Coated steel sheet and high-strength press-hardened steel part, and manufacturing method thereof

Abstract

The present invention relates to a coated steel sheet and a press-hardened steel part having a composition, in weight percent, of 0.15-0.25% C, 0.5-1.8% Mn, 0.1-1.25% Si, 0.01-0.1% Al, 0.1-1.0% Cr, 0.01-0.1% Ti, 0.001-0.004% B, P≦0.020%, S≦0.010%, and N≦0.010%, with the remainder of the composition being iron and unavoidable impurities resulting from refining. The press-hardened steel part comprises a bulk having a microstructure containing, by surface fraction, more than 95% martensite and less than 5% bainite, a coating layer on the surface of the steel part, a ferrite interdiffusion layer between the coating layer and the bulk, and a composition represented by the following formula: (GW int / PAGS bulk )-1≧30% of the prior austenite grain size in the bulk. bulk Width of ferrite grains in the interdiffusion layer GW int Includes the ratio of

Description

[Technical Field]

[0001] The present invention relates to coated steel sheets 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 motor vehicles for anti-intrusion or energy absorbing functions.

[0003] For these types of applications, it is desirable to manufacture steel components that combine high mechanical strength, high impact resistance, and good corrosion resistance. Furthermore, one of the major challenges in the automotive industry is to reduce vehicle weight to improve fuel efficiency while maintaining safety requirements and protecting the global environment.

[0004] This weight reduction can be achieved, inter alia, through the use of steel components with a martensitic or bainitic / martensitic microstructure.

[0005] The publication, WO 2016 / 104881, relates to a hot-press-formed part used as a structural component for vehicles and the like, which requires impact resistance, more specifically, a tensile strength of 1300 MPa or more, and a manufacturing method thereof. It also relates to a method in which a steel material is heated to a temperature at which a single austenite phase can be formed, and then quenched and hot-formed using a die. To achieve these properties, the base steel sheet must contain a thin ferrite layer of less than 50 μm on the surface, and the size and density of carbides must be controlled. This ferrite layer in the substrate prevents microcracks formed on the coating layer from propagating to the base sheet, but results in poor bendability at bending angles of less than 70°.

[0006] WO 2018 / 179839 relates to a hot-pressed part obtained by hot-pressing a steel sheet having a microstructure that varies through its thickness, the hot-pressed part comprising a soft layer made of at least 90% ferrite, a transition layer made of ferrite and martensite, and a hard layer mainly composed of martensite, and having both high strength and high bendability. To obtain these properties, the cold-rolled steel sheet is annealed in an atmosphere with a dew point temperature of 50°C to 90°C, which may be harmful to aluminum alloy coatings. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2016 / 104881 [Patent Document 2] International Publication No. 2018 / 179839 Summary of the Invention [Problem to be solved by the invention]

[0008] It is therefore an object of the present invention to solve the above-mentioned problems and to provide a press-hardened steel part having a combination of high mechanical properties, such as a tensile strength TS of 1350 MPa or more and a bending angle of greater than 70°. Preferably, the press-hardened steel part according to the present invention has a yield strength YS of 1000 MPa or more.

[0009] Another object of the present invention is to provide a coated steel sheet that can be transformed into such press-hardened steel parts by hot forming. [Means for solving the problem]

[0010] The object of the present invention is achieved by providing a steel sheet as defined in claim 1. Another object is achieved by providing a method as defined in claim 2. Another object of the present invention is achieved by providing a press-hardened steel part as defined in claim 3. The steel part may also include the properties defined in any one of claims 4 to 6. Another object is achieved by providing a method as defined in claim 7.

[0011] The invention will now be described in detail and illustrated by way of example without introducing limitations, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0012] [Figure 1a] 1 shows a schematic cross-section of a coated steel sheet from Test 4, which is not according to the invention. [Figure 1b] 1 shows a schematic cross section of a press-hardened steel part from Test 4, which is not according to the present invention. [Figure 2a] 1 shows a schematic cross-section of a coated steel sheet from Test 5, which is not according to the invention. [Figure 2b] 1 shows a schematic cross-section of a press-hardened steel part from test 5, which is not according to the present invention. [Figure 3a] 1 shows a schematic cross-sectional view of coated steel sheets from tests 1 and 2, according to the present invention. [Figure 3b] 1 shows a schematic cross-section of press-hardened steel parts from Tests 1 and 2, according to the present invention. [Figure 4a] 1 shows a schematic cross-sectional view of a coated steel sheet from Test 3, according to the present invention. [Figure 4b] FIG. 1 shows a schematic cross-section of a press-hardened steel part from Test 3, according to the present invention. [Figure 5a] 1 shows a schematic cross section of a coated steel sheet from Test 9, which is not according to the invention. [Figure 5b] 1 shows a schematic cross section of a press-hardened steel part from test 9, which is not according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] The composition of the steel according to the invention will now be described, the contents being expressed in weight percent.

[0014] According to the present invention, the carbon content is 0.15% to 0.25% to ensure satisfactory strength. If the carbon content exceeds 0.25%, the weldability and bendability of the steel plate may be reduced. If the carbon content is less than 0.15%, the tensile strength will not reach the target value.

[0015] The manganese content is in the range of 0.5% to 1.8%. If the amount added exceeds 1.8%, the risk of center segregation increases, impairing bendability. If the amount added is less than 0.5%, the hardenability of the steel sheet decreases. Preferably, the manganese content is in the range of 0.8% to 1.5%.

[0016] According to the present invention, the silicon content is comprised between 0.1% and 1.25%. Silicon is an element involved in hardening in solid solution. Silicon is added to limit the formation of carbides. If the silicon content exceeds 1.25%, silicon oxide is formed on the surface, which impairs the coatability of the steel. Furthermore, the weldability of the steel sheet may be reduced. Preferably, the silicon content is between 0.2% and 1.25%. More preferably, the silicon content is between 0.3% and 1.25%.

[0017] The aluminum content is between 0.01% and 0.1%, as it is a very effective element for deoxidizing steel in the liquid phase during refining. When 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 between 0.01% and 0.05%.

[0018] According to the present invention, the chromium content is in the range of 0.1% to 1.0%. Chromium is a hardening element in solid solution and must be greater than 0.1%. The chromium content is less than 1.0% to limit workability issues and costs.

[0019] The titanium content is included between 0.01% and 0.1% to protect boron from the formation of BN. The titanium content is limited to 0.1% to avoid TiN formation.

[0020] According to the present invention, the boron content is comprised between 0.001% and 0.004%. Boron improves the hardenability of the steel. The boron content is not more than 0.004% to avoid the risk of breaking the slab during continuous casting.

[0021] Some elements can be optionally added.

[0022] Molybdenum content can be added optionally up to a maximum of 0.40%. Like boron, molybdenum improves the hardenability of the steel. Molybdenum is limited to 0.40% or less to limit costs.

[0023] According to the invention, niobium can be optionally added up to a maximum of 0.08% to improve the ductility of the steel. Additions above 0.08% increase the risk of forming NbC or Nb(C,N) carbides, which impairs the bendability. Preferably, the niobium content is not more than 0.05%.

[0024] Calcium can also be added as an optional element up to a maximum of 0.1%. The addition of Ca at the liquid stage allows for the formation of fine oxides that promote castability in continuous casting.

[0025] The remainder of the steel composition is iron and impurities resulting from refining. In this respect, P, S and N are considered as residual elements, which are unavoidable impurities. Their contents are less than 0.010% for S, less than 0.020% for P and less than 0.010% for N.

[0026] Next, the microstructure of the coated steel sheet according to the present invention will be described.

[0027] The cross section of the coated steel sheet of the present invention is shown schematically in Figures 3a and 4a. The coated steel sheet is covered with a decarburized layer (3) on top, which includes a ferrite layer (4) having a thickness of 1 µm to 100 µm, and includes a coating layer (1) and a bulk (2). Preferably, the thickness of the ferrite layer is 20 µm to 100 µm. More preferably, the thickness of the ferrite layer is 25 µm to 100 µm. More preferably, the thickness of the ferrite layer is 25 µm to 80 µm.

[0028] The bulk (2) of the coated steel sheet has a microstructure containing 60% to 90% ferrite in terms of surface fraction, with the remainder consisting of island martensite-austenite, pearlite, or bainite.

[0029] This ferrite is formed during intercritical annealing of cold rolled steel sheet. At the end of the soak, the remainder of the microstructure is austenite, which transforms to martensite-austenite islands, pearlite, or bainite during cooling of the steel sheet.

[0030] The decarburized layer present on top of the bulk is obtained during annealing of cold-rolled steel sheets by controlling the atmosphere in the furnace so that the dew point temperature is strictly set above -10°C and below 20°C.

[0031] The coated steel sheet according to the invention can be produced by any suitable production method, which can be defined by a person skilled in the art. However, it is preferable to use a method according to the invention, which comprises the following steps: The above-mentioned steel composition is provided to a semi-finished product which can be further hot rolled. The semi-finished product is reheated to a temperature in the range of 1150°C to 1300°C.

[0032] The steel sheet is then hot rolled at a finish hot rolling temperature in the range of 800°C to 950°C.

[0033] The hot rolled steel is then cooled and coiled at a temperature Tcoil below 670°C and optionally pickled to remove oxidation.

[0034] The coiled steel sheet is then optionally cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction is preferably between 20% and 80%. If it is less than 20%, recrystallization during the subsequent heat treatment is unfavorable, and the ductility of the steel sheet may be impaired. If it exceeds 80%, there is a risk of edge cracks occurring during cold rolling.

[0035] Next, the steel sheet is annealed at an annealing temperature T in the range of 700°C to 850°C in an HNx atmosphere containing 0% to 15% H. A Annealing to 10 seconds to 1200 seconds, including holding time t A over the annealing temperature T A The temperature is maintained at 700°C to obtain an annealed steel sheet. If the temperature is lower than 700°C, the rate of formation of the decarburized layer is too slow, and it is not possible to obtain a ferrite layer on top of it. A is 10 seconds or more to allow the formation of a ferrite layer, and 1200 seconds or less to limit the thickness of this ferrite layer.

[0036] During this annealing, the atmosphere in the furnace must be kept at a dew point temperature T strictly above -10 ° C and below +20 ° C in order to form the decarburized layer according to the invention. DP1 T DP1 If the temperature is below -10°C, the formation of the decarburized layer will be slow and the ferrite layer will not form on top of it. The bendability of the steel part will be too low. DP1 If the temperature is higher than 20°C, the surface of the steel sheet may be completely oxidized, which may impair the coating properties and mechanical properties of the steel sheet.

[0037] In one embodiment of the present invention, the annealed steel sheet is heated to an annealing temperature T2 comprised between 700 ° C and 850 ° C and maintained at said temperature T2 for a holding time t2 comprised between 10 seconds and 1200 seconds, and the atmosphere has a dew point T2 strictly higher than -10 ° C and lower than +20 ° C. DP2 It has.

[0038] The steel sheet is then coated with an aluminum alloy coating.

[0039] The microstructure of a press-hardened steel part according to the invention will now be described: A cross section of a press-hardened steel part is represented diagrammatically in Figures 3b and 4b.

[0040] The steel parts are continuously subjected to the following from the bulk to the surface of the steel parts: - a bulk (7) having a microstructure containing more than 95% martensite and less than 5% bainite at the surface fraction, -ferrite interdiffusion layer (6), -aluminum-based coating layer (5) Includes.

[0041] During heating of a steel blank cut from the steel plate according to the present invention, all microstructural elements in the bulk transform to austenite, and the ferrite in the decarburized layer transforms to austenite with a grain size wider than that of the bulk austenite. After hot forming, the steel part is then die-quenched. The interdiffusion layer grows from the former wide-grained austenite layer and therefore has a grain width larger than that of the prior austenite grain in the bulk. To improve the bendability of the steel plate without degrading its mechanical properties, PAGS is used. bulk Width of ferrite grains in the interdiffusion layer GW int The ratio is given by the following formula: (Golden Week int / PAGS bulk )-1≧30% Meet the following.

[0042] The width of the ferrite grain is the average distance between two parallel grain boundaries, and the grain boundaries are oriented in the thickness direction of the steel sheet. A , annealing time t A and dew point temperature T DP1 This combination allows for a large grain width in the interdiffusion layer. Furthermore, heating the steel blank before pressing allows for small PAGS in the bulk.

[0043] In one embodiment, the press-hardened steel part may further include a martensite layer with a carbon gradient between the bulk and the interdiffusion layer, as represented by (8) in Figure 4b. During heating of the steel blank, carbon diffuses from the bulk to the surface. The ferrite upper part of the decarburized layer then transforms into an austenite layer with a carbon gradient. During die quenching, this austenite layer with a carbon gradient transforms into a martensite layer with a carbon gradient.

[0044] The press-hardened steel parts according to the invention have a tensile strength TS of 1350 MPa or more and a bending angle of greater than 70°. The bending angle has been determined on the press-hardened parts according to the VDA 238-100 bending standard (normalized to a thickness of 1.5 mm).

[0045] In a preferred embodiment of the present invention, the yield strength YS is 1000 MPa or more.

[0046] TS and YS are measured according to ISO standard ISO 6892-1.

[0047] The press-hardened steel parts according to the invention can be produced by any suitable production method, which can be defined by a person skilled in the art. However, it is preferred to use a method according to the invention comprising the following steps: The coated steel sheet according to the present invention is cut to a predetermined shape to obtain a steel blank. The steel blank is then heated to a temperature in the range of 880°C to 950°C for 10 to 900 seconds to obtain a heated steel blank. The heated blank is then transferred to a forming press before being hot formed and die quenched.

[0048] The present invention will now be illustrated by the following examples, which are not intended to be limiting in any way. [Example]

[0049] Seven grades, whose compositions are summarized in Table 1, were cast into semi-finished products and processed into steel plates and then steel parts according to the processing parameters summarized in Table 2.

[0050] Table 1 - Composition The compositions tested are summarized in the table below, with elemental contents expressed as weight percent.

[0051] [Table 1]

[0052] Table 2 - Processing parameters The cast steel semi-finished products were reheated at 1200°C, hot rolled at a finish hot rolling temperature of 800-950°C, coiled at 550°C, and cold rolled at a reduction of 60%. A and heated to 500 K for a holding time of t in an HNx atmosphere containing 5% H with a controlled dew point. A The steel sheet was then cooled to a temperature of 560-700°C and then hot-dip coated with an aluminum-silicon coating containing 10% silicon.

[0053] Sample 3 was subjected to a second annealing at temperature T2 before coating, at which the steel sheet was maintained for a holding time t2 in an HNx atmosphere with 5% H2 and a controlled dew point. The following specific conditions were applied:

[0054] [Table 2]

[0055] The coated steel sheets were analyzed and the corresponding properties of the decarburized layer are summarized in Table 3.

[0056] Table 3 - Characteristics of the decarburized layer of coated steel sheets

[0057] [Table 3]

[0058] The coated steel sheet was then cut to obtain steel blanks, which were then heated at 900°C for 6 minutes and hot-formed. The steel parts were analyzed to determine the corresponding microstructures, the width (GW) of the ferrite grains in the interdiffusion layer, and the thickness (W). int and prior austenite grain size in the bulk PAGS bulk are summarized in Table 4. The mechanical properties are summarized in Table 5.

[0059] Table 4 - Microstructure of press-hardened steel parts

[0060] [Table 4]

[0061] The surface fraction, the width of the ferrite grains in the interdiffusion layer, and the PAGS are determined by the following method: specimens are cut from the press-hardened steel parts, polished, and etched with reagents known per se to reveal the microstructure, and then the cross-sections are examined using an optical microscope or a scanning electron microscope, for example a scanning electron microscope equipped with a field emission electron gun ("FEG-SEM") at a magnification of more than 5000 times combined with a BSE (backscattered electron) device.

[0062] Table 5 - Mechanical properties of press-hardened steel parts The mechanical properties of the tested samples were measured and are summarized in the table below:

[0063] [Table 5]

[0064] The examples show that the steel parts according to the invention, i.e. Examples 1 to 3, are the only steel parts that exhibit all the target properties thanks to their specific composition and microstructure.

[0065] Figure 3a shows a schematic cross-sectional view of the coated steel sheets of Tests 1 and 2. The processing parameters of the present invention, annealing temperature T A , annealing time t A and dew point temperature T DP1This combination makes it possible to obtain a decarburized layer (3) on which a ferrite layer (4) is formed.

[0066] The coated steel sheet is then hot formed. Figure 3b shows a schematic cross-section of the press-hardened steel parts of tests 1 and 2.

[0067] The grain width of the ferrite formed in the interdiffusion layer (6) is a legacy of the pure ferrite layer where austenite formation occurs during heating, and has a larger grain size. The interdiffusion layer grows with this larger austenite grain size. The grain width of the ferrite in the interdiffusion layer (6) is larger than the prior austenite grain size in the bulk (7), resulting in good bendability with a bending angle greater than 70°.

[0068] Figure 4a shows a schematic cross-sectional view of the coated steel sheet of Test 3. The processing parameters of the present invention, annealing temperature T A , annealing time t A and dew point temperature T DP1 This combination results in the formation of a decarburized layer (3) on top of which is a deeper layer of ferrite (4) than in Tests 1 and 2 due to the longer annealing time.

[0069] The coated steel sheet is then hot formed. Figure 4b shows a schematic cross section of the press-hardened steel from test 3.

[0070] The ferrite grain size in the interdiffusion layer (6) is larger than that of the pure ferrite layer where austenite formation occurs during heating of the steel part. The interdiffusion layer grows with these larger austenite grain sizes. The width of the ferrite grains in the interdiffusion layer (6) is larger than that of the prior austenite grain size in the bulk (7), resulting in good bendability with bending angles greater than 70°. Furthermore, due to the thick ferrite layer (3) in the coated steel sheet, a layer of martensite with a carbon gradient forms between the bulk and the interdiffusion layer in the press-hardened steel part, resulting in a tensile strength greater than 1350 MPa.

[0071] In test 4, the composition of the steel sheet is the same as in test 1 and according to the invention. In comparison with test 1, the dew point temperature during annealing of the steel sheet is too low to obtain a decarburized layer with an upper ferrite portion in the coated steel sheet. Figure 1a shows a schematic cross section of the coated steel sheet of these tests, with the coating layer (1) and the bulk (2).

[0072] The coated steel sheet is then hot formed. Figure 1b shows a schematic cross section of the press-hardened steel part from Test 4. Due to the absence of a ferrite layer, the width of the ferrite grains in the interdiffusion layer (6) is comparable to the prior austenite grain size in the bulk (7), resulting in a low bending angle of less than 70°.

[0073] In Test 5, the coated steel sheet has a decarburized layer without a ferrite layer on top, as shown in Figure 2a. The absence of the ferrite layer is due to the low dew point temperature T of -10°C. DP1 This is due to the fact that the rate of decarburization is slowed down.

[0074] The coated steel sheet is then hot formed. Figure 2b shows a schematic cross section of the press-hardened steel part from test 5. Due to the absence of a ferrite layer, the width of the ferrite grains in the interdiffusion layer (6) is comparable to the prior austenite grain size in the bulk (7), resulting in a low bending angle of less than 70°.

[0075] In Tests 6 and 7, the steel plates have a low carbon level of 0.14%. In Test 6, a low dew point temperature T of -35°C was used. DP1 This does not allow the growth of a decarburized layer and a ferrite layer in the coated steel sheet. In comparison, in Test 7, the steel sheet was annealed at the same temperature and for the same time as in Test 6, but at a dew point of -10°C. This higher dew point temperature allows the formation of a decarburized layer with a ferrite layer due to the low carbon level of the steel sheet. However, this low carbon level does not allow the desired mechanical properties to be obtained in the press-hardened steel part. In particular, the tensile strength is less than 1350 MPa.

[0076] In Test 8, the steel sheet has a low carbon level of 0.08%. This low carbon content, combined with the processing parameters, results in a decarburized layer of the coated steel sheet, without a ferrite layer. Nevertheless, the yield strength and tensile strength of the press-hardened steel part are not achieved due to the low carbon level.

[0077] In Test 9, the steel sheet was maintained at the soaking temperature for 3600 seconds, which resulted in the formation of a thicker ferrite layer in the decarburized layer on the coated steel sheet than in the previous tests. Figure 5a shows a schematic cross-section of the coated steel sheet in Test 9, with the coating layer (1), the decarburized layer (3), a thicker ferrite layer with a coarser grain size (4), and the bulk (2).

[0078] The coated steel sheet was then hot-formed. Figure 5b shows a schematic cross-section of the press-hardened steel part from Test 9. During heating, the bulk microstructure was austenitic, and the thick ferrite layer transformed into a layer of austenite with a carbon gradient. However, for ferrite layer thicknesses greater than 100 μm, a layer of ferrite remained between the interdiffused layer and the layer of austenite with a carbon gradient.

[0079] During die quenching of steel parts, the ferrite layer still exists and the austenite layer with a carbon gradient transforms into a martensite layer with a carbon gradient, resulting in a multiphase layer, which causes a decrease in yield strength and tensile strength.

[0080] In test 10, the steel plate has a carbon content higher than 0.25%. DP1 does not allow the growth of a decarburized layer, resulting in the absence of a ferrite layer in the coated steel sheet and low bending angles of less than 70° in the press-hardened parts.

Claims

1. In weight percent: C: 0.15-0.25% Mn: 0.5-1.8% Si: 0.1-1.25% Al: 0.01~0.1% Cr: 0.1-1.0% Ti: 0.01~0.1% B: 0.001-0.004% P≦0.020% S≦0.010% N≦0.010% Including, and optionally, in weight percent, Mo≦0.40% Nb≦0.08% Ca≦0.1% and The remainder consists of iron and unavoidable impurities resulting from smelting. A coated steel sheet made of steel having a composition, The coated steel sheet has the following properties from the bulk to the surface of the coated steel sheet: - a bulk with a microstructure containing a surface fraction of 60% to 90% ferrite, the remainder being islands of martensite-austenite, pearlite or bainite; - the bulk is covered with a decarburized layer on top of which is a ferrite layer having a thickness of 1 μm to 100 μm, - a coating layer made of aluminium or aluminium alloy Coated steel sheets, including:

2. 1. A method for producing coated steel sheet, comprising: The following successive steps: - casting a steel having the composition according to claim 1 to obtain a slab, - the slab is heated to a temperature T ｒｅｈｅａｔ reheating with - hot rolling the reheated slab at a finish hot rolling temperature of 800°C to 950°C to obtain a hot rolled steel sheet; Winding temperature T below −670°C ｃｏｉｌ coiling the hot-rolled steel sheet to obtain a coiled steel sheet; - optionally pickling the coiled steel sheet; - optionally cold rolling the coiled steel sheet to obtain a cold-rolled steel sheet; - Hot-rolled or cold-rolled steel sheet is annealed at a temperature T of 700 ° C to 850 ° C. Ａ and holding time t Ａ The steel sheet is heated to the annealing temperature T Ａ to obtain an annealed steel sheet, wherein the atmosphere is more than 0% and up to 15% H. ２ Including dew point T above -10°C and below +20°C ＤＰ１ 2. The steps of - cooling the annealed steel sheet to a temperature range of 560°C to 700°C; - coating the annealed steel sheet with an aluminum or aluminum alloy coating; - cooling the coated steel sheet to room temperature; A method comprising:

3. 1. A press-hardened steel part comprising, in weight percent: C: 0.15-0.25% Mn: 0.5-1.8% Si: 0.1-1.25% Al: 0.01~0.1% Cr: 0.1-1.0% Ti: 0.01~0.1% B: 0.001-0.004% P≦0.020% S≦0.010% N≦0.010% Including, and optionally, in weight percent, Mo≦0.40% Nb≦0.08% Ca≦0.1% and The remainder consists of iron and unavoidable impurities resulting from smelting. having a composition The steel part is provided with the following continuously from the bulk to the surface of the steel part: - a bulk with a microstructure comprising, by surface fraction, more than 95% martensite and less than 5% bainite; - ferrite interdiffusion layer, - an aluminium-based coating layer, Including, Prior austenite grain size in the bulk PAGS ｂｕｌｋ the width GW of the ferrite grains in the interdiffusion layer relative to ｉｎｔ The ratio of (GW ｉｎｔ / PAGS ｂｕｌｋ )-1≧30% Meet, press hardened steel parts.

4. 4. The press hardened steel part of claim 3, including a layer of martensite having a carbon gradient between the bulk and the ferrite interdiffused layer.

5. 5. Press-hardened steel part according to claim 3 or 4, having a tensile strength TS of 1350 MPa or more and a bending angle of more than 70°.

6. 6. A press hardened steel part according to claim 5, having a yield strength YS of 1000 MPa or greater.

7. A method for manufacturing a press hardened steel part according to any one of claims 3 to 6, comprising the following successive steps: - providing a coated steel sheet according to claim 1 or a coated steel sheet produced by the method according to claim 2, - cutting said coated steel sheet into a predetermined shape to obtain steel blanks; - heating the steel blank to a temperature of 880°C to 950°C for 10 seconds to 900 seconds to obtain a heated steel blank; - transferring the heated steel blank into a forming press; - hot forming the heated steel blank in said forming press to obtain a formed part, - die-quenching the molded part; Including, method.

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