Steel sheet and high strength press hardened steel part, and method of manufacturing the same

Abstract

To provide steel sheets, and high strength press hardened steel parts having excellent bending properties.SOLUTION: A steel sheet, made of a steel having a composition comprising C: 0.3-0.4%, Mn: 0.5-1.0%, Si: 0.4-0.8%, Cr: 0.1-1.0%, Mo: 0.1-0.5%, Nb: 0.01-0.1%, Al: 0.01-0.1%, Ti: 0.008-0.03%, B: 0.0005-0.003%, P: ≤0.020%, Ca: ≤0.001%, S: ≤0.004%, N: ≤0.005% and comprising optionally Ni<0.5%, having a microstructure comprising, in surface fraction, from 60% to 95% of ferrite, the rest being martensite-austenite islands, pearlite or bainite, and comprising a bulk and a skin layer occupying the outermost 10% of the thickness on either sides of the bulk, the skin layer having a skin layer inclusion population wherein the surface fraction of oxides is equal to or below 60*10-6.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

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

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

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

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

[0006] The object of the present invention is to address the above-mentioned problems and to provide a press-hardened steel part having a combination of high mechanical properties, a tensile strength after hot stamping of 1800 MPa or more and a bending angle in the rolling direction normalized to 1.5 mm of 50° or more as measured by the VDA-238 standard.

[0007] Another object of the present invention is to obtain a steel sheet which can be transformed into such a press hardened steel part by hot forming. [Means for solving the problem]

[0008] The object of the invention is achieved by providing a steel sheet according to claim 1, optionally with the features of claim 2. Another object of the invention is achieved by providing a press-hardened steel part according to claim 3. The steel part may also have the characteristics according to claim 4. Another object is achieved by providing a method according to claim 5.

[0009] The invention will now be explained in more detail and illustrated by means of examples, without introducing limitations, with reference to FIG. 1, which is a schematic cross-sectional view of a steel sheet according to the invention. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view of a steel sheet according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] A steel blank refers to a flat piece of steel cut into any shape suitable for its use. The blank has an upper and lower surface, also called the upper and lower surfaces or the upper and lower surfaces. The distance between said surfaces is designated as the thickness of the blank. The thickness can be measured, for example, using a micrometer, whose spindle and anvil are placed on the upper and lower surfaces. Similarly, the thickness can also be measured on a formed part.

[0012] Hot stamping is a forming technique that involves heating a blank to a temperature at which the steel microstructure is at least partially transformed into austenite, forming the blank at high temperature by stamping the blank, and quenching the formed part to obtain a microstructure with very high strength. Hot stamping makes it possible to obtain very high-strength parts with complex shapes, presenting many technical advantages. It is understood that the heat treatment that the part undergoes includes not only the above-mentioned thermal cycle of the hot stamping process itself, but also other subsequent heat treatment cycles, which may possibly be performed after the part has been painted, such as for example a paint baking step, to bake the paint. The following mechanical properties of hot stamped parts are measured after the complete thermal cycle, optionally including for example a paint baking step, if a paint baking has actually been performed.

[0013] Ultimate tensile strength is measured according to ISO standard ISO6892-1 published in October 2009. Tensile specimens are cut from the flat area of ​​the hot stamped part. If necessary, smaller size tensile test samples are taken to accommodate the total available flat area 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 the present invention refers to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle value should be normalized to 1.5 mm by the following calculation, where α1.5 is the bending angle normalized at 1.5 mm, t is the thickness, and αt is the bending angle for thickness t: α1.5=(αt×√t) / √1.5 In the present invention, the bend angle was measured in the rolling direction, i.e. the direction in which the steel sheet moved during the hot rolling step. The bend angle was measured using a laser measuring device. When performing bend tests on hot stamped parts, samples are cut from the flat area of ​​the part. If necessary, small size samples are taken to accommodate the entire available flat area on the part. If the rolling direction on a hot stamped part is unknown, it can be determined using electron backscatter diffraction (EBSD) analysis over the cross section of the sample in a scanning electron microscope (SEM). The rolling direction is determined according to the intensity of the orientation distribution function (ODF) representing the main fibers at φ2=45°, where φ2 is the Euler angle defined in "H.-J.Bunge:Texture Analysis in Materials Science-Mathematical Methods.1st English Edition by Butterworth Co(Publ.)1982" (see Figures 2.2 and 2.3 for the definition of φ2).

[0015] The bend angle of a part represents the ability of the part to withstand deformation without forming cracks.

[0016] The composition of the steel according to the invention will now be described, the contents being expressed in percentages by weight. The chemical composition is given in terms of lower and upper limits of a compositional range, said limits being included within the possible compositional range according to the invention.

[0017] According to the present invention, the carbon content is in the range of 0.3% to 0.4% to ensure a satisfactory strength. If the carbon content exceeds 0.4%, the weldability and bendability of the steel plate may be deteriorated. If the carbon content is less than 0.3%, the tensile strength does not reach the target value.

[0018] The manganese content is in the range of 0.5% to 1.0%. If the addition amount exceeds 1.0%, the risk of MnS formation increases the loss of bendability. If it is less than 0.5%, the hardenability of the steel sheet decreases.

[0019] The silicon content ranges from 0.4% to 0.8%. Silicon is an element involved in hardening in solid solution. Silicon is added to limit the formation of carbides. Above 0.8%, silicon oxide forms on the surface, which impairs the coatability of the steel. Also, the weldability of the steel plate may decrease.

[0020] The chromium content is in the range of 0.1% to 1.0%. Chromium is the element responsible for hardening in solid solution and must be higher than 0.1% to ensure sufficient strength. The chromium content is preferably less than 0.4% to limit workability problems and costs. Preferably, the chromium content is in the range of 0.1% to 0.4%.

[0021] The molybdenum content is in the range of 0.1% to 0.5%. Molybdenum improves the hardenability of the steel. Below 0.1%, the tensile strength is not reached. Molybdenum is preferably not more than 0.4% in order to limit the cost.

[0022] Niobium is in the range of 0.01% to 0.1%. Niobium improves the ductility of the steel. Above 0.1%, the risk of the formation of NbC or Nb(C,N) carbides increases the loss of bendability. Preferably, the niobium content is in the range of 0.03% to 0.06%.

[0023] According to the present invention, the aluminum content is in the range of 0.01%-0.1% since it is a very effective element for deoxidizing the steel in the liquid phase during smelting. Aluminum can protect boron when the titanium content is not sufficient. The aluminum content is less than 0.1% to avoid oxidation problems and ferrite formation during press hardening. Preferably, the aluminum content is in the range of 0.03%-0.05%.

[0024] According to the present invention, the titanium content is in the range of 0.008%-0.03% to protect boron from being trapped in BN precipitates. The titanium content is limited to 0.03% to avoid excessive TiN formation. As will be explained in more detail further, by measuring the N level of the molten steel before adding Ti, it is possible to add the appropriate amount of Ti to capture the residual N content.

[0025] According to the present invention, the boron content is in the range of 0.0005% to 0.003%. Boron improves the hardenability of the steel. The boron content is not more than 0.003% to avoid the problem of slab fracture during continuous casting.

[0026] Phosphorus is controlled to less than 0.020% as it introduces brittleness and weldability problems.

[0027] Calcium is controlled to less than 0.001% since its presence in the liquid steel can result in the formation of coarse precipitates which are detrimental to bendability.

[0028] Since the presence of sulfur in molten steel can result in the formation of MnS precipitates which are detrimental to bendability, sulfur is controlled to less than 0.004%.

[0029] Nitrogen is controlled to less than 0.005%, preferentially less than 0.004%, and even more preferentially less than 0.003%. The presence of nitrogen can lead to the formation of precipitates such as TiN or TiNbCN which are detrimental to bendability.

[0030] Nickel is optionally added up to a level of 0.5%. Nickel can be used to protect the steel against delayed cracking.

[0031] The remainder of the steel composition is iron and impurities resulting from smelting.

[0032] Next, the microstructure of the coated steel sheet according to the present invention will be described.

[0033] The steel plate has a microstructure containing 60% to 95% ferrite at the surface, with the remainder being martensite-austenite islands, pearlite or bainite.

[0034] Ferrite is formed during intercritical annealing of cold rolled steel sheets. The remainder of the microstructure is austenite at the end of soaking, which transforms to martensite-austenite islands, pearlite or bainite during cooling of the sheet.

[0035] The total amount of ferrite in the steel sheet microstructure is a function of the chemical composition, the annealing temperature TA and the soaking time tA. More austenite is formed during annealing when the annealing temperature TA is the highest, in the range of 700°C to 850°C, and the time tA is the longest, in the range of 10 seconds to 20 minutes. After annealing, the transformation of the formed austenite into martensite, bainite or ferrite mainly depends on the cooling rate. Preferably, the cooling rate is less than 10°C / s in order to form as many soft phases (ferrite, bainite) as possible, which allows good workability of the steel sheet before hot stamping.

[0036] With reference to Figure 1, a steel plate 1 according to the invention comprises a bulk portion 3 and upper and lower skin layers 2. The total thickness of the steel plate 1 is t0 and the thickness ts of the skin layers 2 is such that ts = t0 * 10%. In other words, the skin layers 2 occupy the outermost 10% of the thickness on each side of the bulk.

[0037] The skin layer 2 has a surface fraction of oxide of 60*10 -6 The method used to measure the inclusion population is described in further detail below.

[0038] The inventors have found that there is a correlation between bend angle and skin layer inclusion population, particularly the oxide population, and by controlling the skin inclusion population, it is possible to improve bend angle without adversely affecting other product properties such as tensile strength.

[0039] Below is a description of the methodology used to characterize inclusions in steel plates and steel parts, it should be understood that this is only one possible methodology and that other protocols may also be implemented.

[0040] The inclusions present in the steel sheets have been characterized using a Scanning Electron Microscope (SEM) equipped with a Field Effect Gun (FEG). A Tescan Mira 3 SEM was used with a power setting of 14 kV. Additionally, the inclusions were analyzed using Energy Dispersive Spectroscopy (EDS). 2 A Bruker EDS probe was used.

[0041] The sample is divided into three regions (top skin, bottom skin and bulk as described above). Each region is divided into fields. In each field, inclusions are detected. Each inclusion is zoomed in to capture its morphological characteristics and EDS analysis is performed. A dual-tone threshold is set to capture the particles (on a scale from 0 to 255, where 0 is black and 255 is white): - Classic dark particles such as oxides with a gradient of <150 -Bright particles such as NbC particles with a gradation >220 Using the EDS probe information, shape and brightness level, each particle is then classified into one of the following categories: TiN, NbC, TiNbCN, alumina, complex oxides, sulfides, MnS.

[0042] The next step is to calculate the following properties for the entire set of inclusions and for each particle family: -Average diameter in microns -Inclusion count density / mm 2 - Surface fraction of inclusions, defined as the grand sum over all analyzed fields of the surface area occupied by a given inclusion family divided by the total surface of all analyzed areas. The surface fraction of inclusions can be calculated using the following formula (for a particle type referred to here as "X"):

[0043]

number

[0044] The surface fraction of inclusions combines in a single parameter information about both the density level of the particles and their average size. The inventors have found that the surface fraction of inclusions is a good indicator of cleanliness and, for certain inclusions, correlates well with several important in-service properties, such as bend angle.

[0045] The coated steel sheet according to the invention can be produced by any suitable manufacturing method, which the skilled person can define, however it is preferred to use the method according to the invention, which comprises the steps described below.

[0046] In the following description, the term ladle refers to the vessel used to contain molten steel during the refining process. The term tundish refers to the vessel into which the molten steel is poured before being cast into the mould - tundishes are used in continuous casting: it makes it possible to have a buffer of molten steel available for casting between the finishing of pouring one ladle and the opening of the next one.

[0047] A semi-finished product, which can be further hot rolled, is provided with the above mentioned steel composition. Particular attention should be paid during the refining of said semi-finished product, in particular during the liquidus and casting, to control the inclusion population.

[0048] In a first embodiment, a molten steel refining process includes the following steps: -After the pig iron has been decarburized in the converter by oxygen blowing, the liquid steel is tapped into the ladle without adding Al or any other deoxidizing elements such as Si or Mn which would deoxidize the crude steel at this stage, which allows to minimize the subsequent nitrogen pick-up in the liquid steel.

[0049] - The main alloying elements, especially Mn, Si, Cr, Mo, Nb and B, but not Ti, are incorporated into the liquid steel under vacuum, for example using the Ruhrstahl Heraeus (RH) vacuum degassing system or a vacuum tank degasser (VTD), which makes it possible, among other advantages, to ensure a low nitrogen content.

[0050] - In order to reach the desired very low sulfur levels, a desulfurization step is carried out after said vacuum degassing step. The desulfurization step involves an exchange between the liquid steel and slag formed by adding fluxes to the heat, for example CaO-based fluxes. These fluxes can be added before the desulfurization step, for example during tapping after the converter.

[0051] - Adding Ti after the desulfurization step. Ti is added, for example, by using the measured nitrogen composition to add just the right amount of Ti to precipitate N in the semi-finished product in the form of TiN. For example, the amount of Ti added is equal to or slightly more than 3.42 times the amount of nitrogen measured after desulfurization, in weight percent.

[0052] - No Ca-containing additions (such as SiCa, FeCa or pure Ca) are implemented to minimize the amount of calcium aluminates that may be detrimental to the bending performance of press hardened parts. Due to the very low S levels achieved using the targeted compositions and methods, the produced steel contains a low MnS population.

[0053] -A minimum time is provided to promote the flotation of inclusions. Inclusion flotation refers to the phenomenon whereby inclusions in molten steel float to the slag that covers the molten steel, thanks to its lower density than the steel. Once the inclusions are collected in the slag, they are removed from the molten steel and are not cast into the semi-finished product, thereby reducing the inclusion population. The inventors have found that the inclusion flotation time correlates with the surface fraction of oxides in the skin layer of the steel plate. The determination of the inclusion flotation time depends on the specific process route and equipment used to manufacture the steel. For example, in the above case where a vacuum degasser is used to add Mn, Si, Cr, Mo, Nb and B, and the molten steel is further desulphurized after the vacuum degasser, the inclusion flotation time is: - time spent in the vacuum degasser after adding Mn, Si, Cr, Mo, Nb and B (time is measured after adding alloying elements since the addition of said elements itself can nucleate inclusion particles that need to be suspended in the slag), - the time spent in the desulfurization step, - the sum of the holding time between the desulfurization step and the continuous casting operation itself, which may include gentle stirring using controlled inert gas injection after desulfurization, a ladle transport step between the desulfurization station and the continuous casting operation, waiting time at the continuous casting step, etc. The continuous casting step starts when the ladle is opened to start pouring into the casting tundish.

[0054] In a second embodiment, a molten steel refining process includes the following steps: After the pig iron has been decarburized in the converter by oxygen blowing, the molten steel is tapped into a ladle. Optionally, at this stage, some of the alloying elements can be added, for example at least a part of the Mo, Cr and Mn content of the steel.

[0055] - A desulphurisation step is then carried out to reach the desired very low sulphur level. The desulphurisation step involves an exchange between the liquid steel and slag formed by adding fluxes to the heat, for example CaO-based fluxes. These fluxes can be added before the desulphurisation step, for example during tapping after the converter.

[0056] - The main alloying elements, especially Mn, Si, Cr, Mo, Nb and B, but not Ti at this stage, are incorporated into the molten steel under vacuum, for example using a RH vacuum degassing system or a VTD. After adding the main alloying elements, the steel is stirred under vacuum, known as the stirring step. When using, for example, a RH vacuum degassing system, stirring is naturally induced in the system by the circulation of the molten steel in the snorkel of the vacuum vessel. When using a VTD, stirring can be induced, for example, by bubbling argon inside the molten steel. This stirring step serves both to distribute the alloying elements uniformly in the molten steel and to promote the flotation of inclusions.

[0057] - Ti is added at the end of the vacuum degassing step. For example, the amount of Ti added is determined using the measured nitrogen composition to add just the right amount of Ti to precipitate N in the semi-finished product in the form of TiN. For example, the amount of Ti added is equal to or slightly more than 3.42 times the amount of nitrogen measured at the end of the stirring step, in weight percent.

[0058] - No Ca-containing additions (such as SiCa, FeCa or pure Ca) are implemented to minimize the amount of calcium aluminates that may be detrimental to the bending performance of press hardened parts. Due to the very low S levels achieved using the targeted composition and method, the produced steel contains low MnS populations. The inventors have found that at such low MnS content, the bending performance is very good even without Ca additions to spheroidize the MnS populations.

[0059] - As in the first embodiment, a minimum time is provided to promote the flotation of the inclusions. In this second embodiment, in which the molten steel is desulphurized before adding the main alloying elements under vacuum, the inclusion flotation time is: - time spent in the vacuum degasser after adding Mn, Si, Cr, Mo, Nb and B, - the holding time between the vacuum degasser and the continuous casting operation itself, which may include the ladle transport step between the vacuum degasser and the continuous casting operation, the waiting time at the continuous casting step, etc. The continuous casting step starts when the ladle is opened to start pouring into the casting tundish.

[0060] More generally, it is preferred to refine the steel by carrying out the main additions of Mn, Si, Cr, Mo, Nb and B under vacuum, for example using a vacuum degasser, which allows the nitrogen content in the steel to be low and thus allows better control of the nitrogen-containing inclusions in the steel.

[0061] More generally, the inclusion flotation time is defined as the amount of time the molten steel spends after Mn, Cr, Si, Mo, Nb and B have been added and before the casting step begins.

[0062] To control the inclusion surface fraction in the skin of the steel plate, the inclusion flotation time should be controlled beyond the minimum inclusion flotation time tf. The value of tf depends on the specific industrial setup used to produce the steel. It depends on the production path in the steel mill and the geometric configuration of the ladle used to process the molten steel. Since the inclusion flotation time is related to the hydrodynamics and the movement of small particles in the molten steel, the minimum inclusion flotation time required to reach the desired level of a particular inclusion in the skin of the steel depends on the size of the ladle, its diameter, height, volume, etc. For example, the minimum inclusion flotation time is 60 minutes. For example, the minimum inclusion flotation time is 53 minutes.

[0063] To determine the minimum inclusion flotation time tf for a given steel composition and a given industrial installation and production route, it is recommended to apply the following method: -Perform several heatings using the same chemical composition target The heats are produced using different inclusion flotation times. For example, a series of heats is carried out using inclusion flotation times ranging from a minimum inclusion flotation time corresponding to the minimum feasible inclusion flotation time of an industrial route, and then incrementally longer inclusion flotation times are applied, for example using time increments of 10 minutes. For example, five different inclusion flotation times are applied to five different heats.

[0064] said heat is processed according to the industrial route described below and the inclusion population of the steel is characterized using the method described above.

[0065] - The surface fraction of the skin layer oxide and the respective inclusion flotation time are recorded. The inventors have found that there is a correlation between the surface fraction of the skin layer oxide and the inclusion flotation time. The longer the inclusion flotation time, the lower the surface fraction of the skin layer oxide. The minimum inclusion flotation time tf is the time beyond which the surface fraction of the skin layer oxide falls below 60*10 -6 For example, the inventors found that using the particular industrial equipment that they had available and applying the process route of the first embodiment, the minimum inclusion flotation time was 60 minutes, preferentially 53 minutes. This is shown in the examples below.

[0066] After the steel refining step, the method for producing a steel sheet according to the invention preferably comprises the following steps: -Continuous casting of molten steel into semi-finished products suitable for hot rolling. During the casting process, particular care should be taken to avoid oxygen pick-up and therefore higher oxide levels in the semi-finished products. For example, in the case of a continuous casting process where the semi-finished products are slabs produced in successive sequences by casting in a mould the products of several heats poured into a tundish, specific refractories and linings can be used in the tundish, specific allocation rules can be used for the first slab of the sequence and for the transitional slabs between two different heats, etc.

[0067] The semi-finished product is then optionally reheated at a temperature inclusive of 1150°C to 1300°C.

[0068] The steel sheet is then hot rolled at a finish hot rolling temperature comprised between 800°C and 950°C.

[0069] -The hot rolled steel is then cooled and coiled at a temperature T below 670 °C and optionally pickled to remove oxidation.

[0070] The coiled steel sheet is then optionally cold rolled to obtain a cold rolled steel sheet. The cold rolling reduction is preferably in the range of 20% to 80%. Below 20%, recrystallization during the subsequent heat treatment is not favorable, which may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling.

[0071] In one embodiment of the present invention, the annealed steel sheet is heated to an annealing temperature TA comprised between 700° C. and 850° C. and maintained at said temperature TA for a holding time tA comprised between 10 seconds and 20 minutes.

[0072] In an embodiment 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.

[0073] In summary, the above method preferably comprises the following successive steps: - producing molten steel having the above chemical composition, during the molten steel refining stage, Mn, Si, Cr, Mo, Nb and B are added using a vacuum degasser to ensure a minimum inclusion flotation time tf, said minimum inclusion flotation time being the amount of time the molten steel spends after the addition of Mn, Si, Cr, Mo, Nb and B and before the casting step is started, said minimum inclusion flotation time tf being defined as the minimum inclusion flotation time required to reach a skin layer oxide surface fraction of 60*10-6 or less; - casting said molten steel to obtain a semi-finished product that can be hot rolled; - Optionally, the semi-finished product is heated to a temperature T of 1100 ° C to 1300 ° C (inclusive). reheat and reheating the mixture with hot rolling the semi-finished product at a finish hot rolling temperature including -800 ° C to 950 ° C; - Hot-rolled steel sheets are coiled at a temperature T of less than 670°C. coil and winding the steel sheet in a coil shape. - optionally pickling the coiled steel sheet; - optionally cold rolling the coiled steel sheet to obtain a cold rolled steel sheet; Optionally, hot-rolled or cold-rolled steel sheets are annealed at a temperature T in the range of 700°C to 850°C. A The steel plate is heated to the temperature T A Hold time t includes 10 seconds to 20 minutes A and maintaining the temperature for a period of time to obtain an annealed steel sheet. - optionally cooling the annealed steel sheet to a temperature range of 400°C to 700°C; - optionally coating the annealed steel sheet with a metallic coating; - optionally cooling the coated steel sheet to room temperature.

[0074] Next, the manufacturing process of the pressed parts and the subsequent properties of the pressed parts will be described in detail.

[0075] A steel blank is cut from the steel sheet according to the invention and heated in an annealing furnace. Preferably, the steel blank is heated to a temperature comprised between 880°C and 950°C for 10 seconds to 15 minutes to obtain a heated steel blank. The heated blank is then transferred to a forming press before being hot formed and die quenched to obtain a pressed part.

[0076] The microstructure of the pressed part comprises, by surface fraction, more than 95% martensite and less than 5% bainite + ferrite.Furthermore, the pressed part according to the present invention comprises a bulk portion and upper and lower skin layers, said skin layers occupying the outermost 10% of said thickness on each side of said bulk.The skin layers have an oxide surface fraction of 60*10 -6 The skin inclusion population is:

[0077] The pressed parts according to the invention have a bending angle in the rolling direction normalized to 1.5 mm of at least 50° and a tensile strength TS of at least 1800 MPa. Such high tensile strengths and bending angles endow the parts with very good mechanical resistance, especially in the case of a crash. They provide very good energy absorption capacity and intrusion resistance capacity, thereby increasing the safety of the vehicle. EXAMPLES

[0078] The present invention is illustrated by the following examples which are not intended to be limiting in any way.

[0079] Eight different samples were tested, originating from eight different heats A, B, C, D, E, F, G, H of steel produced using the industrial production route. Samples I1, I2, I3, I4, I5 and I6 are according to the invention and samples R1, R2 are reference samples.

[0080] Table 1 - Sample composition The compositions tested are summarized in the following table, with elemental contents expressed as weight percent:

[0081] [Table 1]

[0082] Table 2 - Steelmaking process parameters and skin inclusions The following process parameters were applied at the steel mill and the following skin inclusion surface fractions were observed - underlined values ​​are not in accordance with the present invention:

[0083] [Table 2] *RH = RH vacuum degassing process time after adding Mn, Si and Cr **DS=Desulfurization process time ***CC = time spent between the end of desulfurization and the start of continuous casting (= ladle opening to pour into the casting tundish).

[0084] Table 3 - Further process conditions The following process parameters were applied along the production route:

[0085] [Table 3]

[0086] Table 4 - Microstructure, bending angle and tensile strength The following microstructures, bend angles and tensile strengths were measured on the samples, the underlined values ​​being not in accordance with the present invention:

[0087] [Table 4]

[0088] Table 4 shows that the samples according to the invention (references I1, I2, I3, I4, I5 and I6), thanks to their specific composition and skin inclusions, have tensile strengths of more than 1800 MPa and bend angles normalized to 1.5 mm in the rolling direction of more than 50°.

[0089] Referring to Table 2, there is a relationship between the inclusion flotation time and the skin oxide surface fraction. The inclusion flotation time represents the amount of time the molten steel spends after adding Mn, Cr, Si, Mo, Nb and B and before the continuous casting step begins.

[0090] The inventors have found that when using the particular composition of the invention and when increasing the inclusion flotation time above the minimum inclusion flotation time tf, it is possible to control the skin oxide fraction below a critical level that ensures good bending resistance. In the industrial configuration used to produce the samples shown in this example, the minimum inclusion flotation time tf is 53 minutes. The value of tf depends on the particular industrial setup used to produce the steel.

[0091] When the inclusion flotation time is tf = 60 min or more, the skin layer oxide surface fraction is 60*10 -6When the inclusion flotation time is less than tf=53 min, the skin layer oxide surface fraction is 60*10 -6 Exceeds.

[0092] The inventors have found that when steels are subjected to bending loads, the inclusion surface fraction in the skin layer plays an important role in improving the material's resistance to crack formation. Surprisingly, this is not true for all types of inclusions. For example, NbC inclusions do not seem to have a significant effect on the bending properties of the steel. On the other hand, it has been found that the oxide surface fraction plays an important role in bending performance. A reduction in the oxide surface fraction helps to improve bending performance.

[0093] Referring to Table 4, all are 60*10 -6 The samples according to the invention (I1, I2, I3, I4, I5 and I6) with the following skin layer oxide surface fractions all have a 1.5 mm normalized bend angle in the rolling direction of at least 50° and a tensile strength of at least 1800 MPa. On the other hand, the reference samples (R1, R2) all have a 1.5 mm normalized bend angle in the rolling direction of less than 50°, while maintaining a tensile strength above 1800 MPa. Thus, the steel produced according to the invention shows a better resistance to crack formation when subjected to load, while showing a very high tensile strength, which improves the impact resistance and safety of parts produced using said material.

Claims

1. A steel plate having, in weight percent: C: 0.3-0.4% Mn: 0.5-1.0% Si: 0.4-0.8% Cr: 0.1-1.0% Mo: 0.1~0.5% Nb: 0.01~0.1% Al: 0.01~0.1% Ti: 0.008~0.03% B: 0.0005-0.003% P≦0.020% Ca≦0.0010% S≦0.004% N≦0.005%; And, optionally, Made from a steel having a composition including: Ni<0.5%; The remainder of the composition is iron and inevitable impurities resulting from smelting, The steel plate has a microstructure containing 60% to 95% ferrite in terms of surface fraction, with the remainder being martensite-austenite islands, pearlite or bainite; The steel sheet comprises, in a bulk to surface direction of the coated steel sheet: - Bulk, Such a bulk is covered on each side of said bulk by a skin layer occupying the outermost 10% of the thickness, said skin layer having an oxide surface fraction of 60*10 －６ 1. A steel plate having a skin layer inclusion population of:

2. The steel sheet of claim 1 further comprising a metallic coating on at least one side.

3. 1. A press hardened steel part, said steel part comprising, in weight percent: C: 0.3-0.4% Mn: 0.5-1.0% Si: 0.4-0.8% Cr: 0.1-0.4% Mo: 0.1~0.5% Nb: 0.01-0.1% Al: 0.01~0.1% Ti: 0.008~0.03% B: 0.0005-0.003% P≦0.020% Ca≦0.001% S≦0.004% N≦0.005%; And, optionally, Ni<0.5%; The remainder of the composition is iron and inevitable impurities resulting from smelting, The steel component has a microstructure comprising a surface fraction of more than 95% martensite and a maximum of 5% bainite or ferrite; The steel part comprises, in a bulk to surface direction of the steel part: - Bulk Such a bulk is covered on each side of said bulk by a skin layer occupying the outermost 10% of the thickness, said skin layer having an oxide surface fraction of 60*10 －６ 1. A press hardened steel part having a skin inclusion population that is:

4. 4. The press hardened steel part of claim 3, wherein the press hardened steel part has a tensile strength TS of at least 1800 MPa and a bend angle in the rolling direction normalized to 1.5 mm of at least 50°.

5. 5. A method for manufacturing a press hardened steel part according to claim 3 or 4, comprising the following successive steps: - providing a steel sheet according to claim 1 or 2, - cutting said steel sheet into a predetermined shape to obtain a steel blank, - heating said steel blank to a temperature of 880°C to 950°C for 10 seconds to 15 minutes to obtain a heated steel blank; - transferring said heated blank to a forming press, - hot forming the heated blank in the forming press to obtain a formed part, - die quenching the molded part; A method comprising:

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