Steel sheet and high-strength press-hardened steel part and method of manufacturing thereof

Abstract

A steel sheet made of a steel having a composition comprising C: 0.3% to 0.4%, Mn: 0.5% to 1.0%, Si: 0.4% to 0.8%, Cr: 0.1% to 1.0%, Mo: 0.1% to 0.5%, Nb: 0.01% to 0.1%, Al: 0.01% to 0.1%, Ti: 0.008% to 0.03%, B: 0.0005% to 0.003%, P < 0.020%, Ca < 0.001%, S < 0.004%, N < 0.005% and optionally comprising Ni < 0.5%, the steel sheet having a microstructure comprising 60% to 95% ferrite in surface fraction, the rest being martensite-austenite islands, pearlite or bainite, and including a bulk and a surface layer occupying the outermost 10% thickness on both sides of the bulk, the surface layer having a population of surface inclusions of oxides, MnS and TiNbCN in which the cumulative surface fraction is equal to or lower than 75*10 ‑6 -2.

Description

TECHNICAL FIELD

[0001] The present invention relates to a steel sheet and to a high-strength press-hardened steel part having good bending properties. BACKGROUND

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

[0003] In such type of applications, it is desirable to produce steel parts having both high mechanical strength and high impact resistance. Moreover, in view of global environmental protection, one of the main challenges in the automotive industry is to reduce the weight of vehicles to improve their fuel efficiency without neglecting safety requirements.

[0004] Such weight reduction can be achieved in particular thanks to the use of steel parts having a microstructure dominated by martensite.

[0005] It is challenging to produce very high-strength steels that are also resistant 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 impact resistance of parts produced with such high-strength steels, because even if 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

[0006] It is an object of the present invention to solve the above challenges and to provide a press-hardened steel part having a combination of high mechanical properties: a tensile strength after hot stamping higher than or equal to 1800 MPa and a bending angle in the rolling direction normalized to 1.5 mm equal to or higher than 50° as measured by the VDA-238 standard.

[0007] It is another object of the present invention to obtain a steel sheet that can be transformed into such a press-hardened steel part by hot forming.

[0008] The object of the present invention is achieved by providing a steel sheet according to claim 1, optionally having the features of claim 2. Another object of the present invention is achieved by providing a press-hardened steel part according to claim 3. This steel part can also comprise the features of claim 4. Another object is achieved by providing a method according to claim 5. BRIEF DESCRIPTION OF DRAWINGS

[0009] The invention will now be described in detail and illustrated by way of non-limiting examples and with reference to the accompanying drawings, in which: Figure 1 Figure 1 is a schematic cross-section of a steel sheet according to the present invention. ​DETAILED DESCRIPTION

[0010] A blank is meant to be a flat steel sheet that has been cut into any shape suitable for its use. A blank has a top face and a bottom face, which are also referred to as top side and bottom side or top surface and bottom surface. The distance between the faces is referred to as the thickness of the blank. This thickness can for example be measured using a micrometer with its main shaft and anvil placed on the top and bottom face. In a similar way, the thickness can also be measured on a shaped part.

[0011] Hot stamping is a shaping technique that involves heating the blank up to a temperature at which the microstructure of the steel is at least partially transformed into austenite, shaping the blank at high temperature by stamping it, and quenching the shaped part to obtain a microstructure with very high strength. Hot stamping allows obtaining parts with complex shapes and very high strength and presents many technical advantages. It should be understood that the heat treatment to which the part is subjected not only includes the heat cycle of the hot stamping process itself as described above, but can also include other subsequent heat treatment cycles, for example a baking step for baking the paint after the part has been painted. The mechanical properties of the hot stamped part are the ones measured after all heat cycles, including optionally the baking step for example if a paint has been applied.

[0012] The ultimate tensile strength is measured according to ISO standard ISO 6892-1 published in October 2009. Tensile test specimens are cut from flat areas of the hot stamped part. If necessary, small size tensile test samples are taken to fit the available flat area on the part.

[0013] The bending angle is measured according to VDA-238 bending standard. For the same material, the bending angle depends on the thickness. For simplicity, the bending angle values of the present invention refer to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle values need to be normalized to 1.5 mm by the following calculation, where a1 5 is the bending angle normalized at 1.5 mm, t is the thickness, and at is the bending angle for a thickness t:

[0014]

[0015] In the present invention, the bend angle is measured along the rolling direction, i.e. the direction along which the steel sheet travels during the hot rolling step. The bend angle is measured using a laser measuring device. 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 on the whole available flat area on the part. If the rolling direction on the hot stamped part is not known, it can be determined with Electron Back-Scattered Diffraction (EBSD) analysis in a Scanning Electron Microscope (SEM) across a cross section of the sample. The rolling direction is determined from 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. First English edition (publication) Butterworth Co 1982" (see Figures 2.2 and 2.3 for the definition of φ2).

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

[0017] 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 by the lower and upper limits of the composition range, said limits being included in the possible composition range according to the present invention.

[0018] According to the present invention, the carbon ranges from 0.3% to 0.4% to ensure satisfactory strength. Above 0.4%, the weldability and bendability of the steel sheet can decrease. If the carbon content is below 0.3%, the tensile strength will not reach the targeted value.

[0019] The manganese content ranges from 0.5% to 1.0%. Above 1.0%, the risk of MnS formation increases, impairing the bendability. Below 0.5%, the hardenability of the steel sheet decreases.

[0020] The silicon content ranges from 0.4% to 0.8%. Silicon is an element that participates in the hardening of the solid solution. Silicon is added to limit carbide formation. Above 0.8%, silicon oxides form at the surface, which impairs the coatability of the steel. In addition, the weldability of the steel sheet can decrease.

[0021] The chromium content ranges from 0.1% to 1.0%. Chromium is an element that participates in the hardening of the solid solution and must be higher than 0.1% to ensure sufficient strength. The chromium content is preferably lower than 0.4% to limit workability problems and costs. Preferably, the chromium content ranges from 0.1% to 0.4%.

[0022] The molybdenum content ranges from 0.1% to 0.5%. Molybdenum improves the hardenability of the steel. Below 0.1%, the tensile strength cannot be reached. Molybdenum is preferably not higher than 0.4% to limit costs.

[0023] Niobium ranges from 0.01% to 0.1%. Niobium improves the ductility of the steel. Above 0.1%, the risk of formation of NbC or Nb(C,N) carbides increases, thereby impairing the bendability. Preferably, the niobium content ranges from 0.03% to 0.06%.

[0024] According to the invention, the aluminium content ranges from 0.01% to 0.1% as it is a very efficient element for deoxidizing the steel in liquid phase during the refining. Aluminium can protect boron if the titanium content is insufficient. The aluminium content is lower than 0.1% to avoid oxidation problems and ferrite formation during press hardening. Preferably, the aluminium content ranges from 0.03% to 0.05%.

[0025] According to the invention, the titanium content ranges from 0.008% to 0.03% to protect boron that will be trapped in BN precipitates. The titanium content is limited to 0.03% to avoid excessive TiN formation. As will be further explained in more details, the appropriate amount of Ti can be added by measuring the N level of the liquid steel before adding Ni to capture the residual N content.

[0026] According to the invention, the boron content ranges from 0.0005% to 0.003%. Boron improves the hardenability of the steel. The boron content is not higher than 0.003% to avoid slab breakage problems during continuous casting.

[0027] Phosphorus is controlled to be lower than 0.020% as it leads to brittleness and weldability problems.

[0028] Calcium is controlled to be lower than 0.001% as the presence of calcium in the liquid steel can lead to the formation of coarse precipitates that are detrimental to the bendability.

[0029] Sulphur is controlled to be lower than 0.004% as the presence of sulphur in the liquid steel can lead to the formation of MnS precipitates that are detrimental to the bendability.

[0030] Nitrogen is controlled to be lower than 0.005%, preferably lower than 0.004%, even more preferably lower than 0.003%. The presence of nitrogen can lead to the formation of precipitates such as TiN or TiNbCN that are detrimental to the bendability.

[0031] Nickel is optionally added up to a level of 0.5%. Nickel can be used to protect the steel from delayed cracking.

[0032] The remainder of the composition of the steel is iron and impurities resulting from the smelting.

[0033] The microstructure of the coated steel sheet according to the application will now be described.

[0034] The steel sheet has a microstructure comprising 60% to 95% ferrite by surface fraction, the remainder being martensite-austenite islands, pearlite or bainite.

[0035] Ferrite forms during intercritical annealing of the cold-rolled steel sheet. The remainder of the microstructure at the end of soaking is austenite which transforms into martensite-austenite islands, pearlite or bainite during cooling of the steel sheet.

[0036] The total amount of ferrite in the microstructure of the steel sheet is a function of the chemical composition, the annealing temperature TA and the soaking time tA. The higher the annealing temperature TA in the range 700°C to 850°C and the longer the time tA in the range 10 seconds to 20 minutes, the more austenite will form during annealing. After annealing, the transformation of the austenite formed into martensite, bainite or ferrite will depend mainly on the cooling speed. Preferably, the cooling speed is lower than 10°C / s in order to form as much soft phase (ferrite, bainite) as possible. This allows good workability of the steel sheet before hot stamping.

[0037] Reference Figure 1 The steel sheet 1 according to the application comprises a bulk portion 3 and top and bottom surface layers 2. The total thickness of the steel sheet 1 is to and the thickness of the surface layers 2 is ts such that ts = to * 10%. In other words, the surface layers 2 occupy the outermost 10% of thickness on both sides of the bulk.

[0038] The surface layers 2 have a surface layer inclusion population in which the cumulative surface fraction of oxides, MnS and TiNbCN is equal to or less than 75*10 -6 The method for measuring the inclusion population will be further detailed below.

[0039] The inventors have found a correlation between the bending angle and the surface layer inclusion population, in particular the population of oxides, MnS and TiNbCN. By controlling the surface layer inclusion population, it is possible to improve the bending angle without adversely affecting other product properties, such as tensile strength.

[0040] The following is a description of the method used in order to characterize the inclusions in the steel sheet and steel parts. It will be understood that this is only one possible method and that other solutions can also be implemented.

[0041] The 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. In addition, the inclusions are analyzed using Energy Dispersive Spectrometry (EDS). A Bruker EDS probe of 120 mm 2 is used.

[0042] The sample is divided into 3 zones (top skin, bottom skin, bulk as previously described). Each zone is divided into fields. In each field, inclusions are detected. Each inclusion is zoomed to capture the morphological features and EDS analysis is performed. Double grayscale thresholds are set to capture the particles (on a scale from 0 to 255, 0 is black and 255 is white):

[0043] - classic dark particles, e.g. oxides, with a grayscale < 150

[0044] - bright particles, e.g. NbC particles, with a grayscale > 220

[0045] Using the information from the EDS probe, the shape and the brightness level, each particle is then classified in one of the following categories: TiN, NbC, TiNbCN, alumina, complex oxide, oxysulfide, MnS.

[0046] The next step is to calculate the following features for the whole set of inclusions and for each particle family:

[0047] - average diameter in microns

[0048] - density in number of inclusions / mm2 2

[0049] - surface fraction of inclusions, defined as the sum of all analyzed fields with respect to the surface area occupied by a given inclusion family divided by the total surface of all analyzed zones. The surface fraction of inclusions can be calculated using the following formula (here for a particle type called "X"):

[0050]

[0051] The surface fraction of inclusions combines both the density level of particles and their average size in one single parameter information. The inventors found that the surface fraction of inclusions is a good indicator of cleanliness and, in the case of specific inclusions, is very relevant to some key use properties such as the bending angle.

[0052] ​The coated steel sheet according to the application can be produced by any suitable manufacturing method and the person skilled in the art can define the method. However, it is preferred to use a method according to the application comprising the following steps.

[0053] In the following description, the term ladle refers to a vessel for containing the molten steel during the refining process. The term tundish refers to a vessel into which the molten steel is poured before being cast into a mould - the tundish is used for continuous casting: it allows having a buffer of molten steel available for casting between the completion of the pouring of one ladle and the opening of the next ladle.

[0054] A semi-finished product capable of being further hot-rolled having the above steel composition is provided. Particular attention should be paid during the refining and casting of said semi-finished product, in particular in the liquid phase, in order to control the inclusion population.

[0055] In a first embodiment, the molten steel refining process comprises the following steps:

[0056] - After decarburization of the pig iron by blowing oxygen under the converter, the molten steel is tapped into a ladle, no Al or any other deoxidizing element such as Si or Mn that would deoxidize the crude steel is added at this stage. This allows minimizing the subsequent nitrogen pick-up of the molten steel.

[0057] - The main alloying elements (in particular Mn, Si, Cr, Mo, Nb and B, but not Ti) are incorporated into the molten steel under vacuum, for example using a Ruhrstahl Heraeus (RH) vacuum degassing system or a Vacuum Tank Degasser (VTD). This allows, among other advantages, ensuring a low nitrogen content.

[0058] - A desulphurization step is carried out after said vacuum degassing step to reach the desired very low sulphur level. The desulphurization step involves an exchange between the molten steel and a molten slag formed by adding a flux (for example a CaO-based flux) to the heat. These fluxes can be added before the desulphurization step, for example during tapping after the converter.

[0059] - Ti is added after the desulphurization step. Ti is added, for example, by taking the measured nitrogen composition in order to add just the right amount of Ti to precipitate N in the form of TiN in the semi-finished product. For example, the amount of Ti added is equal to or slightly higher than 3.42 times the measured nitrogen amount after desulphurization, in weight percent.

[0060] - In order to minimize the amount of calcium aluminate which can be detrimental to the bending properties of the press hardened part, no Ca-containing addition (for example SiCa, FeCa or pure Ca) is made. Since a very low S level is achieved using the targeted composition and process, the produced steel will contain low MnS population.

[0061] - providing a minimum amount of time promoting the floating of inclusions. Inclusion floating refers to the phenomenon according to which inclusions in the liquid steel float on top of the slag covering the liquid steel due to their lower density than the steel. Once inclusions are trapped in the slag, they are removed from the liquid steel and will not be cast into semi-finished products, thus reducing the population of inclusions. The inventors found that the inclusion floating time is correlated to the cumulative surface fraction of oxides, MnS and TiNbCN in the surface layer of the steel sheet. The determination of the inclusion floating time depends on the specific process route and equipment used to manufacture the steel. For example, in the case of the use of a vacuum degasser for the addition of Mn, Si, Cr, Mo, Nb and B and further desulphurization of the liquid steel after the vacuum degasser, the inclusion floating time is the sum of:

[0062] - the time spent in the vacuum degasser after the addition of Mn, Si, Cr, Mo, Nb and B (this time is measured after the addition of the alloying elements because the addition of the elements itself can nucleate inclusion particles that will need to float to the slag),

[0063] - the time spent in the desulphurization step,

[0064] - the holding time between the desulphurization step and the continuous casting operation itself. The holding can include soft stirring with controlled inert gas injection after desulphurization, ladle transportation steps between the desulphurization station and the continuous casting operation, waiting time under the continuous casting step, etc. The continuous casting step starts when the ladle is opened to start pouring in the casting tundish.

[0065] In a second embodiment, the liquid steel refining process comprises the following steps:

[0066] - decarburization of the pig iron by blowing oxygen under the converter, followed by tapping of the liquid steel into a ladle. Optionally, a part of the alloying elements can be added at this stage, for example at least a part of the Mo, Cr and Mn content in the steel can be added.

[0067] - a desulphurization step is then performed to reach the desired very low sulphur level. The desulphurization step involves an exchange between the liquid steel and a slag formed by the addition of fluxes (for example CaO-based fluxes) to the melt. These fluxes can be added before the desulphurization step, for example during tapping after the converter.

[0068] - At this stage, the main alloying elements (in particular Mn, Si, Cr, Mo, Nb and B, but not Ti) are incorporated into the steel bath under vacuum, for example using a RH vacuum degassing system or a VTD. After the addition of the main alloying elements, the steel is stirred under vacuum, which is referred to as a stirring step. For example, when a RH vacuum degassing system is used, the stirring is naturally initiated in the system by the circulation of the steel within the snorkel of the vacuum vessel. When a VTD is used, the stirring can be initiated for example by bubbling argon within the steel. This stirring step serves both to homogeneously distribute the alloying elements within the steel and to promote the floating of inclusions.

[0069] - Ti is added at the end of the vacuum degassing process. For example, the amount of Ti added is added in order to add just the right amount of Ti to precipitate N in the form of TiN in the semi-finished product. For example, the amount of Ti added is equal to or slightly higher than 3.42 times the amount of nitrogen measured at the end of the stirring step, in weight percent.

[0070] - In order to minimize the amount of calcium aluminate, which can be detrimental to the bending properties of the press hardened part, no Ca-containing addition (for example SiCa, FeCa or pure Ca) is made. Since very low S levels are achieved using the targeted composition and process, the produced steel will contain low MnS clusters. The inventors found that with such low MnS content, the bending properties are very good even without Ca addition made in order to spheroidize the MnS clusters.

[0071] - As in the first embodiment, a minimum amount of time is provided to promote the floating of inclusions. In this second embodiment, where the steel is desulphurized under vacuum before the addition of the main alloying elements, the inclusion floating time is the sum of:

[0072] - the time spent in the vacuum degasser after the addition of Mn, Si, Cr, Mo, Nb and B,

[0073] - the holding time between the vacuum degasser and the continuous casting operation itself. Said holding can include a ladle transportation step between the vacuum degasser and the continuous casting operation, waiting time under the continuous casting step, etc. The continuous casting step starts when the ladle is opened to start pouring in the casting tundish.

[0074] More generally, it is preferred to refine the steel by using the main addition of Mn, Si, Cr, Mo, Nb and B under vacuum, for example using a vacuum degasser. This allows a low nitrogen content in the steel, which in turn allows a better control of the nitrogen-containing inclusions in the steel.

[0075] More generally, the inclusion floating time is defined as the total amount of time spent by the steel after the addition of Mn, Cr, Si, Mo, Nb and B and before the start of the casting step.

[0076] To control the inclusion surface fraction in the surface layer of the steel sheet, the inclusion floatation time should be controlled to be higher than a minimum inclusion floatation time tf. The value of tf will depend on the specific industrial setup used to produce the steel. It will depend on the production route in the steelmaking plant and on the geometry of the ladle used to handle the liquid steel. Since the inclusion floatation time is related to the fluid dynamics and the movement of small particles within the liquid steel, the minimum inclusion floatation time needed to reach the desired level of a specific inclusion in the surface layer of the steel will depend on the size of the ladle, its diameter, height, volume, etc. For example, the minimum inclusion floatation time is 60 minutes. For example, the minimum inclusion floatation time is 53 minutes.

[0077] To determine the minimum inclusion floatation time tf for a given steel composition and for a given industrial equipment and production route, it is proposed to use the following method:

[0078] - Perform several melts with the same chemical composition target.

[0079] - Generate said heats with different inclusion floatation times. For example, perform a set of melts with inclusion floatation times ranging from a minimum inclusion floatation time corresponding to the minimum feasible inclusion floatation time for the industrial route, and then apply incrementally longer inclusion floatation times, for example using a time increment of 10 minutes. For example, apply five different inclusion floatation times to five different heats.

[0080] - Process said heats according to the industrial route described below and characterize the inclusion population of the steel using the method described above.

[0081] - Record the surface layer cumulative surface fraction of oxides, MnS and TiNbCN and the corresponding inclusion floatation time. The inventors have found a correlation between the surface layer cumulative surface fraction of said oxides, MnS and TiNbCN and said inclusion floatation time. The longer the inclusion floatation time, the lower the surface layer cumulative surface fraction of oxides, MnS and TiNbCN. Determine the minimum inclusion floatation time tf as the inclusion floatation time above which the surface layer cumulative surface fraction of oxides, MnS and TiNbCN is equal to or lower than 75*10 -6 For example, the inventors have found that, when using a specific industrial equipment available to the inventors and applying the processing route of the first embodiment, the minimum inclusion floatation time is 60 minutes, preferably 53 minutes. This will be exemplified in the examples below.

[0082] After the liquid steel refining step, the method for manufacturing a steel sheet according to the application preferably comprises the following steps:

[0083] - casting the liquid steel into a semi-product suitable to be hot-rolled. During the casting step, particular attention should be paid to avoid oxygen pick-up and thus to avoid a high level of inclusions in the semi-product. For example, in the case of a continuous casting process in which the semi-product is a slab produced in a continuous sequence by casting in a mould the product of a plurality of heats poured into tundishes, specific refractory materials and linings can be used in the tundishes, specific distribution rules can be used for the first of the sequential slabs and for the transition slab between two different heats, etc.

[0084] - then optionally reheating the semi-product at a temperature comprised between 1150°C and 1300°C.

[0085] - then hot-rolling the steel sheet at a finish hot-rolling temperature comprised between 800°C and 950°C.

[0086] - then cooling the hot-rolled steel and coiling it at a temperature T 卷取 below 670°C, and optionally pickling to remove the oxidation.

[0087] - then optionally cold-rolling the coiled steel sheet to obtain a cold-rolled steel sheet. The cold-rolling reduction is preferably comprised between 20% and 80%. Below 20%, the recrystallization during the subsequent heat treatment is not favored, which can impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during the cold-rolling.

[0088] - in one embodiment of the application, the annealed steel sheet is heated to an annealing temperature TA comprised between 700°C and 850°C, and is maintained at said temperature TA for a holding time tA comprised between 10 seconds and 20 minutes.

[0089] - in one embodiment of the application, said annealed steel sheet is cooled to a temperature comprised between 400°C and 700°C, and is further coated with a metallic coating.

[0090] In summary, the above-described method preferably comprises the following sequential steps:

[0091] - producing a liquid steel having the above-described chemical composition, wherein during the liquid steel refining phase, Mn, Si, Cr, Mo, Nb and B are added using a vacuum degasser, and wherein a minimum inclusions floating time tf is ensured, said inclusions floating time being the total amount of time spent by the liquid steel after the addition of Mn, Si, Cr, Mo, Nb and B and before the beginning of the casting step, and said minimum inclusions floating time tf being defined as the minimum inclusions floating time required to reach a surface fraction of surface layer of oxides, MnS and TiNbCN equal to or lower than 75*10 -6 - casting said liquid steel to obtain a semi-product able to be hot-rolled,

[0092] - casting said liquid steel to obtain a semi-product able to be hot-rolled,

[0093] - optionally hot rolling the semi-finished product at a finish rolling temperature T of 800°C to 950°C, 再加热 - reheating the semi-finished product at a temperature T of 1100°C to 1300°C,

[0094] - hot rolling the semi-finished product at a finish rolling temperature T of 800°C to 950°C,

[0095] - coiling the hot-rolled steel sheet at a coiling temperature T of less than 670°C to obtain a coiled steel sheet, 卷取 - coiling the hot-rolled steel sheet at a coiling temperature T of less than 670°C to obtain a coiled steel sheet,

[0096] - optionally pickling the coiled steel sheet,

[0097] - optionally cold rolling the coiled steel sheet to obtain a cold-rolled steel sheet,

[0098] - optionally heating the hot-rolled steel sheet or the cold-rolled steel sheet to an annealing temperature T of 700°C to 850°C, A and holding the steel sheet at said temperature T for a holding time t of 10 seconds to 20 minutes, A to obtain an annealed steel sheet, A to obtain an annealed steel sheet,

[0099] - optionally cooling the annealed steel sheet to a temperature in the range of 400°C to 700°C,

[0100] - optionally coating the annealed steel sheet with a metallic coating,

[0101] - optionally cooling the coated steel sheet to room temperature.

[0102] The pressing part manufacturing process and the features of the pressed part that follows will now be detailed.

[0103] A steel blank is cut from the steel sheet according to the application and heated in an annealing furnace. Preferably, the steel blank is heated to a temperature of 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, then hot formed and subjected to a press hardening to obtain a pressed part.

[0104] The microstructure of the pressed part comprises more than 95% of martensite and less than 5% of bainite + ferrite in surface fraction. Moreover, the pressed part according to the application comprises a bulk portion and a top and bottom skin, where the skin occupies the outermost 10% of thickness on both sides of the bulk, said skin having a skin inclusion population where the cumulative surface fraction of oxides, MnS and TiNbCN is equal to or less than 75*10 -6 .

[0105] The compacted part according to the application has 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 strength and bending angle impart very good mechanical resistance to the part, especially in the event of a crash. They provide very good energy absorption capacity and intrusion resistance, thus improving the safety of the vehicle.

[0106] The application will now be illustrated by the following examples which are in no way limiting.

[0107] Tests were carried out on 5 different samples from 8 different heats A, B, C, D, E, F, G and H produced using an industrial production route. Samples II, I2, I3, I4, I5 and I6 according to the application, samples R1, R2 being reference samples.

[0108] Table 1 - Sample composition

[0109] The compositions tested are summarized in the following table, in which the element contents are expressed in weight percent:

[0110] Steel reference A B C D E F G H C 0.4 0.4 0.3 0.4 0.4 0.3 0.4 0.4 Mn 0.6 0.7 0.6 0.6 0.7 0.6 0.8 0.8 Si 0.5 0.5 0.5 0.6 0.5 0.5 0.6 0.5 Cr 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 Mo 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Nb 0.06 0.05 0.05 0.05 0.05 0.05 0.05 0.04 Al 0.03 0.03 0.05 0.04 0.03 0.04 0.05 0.03 Ti 0.013 0.009 0.010 0.011 0.009 0.012 0.013 0.014 B 0.0016 0.0018 0.0022 0.0020 0.0017 0.0016 0.0020 0.0018 P 0.012 0.012 0.011 0.011 0.012 0.011 0.009 0.012 Ca 0.0003 <0.0003 0.0004 0.0003 <0.0003 <0.0003 <0.0003 <0.0003 S 0.004 0.002 0.001 0.001 0.003 0.002 0.002 0.002 N 0.002 0.004 0.003 0.003 0.003 0.003 0.004 0.003 Ni 0.4 0.4 0.4 0.4 0.4 0.02 0.01 0.01

[0111] Table 2 - Steel plant process parameters and surface inclusions

[0112] The following process parameters were applied in the steelmaking shop and the following surface fraction of surface inclusions was observed - the values underlined are not according to the application:

[0113]

[0114] * RH = RH vacuum degasser process time after addition of Mn, Si and Cr

[0115] ** DS = desulphurization process time

[0116] *** CC = time spent between the end of desulphurization and the start of continuous casting (= ladle opening to pour into the casting tundish).

[0117] Table 3 - Additional process conditions

[0118] The following process parameters were applied along the production route:

[0119]

[0120] Table 4 - Microstructure, bending angle and tensile strength

[0121] The following microstructure, bending angle and tensile strength were measured on the samples, the values underlined are not according to the application:

[0122]

[0123] Table 4 shows that the samples according to the application (references II, I2, I3, I4, I5 and I6) have a tensile strength higher than 1800 MPa and a bending angle in the rolling direction normalized to 1.5 mm higher than 50° due to their specific composition and surface inclusions.

[0124] Referring to Table 2, there is a relationship between the inclusion floatation time and the surface fraction of oxides, MnS and TiNbCN in the surface layer. The inclusion floatation time represents the total amount of time the steel spends after the addition of Mn, Cr, Si, Mo, Nb and B and before the start of the continuous casting step.

[0125] The inventors found that when using the specific composition of the application and when the inclusion floatation time is increased above a minimum inclusion floatation time tf, the surface fraction of oxides, MnS and TiNbCN in the surface layer can be controlled below a critical level, which ensures a good bending resistance. In the industrial configuration used to produce the samples given in the current examples, the minimum inclusion floatation time tf is 53 minutes. The value of tf will depend on the specific industrial setup used to produce the steel.

[0126] When the inclusion floatation time is equal to or higher than tf = 60 minutes, the surface fraction of the oxides, MnS and TiNbCN fraction in the surface layer is equal to or lower than 75*10 -6 When the inclusion floatation time is lower than tf = 53 minutes, the surface fraction of the oxides, MnS and TiNbCN fraction in the surface layer is higher than 75*10 -6 .

[0127] The inventors found that the surface fraction of inclusions in the surface layer plays an important role in improving the resistance of the material to crack formation when the steel is subjected to bending loads. Surprisingly, this is not the case for all types of inclusions. For example, NbC inclusions seem to have no significant influence on the bending properties of the steel. On the other hand, the cumulative surface fraction of oxides, MnS and TiNbCN was found to play an important role in the bending properties. Reducing the cumulative surface fraction of oxides, MnS and TiNbCN helps to improve the bending properties.

[0128] Referring to Table 4, all the samples according to the application have a surface fraction of oxides, MnS and TiNbCN in the surface layer equal to or lower than 75*10 -6The surface fraction of the surface layer accumulation of oxides of Mn, MnS and TiNbCN of the samples (I1, I2, I3, I4, I5 and I6) according to the present application all have a bending angle in the rolling direction normalized to 1.5 mm of at least 50° and also have a tensile strength of at least 1800 MPa. On the other hand, the reference samples (R1, R2), while maintaining a tensile strength higher than 1800 MPa, all have a bending angle in the rolling direction normalized to 1.5 mm of less than 50°. Thus, the steel produced according to the present application will exhibit a better resistance to crack formation when subjected to loads, while exhibiting a very high tensile strength, which will improve the crash resistance and safety of the components produced using said material.

Claims

1. A steel sheet made of a steel having a composition comprising in weight percent: C: 0.3% to 0.4% Mn: 0.5% to 1.0% Si: 0.4% to 0.8% Cr: 0.1% to 1.0% Mo: 0.1% to 0.5% Nb: 0.01% to 0.1% Al: 0.01% to 0.1% Ti: 0.008% to 0.03% B: 0.0005% to 0.003% P≤0.020％ Ca < 0.0010% S≤0.004％ N≤0.005％ and optionally comprising: Ni < 0.5% the remainder of the composition being iron and unavoidable impurities resulting from the melting, said steel sheet having a microstructure comprising 60% to 95% ferrite in surface fraction, the rest being martensite-austenite islands, pearlite or bainite, said steel sheet comprising from the bulk of the coated steel sheet to the surface: - a bulk - the top of such a body is a skin occupying the outermost 10% thickness on both sides of the body, said skin having a population of surface inclusions of which the cumulative surface fraction of oxides, MnS and TiNbCN is equal to or less than 75*10 -6 -2.

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

3. A press-hardened steel part, said steel part having a composition comprising in weight percent: C: 0.3% to 0.4% Mn: 0.5% to 1.0% Si: 0.4% to 0.8% Cr: 0.1% to 0.4% Mo: 0.1% to 0.5% Nb: 0.01% to 0.1% Al: 0.01% to 0.1% Ti: 0.008% to 0.03% B: 0.0005% to 0.003% P≤0.020％ Ca < 0.001% S≤0.004％ N≤0.005％ and optionally comprising: Ni < 0.5% the remainder of the composition being iron and unavoidable impurities resulting from the melting, said steel part having a microstructure comprising more than 95% martensite and up to 5% bainite or ferrite in surface fraction, said steel part comprising from the bulk of the steel part to the surface: - a bulk - the top of such a body is a skin occupying the outermost 10% thickness on both sides of the body, said skin having a population of surface inclusions of which the cumulative surface fraction of oxides, MnS and TiNbCN is equal to or less than 75*10 -6 -2.

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

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

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