A HOT FORMING METHOD

Abstract

The invention relates to a hot forming method comprising the following steps: A. supplying a heat-treatable steel sheet, pre-coated with a zinc- or aluminum-based pre-coating for anti-corrosion purposes, B. deposition of a hydrogen barrier pre-coating over a thickness of 10 nm to 550 nm, C. discontinuous annealing of the pre-coated steel sheet in an inert atmosphere to obtain a pre-alloyed steel sheet, D. cutting the pre-alloyed steel sheet to obtain a blank, E. heat-treating the blank to obtain a fully austenitic microstructure in the steel, F. transferring the blank to a pressing tool, G. hot forming the blank to obtain a part, H.Cooling the part obtained in step G) to obtain a steel microstructure that is martensitic or martensitic-bainitic or that is composed of at least 75% in terms of volume fraction of equiaxed ferrite, 5% to 20% by volume of martensite and bainite in an amount less than or equal to 10% by volume.

Description

A HOT FORMING METHOD The present invention relates to a hot forming method comprising supplying a steel sheet coated with an anti-corrosion pre-coating, which is directly finished with a hydrogen barrier pre-coating that further inhibits hydrogen absorption, resulting in a part that has excellent resistance to late cracking. The invention is particularly suitable for the manufacture of automotive vehicles. Coated steel sheet for hot forming is sometimes called pre-coated; this prefix indicates that a transformation of the pre-coating's nature will take place during heat treatment prior to stamping. There may be more than one pre-coating. This invention discloses two pre-coatings. It is well known that certain applications, especially in the automotive sector, require metal structures that are lighter and more resistant to impact, while also possessing good tensile strength. For this purpose, steels with enhanced mechanical properties, such as hot and cold stamped steel, are commonly used. However, it is known that susceptibility to late cracking increases with mechanical strength, particularly after certain hot and cold forming operations, as high residual stresses may persist after deformation. In combination with the atomic hydrogen possibly present in the steel sheet, these stresses can lead to late cracking, meaning that cracking occurs at a certain point after the deformation itself. Hydrogen can progressively accumulate by diffusion in crystal lattice defects, such as matrix / inclusion interfaces, twin boundaries, and grain boundaries. In these latter defects, hydrogen can become damaging when it reaches a critical concentration after a certain time.This delay stems from the residual stress distribution field and the kinetics of hydrogen diffusion, given the low hydrogen diffusion coefficient at room temperature. Furthermore, hydrogen located at grain boundaries weakens cohesion and promotes the development of late intergranular cracks. Some parts are produced by pre-alloying an aluminum-based coated steel sheet and then hot forming the pre-alloyed coated steel sheet. These parts typically exhibit very poor hydrogen absorption during batch annealing and hot stamping. In fact, because batch annealing is performed over several hours, a significant amount of hydrogen can be absorbed, especially during this phase. Patent application EP3396010 discloses a method for manufacturing an Al-Fe alloy coated steel sheet for hot forming, the Al-Fe alloy coated steel sheet having high resistance to late hydrogen fracture and coating layer separation and high weldability, the method comprising: - to form an Al-Si coating layer on a surface of a base steel sheet, - heating the Al-Si coated base steel sheet to a maximum heat treatment temperature that varies between 450 °C and 750 °C at a heating rate of 1 °C / h to 500 °C / h in a heating furnace in which there is an atmosphere having a dew point of less than 10 °C; and - form an Al-Fe alloy coating layer on the surface of the base steel sheet by holding the base steel sheet coated with AI-SI at the maximum heat treatment temperature for 1 to 100 hours. The atmosphere of the batch annealing process and the heat treatment conditions are adjusted to obtain a specific microstructure and characteristics of Al-Fe to prevent late hydrogen fracture. In fact, this patent application discloses an aluminum-iron (Al-Fe) alloy coated steel sheet for hot forming, which has high resistance to late hydrogen fracture and coating layer separation and high weldability, the Al-Fe alloy coated steel sheet comprising a base steel sheet and an alloy coating layer formed between the base steel sheet and an oxide layer, wherein the alloy coating layer comprises: an Al-Fe alloy I layer formed on the base steel sheet and having a Vickers hardness of 200 Hv to 800 Hv; an Al-Fe alloy III layer formed in the Al-Fe alloy I layer and having a Vickers hardness of 700 Hv to 1200 Hv; and an Al-Fe alloy II layer formed in the Al-Fe alloy III layer continuously or discontinuously in a length direction of the steel sheet, and having a Vickers hardness of 400 Hv to 900 Hv, wherein an average oxygen content at a depth of 0.1 pm from a surface of the oxide layer is 20% or less by weight. However, in practice, obtaining a steel sheet coated with an aluminum-iron alloy that possesses the specific microstructure and characteristics is very difficult. In fact, a wide range of dew point and heating rates is reported. Consequently, there is a risk that the specific Al-Fe alloy coating will not be achievable across the entire range, resulting in significant research efforts to determine the correct parameters. Patent application EP2312005 discloses a method for producing aluminum-clad steel sheet for hot stamping with rapid heating, characterized by annealing the aluminum-clad steel sheet having an amount of aluminum plating deposition per side of 30 g / m2 to 100 g / m2 in a box annealing furnace as it is in a coil state, during which the annealing is by a combination of a holding time and annealing temperature in an interior region that includes the sides of a pentagon having five coordinate points (600 °C, 5 hours), (600 °C, 200 hours), (630 °C, 1 hour), (750 °C, 1 hour) and (750 °C, 4 hours) as vertices in an XY plane having the holding time and annealing temperature as its X and Y axes, and with the X axis expressed logarithmically.This patent application also discloses the aluminum-clad steel sheet for hot stamping with rapid heating obtained by the above method. The patent recommends conditions for performing batch annealing at 600 °C to 750 °C in an air atmosphere to reduce hydrogen in steel. However, the amount of hydrogen absorbed during batch annealing remains high. Therefore, the objective of the invention is to provide an easy-to-implement hot forming method that prevents hydrogen absorption in the pre-alloyed aluminum-based steel sheet and, consequently, in the hot-formed part. It aims to produce a part with excellent resistance to late cracking that can be achieved by hot forming methods that include hot forming. This objective is achieved by providing a hot forming method comprising the following steps: A. the supply of a steel sheet for heat treatment, pre-coated with a zinc or aluminum-based pre-coating for anti-corrosion purposes, B. the deposition of a hydrogen barrier precoat over a thickness of 10 nm at 550 nm, C. the discontinuous annealing of the pre-coated steel sheet in an inert atmosphere to obtain a pre-alloyed steel sheet, D. cutting the pre-alloyed steel sheet to obtain a blank, E. the heat treatment of the raw piece to obtain a fully austenitic microstructure in the steel, F. the transfer of the blank to a pressing tool, G. the hot forming of the raw piece to obtain a finished piece, H. Cooling the part obtained in step G) to obtain a steel microstructure that is martensitic or martensitic-bainitic or that is composed of at least 75% in terms of volume fraction of equiaxed ferrite, 5% to 20% by volume of martensite and bainite in an amount less than or equal to 10% by volume. In fact, without wishing to be limited by any theory, the inventors have surprisingly discovered that when the steel sheet is pre-coated with a hydrogen barrier pre-coating and when the batch annealing is performed in an inert atmosphere, hydrogen absorption in the steel sheet is reduced. It is believed that, due to the hydrogen barrier pre-coating, thermodynamically stable oxides form on its surface with low diffusion kinetics. These thermodynamically stable oxides reduce H2 absorption. Furthermore, it appears that when the batch annealing atmosphere is not oxidized, it further prevents hydrogen absorption because the pre-coating diffuses and oxidizes on the surface of the pre-coated steel sheet.Therefore, the zinc or aluminum-based precoatings and the hydrogen barrier oxidize on the surface of the pre-coated steel sheet, both acting as hydrogen barriers. In step A), the steel sheet used is composed of heat-treated steel such as cq Lcnn / zznz / E / YiAi as described in European standard EN 10083. It can have a tensile strength greater than 500 MPa, advantageously between 500 MPa and 2000 MPa before or after heat treatment. The composition by weight of the steel sheet is preferably as follows: 0.03% < C < 0.50%; 0.3% < Mn < 3.0%; 0.05% < Si < 0.8%; 0.015% < Ti < 0.2%; 0.005% < Al < 0.1%; 0% < Cr < 2.50%; 0% < S < 0.05%; 0% < P < 0.1%; 0% < B < 0.010%; 0% < Ni < 2.5%; 0% < Mo < 0.7%; 0% < Nb < 0.15%; 0% < N < 0.015%; 0% < Cu < 0.15%; 0% < Ca < 0.01%; 0% < W < 0.35%, the remainder being iron and unavoidable impurities from steelmaking. For example, the steel sheet is 22MnB5 with the following composition: 0.20% < C < 0.25%; 0.15% < Si < 0.35%; 1.10% < Mn < 1.40%; 0% < Cr < 0.30%; 0% < Mo < 0.35%; 0% < P < 0.025%; 0% < S < 0.005%; 0.020% < Ti < 0.060%; 0.020% < Al < 0.060%; 0.002% < B < 0.004%, the remainder being iron and unavoidable impurities from steelmaking. The steel sheet may be Usibor®2000 with the following composition: 0.24% < C < 0.38%; 0.40% < Mn < 3%; 0.10% < Si < 0.70%; 0.015% < Al < 0.070%; 0% < Cr < 2%; 0.25% < Ni < 2%; 0.020% < Ti < 0.10%; 0% < Nb < 0.060%; 0.0005% < B < 0.0040%; 0.003% < N < 0.010%; 0.0001% < S < 0.005%; 0.0001% < P < 0.025%; it being understood that the titanium and nitrogen content satisfies Ti / N > 3.42; and the contents of carbon, manganese, chromium and silicon satisfy: Mn Cr If the composition optionally comprises one or more of the following elements: 0.05% < Mo < 0.65%; 0.001% < W < 0.30%; 0.0005% < Ca < 0.005%, the remainder being iron and unavoidable impurities from steelmaking. cq Lcnn / zznz / E / YiAi For example, the steel sheet is Ductibor®500 with the following composition: 0.040% < C < 0.100%; 0.80% < Mn < 2.00%; 0% < Si < 0.30%; 0% < S < 0.005%; 0% < P < 0.030%; 0.010% < Al < 0.070%; 0.015% < Nb < 0.100%; 0.030% < Ti < 0.080%; 0% < N < 0.009%; 0% < Cu < 0.100%; 0% < Ni < 0.100%; 0% < Cr < 0.100%; 0% < Mo < 0.100%; 0% < Ca < 0.006%, the remainder being iron and unavoidable impurities from steelmaking. The steel sheet can be obtained by hot rolling and optionally cold rolling depending on the desired thickness, which can be, for example, between 0.7 mm and 3.0 mm. Optionally, in step A) the hydrogen barrier precoat comprises optional elements selected from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or B1, the weight content of each additional element being less than 0.3% by weight. Preferably in step A), the hydrogen barrier precoat comprises at least one element selected from: nickel, chromium, aluminum, magnesium and yttrium. Preferably in step A), the hydrogen barrier precoat consists of nickel and chromium; that is, the barrier precoat comprises nickel, chromium, and unavoidable impurities. Advantageously, the Ni / Cr weight ratio is between 1.5 and 9. In fact, without wishing to be limited by any theory, it is believed that this specific ratio further reduces hydrogen absorption during the austenitizing treatment. In another preferred embodiment, the hydrogen barrier precoating consists of nickel and aluminum, i.e., the hydrogen barrier precoating comprises Ni, Al and unavoidable impurities. In another preferred embodiment, the hydrogen barrier precoating consists of 50%, 75%, or 90% chromium by weight. More preferably, it consists of chromium; that is, the hydrogen barrier precoating comprises only Cr and unavoidable impurities. In another preferred embodiment, the hydrogen barrier precoat consists of magnesium at 50%, 75%, or 90% by weight. More preferably, it consists of magnesium; that is, the hydrogen barrier precoat comprises only Mg and unavoidable impurities. In another preferred embodiment, the hydrogen barrier precoating consists of nickel, aluminum and triium, i.e., the hydrogen barrier precoating comprises Ni, Al and Y, and unavoidable impurities. Preferably, in step A), the hydrogen barrier precoat has a thickness between 10 nm and 90 nm or between 150 nm and 250 nm. For example, the thickness of the hydrogen barrier precoat is 50 nm, 200 nm, or 400 nm. Without wishing to limit ourselves to any one theory, it appears that when the hydrogen barrier precoating is less than 10 nm, there is a risk of hydrogen being absorbed into the steel because the hydrogen barrier precoating does not sufficiently cover the steel sheet. When the hydrogen barrier precoating is above 550 nm, there appears to be a risk that the hydrogen barrier precoating will become more brittle and that hydrogen absorption will begin due to the brittleness of the barrier coating. In a preferred embodiment, the zinc- or aluminum-based precoating is aluminum-based and comprises less than 15.7% Si, less than 5.0.7% Fe, optionally 0.1.7% to 8.0.7% Mg, and optionally 0.1.7% to 30.0.7% Zn, the remainder being Al. For example, the zinc- or aluminum-based precoating is AluSi®. In another preferred embodiment, the zinc or aluminum precoat is zinc-based and comprises less than 6.07% Al, less than 6.07% Mg, the remainder being Zn. For example, the zinc or aluminum-based precoat is a zinc coating to obtain the following product: Usibor® GL The zinc or aluminum-based precoating may also comprise impurities and residual elements such as iron with a content of up to 5.0 7o, preferably 3.0 7o, by weight. Preferably, the precoats of step A) are deposited by physical vapor deposition, electrogalvanizing, hot-dip galvanizing, or roll coating. Preferably, the hydrogen barrier precoat is deposited by electron beam induced deposition or roll coating. Preferably, the zinc- or aluminum-based precoat is deposited by hot-dip galvanizing. Optionally, after the application of the pre-coatings, a finishing roll can be performed. This operation hardens the steel sheet and provides a surface roughness that facilitates subsequent forming. Degreasing and surface treatment can then be applied to improve, for example, adhesive bonding or corrosion resistance. Preferably, in step C), the discontinuous annealing is carried out at a temperature between 450 °C and 750 °C, preferably between 550 °C and 750 °C. Preferably, in step C), the inert gas is chosen from helium (He), neon (Ne), argon (Ar), nitrogen, hydrogen, or a mixture thereof. Advantageously, in step C), the heating rate of the batch annealing is above or equal to 5000 °C.h'1, more preferably between 10000 °Ch'1 and 15000 °C.It1 or between 20000 °C.lr1 and 35000 °Ch-1. Preferably, in step C), the cooling rate is less than or equal to 100°C.h'1. Preferably, the cooling rate has three cooling rates that vary from 1 °C.h'1 to 100 °C-1. Preferably, in step C), the discontinuous annealing is carried out for 1 to 100 hours. Next, the pre-alloyed steel sheet is cut to obtain a blank. The raw piece is subjected to heat treatment in a furnace with an inert atmosphere. Preferably, in steps C) and / or E), the dew point is below or equal to -10 °C, more preferably between -30 °C and -60 °C. In fact, without wishing to be limited by any theory, it is believed that when the dew point is in the above range, the thermodynamically stable oxide layer further reduces H2 absorption during heat treatment. Preferably, the heat treatment is carried out at a temperature between 800 °C and 970 °C. More preferably, the heat treatment is carried out at an austenitizing temperature (Tm) typically between 840 °C and 950 °C, preferably between 880 °C and 930 °C. Advantageously, the workpiece is held at this temperature for a holding time (Tm) of between 1 and 12 minutes, preferably between 3 and 9 minutes. During the heat treatment prior to hot forming, pre-coatings form an alloy layer that has high resistance to corrosion, abrasion, wear, and fatigue. At room temperature, the hydrogen absorption mechanism in steel differs from that at high temperatures, particularly during austenitizing. In fact, at high temperatures, water in the furnace typically dissociates on the steel sheet surface into hydrogen and oxygen. Without wishing to be limited by any single theory, it is believed that the hydrogen barrier precoating and the inert atmosphere of batch annealing can prevent the dissociation of water on the surface of the hydrogen barrier precoating and, consequently, prevent hydrogen diffusion through both precoatings. After heat treatment, the blank is transferred to a hot forming tool and hot formed at a temperature between 600°C and 830°C. Hot forming can be by hot stamping or rolling. Preferably, the blank is hot stamped. The blank is then cooled in the hot forming tool or subsequently transferred to a specific cooling tool. The cooling rate is controlled depending on the composition of the steel, so that the final microstructure after hot forming comprises mostly martensite, preferably contains martensite, or martensite and bainite, or is composed of at least 75% equiaxed ferrite, 5% to 20% martensite and bainite in an amount less than or equal to 10%. A hardened part having excellent resistance to late cracking according to the invention, therefore, is obtained by hot forming. Preferably, the part comprises a steel sheet pre-coated with a zinc- or aluminum-based pre-coating. This first pre-coating layer is directly finished with a hydrogen barrier coating and an oxide layer comprising thermodynamically stable oxides. This hydrogen barrier coating is alloyed by diffusion with the zinc- or aluminum-based pre-coating, which in turn alloys with the steel sheet. In fact, without wishing to be limited by any theory, it appears that iron from the steel sheet diffuses onto the surface of the hydrogen barrier pre-coating during heat treatment. Preferably, the thermodynamically stable oxides may respectively comprise Cr2O3; FeO; NiO; Fe2O3; FesCU, MgO, Y2O3 or a mixture thereof. If the zinc- or aluminum-based precoating is zinc-based, the oxides may also include ZnO. If the zinc- or aluminum-based precoating is aluminum-based, the oxides may also include Al₂O₃ and / or MgA₂ECU. Preferably, the thickness of the oxide layer is between 10 nm and 550 nm. Preferably, the part is a front rail, seat crossmember, side step, fender crossmember, front floor brace, rear floor crossmember, rear rail, B-pillar, door frame or underride guard. For automotive applications, after the phosphating step, the part is immersed in an electrophoretic coating bath. Typically, the phosphate layer is between 1 and 2 µm thick, and the electrophoretic coating layer is between 15 and 25 µm thick, preferably 20 µm or less. The electrophoretic layer provides additional corrosion protection. After the electrophoretic coating step, further paint layers can be applied, such as a primer, base coat, and topcoat. Before applying the electrophoretic coating to the part, the part is degreased and subjected to a phosphating process to ensure the adhesion of the cataphoresis. The invention will be explained from now on in essays carried out for informational purposes only. These are not limiting. Examples For all samples, the steel sheets are 22MnB5. The composition of the steel is as follows: C = 0.2252%; Mn = 1.1735%; P = 0.0126%; S = 0.0009%; N = 0.0037%; Si = 0.2534%; Cu = 0.0187%; Ni = 0.0197%; Cr = 0.180%; Sn = 0.004%; Al = 0.0371%; Nb = 0.008%; Ti = 0.0382%; B = 0.0028%; Mo = 0.0017%; As = 0.0023%; and V = 0.0284%. All steel sheets are pre-coated with a first anti-corrosion coating, hereinafter referred to as AluSi®. This pre-coating comprises 9% silicon by weight, 3% iron by weight, and the remainder is aluminum. It is applied by hot-dip galvanizing. Subsequently, two tests were pre-coated with a 2nd pre-coating comprising 80% Ni and 20% Cr deposited by magnetron sputtering. cq Lcnn / zznz / E / YiAi Example 1: Hydrogen Test This test is used to determine the amount of hydrogen absorbed during the austenitizing heat treatment of a hot forming method. Test 1 is a steel sheet pre-coated with a first pre-coating of AluSi® (25 ppm). It was then subjected to batch annealing at a temperature of 650 °C for 5 hours. The heating rate was 108,000 °C / h. The atmosphere for the batch annealing was nitrogen. Cooling after the batch annealing was carried out at a rate of 85 °C / h for 2 hours 20 minutes, 19 °C / h for 17 hours, and 2.5 °C / h for 8 hours. Test 2 is a steel sheet pre-coated with a 1st pre-coating of AluSi® (25 ppm) and a 2nd pre-coating comprising 80% Ni and 20% Cr. It was then subjected to batch annealing at a temperature of 650 °C for 5 hours. The heating rate was 10,800 °C / h. The atmosphere for the batch annealing was nitrogen. Cooling after the batch annealing was carried out at a rate of 85 °C / h for 2 hours and 20 minutes, 190 °C / h for 17 hours, and 2.5 °C / h for 8 hours. Test 3 involves a steel sheet pre-coated with a first pre-coating of AluSi® (25 ppm). It was then subjected to batch annealing at a temperature of 650 °C for 5 hours. The heating rate was 10,800 °C / h. The annealing atmosphere was air. Cooling after batch annealing was performed at a rate of 85 °C / h for 2 hours 20 minutes, 19 °C / h for 17 hours, and 2.5 °C / h for 8 hours. Test 4 is a steel sheet pre-coated with a 1st pre-coating of AluSi® (25 ppm) and a 2nd pre-coating comprising 80% Ni and 20% Cr. It was then subjected to batch annealing at a temperature of 650 °C for 5 hours. The heating rate was 10,800 °C / h. The atmosphere during batch annealing was air. Cooling after batch annealing was performed at a rate of 85 °C / h for 2 hours 20 minutes, 19 °C / h for 17 hours, and 2.5 °C / h for 8 hours. All samples were then cut and heated to 900 °C for 3 minutes. The atmosphere during heat treatment was air. The blanks were transferred to a pressing tool and hot-stamped to obtain parts of varying thickness. The parts were then cooled by immersion in warm water to achieve martensitic transformation hardening. Finally, the amount of hydrogen absorbed by the assays during heat treatment was measured by thermal desorption using a thermal desorption analyzer (TDA). For this purpose, each assay was placed in a quartz chamber and slowly heated in an infrared furnace under a nitrogen flow. The hydrogen / nitrogen released from the mixture was captured with a leak detector, and the hydrogen concentration was measured with a mass spectrometer. The results are shown in Table 1 below. cq Lcnn / zznz / E / YiAi PDO Precoating Tests Atmosphere Ni / Cr Ratio Precoat Thickness (nm) H2 Content (ppm by mass) 1 - N2 - - 0.6 2* Ni / Cr 80 / 20 n2 4 200 0.35 3 - air - - 0.9 4 Ni / Cr 80 / 20 air 4 200 0.6 *: example according to the invention. Test 2 according to the present invention releases a significantly lower amount of hydrogen compared to the comparative examples.

Claims

1. A hot forming method comprising the following steps: A. supplying a heat-treatable steel sheet pre-coated with a zinc- or aluminum-based anti-corrosion pre-coating, B. deposition of a hydrogen barrier pre-coating over a thickness of 10 nm to 550 nm, C. discontinuous annealing of the pre-coated steel sheet in an inert atmosphere to obtain a pre-alloyed steel sheet, D. cutting the pre-alloyed steel sheet to obtain a blank, E. heat-treating the blank to obtain a fully austenitic microstructure in the steel, F. transferring the blank to a pressing tool, G. hot forming the blank to obtain a part, H.Cooling the part obtained in step G) to obtain a steel microstructure that is martensitic or martensitic-bainitic or that is composed of at least 75% in terms of volume fraction of equiaxed ferrite, 5% to 20% by volume of martensite and bainite in an amount less than or equal to 10% by volume.

2. A hot forming method according to claim 1, wherein in step B), the hydrogen barrier precoat comprises at least one element selected from: nickel, chromium, magnesium, aluminum and yttrium.

3. A hot forming method according to claim 1 or 2, wherein in step B), the hydrogen barrier precoating consists of nickel and chromium; or nickel and aluminum; or magnesium; or chromium; or nickel, aluminum and triium.

4. A hot forming method according to any of claims 1 to 3, wherein in step A), the zinc or aluminum-based precoating is zinc-based and comprises less than 6.0% Al, less than 6.0% Mg, the remainder being Zn.

5. A hot forming method according to any of claims 1 to 3, wherein in step A), the zinc or aluminum-based precoating is aluminum-based and comprises less than 15% Si, less than 5.0% Fe, optionally 0.1% to 8.0% Mg and optionally 0.1% to 30.0% Zn, the remainder being Al.

6. A hot forming method according to any of claims 1 to 5, wherein in step C), the discontinuous annealing is performed at a temperature between 450 °C and 750 °C.

7. A hot forming method according to any of claims 1 to 6, wherein in step C), the discontinuous annealing heating rate is greater than or equal to 5000 °C.h1.

8. A hot forming method according to any of claims 1 cq Lcnn / zznz / E / YiAi to 7, wherein in step C), the cooling rate is less than or equal to 100 °Ch·1.

9. A hot forming method according to any of claims 1 to 8, wherein in step C), the discontinuous annealing is performed for 1 to 100 hours.

10. A hot forming method according to any of claims 1 to 9, wherein the inert gas is selected from helium (He), neon (Ne), argon (Ar), nitrogen, hydrogen or a mixture thereof.

11. A hot forming method according to any one of claims 1 to 10, wherein in step E) independently of each other, the atmosphere is inert or has an oxidizing power equal to or greater than that of an atmosphere consisting of 1% by volume of oxygen and equal to or less than that of an atmosphere consisting of 50% by volume of oxygen.

12. A method according to any of claims 1 to 11, wherein in step E), the atmosphere has a dew point below or equal to -10 °C.

13. A hot forming method according to any of claims 1 to 12, wherein in step E), the heat treatment is carried out at a temperature between 800 °C and 970 °C.

14. A hot forming method according to any of claims 1 to 13, wherein during step G), the hot forming of the blank at a temperature between 600 °C and 830 °C.