A HOT FORMING METHOD.

Abstract

The present invention relates to a hot forming method comprising the following steps: A. supplying a steel sheet for heat treatment, which is optionally coated with a zinc- or aluminum-based pre-coating, B. depositing a hydrogen barrier pre-coating comprising chromium and not nickel over a thickness of 10 nm to 550 nm, C. cutting the pre-coated steel sheet to obtain a blank, D. heat-treating the blank at a furnace temperature of 800 °C to 970 °C, for a residence time of 1 to 12 minutes, in an atmosphere with 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, the atmosphere having a dew point of between -30 °C and +30 °C, E. transferring the blank to a pressing tool, F.The hot forming of the blank at a temperature of 600 °C to 830 °C to obtain a part, G. the cooling of the part obtained in step E) 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, from 5% to 20% by volume of martensite and of 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 heat-treatable steel sheet coated with a barrier coating. This hydrogen barrier pre-coating inhibits further hydrogen absorption and improves late fracture resistance. The invention is particularly suitable for the manufacture of automotive parts. The 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 one pre-coating, optionally 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, and that have good tensile strength. For this purpose, steels with improved mechanical properties, such as hot and cold stamped steel, are commonly used. However, it is known that susceptibility to late fracture increases with mechanical strength 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 result in late fracture, cracking that occurs some time 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 arises from the residual stress distribution field and the kinetics of hydrogen diffusion, with the hydrogen diffusion coefficient being low at room temperature.Furthermore, hydrogen located at grain boundaries weakens their cohesion and favors the appearance of late intergranular cracks. Hot forming is known to be critical for hydrogen absorption, which increases susceptibility to late fracture. Absorption can occur during the austenitizing heat treatment, the heating step prior to hot press forming itself. In fact, hydrogen absorption in steel depends on the metallurgical phase. Furthermore, at high temperatures, water in the furnace dissociates on the surface of the steel sheet into hydrogen and oxygen. Document WO2017 / 187255 discloses a precoating that acts as a barrier to prevent hydrogen absorption, particularly during heat treatment prior to hot forming. This hydrogen barrier precoating comprises nickel and chromium, where the Ni / Cr weight ratio is between 1.5 and 9. This patent application discloses that a heat treatment atmosphere is either an inert atmosphere or an atmosphere comprising air. All examples are performed in an atmosphere consisting of nitrogen. According to document WO2020 / 070545, the heat treatment prior to hot forming can occur in an atmosphere with 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; the atmosphere has a dew point between -30°C and +30°C, to further reduce hydrogen absorption. In both patent applications, although hydrogen absorption is improved during the austenitizing heat treatment, obtaining a part with excellent late fracture resistance is not enough. In fact, even if the pre-coating barrier reduces hydrogen absorption, some hydrogen molecules are still absorbed by the steel sheet. Therefore, the objective of the invention is to provide a hot forming method that prevents hydrogen absorption in the steel sheet. It aims to produce a part with excellent late fracture resistance achievable by this hot forming method, which includes hot forming. This is achieved by providing a hot forming method comprising the following steps: A. the supply of a steel sheet for heat treatment, which is optionally coated with a zinc- or aluminum-based pre-coating, B. the deposition of a hydrogen barrier precoat comprising chromium and not comprising nickel over a thickness of 10 nm to 550 nm, C. cutting the pre-coated steel sheet to obtain a blank, D. the heat treatment of the raw piece at a furnace temperature of 800 °C to 970 °C, for a holding time of 1 to 12 minutes, in an atmosphere with 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, the atmosphere having a dew point of between -30 °C and +30 °C, E. the transfer of the blank to a pressing tool, F. Hot forming of the blank at a temperature of 600 °C to 830 °C to obtain a part, G. Cooling the part obtained in step E) 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, the inventors have surprisingly discovered that when the steel sheet is pre-coated with a hydrogen barrier pre-coating containing chromium but not nickel, and when the austenitizing heat treatment is performed in the aforementioned atmosphere, this barrier effect of the pre-coating is further enhanced, preventing even more hydrogen absorption into the steel sheet. Unlike a nitrogen atmosphere, which results in the formation of a thinner layer of selective oxides on the surface of the hydrogen barrier pre-coating during the austenitizing heat treatment, it is believed that thermodynamically stable oxides with low kinetics form on the surface of the barrier pre-coating. In the specific atmosphere described above, it is believed that a hydrogen barrier precoat comprising chromium but not nickel allows for a greater reduction in hydrogen absorption than a hydrogen barrier precoat comprising both nickel and chromium. In fact, chromium is believed to form a thicker oxide layer than that formed by nickel and chromium. Without wishing to be limited to any one theory, it is believed that a hydrogen barrier precoat comprising chromium but not nickel can prevent water dissociation on the hydrogen barrier precoat surface and also prevent hydrogen diffusion through the hydrogen barrier precoat.With an atmosphere that 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, it is believed that oxides that are thermodynamically stable further inhibit the dissociation of water. One of the essential features of the method according to the invention consists in choosing the atmosphere that has an oxidation 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. In step A), the steel sheet used is composed of heat-treated steel 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 Si2'6C+T3% the composition which 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. 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. In step A), the steel sheet can be finished directly with a zinc- or aluminum-based precoat for corrosion protection. In a preferred embodiment, the zinc- or aluminum-based precoat 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. For example, the zinc- or aluminum-based precoat is AluSi®. In another preferred embodiment, 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. For example, the zinc- or aluminum-based precoating is a zinc coating to obtain the following product: Usibor® Gl. The aluminum or zinc-based precoating may also comprise 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 precoat hydrogen barrier 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 less than 0.3% by weight. In a preferred embodiment, in step A), the hydrogen barrier precoat does not comprise at least one of the elements selected from Al, Fe, Si, Zn, and N. In fact, without wishing to be limited to any theory, there is a risk that the presence of at least one of these elements may reduce the barrier effect of the hydrogen precoat. Preferably, in step A), the hydrogen barrier precoat consists of Cr at 50%, 75%, or 90% by weight. More preferably, it consists of Cr, i.e., the hydrogen barrier precoat comprises only Cr and additional elements. Preferably, in step A), no other precoat is deposited on the hydrogen barrier precoat before steps B) to F). Preferably, in step A), the hydrogen barrier precoat has a thickness of between 10 nm and 90 nm or 150 nm and 250 nm. For example, the thickness of the barrier precoat is 50 nm, 200 nm, or 400 nm. Without wishing to limit ourselves to any one theory, it appears that when the barrier precoating is less than 10 nm thick, there is a risk of hydrogen absorption into the steel because the barrier precoating does not sufficiently cover the steel sheet. When the barrier precoating is greater than 550 nm thick, there appears to be a risk that the barrier precoating will become more brittle, and hydrogen absorption will begin due to this brittleness. Precoatings can be deposited by any method known to a skilled technician, such as hot-dip galvanizing, roll coating, electrogalvanizing, physical vapor deposition (such as jet vapor deposition), magnetron spraying, or electron beam-induced deposition. Preferably, the hydrogen barrier precoat is deposited by electron beam-induced deposition or roll coating. After precoating, a fit rolling operation can be performed, hardening the steel sheet and providing a roughness that facilitates subsequent forming. Degreasing and surface treatment can be applied to improve, for example, adhesive bonding or corrosion resistance. After the steel sheet is supplied pre-coated with the metallic pre-coating according to the present invention, the pre-coated steel sheet is cut to obtain a blank. The blank is then heat-treated in a furnace. Preferably, the heat treatment is carried out in a non-protective atmosphere or in a protective atmosphere at a temperature between 800°C and 970°C. More preferably, the heat treatment is carried out at an austenitizing temperature Tm commonly between 840°C and 950°C, preferably 880°C to 930°C. Advantageously, the blank is held 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, the pre-coating forms an alloy layer that has high resistance to corrosion, abrasion, wear, and fatigue. Preferably, in step C), the atmosphere has an oxidizing power equal to or greater than that of an atmosphere consisting of 10% oxygen by volume and equal to or less than that of an atmosphere consisting of 30% oxygen by volume. For example, the atmosphere is air, that is, it consists of approximately 78% N2, approximately 21% O2, and other gases such as rare gases, carbon dioxide, and methane. Preferably, in step C), the dew point is between -20 °C and +20 °C, and advantageously between -15 °C and +15 °C. In fact, without wishing to limit ourselves to any theory, it is believed that when the dew point is within the above range, the thermodynamically stable oxide layer further reduces H2 absorption during heat treatment. The atmosphere can consist of N2 or Ar or mixtures 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 blank is transferred to a hot forming tool and hot formed at a temperature between 600 °C and 830 °C. Hot forming can be either 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 dedicated 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 late fracture resistance according to the invention is obtained by hot forming. Optionally, the part comprises a steel sheet pre-coated with a zinc- or aluminum-based pre-coating for anti-corrosion purposes. Preferably, the part comprises a steel sheet pre-coated with a hydrogen barrier pre-coating comprising chromium but not nickel and an oxide layer comprising thermodynamically stable iron and chromium oxides but not nickel oxides, the hydrogen barrier pre-coating alloying with the steel sheet through diffusion. More preferably, the steel sheet is directly finished with a zinc- or aluminum-based precoating. This zinc- or aluminum-based coating layer is directly finished with a hydrogen barrier precoating comprising chromium but not nickel. The hydrogen barrier precoating includes an oxide layer comprising thermodynamically stable iron and chromium oxides but not nickel oxides. The hydrogen barrier precoating is diffusion-alloyed with the zinc- or aluminum-based precoating, and the zinc- or aluminum-based precoating is also alloyed with the steel sheet. Without wishing to be limited to any one theory, it appears that iron from the steel diffuses onto the surface of the hydrogen barrier precoating during heat treatment.With the atmosphere of step C), it is believed that iron and chromium slowly oxidize, forming thermodynamically stable oxides that prevent hydrogen absorption in the steel sheet. Preferably, the thermodynamically stable chromium and iron oxides may comprise Cr2O3, FeO, Fe2O3 and / or FesCU or a mixture thereof. If a zinc-based precoating is present, the oxides may also comprise ZnO. If an aluminum-based precoating is present, the oxides may also comprise Al2O3. For automotive applications, after the phosphating step, the part is immersed in an electrophoretic coating bath. Typically, the phosphate layer thickness is between 1 and 2 µm, and the pre-coat thickness is between 15 and 25 µm, 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 steel composition is as follows: C = 0.2252%; Mn = 1.1735%; P = 0.0126%; S = 0.0009%; N = 0.0037%; Si = 0.2534%; Cu = iviA / a / zuzz / uuD 1 o / 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%. Some steel sheets are pre-coated with a first coating, an anti-corrosive pre-coating referred to hereafter 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. Some steel sheets are coated with a 2d0 precoating deposited by magnetron spraying. 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. The test elements are steel sheets pre-coated with a 1st pre-coating which is AluSi® (25 pm) and a 2nd pre-coating comprising 80% Ni and 20% Cr or consisting of Cr. After the pre-coatings were applied, the coated test specimens were cut to obtain blanks. These blanks were then heated to 900 °C for a holding time of 5 to 10 minutes. The atmosphere during heat treatment was either air or nitrogen with a dew point between -15 °C and +15 °C. The blanks were then transferred to a press tool and hot-stamped to produce horseshoe-shaped blanks. Finally, the blanks were quenched by immersion in warm water to achieve martensitic transformation hardening. Finally, the amount of hydrogen adsorbed by the test elements during heat treatment was measured by thermal desorption using a thermal desorption analyzer (TDA). For this purpose, each test element 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. ινΐΛ / a / zuzz / uuo i or The results are shown in Table 1 below: Tests Atmosphere Dew Point (°C) 2nd Precoating Ni / Cr Ratio 2nd Precoating Thickness (nm) H2 Content (in 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 * : examples according to the invention. Test element 4 according to the present invention releases a very low amount of hydrogen compared to the comparative examples. After heat treatment and hot forming, the surface of test element 4 has been analyzed. It comprises the following oxides on the surface: Cr2Os, Fe2O3, Fe3O4 and Al2O3. From the steel sheet to the outer surface, the test element 4 comprises the following layers: • an interdiffusion layer comprising iron from the steel sheet, aluminum, silicon and other elements, with a thickness of 10 pm to 15 pm, • an alloyed layer containing aluminum, silicon and iron from the steel sheet in a smaller amount than the lower layer and other elements, with a thickness of 20 pm to 35 pm, • a thin layer containing less iron and more oxides than the lower layers, with a thickness of 100 nm to 300 nm, • a thinner layer containing the largest amount of oxides compared to the lower layers, especially Cr and Al oxides, and located directly below the surface, with a thickness of 50 nm to 150 nm.

Claims

1. A hot forming method comprising the following steps: A. supplying a steel sheet for heat treatment, which is optionally pre-coated with a zinc- or aluminum-based pre-coating, B. depositing a hydrogen barrier pre-coating comprising chromium and not comprising nickel over a thickness of 10 nm to 550 nm, C. cutting the pre-coated steel sheet to obtain a blank, D. heat-treating the blank at a furnace temperature of 800 °C to 970 °C, for a residence time of 1 to 12 minutes, in an atmosphere having 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, the atmosphere having a dew point of between -30 °C and +30 °C, E. transferring the blank to a pressing tool, F.The hot forming of the blank at a temperature of 600 °C to 830 °C to obtain a part, G. the cooling of the part obtained in step E) to obtain a microstructure in steel that is martensitic or martensito-bainitic or 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 precoating does not comprise at least one of the elements selected from Al, Fe, Si, Zn and N.

3. A hot forming method according to any of claim 1 or 2, wherein in step A), the hydrogen barrier precoating consists of chromium.

4. A hot forming method according to any of claims 1 to 3, wherein no additional precoat is deposited over the hydrogen barrier precoat between steps C and G.

5. A hot forming method according to any of claims 1 to 4, 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 4, 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.

7. A hot forming method according to any of claims 1 to 6, wherein the hydrogen barrier precoating of step A) is deposited by physical vapor deposition, electro-galvanizing or rolling coating.

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

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

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

11. A part obtainable from the method according to any one of claims 1 to 10, comprising a steel sheet, a hydrogen barrier precoating containing chromium and not containing nickel and alloyed by diffusion of iron from the steel sheet, and finished by an oxide layer including iron oxides from the steel sheet, chromium oxides and not including nickel oxides from the hydrogen barrier precoating.

12. A part obtainable from the method according to any of claims 1 to 10, comprising a steel sheet, a zinc-based precoating, a hydrogen barrier precoating containing chromium and not containing nickel and alloyed by diffusion of iron from the steel sheet and diffusion of zinc and other elements from the zinc-based precoating, and finished by an oxide layer including iron oxides from the steel sheet, zinc oxides from the zinc-based precoating, chromium oxides from the hydrogen barrier precoating and not including nickel oxides.

13. A part obtainable from the method according to any one of claims 1 to 10, comprising a steel sheet, an aluminum-based precoating, a hydrogen barrier precoating containing chromium and not containing nickel and alloyed by diffusion of iron from the steel sheet and diffusion of aluminum and other elements from the aluminum-based precoating, and finished by an oxide layer including iron oxides from the steel sheet, aluminum oxides as Al2O3 from the aluminum-based precoating, chromium oxides from the hydrogen barrier precoating and not including nickel oxides.

14. A part according to any of claims 11 to 13, wherein the thermodynamically stable chromium and iron oxides may respectively comprise Cr2Os; FeO, Fe2O3 and / or Fe3U4 or a mixture thereof.

15. Use of a part according to any of claims 11 to 14 or obtainable from the method according to any of claims 1 to 10, for the manufacture of a motor vehicle.