Steel plates with excellent bending properties, high-strength press-hardened steel parts, and methods for manufacturing the same.

Abstract

Steel plate with excellent bending properties, high-strength press-hardened steel parts, and manufacturing method thereof 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 plate 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 plate, such bulk is covered by a skin layer occupying the outermost 10% of the thickness on both sides of the bulk, and the density of TiN / Ti(C,N) inclusions in the skin is 240 particles / mm 2 and the clustering index of MnS inclusions in the skin is less than 110 μm / mm 2 This makes it possible to produce hot pressed parts with tensile strengths of 1300 MPa or more and bend angles strictly greater than 48°, normalized to 1.5 mm and measured in the transverse direction.

Description

[Technical Field]

[0001] This 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 automobile vehicles for intrusion prevention or energy absorption functions.

[0003] For such applications, it is desirable to manufacture steel components that combine high mechanical strength with high impact resistance. Furthermore, one of the major challenges in the automotive industry is to reduce vehicle weight to improve fuel efficiency from an environmental perspective, without neglecting safety requirements.

[0004] This weight reduction can be achieved, in particular, by using steel components that primarily have a martensitic microstructure.

[0005] It is difficult to manufacture very high-strength steel that also has good resistance to crack formation under bending. In fact, very high-strength steel tends to crack prematurely when subjected to bending loads. This is detrimental to the impact resistance of parts made from such high-strength steel, because even though the material can withstand very high loads due to its high tensile strength, once cracks begin to appear in the part, these cracks propagate rapidly under continuous load, causing the part to fail prematurely. [Overview of the project]

[0006] The object of the present invention is to address the above-mentioned problems and provide a press-hardened steel part that has a combination of high mechanical properties, a tensile strength of 1300 MPa or more after hot stamping, and a high bending angle measured transversely at more than 48° when normalized to a thickness of 1.5 mm.

[0007] Another object of the present invention is to provide a steel sheet that can be deformed into such press-hardened steel parts by hot forming, and a method for manufacturing such a steel sheet.

[0008] An object of the present invention is achieved by providing a steel sheet according to claim 1 having any of 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 have the features of claims 6 to 7. A further object of the present invention is a method for manufacturing the hot-stamped part according to claim 8, which optionally includes the features of claim 9.

[0009] Here, the present invention will be described in detail without introducing limitations, with reference to Figure 1, and illustrated by examples. Figure 1 is a schematic cross-sectional view of a steel plate according to the present invention. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic cross-sectional view of the steel plate according to the present invention. [Modes for carrying out the invention]

[0011] A steel blank refers to a flat sheet of steel cut into any shape suitable for its application. The blank has a top and a bottom surface, also referred to as the top and bottom surfaces or top and bottom surfaces. The distance between these surfaces is specified as the thickness of the blank. The thickness can be measured, for example, using a micrometer, with the spindle and anvil positioned on the top and bottom surfaces. Similarly, the thickness can also be measured on a molded part.

[0012] Hot stamping is a forming technique that involves heating a blank to a temperature at which the steel microstructure transforms at least partially into austenite, shaping the blank at high temperatures by stamping it, 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 the part undergoes includes not only the thermal cycle of the hot stamping process itself as described above, but also, in some cases, other subsequent heat treatment cycles performed after the part has been painted to cure the paint, such as a paint curing step. The mechanical properties of the hot-stamped parts below were measured after the complete thermal cycle, and optionally include, for example, a paint curing step if paint curing was actually performed.

[0013] The ultimate tensile strength is measured according to the ISO standard ISO 6892-1, published in October 2009. Tensile test specimens are cut from the flat areas of the hot-stamped parts. Smaller sized tensile test samples are taken to accommodate all available flat areas on the part, if necessary.

[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 values ​​in this invention refer to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle value must be normalized to 1.5 mm by the following calculation, where α1.5 is the bending angle normalized to 1.5 mm, t is the thickness, and αt is the bending angle relative to the thickness t. α1.5 = (αt × √t) / √1.5

[0015] In the present invention, the bending angle is measured in the transverse direction, that is, the transverse direction relative to the rolling direction in which the steel sheet moves during the hot rolling step. The bending angle is measured using a laser measuring device. The values reported are values after springback. When performing a bending test on a hot-stamped part, the sample is cut from a flat region of the part. If necessary, small-sized samples are taken to accommodate all available flat regions of the part. If the rolling direction of the hot-stamped part is unknown, it can be determined using electron backscatter diffraction (EBSD) analysis across the cross-section of the sample with a scanning electron microscope (SEM). The rolling direction is determined according to the intensity of the orientation density function (ODF) representing the main fiber 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 Figures 2.2 and 2.3 for the definition of φ2).

[0016] The bending angle of a part represents the ability of the part to withstand deformation without crack formation.

[0017] Next, the composition of the steel according to the present invention will be described, wherein the content is expressed in weight percent. The chemical composition is indicated by the lower limit and upper limit of the composition range, and said limits are included within the possible composition range according to the present invention. Where preferred ranges for a given element are disclosed, the present invention also discloses all possible combinations of these preferred ranges for each individual element.

[0018] According to the present invention, carbon is in the range of 0.2% to 0.4% to ensure sufficient strength. When carbon exceeds 0.4%, the weldability and bendability of the steel sheet may be deteriorated. If the carbon content is less than 0.2%, the tensile strength cannot reach the target value. In certain embodiments, carbon is in the range of 0.2% to 0.3% to ensure sufficient strength while further controlling excellent weldability and bendability. In certain embodiments, carbon is in the range of 0.2% to 0.25% to ensure sufficient strength while further controlling excellent weldability and bendability.

[0019] The manganese content is in the range of 0.8% to 2.0%. When the added amount exceeds 2.0%, the risk of MnS formation increases, and bendability is impaired. If it is less than 0.8%, the hardenability of the steel sheet during the hot stamping process decreases.

[0020] In certain embodiments, the manganese content is in the range of 1.0% to 1.4% to further improve the hardenability of steel, further limit the formation of MnS, and thereby improve bendability.

[0021] The silicon content is in the range of 0.1% to 0.5%. Silicon is an element involved in hardening in solid solution. Silicon is added to limit the formation of carbides. When it exceeds 0.5%, silicon oxide is formed on the surface, which impairs the coatability of steel. Furthermore, the weldability of parts produced from the steel sheet may be deteriorated. In certain embodiments, the silicon content is in the range of 0.1% to 0.4% to further improve coatability and weldability. In certain embodiments, the silicon content is in the range of 0.15% to 0.35% to further harden the steel and further improve coatability and weldability.

[0022] According to the present invention, the aluminum content is in the range of 0.01% to 0.1%, as aluminum is a very effective element for deoxidizing steel in the liquid phase during smelting. If the titanium content is insufficient, aluminum can protect boron. 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% to 0.06% to further ensure good deoxidation of steel in the liquid phase while further avoiding oxidation problems and ferrite formation during compression hardening.

[0023] According to the present invention, the titanium content is in the range of 0.01% to 0.1% to protect boron; otherwise, it will be trapped within the BN precipitate. The titanium content is limited to 0.1% to avoid excessive TiN formation. In certain embodiments, the Ti content is in the range of 0.02% to 0.06% to further protect boron while further avoiding excessive TiN formation.

[0024] According to the present invention, the boron content is in the range of 0.0005% to 0.005%. Boron improves the hardenability of steel. The boron content is 0.005% or less to avoid the problem of slab fracture during continuous casting. In certain embodiments, the boron content is in the range of 0.002% to 0.004% to further ensure the hardenability of steel and further avoid the problem of slab fracture.

[0025] Phosphorus is controlled to 0.040% or less because it causes brittleness and weldability problems. In certain embodiments, the P content is controlled to 0.020% or less to further avoid brittleness and weldability problems.

[0026] The presence of calcium in molten steel can lead to the formation of coarse inclusions that are detrimental to flexibility; therefore, calcium content is controlled to 0.01% or less. In certain embodiments, the Ca content is controlled to 0.005% or less to further avoid the problems associated with coarse inclusions.

[0027] The presence of sulfur in molten steel can lead to the formation of MnS precipitates, which are detrimental to bendability; therefore, sulfur content is controlled to 0.006% or less. In certain embodiments, the sulfur content is controlled to 0.005% or less to further avoid the formation of MnS precipitates.

[0028] The nitrogen content is controlled to less than 0.01%, preferably 0.008%, and more preferably less than 0.005%. The presence of nitrogen can lead to the formation of precipitates such as TiN or TiNbCN, which adversely affects flexibility.

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

[0030] Molybdenum may be added optionally, up to a maximum of 0.3%. Molybdenum improves the hardenability of steel. The amount of molybdenum is limited to 0.3% to limit costs and avoid processing problems.

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

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

[0033] If one or more of the above elements are added, the following formula is further examined to limit costs and avoid processing problems: Cr + Mo + Nb + V ≤ 0.5%.

[0034] In certain embodiments, 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 for further control of the inclusion cluster in the steel sheet, and therefore further improvement in bendability.

[0037] The remainder of steel composition consists of iron and impurities arising from the smelting process. The level of impurities arising from the smelting process depends on the manufacturing route used and the level of scrap used in the molten steel. For example, when using a basic oxygen furnace route with low-level steel scrap (recycled steel), the level of impurities remains very low. However, it is also possible to add a large amount of scrap in the converter to the pig iron produced in the basic oxygen furnace, which increases the level of impurities. Furthermore, for example, when smelting steel using an electric furnace with a very high proportion of recycled scrap steel, the level of impurities increases significantly. When using high-level 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] Next, the microstructure of the steel sheet according to the present invention will be described.

[0039] The steel sheet has a microstructure in which the surface fraction of any analytical cross-section includes the following: -75%~90% ferrite, - The remaining portion consists of carbide Fe3C and hard phases such as martensite and bainite.

[0040] Referring to Figure 1, the steel sheet 1 according to the present invention comprises a bulk portion 3 and upper and lower skin layers 2. The total thickness of the steel sheet 1 is t0, and the thickness ts of the skin layer 2 is such that ts = t0 * 10%. In other words, the skin layer 2 accounts for the outermost 10% of the thickness on both sides of the bulk, and the bulk of the steel sheet corresponds to 80% of the thickness of the steel sheet.

[0041] The inventors have found a correlation between the bending angle and the inclusion clusters in the skin portion of the steel sheet. In particular, the density of TiN / Ti(C,N) inclusions in the skin is 240 particles / mm³. 2 The clustering index of MnS inclusions in the steel skin is less than 110 μm / mm². 2 By controlling both so that they remain below a certain value, it becomes possible to ensure that the lateral normalized bending angle α1.5 is strictly greater than 48°.

[0042] The following is a description of the methodology used to characterize inclusions in steel plates and steel components. Please understand that this is only one possible methodology, and other protocols can also be implemented.

[0043] The cross-section of the steel plate in which the inclusions are observed is taken in the direction of the steel's rolling. In other words, the plane of the observed cross-section has a transverse direction as its normal direction.

[0044] Inclusions present in the steel plates were 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 allows for the detection of small particles of approximately 0.5 μm. Using the FEGSEM setting allows for obtaining stable images with excellent resolution over long periods, which may be necessary to complete extensive image analysis. The FEGSEM setting allows for acquiring image fields over periods of up to 48 hours, which may be necessary for the analysis of multiple samples. 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), obtain high counting rates, and thus enable precise quantification. 2 A Bruker EDS probe was used. Accurate quantification was obtained using the phi-rho-Z method.

[0045] Using the RJ Lee Group's Automated Steel Cleanliness Analysis Tool (ASCAT), we will pilot the SEM and associated EDS 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 (upper skin, lower skin, and bulk, as previously mentioned). 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 matrix noise in the SEM image. As you would expect, only objects with a diameter greater than 0.5 μm are actually considered. The initial selection of objects, referred to as particles, is done by selecting objects that form a solid 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).

[0046] Next, each individual particle is zoomed in on to capture its morphological features, and EDS analysis is performed. A database of all particles is created using ASCAT, taking into account the chemical and morphological characteristics of all analyzed particles for all acquired images.

[0047] Of the entire set of particles analyzed, only those with a size greater than 0.5 μm and an iron content of less than 80% are retained for subsequent analysis and are called inclusions, while the other particles are considered to be part of the matrix and are not relevant to subsequent analysis.

[0048] Next, using information from the EDS probe, each inclusion is classified into one of the following families: TiN, alumina, complex oxides, oxysulfide particles, MnS, etc. For example, Table 1 details the strict rules used by the inventors to classify MnS and TiN / Ti(C,N) inclusions. Oxygen quantification is made possible by a high-performance EDS detector. Oxygen levels are checked to separate TiN from TiO2 and MnS from complex oxysulfide inclusions.

[0049] Table 1 - Criteria for classifying inclusions by weight percentage of Ti, M, SO, and Nb

[0050] [Table 1]

[0051] Next, the following characteristics are calculated for each inclusion family: -Average diameter in microns, -mm 2 Density of the number of inclusions per unit, The method for calculating the clustering index is based on the DBSCAN (Density-Based Spatial Clustering in Noisy Applications) 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.

[0052] The clustering index is determined using 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 more than Min_point individual inclusions.

[0053] With regard to the present invention, the inventors have found that good detection of clusters can be obtained when the maximum distance Max_distance is 30 μm and the minimum number of inclusions per cluster Min_points is 4.

[0054] The length L of a given cluster is calculated as follows: - First determine the convex hull of a cluster using a known algorithm (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) - Then, determine the maximum Feret diameter of said convex hull, called Dmax, and also determine the Feret diameter taken in the direction perpendicular to Dmax, called Dperp. Information regarding Feret diameter measurement can be obtained, for example, from "Particle Size Measurements: Fundamentals, Practice, Quality", Springer, Henk G. Merkus (January 1, 2009).

[0055] - The length L of said cluster is calculated as follows

[0056] [Math.]]

[0057] For each type of inclusion, the average length L_average of all clusters is calculated.

[0058] The cluster density C_density of a given type of inclusion is the number of clusters per mm 2 .

[0059] 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 μm / mm 2 . The present inventors have found that said cluster index makes it possible to compare samples having different properties using a unique value, and that this value sufficiently correlates with the bending behavior of said samples.

[0060] The steel sheet according to the present invention can be manufactured by any suitable manufacturing method, which can be defined by those skilled in the art. However, it is preferable to use the method according to the present invention, which includes the steps described below.

[0061] In the following description, the term ladle refers to the vessel used to hold molten steel during the refining process. The refining process is referred to as the step of adjusting the final chemical composition and temperature of the molten steel before casting it into its first solidified form (for example, before casting it into a slab that will later be hot-rolled).

[0062] To successfully control the inclusion clusters in steel, for example, the following steps can be implemented: - Molten steel is tapped into a ladle from the previous steelmaking process step. For example, in the case of an electric arc furnace manufacturing process, the previous process step is the electric arc furnace process itself. For example, in the case of a blast furnace and converter process (or a direct reduced iron and converter process), the previous process step is the converter.

[0063] - The sulfur content of the molten steel before the refining step is measured, for example, by taking a sample of the molten steel and analyzing it using a spark spectrometer. The sulfur content is measured, for example, by directly sampling the molten steel in the ladle, or by taking a sample while the molten steel is being tapped into the ladle. The sulfur content before the refining step is measured in weight percent and is referred to as S_start in the following description.

[0064] - To deoxygenate the molten steel, aluminum is added to the ladle at the start of the refining process. The addition of Al is carried out, for example, at the same time that the steel is poured into the ladle, which is advantageous as it saves time, thus improving productivity and allowing the molten steel to remain at a sufficiently high temperature. The amount of Al added to the molten steel at the start of the refining process is expressed in kilograms of aluminum per ton of molten steel (kg / ton), and is referred to as Al_added in the following description.

[0065] -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 (e.g., continuous casting) is expected to ensure it is heated, the molten steel is reheated by aluminosermic heating. This is done by adding a determined amount of aluminum and blowing a determined amount of oxygen into the molten steel, corresponding to the stoichiometric ratio required to form Al2O3 with the added aluminum. The strong exothermic reaction between Al and O2 allows the temperature of the molten steel to rise. The amount of O2 injected during this optional step is called O2_inj, and is the standard cubic meter (Nm³) of O2 per ton of molten steel. 3 It is expressed as ( / ton). Since there is a direct stoichiometric relationship between O2_inj and the associated Al injection for aluminothermic reheating, the amount of Al injected for aluminothermy is not considered separately in this explanation. Note that the Al injected for aluminothermy is different from the Al_added mentioned above.

[0066] - The slag ratio (%CaO / %Al2O3) is greater than 1, the amount of slag per ton of molten steel is at least 10 kg / ton of molten steel, and the slag remains liquid to facilitate chemical exchange with the steel, and the steel beneath the slag is accessible, allowing the steel and / or slag to be tapped separately. The slag composition on top of the molten steel is adjusted by adding an appropriate amount of minerals (the liquid state of the slag is confirmed using visual and / or thermodynamic rules based on its composition and temperature). -In a subsequent step, the molten steel is stirred by blowing in an inert gas, for example, by blowing in Ar. This is done to promote exchange between the molten steel and the slag, and to reduce the sulfur content of the molten steel.

[0067] -In a further step, Ca is added to the ladle to spheroidize inclusions present in the molten steel. For example, Ca may be added in the form of silicon calcium (SiCa), or in the form of strong calcium (FeCa), or as pure calcium. For example, the addition may be carried out by adding SiCa or FeCa to the ladle in the form of a cored wire, which advantageously allows for easy control of the amount of Ca added by controlling the length of the cored wire inserted into the molten metal and the injection rate. The amount of Ca added to the molten steel is measured in weight percent of the molten steel and will be referred to as Ca_added in the following description.

[0068] Taking the above process into consideration, the inventors have found that a satisfactory level of inclusions can be obtained to reach a desired bending level after hot stamping by controlling the above-mentioned measured level of sulfur at the start of the refining process (S_start measured in wt%), the amount of Al added at the start of the refining process (Al_added measured in kg / ton), the amount of Ca added during the refining process (Ca_added measured in kg / ton), and the amount of O2 injected (O2_inj measured in Nm3 / ton) to verify that the following combination (referred to as C1 for the remainder of this specification) remains below a given cutoff value.

[0069] 217.8-315.1*Ca_added+41.5*O2_inj+18700*S_start-40*Al_added(C1) In practice, the specific cutoff value that needs to be controlled for combination C1 depends on the specific industrial equipment used to manufacture the steel. It depends on the manufacturing process in the steelworks, the geometric configuration of the ladle used to process the molten steel, the equipment used to add different additives, the oxygen blowing configuration, and so on.

[0070] To determine the relationships between these parameters for a given industrial machine and manufacturing route, it is recommended to apply the following method: - Perform several heating steps using the aforementioned chemical composition range.

[0071] - The heat is processed using different refining process parameters, particularly different levels of measured sulfur 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 injected. The range of refining process parameters tested is selected to represent the industrial variation of these parameters. For example, six different sets of heat are selected, each having six different sets of refining process parameters. For example, eight different sets of heat are selected, each having eight different sets of refining process parameters.

[0072] -The heat is applied according to the industrial pathway described below, and the inclusion clusters in the steel are characterized using the method described above.

[0073] Next, the density of TiN / Ti(C,N) inclusions in the skin, the clustering index of MnS inclusions in the steel skin portion, and related refining process parameters are recorded. The combination C1 of the refining process parameters is calculated. As a general trend, it can be seen that the higher the combination C1, the higher the density of TiN / Ti(C,N) inclusions in the skin and the higher the clustering index of MnS inclusions in the steel skin portion.

[0074] - Using the above dataset which correlates skin inclusion properties with refining process parameters, the density of TiN / Ti(C,N) inclusions in the skin is 240 particles / mm³. 2 The clustering index of MnS inclusions in the steel skin portion is less than 110 μm / mm². 2 A cutoff value less than the specified value is determined. The cutoff value of combination C1 determines how to control the refining process of the specific industrial equipment under consideration. By controlling C1 to less than the cutoff value, 240 particles / mm 2 Density of TiN / Ti(C,N) inclusions in skins less than 110 μm / mm 2It becomes possible to manufacture steel sheets having both clustering indices of MnS inclusions in the steel skin portion less than 1.5. Thus, it becomes possible to achieve the relevant excellent bending level where the lateral normalized bending angle α1.5 is exactly 48°.

[0075] For example, in the case of a specific industrial equipment in which the inventors conducted experiments, the cutoff value is equal to 270.

[0076] After the molten steel refining step, the method for producing steel sheets according to the present invention preferably includes the following steps.

[0077] - 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 pickup and therefore higher inclusion levels in the semi-finished products. For example, in a continuous casting process where the semi-finished products are slabs produced in a continuous sequence by casting products of multiple heats in a mold into a tundish, certain refractories and linings can be used in the tundish, and certain assignment rules can be used for sequence slabs and transient slabs between two different heats.

[0078] Next, the semi-finished product is optionally reheated at a temperature including 1150°C to 1300°C.

[0079] Next, the steel sheet is hot-rolled at a finishing hot-rolling temperature range of 800°C to 950°C.

[0080] Next, the hot-rolled steel is cooled, wound up in a Tcoil at a temperature of less than 670°C, and optionally pickled to remove oxidation.

[0081] Next, the coiled steel sheet is optionally cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction ratio is preferably in the range of 20% to 80%. If it is less than 20%, recrystallization during subsequent heat treatment is undesirable and may impair the ductility of the steel sheet. If it exceeds 80%, there is a risk of edge cracking during cold rolling.

[0082] -In an embodiment of the present invention, the steel plate is heated in an annealing furnace to a soaking temperature including 700°C to 850°C and maintained at the soaking temperature for a soaking time including 10 seconds to 20 minutes.

[0083] -In embodiments of the present invention, the annealed steel sheet is cooled to a temperature range of 400°C to 700°C and further coated with a metal coating. The metal coating is, for example, an aluminum-based metal coating containing at least 50% by weight of aluminum. The metal coating is, for example, a zinc-based metal coating containing at least 50% by weight of zinc.

[0084] -In this embodiment of the present invention, the steel plate is then cooled to room temperature.

[0085] In summary, the method described above preferably includes the following sequence of steps:

[0086] - A step of producing molten steel having the above chemical composition, wherein during the molten steel refining step, the sulfur level 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 injected are controlled to verify that the combination 217.8-315.1*Ca_added+41.5*O2_inj+18700*S_start-40*Al_added(C1) remains below a predetermined cutoff value. The cutoff value is such that if C1 is below the cutoff value, the density of TiN / Ti(C,N) inclusions in the skin is 240 particles / mm 2 The clustering index of MnS inclusions in the steel skin is less than 110 μm / mm². 2 It is determined for the specific industrial equipment being used that the value should be less than [amount missing].

[0087] - A step of casting the molten steel to obtain a semi-finished product that can be hot-rolled, -Optionally, the semi-finished product is contained within a temperature range of 1100°C to 1300°C. reheat Then, the step of reheating, - A step in which the semi-finished product is hot-rolled at a finishing hot-rolling temperature within the range of 800°C to 950°C. - Hot-rolled steel sheet with a winding temperature of less than 670°C T coil The step of winding it up to obtain a coiled steel sheet, -Optionally, a step of pickling the coiled steel sheet, -Optionally, cold-roll a coiled steel sheet at a reduction ratio ranging from 20% to 80% to obtain a cold-rolled steel sheet. -Optionally, a step of heating a hot-rolled or cold-rolled steel sheet to a soaking temperature within 700°C to 850°C, and maintaining the steel sheet at the said temperature for a soaking time within 10 seconds to 20 minutes to obtain an annealed steel sheet. -Optionally, the step of cooling the annealed steel sheet to a temperature range of 400°C to 700°C. -Optionally, the step of coating the annealed steel sheet with a metal coating. -Optionally, a step of cooling the covered steel plate to room temperature.

[0088] Next, we will describe in detail the manufacturing process of pressed parts and the subsequent characteristics of those pressed parts.

[0089] A steel blank is cut from a steel sheet according to the present invention and heated in an austenitizing furnace. Preferably, the steel blank is heated to a temperature within 880°C to 950°C for 10 seconds to 15 minutes to obtain a heated steel blank. The heated blank is then transferred to a forming press, where it is hot-formed and die-quenched to obtain a pressed part.

[0090] Optionally, the hot-stamped parts are further subjected to a paint curing step in which the parts are heated to a temperature of 150°C to 250°C for a duration of 10 minutes to 2 hours.

[0091] The microstructure of the pressed part contains more than 95% martensite and less than 5% bainite + ferrite in surface fraction on any analyzed cross-section. 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 skin layers have a particle size of 240 particles / mm². 2Density of TiN / Ti(C,N) inclusions less than 110 μm / mm 2 It has a clustering index of MnS inclusions in the steel skin portion less than .

[0092] The pressed part according to the present invention has a tensile strength of over 1300 MPa, preferably over 1350 MPa, preferably over 1400 MPa, and a lateral normalized bending angle α1.5 that is strictly over 48°. Such high tensile strength and high bending angle impart very good mechanical resistance to the part, especially in the event of a collision. These properties provide very good energy absorption and penetration resistance, thereby enhancing the safety of the vehicle. [Examples]

[0093] The present invention will be explained by the following examples, but these are by no means limiting.

[0094] Eleven different samples derived from eleven different heats A, B, C, D, E, F, G, H, I, J, and K of steel produced using industrial production routes were tested. Samples I1, I2, I3, I4, I5, I6, and I7 are according to the present invention, while samples R1, R2, R3, and R4 are reference samples.

[0095] All manufactured samples followed the same industrial production process at the steelworks. All samples were coated after annealing using an AlSi-based coating containing 8–12 wt% Si, 2–4 wt% Fe, and the remainder being Al.

[0096] Table 2 - Sample Composition The tested compositions are summarized in the table below, with elemental content expressed as weight percentage. The remainder of the composition consists of iron and unavoidable impurities resulting from the smelting process.

[0097] [Table 2]

[0098] Table 3 - Steel mill process parameters, density of TiN / Ti(C,N) inclusions in the skin, and clustering index of MnS inclusions in the skin. The following process parameters were applied in the steel mill, and the following densities of TiN / Ti(C,N) inclusions in the skin and the clustering index of MnS inclusions in the steel skin portion were observed - the underlined values ​​are not according to the present invention.

[0099] [Table 3]

[0100] *C1=217.8-315.1*Ca_added+41.5*O2_inj+18700*S_start-40*Al_added

[0101] As can be seen, under the tested industrial conditions, the refining process parameters were properly controlled to keep C1 below 270, thereby achieving a density of 240 particles / mm³ of TiN / Ti(C,N) inclusions in the skin. 2 It can be controlled to less than 110 μm / mm², and the clustering index of MnS inclusions in the steel skin portion can be controlled to less than 110 μm / mm². 2 It can be controlled to less than 270. As mentioned above, this cutoff value of 270 is specific to the industrial equipment on which the trial was conducted, and it is necessary to determine an appropriate cutoff coefficient for a given industrial equipment, for example, according to the methodology described above.

[0102] Table 4 - Further process conditions The following process parameters were applied along the production route.

[0103] [Table 4]

[0104] Table 5 - Microstructure, bending angle, and tensile strength The following microstructure (surface fraction), bending angle, bending angle anisotropy, and tensile strength were measured on the sample, but the underlined values ​​are not those according to the present invention.

[0105] [Table 5]

[0106] Table 5 shows that the sample according to the present invention has a tensile strength exceeding 1300 MPa in the transverse direction, while also having a normalized bending angle α1.5 measured in the transverse direction that is strictly greater than 48°. On the other hand, the reference samples have a comparable tensile strength level exceeding 1300 MPa, but all have a normalized bending angle α1.5 measured in the transverse direction that is 48° or less.

[0107] The inventors have found that this very good level of flexibility correlates with the density of TiN / Ti(C,N) inclusions in the skin and the clustering index of MnS inclusions in the steel skin portion.

[0108] The clustering index of MnS inclusions in the skin is 110 μm / mm². 2 In the above cases, the bending angle becomes small, as in the case of reference samples R1, R2, and R3, and the clustering index of MnS inclusions in the skin of all reference samples R1, R2, and R3 is 110 μm / mm². 2 The above conditions are met, and the normalized bending angle of 1.5 mm in the lateral direction is 48° or less.

[0109] The density of TiN / Ti(C,N) inclusions in the skin is 240 μm / mm². 2 In this case, as with reference sample R1, the bending angle becomes small, and reference sample R1 has a density of TiN / Ti(C,N) inclusions in the skin of 246 μm / mm². 2 The normalized bending angle of 1.5 mm in the lateral direction is 46° or less.

[0110] By controlling the skin inclusion population to the aforementioned range, the manufactured steel sheet can be used to produce hot-stamped parts with very good, robust, and stable impact resistance for use in industries such as the automotive industry.

Claims

1. A steel plate made of steel, wherein the steel is present in a weight percentage of, 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% Cr ≤ 0.4% Mo ≤ 0.3% Nb ≤ 0.1% V ≤ 0.3% It has a composition containing (Cr + Mo + Nb + V ≤ 0.5%), The remainder of the aforementioned composition consists of iron and unavoidable impurities arising from the smelting process. The above composition is subject to the following conditions: 5.22 * (S - Ca * 32 / 40) * 10 4 + 11.4 * (Ti 2 * N) * 10 6 + 136.5 < 280, where all elements are expressed in weight percent, further adhering to this condition. The steel sheet has a microstructure containing 75% to 90% ferrite in surface fraction, with the remaining portion being Fe ３ It is composed of C and a hard phase consisting of martensite and bainite. The steel plate extends from the bulk to the surface of the steel plate. - Equipped with bulk equivalent to 80% of the thickness of the steel plate, - The bulk is covered by an upper skin layer and a lower skin layer that occupy the outermost 10% of the thickness on both sides of the bulk, and the density of TiN / Ti(C,N) inclusions in the skin is exactly 240 particles / mm³. ２ The clustering index of MnS inclusions in the skin is less than 110 μm / mm². ２ A steel plate that is less than [a certain value].

2. C: 0.2–0.25%, and / or Mn: 1.0–1.4%, and / or Si: 0.1–0.4%, and / or Al: 0.02–0.06%, and / or Ti: 0.02–0.06%, and / or B: 0.002–0.004%, and / or P ≤ 0.020%, and / or Ca ≤ 0.005%, and / or S ≤ 0.005%, and / or The steel plate according to claim 1, wherein N ≤ 0.008%.

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

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

5. A press-hardened steel part, wherein the steel part is, by weight percentage, 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% Cr ≤ 0.4% Mo ≤ 0.3% Nb ≤ 0.1% V ≤ 0.3% It has a composition containing (Cr + Mo + Nb + V ≤ 0.5%), The remainder of the aforementioned composition consists of iron and unavoidable impurities arising from the smelting process. The above composition is subject to the following conditions: 5.22 * (S - Ca * 32 / 40) * 10 4 + 11.4 * (Ti 2 * N) * 10 6 + 136.5 < 280, where all elements are expressed in weight percent, further adhering to this condition. The steel part has a microstructure that, in terms of surface fraction, contains more than 95% martensite and up to 5% bainite or ferrite. The steel part extends from the bulk to the surface of the steel part. - Equipped with bulk, - The bulk is covered by a skin including an upper skin layer and a lower skin layer that occupy the outermost 10% of the thickness on both sides of the bulk, and the density of TiN / Ti(C,N) inclusions in the skin is 240 particles / mm³ ２ The clustering index of MnS inclusions in the skin is less than 110 μm / mm². ２ Press-hardened steel parts that are less than [amount missing].

6. The above composition, in weight percent, C: 0.2–0.25%, and / or Mn: 1.0–1.4%, and / or Si: 0.1–0.4%, and / or Al: 0.02–0.06%, and / or Ti: 0.02–0.06%, and / or B: 0.002–0.004%, and / or P ≤ 0.020%, and / or Ca ≤ 0.005%, and / or S ≤ 0.005%, and / or The press-hardened steel part according to claim 5, wherein N ≤ 0.008%.

7. The press-hardened steel part according to claim 5 or 6, wherein the press-hardened steel part has a tensile strength TS measured in the transverse direction of 1300 MPa or more and is normalized to 1.5 mm, and has a bending angle measured in the transverse direction of exactly 48° or more.

8. A method for manufacturing press-hardened steel parts, comprising the following series of steps: - A step of providing a steel plate made of steel, Here, the steel is, by weight percentage, 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% Cr ≤ 0.4% Mo ≤ 0.3% Nb ≤ 0.1% V ≤ 0.3% It has a composition containing (Cr + Mo + Nb + V ≤ 0.5%), The remainder of the aforementioned composition consists of iron and unavoidable impurities arising from the smelting process. The above composition is subject to the following conditions: 5.22 * (S - Ca * 32 / 40) * 10 4 + 11.4 * (Ti 2 * N) * 10 6 + 136.5 < 280, where all elements are expressed in weight percent, further adhering to this condition. The steel sheet has a microstructure containing 75% to 90% ferrite in surface fraction, with the remaining portion being Fe ３ It is composed of C and a hard phase consisting of martensite and bainite. The steel plate extends from the bulk to the surface of the steel plate. - Equipped with bulk equivalent to 80% of the thickness of the steel plate, - Such a bulk is covered by a skin comprising an upper skin layer and a lower skin layer that occupy the outermost 10% of the thickness on both sides of the bulk, and the density of TiN / Ti(C,N) inclusions in the skin is exactly 240 particles / mm³ ２ The clustering index of MnS inclusions in the skin is less than 110 μm / mm². ２ It is less than, - In order to obtain a steel blank, the steps include cutting the steel plate into a predetermined shape, - The step of heating the steel blank to a temperature of 880°C to 950°C for 10 seconds to 15 minutes to obtain a heated steel blank, - The step of transferring the heated blank to a forming press, - The step of hot-forming the heated blank in the molding press to obtain a molded part, - Including the step of die-quenching the molded part, Here, the press-hardened steel part has a microstructure that, in terms of surface fraction, contains more than 95% martensite and up to 5% bainite or ferrite. The steel part extends from the bulk to the surface of the steel part. - Equipped with bulk, - Such a bulk is covered by a skin including an upper skin layer and a lower skin layer that occupy the outermost 10% of the thickness on both sides of the bulk, and the density of TiN / Ti(C,N) inclusions in the skin is 240 particles / mm ２ or less, and the clustering index of MnS inclusions in the skin is 110 µm / mm ２ or less, method.

9. The method according to claim 8, further comprising a paint baking step in which the molded part is heated to a temperature of 150°C to 250°C for a duration of 10 minutes to 2 hours.

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