A hot molding method

Abstract

The present disclosure relates to a hot forming method comprising the following steps: a. providing a steel sheet for heat treatment which is optionally pre-coated with a zinc- or aluminum-based pre-coat, b. depositing a hydrogen barrier pre-coat comprising chromium and free of zinc nickel to a thickness of from 10 to 550 nm, c. cutting the pre-coated steel sheet to obtain a blank sheet, d. Thermally treating the raw sheet at a furnace temperature of 800 to 970 degrees Celsius, during a dwell time of 1 to 12 minutes, in an atmosphere having an oxidizing power equal to or greater than an atmosphere containing 1% by volume of oxygen and equal to or less than an atmosphere containing 50% by volume of oxygen, such atmosphere having a dew point between -30 and +30 degrees Celsius,\ne. Transferring said sheet to a forming machine,\nand. Hot forming the sheet at a temperature between 600 and 830 degrees Celsius to obtain the part,\ng. Cooling the piece obtained in step (f) to obtain a microstructure in the steel that is martensitic or martensito-bainite or made of at least 75% by volume of equiaxed ferrite, from 5 to 20% by volume of martensite and bainite in an amount less than or equal to 10% by volume.

Description

A hot molding method The present invention relates to a hot forming method comprising providing a steel sheet for heat treatment which is coated with a barrier coating. This hydrogen barrier pre-coating more effectively prevents hydrogen absorption and improves the resistance to delayed fracture. The invention is particularly suitable for the production of automobiles and vehicles. Coated steel sheet for hot forming is sometimes called "precoated". This prefix indicates that a change in the nature of the precoat occurs during heat treatment and prior to forming. More than one precoat is possible. The present invention discloses one and optionally two precoats. We know that some applications, especially in the automotive sector, require metal structures to be lighter, more robust and have the desired ductility for crashworthiness. For this purpose, steels with improved mechanical properties are usually used, and such steels are formed by hot or cold forming. However, it is known that following some cold forming and hot forming operations, the susceptibility to delayed fracture increases with the mechanical strength because residual stresses tend to remain after deformation. In combination with atomic hydrogen that may be present in the steel sheet, these stresses may lead to delayed fracture that causes cracking at a certain time after the deformation itself. Hydrogen may accumulate progressively by penetrating into crystal lattice defects such as matrix / void interfaces, twin boundaries and grain boundaries. In these defects, hydrogen may become harmful if a critical concentration is reached and after a certain time has passed. This delay is due to the residual stress distribution area and also to the kinetics of hydrogen diffusion, given that the hydrogen diffusion coefficient is low at ambient temperature. In addition, hydrogen located at the grain boundaries weakens their cohesion and causes delayed bond cracks to appear. We know that hot forming is critical for hydrogen absorption and increases susceptibility to delayed fracture. Absorption may occur in the austenitizing heat treatment, which is the thermal step prior to hot forming itself. The absorption of hydrogen into the steel is actually dependent on the metallurgical phase. Furthermore, at high temperatures, water in the furnace dissociates into hydrogen and oxygen at the surface of the steel sheet. Document WO2017 / 187255 discloses a pre-coating having a barrier effect for preventing hydrogen absorption, in particular during heat treatment prior to hot forming. This hydrogen barrier pre-coating comprises nickel and chromium, in which the weight ratio of Ni / Cr is between 1.5 and 9. This patent application discloses a heat treatment atmosphere which is an inert atmosphere or an atmosphere comprising air. All examples are carried out in an atmosphere containing nitrogen. According to WO2020 / 070545, the thermal treatment prior to hot forming may be carried out in an atmosphere having an oxidizing power equal to or greater than an atmosphere containing 1% by volume of oxygen and equal to or less than an atmosphere containing 50% by volume of oxygen, such atmosphere having a dew point between -30 and +30°C to further reduce hydrogen absorption. In both patent applications, although hydrogen absorption is improved during austenitizing heat treatment, this is not sufficient to achieve a part with excellent resistance to delayed fracture. In fact, even if the pre-coating barrier reduces hydrogen absorption, a small amount of hydrogen molecules are still absorbed by the steel sheet. Therefore, the object of the present invention is to provide a hot forming method in which hydrogen absorption into the steel sheet is prevented. The object of the present invention is to provide a part with excellent resistance to delayed fracture, which is achieved through the said hot forming method including hot forming. This goal is achieved by providing a hot molding method that includes the following steps: A. Providing a steel sheet for heat treatment which is optionally pre-coated with a zinc- or aluminum-based pre-coating, B. Deposition of a chromium-containing, nickel-free hydrogen barrier pre-coating with a thickness of 10 to 550 nm, C. Cutting pre-coated steel sheet to obtain a raw sheet, D. Thermal treatment of the raw sheet at a furnace temperature of 800 to 970°C, for a dwell time of 1 to 12 minutes, in an atmosphere having an oxidizing power equal to or greater than an atmosphere containing 1% oxygen by volume and equal to or less than an atmosphere containing 50% oxygen by volume, such atmosphere having a dew point between -30 and +30°C. E. Transferring the said sheet to a molding machine F. Hot forming of the sheet at a temperature between 600 and 830 degrees Celsius to obtain the part, G. Cooling the part obtained in step (f) to obtain a microstructure in the steel that is martensitic or martensito-bainite or made of at least 75% by volume of equiaxed ferrite, from 5 to 20% by volume of martensite and bainite in an amount less than or equal to 10% by volume. In fact, the inventors unexpectedly found that when the steel sheet is precoated with a hydrogen barrier precoat containing chromium and containing no nickel and when the austenitizing heat treatment is carried out in the atmosphere mentioned above, this barrier effect of the precoat is further improved and even further prevents the absorption of hydrogen into the steel sheet. In contrast to the nitrogen atmosphere with which a thinner layer of selective oxides is formed on the surface of the hydrogen barrier precoat during the austenitizing heat treatment, it is believed that thermodynamically stable oxides are formed on the surface of the barrier precoat with a low kinetics. In the above-mentioned atmosphere, it is believed that the hydrogen barrier pre-coating comprising chromium and lacking nickel provides a greater reduction in hydrogen absorption compared to the hydrogen barrier pre-coating comprising nickel and chromium. In fact, it is believed that chromium forms a thicker oxide layer than that formed by nickel and chromium. Without being limited by theory, it is believed that the hydrogen barrier pre-coating comprising chromium and lacking nickel is capable of preventing water decomposition on the surface of the hydrogen barrier pre-coating and also prevents hydrogen from permeating through the hydrogen barrier pre-coating. With an atmosphere having an oxidizing power equal to or higher than an atmosphere containing 1% by volume oxygen and equal to or lower than an atmosphere containing 50% by volume oxygen, it is believed that thermodynamically stable oxides provide greater inhibition of water decomposition. One of the essential features of the method according to the invention consists in selecting an atmosphere having an oxidizing power equal to or greater than an atmosphere containing 1% by volume of oxygen and equal to or less than an atmosphere containing 50% by volume of oxygen. In step (a), the steel sheet used is made of steel used for heat treatment as described in European Standard EN 10083 and can have a tensile strength higher than 500 MPa, advantageously between 500 and 2000 MPa before or after heat treatment. The weight composition of the steel sheet should preferably be 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% Iron residue and unavoidable impurities from steel production. For example, 22MnB5 steel sheet has 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%, Iron residue and unavoidable impurities from steel production. The steel sheet can 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%; With the understanding that the titanium and nitrogen contents satisfy the relationship Ti / N > 3.42 and the carbon, manganese, chromium and silicon contents satisfy the following relationship: The composition optionally includes one or more of the following: 0.05% ≤ Mo ≤ 0.65%; 0.001% ≤ W ≤ 0.30%; 0.0005% ≤ Ca ≤ 0.005% Iron residue and unavoidable impurities from steel production. For example, Ductibor®500 steel sheet has 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% Iron residue and unavoidable impurities from steel production. Steel sheet can be obtained by hot rolling and optionally by cold rolling, depending on the desired thickness, which can be, for example, between 0.7 and 3.0 mm. In step (a) a zinc- or aluminum-based pre-coating for anti-corrosion purposes can be applied directly over the steel sheet. In a preferred embodiment, the zinc- or aluminum-based pre-coating is aluminum-based and contains less than 15% silicon, less than 5.0% iron, optionally 0.1 to 8.0% magnesium, and optionally 0.1 to 30.0 zinc, with the balance being aluminum. For example, the zinc- or aluminum-based pre-coating is AluSi®. In another preferred embodiment, the zinc- or aluminum-based pre-coating is zinc-based and contains less than 6.0% aluminum, less than 6.0% magnesium, and the balance is zinc. For example, the zinc- or aluminum-based pre-coating is a zinc coating to provide the following product: Usibor® GI The zinc or aluminum pre-coating may also contain impurities and residual elements such as iron with a content of up to 5.0%, preferably 3.0% by weight. Optionally, in step (a), the hydrogen barrier pre-coating comprises elements selected from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the weight content of each additional element being lower than 0.3% by weight. In a preferred embodiment, in step (a) the hydrogen barrier precoat does not contain at least one of the following elements: Al, Fe, Si, Zn and N. Indeed, without being limited by theoretical considerations, there is a risk that the presence of at least one of said elements may reduce the hydrogen barrier effect of the precoat. Preferably, in step (a) the hydrogen barrier precoat comprises 50% or 75% or 90% by weight chromium. More preferably, the precoat comprises chromium, i.e. the hydrogen barrier precoat comprises only chromium and complementary elements. Preferably, in step (a) and before steps (b) to (f), no other precoating is deposited on the hydrogen barrier precoating. Preferably, in step (a) the hydrogen barrier precoat has a thickness of between 150 and 250 nm. For example, the thickness of the hydrogen barrier precoat is 50, 200 or 400 nm. Without being limited by theoretical considerations, it seems that when the hydrogen barrier precoat is less than 10 nm, there is a risk that hydrogen will be absorbed into the steel because the barrier precoat does not sufficiently cover the steel sheet. When the barrier precoat is higher than 550 nm, there seems to be a risk that the barrier precoat becomes more brittle and that hydrogen absorption will begin due to the brittleness of the barrier precoat. The barrier precoat can be deposited by any method known to the person skilled in the art, for example a hot dip galvanizing process, a roller coating, an electrogalvanizing process, a physical vapor deposition such as jet vapor deposition, magnetron sputtering or electron beam deposition. Preferably, the hydrogen barrier precoat is deposited by electron beam deposition or roller coating. After the precoat is deposited, a tempering roll can be performed which allows the precoated steel sheet to be hardened and to give it a roughness which facilitates subsequent forming. A degreasing and surface finishing can be performed, for example, to improve the adhesive bond or corrosion resistance. After providing a pre-coated steel sheet with a metallic pre-coating according to the present invention, the pre-coated steel sheet is cut to obtain a raw sheet. A heat treatment is applied to said sheet in a furnace. Preferably, the heat treatment is carried out under a non-protective atmosphere and at a temperature between 800 and 970°C. More preferably, the heat treatment is carried out at an austenitizing temperature Tm typically between 840 and 950°C, preferably between 880 and 930°C. Advantageously, said sheet is held for a dwell time of between 1 and 12 minutes, preferably between 3 and 9 minutes. During the heat treatment prior to hot forming, the pre-coating forms an alloy layer with high resistance to corrosion, wear, erosion and fatigue. Preferably, in step (c), the atmosphere has an oxidizing power equal to or greater than an atmosphere containing 10% by volume of oxygen and equal to or less than 30% by volume of oxygen. For example, the atmosphere is air, i.e., containing about 78% N2, about 21% O2, and other gases such as noble gases, carbon dioxide, and methane. Preferably, in step (c) the dew point is between -20 and +20°C and advantageously between -15 and +15°C. Indeed, without being limited by theoretical considerations, it is believed that when the dew point is in the high range, the thermodynamically stable oxide layer further reduces the H2 absorption during thermal treatment. The atmosphere can be made of N2 or Ar or a mixture of nitrogen or argon and gaseous oxidants such as oxygen, mixtures of CO and CO2, or mixtures of H2 and H2O. It is also possible to use mixtures of CO and CO2 or mixtures of H2 and H2 without the addition of an inert gas. After heat treatment, the sheet is transferred to a hot forming tool and formed at a temperature between 600 and 830 °C. Hot forming can be hot stamping or roll-forming. Preferably, the sheet is hot-formed. The part is then cooled in the hot forming tool or by transfer to a specific cooling device. The cooling rate is controlled depending on the steel composition in such a way that the final microstructure after hot-forming consists predominantly of martensite, preferably containing martensite, or martensite and bainite, or is at least made up of 75% equiaxed ferrite, from 5 to 20% martensite and bainite in an amount less than or equal to 10%. A hardened part with excellent resistance to delayed fracture according to the invention is thus obtained by hot forming. Optionally, the part comprises a steel sheet precoated with a zinc- or aluminum-based precoat for anti-corrosion purposes. Preferably, the part comprises a steel sheet precoated with a hydrogen barrier precoat comprising chromium and free of nickel and an oxide layer comprising thermodynamically stable iron, chromium oxides and free of nickel oxides, and such hydrogen barrier precoat is alloyed by infiltration with the steel sheet. More preferably, the steel sheet is directly under a zinc- or aluminum-based precoat, which zinc- or aluminum-based coating is directly under a hydrogen barrier precoat comprising chromium and free of nickel. The hydrogen barrier precoat comprises an oxide layer comprising thermodynamically stable iron, chromium oxides and free of nickel oxides. The hydrogen barrier precoat is alloyed by infiltration with the zinc- or aluminum-based precoat, which in turn is alloyed with the steel sheet. Without being limited by theory, it is believed that the iron in the steel penetrates the surface of the hydrogen barrier precoat during heat treatment. With the atmosphere of step (c), it is believed that the iron and chromium slowly oxidize and form thermodynamically stable oxides that prevent hydrogen from being absorbed into the steel sheet. Preferably, the thermodynamically stable chromium and iron oxides may include Cr2O3, FeO, Fe2O3 and / or Fe3O4 or a mixture thereof. If there is a zinc-based pre-coating, the oxides can also include ZnO. If there is an aluminum-based pre-coating, the oxides can also include Al2O3. For applications in the automotive industry, after the phosphating step, the part is immersed in an electroplating bath. The thickness of the phosphate layer is usually between 1 and 2 µm and the thickness of the electroplating layer between 15 and 25 µm, preferably less than or equal to 20 µm. The electroplating layer ensures additional corrosion protection. After the electroplating step, other paint layers can be deposited, for example a primer coat, a primer coat and a top coat. Before performing electrodeposition on a part, the part must be degreased and phosphated to ensure electrophoresis adhesion. The invention will now be described by experiments carried out and for informational purposes only. These experiments are not exclusive. Examples For all samples, the steel sheets used are 22MnB5. The steel composition 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% et V = 0.0284%. Some steel sheets are pre-coated with a first pre-coating, an anti-corrosion pre-coating called AluSi®. This pre-coating consists of 9% by weight silicon, 3% by weight iron and the remainder aluminum and is deposited by hot-dip galvanizing. Some steel sheets are coated with a second precoat deposited by magnetron sputtering. Example 1: Hydrogen test This test is used to determine the amount of hydrogen absorbed during austenitizing heat treatment in a hot forging process. The test specimens were precoated steel sheets with a first precoat of AluSi® (25 µm) and a second precoat consisting of 80% nickel and 20% chromium or consisting of chromium. After the deposition of the pre-anodes, the coated test specimens were cut to obtain raw sheets. These sheets were then heated at a temperature of 900°C for a dwell time varying between 5 and 10 minutes. The atmosphere during the heat treatment was air or nitrogen with a dew point between -15 and +15°C. The sheets were transferred into the forming tool and heated to obtain omega-shaped parts. The parts were then quenched by immersing the specimens in hot water to obtain a martensitic transformation hardening. Finally, the amount of hydrogen absorbed by the samples during heat treatment was measured by thermal absorption using a thermal absorption analyzer (TDA). For this purpose, each sample was placed in a quartz chamber and slowly heated in an infrared furnace under a nitrogen flow. The released hydrogen / nitrogen mixture was detected by a leak detector and the hydrogen concentration was measured by a mass spectrometer. The results can be seen in Table 1 below. Samples Atmosphere Dew point (°C) Second pre-coat Ni / Cr ratio Second pre-coat thickness (nm) H2 content (ppm by mass) 1 (PCT / IB2018 / 057719) Air +15°C Ni / Cr 80 / 20 4 200 0.2 2 (PCT / IB2018 / 057719) N2 +15°C Ni / Cr 80 / 20 4 200 0.4 3 (WO2017187255) N2 +15°C Cr - 200 0.4 4* Air +15°C Cr - 200 0.09 *: Samples according to the invention Sample 4 according to the present invention released a very low amount of hydrogen compared to the comparison samples. After thermal finishing and hot forming, the surface of sample 4 was analyzed. The surface contained the following oxides: Cr2O3, Fe2O3, Fe3O4, and Al2O3. From the steel sheet to the outer surface, test piece 4 consists of the following layers: - a cross-penetration layer consisting of iron from steel sheet, aluminum, silicon and other elements, with a thickness between 10 and 15 µm, - An alloy layer containing aluminum, silicon and iron from steel sheet in smaller amounts than the underlying layer and other elements, with a thickness between 20 and 35 µm, - A thin layer containing less iron and more oxides compared to the underlying layers, with a thickness between 100 and 300 nm, - A thinner layer containing the highest amount of oxides compared to the underlying layers, especially chromium and aluminum oxides, and located directly below the surface with a thickness between 50 and 150 nm.

Claims

CLAIMS 1. A press hardening method comprises the following steps: A. the provision of a steel sheet for heat treatment, being optionally precoated with a zinc- or aluminum-based pre-coating, B. The deposition of a hydrogen barrier pre-coating comprising chromium and not comprising nickel over a thickness from 10 to 550 nm , C. the cutting of the precoated steel sheet to obtain a blank, D. the heat treatment of the blank at a furnace temperature from 800 to 970°C, during a dwell time from 1 to 12 minutes, in an atmosphere having an oxidizing power equal or higher than that of an atmosphere consisting of 1% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 50% by volume of oxygen, such atmosphere having a dew point between -30 and +30°C, E. the transfer of the blank into a press tool, F. the hot-forming of the blank at a temperature from 600 to 830°C to obtain a part, G. the cooling of the part obtained at step E) to obtain a microstructure in steel being martensitic or martensito-bainitic or made of at least 75 % in terms of volume fraction of equiaxed ferrite , from 5 to 20 % in volume of martensite and bainite in amount less than or equal to 10 % in volume.

2. A press hardening method according to claim 1, wherein in step B), the hydrogen barrier pre-coating does not comprise at least one of the elements chosen from Al, Fe, Si, Zn, and N.

3. A press hardening method according to anyone of claims 1 or 2, wherein in step A), the hydrogen barrier pre-coating consists of chromium .

4. A press hardening method according to anyone of claims 1 to 3, wherein no further pre-coating is deposited on top of the hydrogen barrier pre-coating between steps C and G.

5. A press hardening method according to anyone of claims 1 to 4, wherein in step A), the zinc-or aluminum-based pre-coating is based on aluminum 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 press hardening method according to anyone of claims 1 to 4, wherein in step A), the zinc-or aluminum-based pre-coating is based on zinc and comprises less than 6.0% Al, less than 6.0% of Mg, the remainder being Zn.

7. A press hardening method according to anyone of claims 1 to 6, wherein the hydrogen barrier pre-coating of step A) is deposited by physical vapor deposition, by electro-galvanization or roll-coating.

8. A press hardening method according to claim 7, wherein in step C), the atmosphere has an oxidizing power equal or higher than that of an atmosphere consisting of 10% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 30% by volume of oxygen.

9. A press hardening method according to claim 8, wherein in step C) the atmosphere is air.

10. A press hardening method according to claim 9, wherein in step C), the heat treatment is performed at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel.

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