Coated steel sheets and high-strength press-hardened steel parts, and their manufacturing methods

Abstract

We provide press-hardened steel parts with a tensile strength of 1350 MPa or more and a bending angle greater than 70°. [Solution] The present invention relates to coated steel sheets and press-hardened steel parts, comprising, by 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%, with the remainder being iron and unavoidable impurities. The press-hardened steel part comprises a bulk having a microstructure containing more than 95% martensite and less than 5% bainite in surface fraction, a coating layer on the surface of the steel part, a ferrite interdiffusion layer between the coating layer and the bulk, and the following formula, (GW int / PAGS bulk Prior austenite particle size PAGS in bulk satisfying -1 ≥ 30% bulk GW (width of ferrite grains in the interdiffusion layer) int It includes the ratio of .

Description

[Technical Field]

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

[0003] For these types of applications, it is desirable to manufacture steel components that possess 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 from an environmental perspective, without neglecting safety requirements.

[0004] This weight reduction can be achieved, in particular, by using steel components having a martensite or bainite / martensite microstructure.

[0005] The publication, International Publication No. 2016 / 104881, relates to hot-pressed parts used as structural components for vehicles and the like, requiring impact resistance, more specifically, having a tensile strength of 1300 MPa or more, and to a method for manufacturing them, as well as a method for heating steel to a temperature at which an austenite single phase can be formed, and then quenching and hot-forming it using a die. To obtain such properties, the base steel sheet must contain a thin ferrite layer of less than 50 μm on its surface, and the size and density of the carbides must be controlled. This ferrite layer in the substrate allows for the suppression of propagation of fine cracks formed on the plating layer to the base sheet, but results in low bendability with a bending angle of less than 70°.

[0006] International Publication No. 2018 / 179839 relates to a hot-pressed part obtained by hot-pressing a steel sheet having a microstructure that changes in the thickness direction, 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 such properties, the cold-rolled steel sheet is annealed in an atmosphere containing a dew point temperature of 50°C to 90°C, which may be detrimental to the aluminum alloy coating. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2016 / 104881 [Patent Document 2] International Publication No. 2018 / 179839 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Therefore, the object of the present invention is to solve the above-mentioned problems and provide a press-hardened steel part having a combination of high mechanical properties, including a tensile strength TS of 1350 MPa or more and a bending angle 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 obtain a coated steel sheet that can be deformed 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 the steel sheet according to claim 1. Another object is achieved by providing the method according to claim 2. Another object of the present invention is achieved by providing the press-hardened steel part according to claim 3. The steel part can also include the characteristics according to any one of claims 4 to 6. Another object is achieved by providing the method according to claim 7.

[0011] Here, referring to the accompanying drawings, the present invention will be described in detail and illustrated by way of examples without introducing any limitations.

Brief Description of the Drawings

[0012] [Figure 1a] A schematic cross-sectional view of the coated steel sheet of Test 4 is shown. This is not according to the present invention. [Figure 1b] A schematic cross-sectional view of the press-hardened steel part from Test 4 is shown. This is not according to the present invention. [Figure 2a] A schematic cross-sectional view of the coated steel sheet of Test 5 is shown. This is not according to the present invention. [Figure 2b] A schematic cross-sectional view of the press-hardened steel part from Test 5 is shown. This is not according to the present invention. [Figure 3a] A schematic cross-sectional view of the coated steel sheet of Tests 1 and 2 is shown. This is according to the present invention. [Figure 3b] A schematic cross-sectional view of the press-hardened steel part from Tests 1 and 2 is shown. This is according to the present invention. [Figure 4a] A schematic cross-sectional view of the coated steel sheet of Test 3 is shown. This is according to the present invention. [Figure 4b] A schematic cross-sectional view of the press-hardened steel part from Test 3 is shown. This is according to the present invention. [Figure 5a] A schematic cross-sectional view of the coated steel sheet of Test 9 is shown. This is not according to the present invention. [Figure 5b] A schematic cross-section of the press-hardened steel part from Test 9 is shown. This is not according to the present invention.

Embodiments for Carrying Out the Invention

[0013] Next, the composition of the steel according to the present invention will be described, with the content expressed as a weight percentage.

[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 decrease. If the carbon content is less than 0.15%, the tensile strength will not reach the target value.

[0015] The manganese content is 0.5% to 1.8%. If the amount added exceeds 1.8%, the risk of central segregation increases and the flexibility is impaired. If it is less than 0.5%, the hardenability of the steel sheet decreases. Preferably, the manganese content is 0.8% to 1.5%.

[0016] According to the present invention, the silicon content is 0.1% to 1.25%. Silicon is an element involved in hardening in solid solutions. Silicon is added to limit the formation of carbides. If the content exceeds 1.25%, silicon oxide forms on the surface, impairing the coating properties of the steel. Furthermore, the weldability of the steel sheet may decrease. Preferably, the silicon content is 0.2% to 1.25%. More preferably, the silicon content is 0.3% to 1.25%.

[0017] The aluminum content is 0.01% to 0.1% because it is a very effective element for deoxygenating the steel in the liquid phase during refinement. If the titanium content is insufficient, aluminum can protect boron. The aluminum content is less than 0.1% to avoid oxidation problems and ferrite formation during press hardening. Preferably, the aluminum content is 0.01% to 0.05%.

[0018] According to the present invention, the chromium content is 0.1% to 1.0%. Chromium is an element involved in hardening in the solid solution and must be higher than 0.1%. The chromium content is less than 1.0% to limit processability issues and costs.

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

[0020] According to the present invention, the boron content is 0.001% to 0.004%. Boron improves the hardenability of steel. The boron content is 0.004% or less to avoid the risk of slab breakage during continuous casting.

[0021] Several elements can be added as desired.

[0022] The molybdenum content can be optionally added up to a maximum of 0.40%. Similar to boron, molybdenum improves the hardenability of steel. Molybdenum content is kept below 0.40% to limit costs.

[0023] According to the present invention, niobium can be optionally added up to a maximum of 0.08% to improve the ductility of the steel. If the amount added exceeds 0.08%, the risk of formation of NbC or Nb(C,N) carbides increases, impairing the bendability. Preferably, the niobium content is 0.05% or less.

[0024] Calcium can also be added as any element up to a maximum of 0.1%. The addition of Ca in the liquid stage allows for the formation of fine oxides, which promotes castability in continuous casting.

[0025] The remainder of the steel's composition consists of iron and impurities resulting from the refining process. In this regard, P, S, and N are considered residual elements that are at least unavoidable impurities. Their content is 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] A cross-section of the coated steel sheet of the present invention is schematically shown in Figures 3a and 4a. The coated steel sheet is covered on the upper part with a decarburized layer (3) containing a ferrite layer (4) having a thickness of 1 μm to 100 μm, and also includes a coating layer (1) and 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 contains 60% to 90% ferrite by surface fraction, and the remainder has a microstructure consisting of island-like martensite-austenite, pearlite, or bainite.

[0029] This ferrite is formed during the transformation section annealing of the cold-rolled steel sheet. The remaining microstructure is austenite at the end of soaking, which transforms into island martensite-austenite, pearlite, or bainite during the cooling of the steel sheet.

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

[0031] The coated steel sheet according to the present invention can be manufactured by any suitable manufacturing method, which can be specified by those skilled in the art. However, it is preferable to use the method according to the present invention, which includes the following steps: The above-mentioned steel composition is provided to the semi-finished product that can be further hot-rolled. The semi-finished product is reheated at a temperature including 1150°C to 1300°C.

[0032] Next, the steel sheet is hot-rolled at a finishing hot-rolling temperature ranging from 800°C to 950°C.

[0033] Next, the hot-rolled steel is cooled, wound up in a Tcoil at a temperature of less than 670°C, and optionally pickled to remove oxidation.

[0034] Next, the rolled steel sheet is optionally cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction ratio is preferably between 20% and 80%. If it is less than 20%, recrystallization during subsequent heat treatment is undesirable and may impair the ductility of the steel sheet. If it exceeds 80%, there is a risk of edge cracking occurring during cold rolling.

[0035] Next, the steel plate is annealed in an HNx atmosphere containing 0% to 15% H2 at an annealing temperature T between 700°C and 850°C. A Anneal until t, holding time including 10 seconds to 1200 seconds. A Over the annealing temperature T A Maintain the temperature to obtain an annealed steel sheet. Below 700°C, the formation rate of the decarburized layer is too slow, and a ferrite layer cannot be obtained on top of it. Holding time t A The time interval is 10 seconds or longer to allow the formation of the ferrite layer, and 1200 seconds or less to limit the thickness of this ferrite layer.

[0036] During this annealing process, the atmosphere inside the furnace is strictly above -10°C and below +20°C, with a dew point temperature T, in order to form the decarburized layer according to the present invention. DP1 It is controlled to have T DP1 If the temperature is below -10°C, the formation of the decarburized layer is delayed, and a ferrite layer does not form on top of it. The bendability of the steel part becomes too low. DP1 If the temperature is higher than 20°C, the surface of the steel sheet may oxidize completely, potentially impairing the coating properties and mechanical characteristics of the steel sheet.

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

[0038] Next, the steel plate is covered with an aluminum alloy coating.

[0039] Next, the microstructure of the press-hardened steel part according to the present invention will be described. The cross-section of the press-hardened steel part is schematically shown in FIGS. 3b and 4b.

[0040] The steel part continuously has the following from the bulk to the surface of the steel part: - A bulk (7) having a microstructure containing more than 95% martensite and less than 5% bainite in surface fraction, - A ferrite interdiffusion layer (6), - An aluminum-based coating layer (5) is included.

[0041] During the heating of the steel blank cut out from the steel sheet according to the present invention, all the microstructure elements of the bulk transform into austenite, and the ferrite in the decarburized layer transforms into austenite having a grain size wider than that of the austenite in the bulk. After hot forming, the steel part is then die quenched. Since the interdiffusion layer grows from the former wide-grained austenite layer, it has a grain width larger than the grain size of the prior austenite in the bulk. In order to improve the bendability of the steel sheet without degrading the mechanical properties, the ratio of the width GW of the ferrite grains in the interdiffusion layer to the prior austenite grain size PAGS in the bulk bulk satisfies the following formula int for (GW int / PAGS bulk ) - 1 ≧ 30% is satisfied.

[0042] The width of the ferrite grains is the average distance between two parallel grain boundaries, and the grain boundaries are oriented in the thickness direction of the steel sheet. The combination of the annealing temperature T A , the annealing time t A and the dew point temperature T DP1 according to the present invention makes it possible to obtain a large grain width in the interdiffusion layer. Furthermore, the heating of the steel blank before press forming makes it possible to obtain a small PAGS in the bulk.

[0043] In one embodiment, the press-hardened steel part may further include a martensite layer having a carbon gradient between the bulk and the interdiffusion layer, as shown in (8) of 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 having a carbon gradient. During die quenching, this austenite layer having a carbon gradient transforms into a martensite layer having a carbon gradient.

[0044] The press-hardened steel parts according to the present invention have a tensile strength (TS) of 1350 MPa or more and a bending angle greater than 70°. The bending angle is determined in the press-hardened parts according to the VDA238-100 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 present invention can be manufactured by any suitable manufacturing method, which can be specified by those skilled in the art. However, it is preferable to use the method according to the present invention, which includes the following steps: The coated steel sheet according to the present invention is cut into a predetermined shape to obtain a steel blank. Next, the steel blank is heated to a temperature including 880°C to 950°C for 10 to 900 seconds to obtain a heated steel blank. Then, the heated blank is transferred to a forming press before hot forming and die quenching.

[0048] The present invention will be explained below with reference to the following examples, but these are by no means limiting. [Examples]

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

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

[0051] [Table 1]

[0052] Table 2 - Processing Parameters The cast steel semi-finished product was reheated at 1200°C, hot-rolled at a finish hot-rolling temperature including 800-950°C, coiled at 550°C, and cold-rolled with a reduction ratio of 60%. The steel sheet was then subjected to temperature T A The sample is heated to a controlled dew point in an HNx atmosphere containing 5% H2, and held for a duration of t. A The temperature was maintained for a period of time. The steel plate was then cooled to a temperature of 560-700°C, and then hot-dip plated with an aluminum-silicon coating containing 10% silicon.

[0053] Sample 3 was subjected to a second annealing at temperature T2 before coating, and the steel sheet was maintained at the T2 temperature 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] Next, the coated steel sheet was cut to obtain a steel blank, which was then heated at 900°C for 6 minutes and hot-formed. The steel parts were analyzed to determine the corresponding microstructure and the width of the ferrite grains in the interdiffusion layer (GW). int and prior austenite particle size PAGS in bulk bulk The above is 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 ferrite grains in the interdiffusion layer, and PAGS are determined by the following method: a specimen is cut from a press-hardened steel part, polished, and etched with a reagent known to the extent of the microstructure. The cross-section is then 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") with a magnification of more than 5000x in combination 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 summarized in the table below:

[0063] [Table 5]

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

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

[0066] Next, the coated steel sheet is hot-formed. Figure 3b shows a schematic cross-sectional view of the press-hardened steel parts from Tests 1 and 2.

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

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

[0069] Next, the coated steel sheet is hot-formed. Figure 4b shows a schematic cross-sectional view of the press-hardened steel from Test 3.

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

[0071] In Test 4, the composition of the steel sheet is the same as in Test 1 and according to the present invention. Compared to Test 1, the dew point temperature during annealing of the steel sheet is too low, making it impossible to obtain a decarburized layer with an upper ferrite portion in the coated steel sheet. Figure 1a shows a schematic cross-sectional view of the coated steel sheet in these tests, having a coating layer (1) and bulk (2).

[0072] Next, the coated steel sheet is hot-formed. Figure 1b shows a schematic cross-sectional view of the press-hardened steel part from Test 4. Because there is no ferrite layer, the width of the ferrite grains in the interdiffusion layer (6) is equivalent to the prior austenite grain size of 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 schematically shown in Figure 2a. The absence of a ferrite layer is due to the low dew point temperature T of -10°C. DP1 This is due to a process that slows down the rate of decarburization.

[0074] Next, the coated steel sheet is hot-formed. Figure 2b shows a schematic cross-sectional view of the press-hardened steel part from Test 5. Because there is no ferrite layer, the width of the ferrite grains in the interdiffusion layer (6) is equivalent to the prior austenite grain size of the bulk (7), resulting in a low bending angle of less than 70°.

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

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

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

[0078] Next, the coated steel sheet was hot-formed, and Figure 5b shows a schematic cross-sectional view of the press-hardened steel part from Test 9. During heating of the steel part, the bulk microstructure is austenite, and the thick ferrite layer transforms into an austenite layer with a carbon gradient. However, due to the thickness of the ferrite layer, which is greater than 100 μm, the ferrite layer remains between the interdiffusion layer and the austenite layer with a carbon gradient.

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

[0080] In Test 10, the steel plate had a carbon content higher than 0.25%. Low dew point temperature T -40℃ DP1 This prevents the growth of the decarburized layer, resulting in the absence of a ferrite layer in the coated steel sheet and a low bending angle of less than 70° in press-hardened parts.

Claims

1. In terms of weight percentage, the following: 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% Includes, And optionally, in weight percentage, the following: Mo ≤ 0.40% Nb ≤ 0.08% Ca ≤ 0.1% Contains one or more of the elements, A coated steel sheet made of steel having a certain composition, The remainder of the composition consists of iron and unavoidable impurities resulting from smelting. The coated steel sheet has the following characteristics from the bulk to the surface: - A bulk material having a microstructure in which 60% to 90% ferrite is contained in surface fraction, with the remainder being island-like martensite-austenite, pearlite, or bainite. - The bulk material is covered with a decarburized layer containing a ferrite layer having a thickness of 1 μm to 100 μm on top. - A coating layer made of aluminum or aluminum alloy. Coated steel sheet, including

2. A method for manufacturing coated steel sheets, The following sequence of steps, - A step of obtaining a slab by casting steel having the composition described in claim 1, - The slab is heated to a temperature T including 1100°C to 1300°C. ｒｅｈｅａｔ Then, the step of reheating, The step of hot-rolling the slab, which has been reheated to a finish hot-rolling temperature including -800°C to 950°C. Winding temperature T below -670℃ ｃｏｉｌ The process involves winding the hot-rolled steel sheet to obtain a wound steel sheet. -Optionally, a step of pickling the rolled steel sheet, -Optionally, cold-roll the rolled steel sheet to obtain a cold-rolled steel sheet. - Hot-rolled or cold-rolled steel sheet annealed at an annealing temperature T including 700°C to 850°C Ａ Heat to a certain temperature, and hold for t including 10 seconds to 1200 seconds. Ａ The steel plate is heated to the temperature T Ａ The step of obtaining an annealed steel sheet by maintaining the atmosphere in such a state, wherein the atmosphere contains 0% to 15% H2 and has a dew point T that is strictly above -10°C and below +20°C. ＤＰ１ Steps, - A step of cooling the annealed steel sheet to a temperature range of 560°C to 700°C. - The step of coating the annealed steel sheet with an aluminum or aluminum alloy coating. - A step of cooling the coated steel plate to room temperature. Methods that include...

3. Press-hardened steel parts, in weight percentage, as follows: 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% Includes, And optionally, in weight percentage, the following: Mo ≤ 0.40% Nb ≤ 0.08% Ca ≤ 0.1% Contains one or more of the elements, Having composition, The remainder of the composition consists of iron and unavoidable impurities resulting from smelting. The steel component is continuous from the bulk to the surface of the steel component as follows: - Bulk having a microstructure containing more than 95% martensite and less than 5% bainite in surface fraction, - Ferrite interdiffusion layer, - Aluminum-based coating layer, Includes, Prior austenite particle size PAGS in the bulk ｂｕｌｋ GW of the width of the ferrite grains in the interdiffusion layer relative to the width of the ferrite grains in the interdiffusion layer. ｉｎｔ The ratio is given by the following formula: (GW ｉｎｔ / PAGS ｂｕｌｋ )-1≧30% Press-hardened steel parts that meet the requirements.

4. The press-hardened steel part according to claim 3, further comprising a layer of martensite having a carbon gradient between the bulk and the ferrite interdiffusion layer.

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

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

7. A method for manufacturing a press-hardened steel part according to any one of claims 3 to 6, comprising the following consecutive steps: - A step of providing a steel sheet having the composition described in claim 1, or manufactured by the method described in claim 2, - In order to obtain a steel blank, the steel plate is cut into a predetermined shape. - A step of heating a steel blank to a temperature including 880°C to 950°C for 10 to 900 seconds to obtain a heated steel blank. - The step of transferring the heated blank to the forming press, - A step of obtaining a molded part by hot forming a blank heated in the molding press, - Step of die-quenching the molded part. Methods that include...

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