .

[0101] - casting the molten steel to obtain a semi-product able to be hot-rolled,

[0102] - optionally reheating the semi-product at a temperature T 再加热 ranging from 1100°C to 1300°C,

[0103] - hot-rolling the semi-product at a finish hot-rolling temperature ranging from 800°C to 950°C,

[0104] - coiling the hot-rolled steel sheet at a coiling temperature T 卷取 below 670°C to obtain a coiled steel sheet,

[0105] - optionally pickling the coiled steel sheet,

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

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

[0108] - optionally cooling the annealed steel sheet to a temperature ranging from 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 press part manufacturing process and the properties of the press part that follow will now be detailed.

[0112] A steel blank is cut from the steel sheet according to the application and is heated in an austenitizing furnace. Preferably, the steel blank is heated to a temperature ranging from 880°C to 950°C during 10 seconds to 15 minutes to obtain a heated steel blank. The heated blank is then transferred to a press, which is then hot-shaped and subjected to a press quenching to obtain a press part.

[0113] 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.

[0114] The microstructure of the pressed part comprises more than 95% of martensite and less than 5% of bainite + ferrite in terms of surface fraction on any section analyzed. Moreover, the pressed part according to the application comprises a bulk portion and a top and a bottom surface layer, where the surface layers occupy the outermost 10% of thickness on both sides of the bulk. Said surface layers have a density of TiN / Ti(C,N) inclusions less than 240 grains / mm 2 and a clustering index of MnS inclusions in the surface portion of the steel lower than 110 pm / mm 2 .

[0115] The pressed part according to the application has a tensile strength higher than 1300 MPa, preferably higher than 1350 MPa, preferably higher than 1400 MPa, and a normalized bending angle a1 5 in the transverse direction strictly greater than 48°. Such high tensile strength and high bending angle confer to the part very good mechanical resistance, especially in case of impact. These characteristics provide very good energy absorption capacity and intrusion prevention, thus improving the safety of the vehicle.

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

[0117] 11 different samples of steel from 11 different melts A, B, C, D, E, F, G, H, I, J and K produced using an industrial production route were tested. Samples II, I2, I3, I4, I5, I6 and I7 are according to the application, samples R1, R2, R3 and R4 are reference samples.

[0118] All produced samples followed the same industrial production process in the steelmaking plant. All samples were coated after annealing with an AlSi based coating comprising 8 to 12 wt% of Si, 2 to 4 wt% of Fe, the rest being Al.

[0119] Table 2 - sample compositions

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

[0121]

[0122] Table 3 - steelmaking shop process parameters, density of TiN / Ti(C,N) inclusions in the surface layer, and clustering index of MnS inclusions in the surface layer Table 4 - additional process conditions

[0123] The following process parameters were applied in a steelmaking plant and the density of TiN / Ti(C,N) inclusions in the surface layer and the clustering index of MnS inclusions in the surface portion of the steel were observed - the underlined values are not according to the present application:

[0124]

[0125] * C1 = 217.8 - 315.1 * Ca_add + 41.5 * O2_injection + 18700 * S_start - 40 * Al_add

[0126] As can be seen, under the industrial conditions tested, by ensuring that the refining process parameters are properly controlled to keep C1 below 270, it is possible to control the density of TiN / Ti(C,N) inclusions in the surface layer to less than 240 particles / mm 2 and the clustering index of MnS inclusions in the surface portion of the steel to less than 110 pm / mm 2 . As previously explained, the cut-off value of 270 is specific to the industrial setup for which the test was performed and an appropriate cut-off factor for a given industrial setup would need to be determined, for example, by following the method described above.

[0127] Table 5 - microstructure, bending angle and tensile strength

[0128] The following process parameters were applied along the production route:

[0129]

[0130] ​

[0131] The following microstructure (in surface fraction), bend angle, bend angle anisotropy and tensile strength were measured on the samples, the underlined values are not according to the present application:

[0132]

[0133] Table 5 shows that the samples according to the present application have a tensile strength in the transverse direction higher than 1300 MPa, while having a normalized bend angle a1.5 measured in the transverse direction strictly greater than 48°. On the other hand, the reference samples, despite having a comparable tensile strength level higher than 1300 MPa, all have a normalized bend angle a1.5 measured in the transverse direction equal to or less than 48°.

[0134] The inventors have found that this very good level of bendability is correlated with the density of TiN / Ti(C,N) inclusions in the surface layer and with the clustering index of MnS inclusions in the surface layer portion of the steel.

[0135] When the clustering index of MnS inclusions in the surface layer is equal to or greater than 110 pm / mm 2 , the bend angle is reduced, for example in the case of reference samples R1, R2 and R3, which all have a clustering index of MnS inclusions in the surface layer greater than or equal to 110 pm / mm 2 and a normalized bend angle in the transverse direction of 1.5 mm equal to or less than 48°.

[0136] When the density of TiN / Ti(C,N) inclusions in the surface layer is equal to or greater than 240 pm / mm 2 , the bend angle is reduced, for example in the case of reference sample R1, which has a density of TiN / Ti(C,N) inclusions in the surface layer of 246 pm / mm 2 and exhibits a bend angle normalized to 1.5 mm in the transverse direction of 46° or less.

[0137] By controlling the population of surface layer inclusions in the ranges described above, the steel sheet produced allows the production of hot-stamped parts with very good, robust and stable crash resistance for use in the automotive industry for example.

[0138] According to an embodiment of the application, the following annexes are also disclosed:

[0139] Annex 1. A steel sheet made of a steel having the following composition comprising, in percent by weight:

[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 comprises:

[0151] Cr < 0.4%

[0152] Mo < 0.3%

[0153] Nb < 0.1%

[0154] V < 0.3%

[0155] wherein Cr + Mo + Nb + V < 0.5%

[0156] the remainder of the composition being iron and unavoidable impurities resulting from the manufacturing process,

[0157] the steel sheet having, in surface fraction, a microstructure comprising 75% to 90% of ferrite, the remainder being a microstructure consisting of Fe3C and hard phases such as martensite and bainite,

[0158] the steel sheet comprising, from the bulk of the steel sheet to the surface:

[0159] - a bulk representing 80% of the thickness of the steel sheet,

[0160] - the top of such bulk being a top and a bottom skin representing the outermost 10% thickness on both sides of the bulk, the density of TiN / Ti(C,N) inclusions in said skins being strictly lower than 240 particles / mm 2 , and the clustering index of MnS inclusions in said skins being strictly lower than 110 pm / mm 2 .

[0161] Addendum 2. Steel sheet according to Addendum 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. Steel sheet according to Note 1 or 2, wherein the steel sheet is coated with a metallic coating comprising at least 50% of Al by weight.

[0173] Note 4. Steel sheet according to Note 1 or 2, wherein the steel sheet is coated with a metallic coating comprising at least 50% of Zn by weight.

[0174] Note 5. Steel sheet according to any one of Notes 1 to 4, wherein the chemical composition also complies with the following conditions, all elements being expressed in wt%: 5.22 * (S - Ca * 32 / 40) * 10 4 + 11,4 * (Ti 2* N) * 10 6 + 136.5 < 280.

[0175] Note 6. Press-hardened steel part, the steel part having the following composition comprising, in weight percent:

[0176] C: 0.2% to 0.3%

[0177] Mn: 0.8% to 2.0%

[0178] Si: 0.1% to 0.5%

[0179] Al: 0.01% to 0.1%

[0180] Ti: 0.01% to 0.1%

[0181] B: 0.0005% to 0.005%

[0182] P < 0.040%

[0183] Ca < 0.01%

[0184] S < 0.006%

[0185] N < 0.01%

[0186] and optionally comprising:

[0187] Cr < 0.4%

[0188] Mo < 0.3%

[0189] Nb < 0.1%

[0190] V < 0.3%

[0191] where Cr + Mo + Nb + V < 0.5%

[0192] the remainder of the composition being iron and unavoidable impurities resulting from the processing,

[0193] the steel part having a microstructure comprising more than 95% of martensite and up to 5% of bainite or ferrite in surface fraction,

[0194] the steel part comprising, from the bulk of the steel part to the surface:

[0195] - a bulk,

[0196] - the top of such bulk being a top and a bottom skin occupying the outermost 10% thickness on both sides of the bulk, the density of TiN / Ti(C,N) inclusions in the skin being less than 240 particles / mm 2 and the clustering index of MnS inclusions in the skin being lower than 110 pm / mm 2 .

[0197] Note 7. Press-hardened steel part according to Note 6, wherein:

[0198] C: 0.2% to 0.25%, and / or

[0199] Mn: 1.0% to 1.4%, and / or

[0200] Si: 0.1% to 0.4%, preferably 0.15% to 0.35%, and / or

[0201] Al: 0.02% to 0.06%, and / or

[0202] Ti: 0.02% to 0.06%, and / or

[0203] B: 0.002% to 0.004%, and / or

[0204] P < 0.020%, and / or

[0205] Ca < 0.005%, and / or

[0206] S < 0.005%, and / or

[0207] N < 0.008%, preferably N < 0.005%.

[0208] Note 8. Press-hardened steel part according to Note 6 or 7, wherein the chemical composition also respects the following conditions, all elements being expressed in weight %: 5.22 *(S-Ca * 32 / 40) * 10 4 +11,4 * (Ti 2* N) * 10 6 +136.5<300.

[0209] Note 9. Press-hardened steel part according to any one of Notes 6 to 8, wherein the press-hardened steel part has a tensile strength TS measured in transverse direction of at least 1300 MPa and a bending angle normalized to 1.5 mm and measured in transverse direction strictly greater than 48°.

[0210] Note 10. A method for manufacturing a steel sheet according to any one of Notes 1 to 5, comprising the following successive steps:

[0211] - providing a molten steel having a chemical composition comprising, in percent by weight:

[0212] C: 0.2% to 0.3%

[0213] Mn: 0.8% to 2.0%

[0214] Si: 0.1% to 0.5%

[0215] Al: 0.01% to 0.1%

[0216] Ti: 0.01% to 0.1%

[0217] B: 0.0005% to 0.005%

[0218] P < 0.040%

[0219] Ca < 0.01%

[0220] S < 0.006%

[0221] N < 0.01%

[0222] and optionally comprising:

[0223] Cr < 0.4%

[0224] Mo < 0.3%

[0225] Nb < 0.1%

[0226] V < 0.3%

[0227] with Cr + Mo + Nb + V < 0.5%

[0228] the remainder of the composition being iron and unavoidable impurities,

[0229] - casting the liquid steel to obtain a semi-product able to be hot rolled,

[0230] - hot rolling the semi-product at a finish hot rolling temperature comprised between 800°C and 950°C,

[0231] - coiling the hot rolled steel sheet at a coiling temperature T 卷取 under 670°C to obtain a coiled steel sheet.

[0232] Note 11. The method according to Note 10, wherein:

[0233] C: 0.2% to 0.25%, and / or

[0234] Mn: 1.0% to 1.4%, and / or

[0235] Si: 0.1% to 0.4%, preferably 0.15% to 0.35%, and / or

[0236] Al: 0.02% to 0.06%, and / or

[0237] Ti: 0.02% to 0.06%, and / or

[0238] B: 0.002% to 0.004%, and / or

[0239] P < 0.020%, and / or

[0240] Ca < 0.005%, and / or

[0241] S < 0.005%, and / or

[0242] N < 0.008%, preferably N < 0.005%.

[0243] Note 12. The method according to Note 10 or 11, in which the step of providing the liquid steel comprises a stage of refining the liquid steel, during which the level of sulphur measured at the beginning of the refining, the Al addition at the beginning of the refining, the Ca addition and the volume of O2 injection during the refining are controlled to verify the combination C1 = 217.8 - 315.1 * Ca_addition + 41.5 * O2_injection + 18700 * S_start - 40 * the Al addition is kept below a predetermined cut-off value,

[0244] Al_addition is the Al added at the beginning of the refining, in kg of aluminium per ton of liquid steel,

[0245] S_start is the sulphur content in weight % before said refining,

[0246] O2_injection is the amount of O2 injected during the optional step of aluminothermic heating, expressed in normal cubic meters of O2 per ton of liquid steel,

[0247] Ca_addition is the amount of Ca added in the liquid steel, measured in weight % in said liquid steel.

[0248] Clause 13. A method for manufacturing a press hardening steel part according to any one of clauses 6 to 9, comprising the following successive steps:

[0249] - providing a steel sheet according to any one of clauses 1 to 5,

[0250] - cutting said steel sheet into a predetermined shape so as to obtain a steel blank,

[0251] - heating said steel blank to a temperature comprised between 880°C and 950°C during 10 seconds to 15 minutes so as to obtain a heated steel blank,

[0252] - transferring said heated blank to a press,

[0253] - hot forming said heated blank in said press so as to obtain a formed part,

[0254] - subjecting said formed part to press quenching.

[0255] Clause 14. The method according to clause 13, further comprising a paint baking step in which said formed part is heated to a temperature comprised between 150°C and 250°C for a duration comprised between 10 minutes and 2 hours.

Claims

1. A steel sheet made of a steel having a composition comprising in weight percent: 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 comprising: Cr≤0.4％ Mo < 0.3% Nb < 0.1% V≤0.3％ wherein Cr + Mo + Nb + V < 0.5% the remainder of the composition being iron and unavoidable impurities resulting from the manufacturing process, said steel sheet having a microstructure comprising in surface fraction: 75% to 90% of ferrite, the rest being a microstructure consisting of Fe3C and hard phases such as martensite and bainite, said steel sheet having a thickness comprised between 1.2 mm and 1.7 mm, said steel sheet comprising from the bulk of the steel sheet to the surface: - the top of such a body is a top skin and a bottom skin occupying the outermost 10% thickness on both sides of the body, the density of TiN / Ti(C,N) inclusions in said skins being strictly below 240 particles / mm 2 , and the clustering index of MnS inclusions in said skins being strictly below 110 pm / mm 2 . - a bulk representing 80% of the thickness of the steel sheet, 2. The steel sheet 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 sheet according to claim 1 or 2, wherein the steel sheet is coated with a metallic coating comprising at least 50% in weight of Al.

5. Steel sheet according to claim 1 or 2, wherein the chemical composition further complies with the following conditions, all elements being expressed in wt%: 5.22 * (S-Ca * 32 / 40) * 10 4 +11,4 * (Ti 2* N) * 10 6 +136.5<280。 4. The steel sheet according to claim 1 or 2, wherein the steel sheet is coated with a metallic coating comprising at least 50% in weight of Zn.

6. A press hardened steel part, said steel part having a composition comprising in weight percent: 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% P≤0.040％ B: 0.0005% to 0.005% S≤0.006％ N≤0.01％ Ca < 0.01% Cr≤0.4％ and optionally comprising: Mo < 0.3% V≤0.3％ Nb < 0.1% wherein Cr + Mo + Nb + V < 0.5% the remainder of the composition being iron and unavoidable impurities resulting from the manufacturing process, said steel part having a microstructure comprising more than 95% of martensite and up to 5% of bainite or ferrite in surface fraction, said steel part comprising from the bulk of the steel part to the surface: - the top of such a body is a top skin and a bottom skin occupying the outermost 10% thickness on both sides of the body, the density of TiN / Ti(C,N) inclusions in said skins being less than 240 particles / mm 2 , and the clustering index of MnS inclusions in said skins being lower than 110 pm / mm 2 . - a bulk, 7. The press hardened steel part 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%. 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. Press hardenable steel part according to claim 6 or 7, wherein the chemical composition further complies with the following conditions, all elements expressed in wt%: 5.22 * (S-Ca * 32 / 40) * 10 4 +11,4 * (Ti 2* N) * 10 6 +136.5<300.

9. Press-hardened steel part according to claim 6 or 7, wherein the press-hardened steel part has a tensile strength TS measured in transverse direction of at least 1300 MPa and a bending angle normalized to 1.5 mm and measured in transverse direction strictly greater than 48°.

10. A method for manufacturing a steel sheet according to claim 1 or 2, comprising the following successive steps: - providing a molten steel having a chemical composition comprising in weight percent: 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 comprising: Cr≤0.4％ Mo < 0.3% Nb < 0.1% V≤0.3％ wherein Cr + Mo + Nb + V < 0.5% the remainder of the composition being iron and unavoidable impurities, - casting the molten steel to obtain a semi-product able to be hot-rolled, - hot-rolling the semi-product at a finish hot-rolling temperature comprised between 800°C and 950°C, - the hot-rolled steel sheet is coiled at a coiling temperature Tc lower than 670°C 卷取 the coiling to obtain a coiled steel sheet.

11. The method according to 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 according to claim 10 or 11, in which the step of providing the molten steel comprises a stage of refining the molten steel, during which the level of sulphur 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 injection are controlled to verify the combination C1 = 217.8 - 315.1 * Ca_addition + 41.5 * O2_injection + 18700 * S_start - 40 * Al_addition remains below a predetermined cut-off value, Al_add is the Al added at the beginning of the refining, in kg of aluminum per ton of molten steel, S_start is the sulfur content before the refining, in weight percent, O2_injection is the amount of O2 injected during the optional step of aluminum-thermic heating, in normal cubic meters of O2 per ton of molten steel, Ca_add is the amount of Ca added in the molten steel, measured in weight percent within the molten steel.

13. A method for manufacturing a press-hardened steel part according to claim 6 or 7, comprising the following successive steps: - providing a steel sheet according to claim 1 or 2, - cutting the steel sheet into a predetermined shape so as to obtain a steel blank, - heating the steel blank to a temperature comprised between 880°C and 950°C during 10 seconds to 15 minutes so as to obtain a heated steel blank, - transferring the heated blank to a press, - hot-forming the heated blank in the press so as to obtain a formed part, - subjecting the formed part to a press-quenching.

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

15. The method of claim 14, wherein the baking step is performed after the curing step.

16. The method of claim 14, wherein the baking step is performed before the curing step.

17. The method of claim 14, wherein the baking step is performed simultaneously with the curing step.

18. The method of claim 14, wherein the baking step

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