A METHOD FOR MANUFACTURING AN ASSEMBLY.

Abstract

The present invention relates to a pre-coated steel substrate coated with: - a first pre-coating comprising titanium, this first coating having a thickness of 40 nm to 1200 nm, - optionally, an intermediate pre-coating layer comprising at least 8 wt% nickel and at least 10 wt% chromium, the remainder being iron or an intermediate pre-coating layer comprising Fe, Ni, Cr and Ti wherein the amount of Ti is greater than or equal to 5 wt% and wherein the following equation is satisfied: 8 wt% < Cr + Ti < 40 wt%, the remainder being Fe and Ni, the intermediate pre-coating layer having a thickness between 2 nm and 30 nm - a second pre-coating which is a zinc-based coating and - the steel substrate comprising more than 0.05 wt% Si.

Description

A METHOD FOR MANUFACTURING AN ASSEMBLY The present invention relates to a pre-coated steel substrate, a method for manufacturing the coated steel substrate, a method for manufacturing an assembly, and an assembly itself. It is particularly suitable for the construction and automotive industries. Zinc-based coatings are commonly used because they provide corrosion protection through barrier and cathodic protection. The barrier effect is achieved by applying a metallic or non-metallic coating to a steel surface. This coating prevents contact between the steel and the corrosive atmosphere. The barrier effect is independent of the coating's composition and the substrate. In contrast, sacrificial cathodic protection relies on the fact that zinc is a more active metal than steel in terms of electromotive force (EMF) series. Therefore, if corrosion occurs, zinc is consumed preferentially to steel. Cathodic protection is essential in areas where steel is directly exposed to the corrosive atmosphere, such as cut edges where the surrounding zinc corrodes before the steel. However, when heating steps are performed on these zinc-coated steel sheets, for example, during hot pressure hardening or resistance spot welding, cracks are observed in the steel that initiate at the steel / coating interface. In fact, occasionally, there is a reduction in mechanical properties due to the presence of cracks in the coated steel sheet after the aforementioned operation. These cracks appear under the following conditions: high temperatures above the melting point of the coating materials; contact between the liquid metal, which has a low melting point (such as zinc), and the substrate, combined with the presence of critical stresses; and diffusion and wetting of the molten metal at the grain boundaries and grain boundaries of the steel substrate.The designation for such a phenomenon is known as liquid metal embrittlement (LME), and also called liquid metal assisted cracking (LMAC). Therefore, the objective of the invention is to provide an assembly comprising at least one steel substrate that is free from LME issues. Its objective is to make available, in particular, an easy-to-implement method for obtaining this LME-free assembly after hot forming and / or welding. To this end, the invention relates to a pre-coated steel substrate according to any of claims 1 to 13. The invention relates to a method for manufacturing this pre-coated steel substrate according to any of claims 14 to 16. The invention also relates to a method for manufacturing an assembly according to claims 17 or 18. The invention relates to an assembly according to claims 19 to 23. Finally, the invention relates to the use of the assembly according to claim 24. The invention will now be illustrated by indicative examples provided for informational purposes only, and without limitation, with reference to the accompanying figures in which: - Figure 1 schematically represents a pre-coated steel substrate according to the invention and - Figure 2 represents an assembly according to the present invention. The designation steel or steel sheet means a sheet of steel, a coil, or a plate having a composition that enables the piece to achieve a tensile strength of up to 2500 MPa and more preferably up to 2000 MPa. For example, the tensile strength is greater than or equal to 500 MPa, preferably greater than or equal to 980 MPa, advantageously greater than or equal to 1180 MPa, and even greater than or equal to 1470 MPa. The invention relates to a pre-coated steel substrate coated with: - a first pre-coating comprising titanium, this first coating has a thickness of 40 nm to 1200 nm, - optionally, an intermediate pre-coating layer comprising at least 8 wt% nickel and at least 10 wt% chromium, the remainder being iron or an intermediate pre-coating comprising Fe, Ni, Cr and Ti wherein the amount of Ti is greater than or equal to 5 wt% and wherein the following equation is satisfied: 8 wt% < Cr + Ti < 40 wt%, the remainder being Fe and Ni, the intermediate layer having a thickness of 2 nm to 30 nm, - a second layer of pre-coating which is a zinc-based coating and - the steel substrate comprises more than 0.05% by weight of Si. In fact, without wishing to be limited by any particular theory, it is believed that, during welding, the molten Zn in the second precoat dissolves the steel until the coating becomes saturated with iron. In standard Zn-coated steel without the first precoat containing Ti, the critical embrittlement phenomenon is observed to occur after this initial rapid dissolution, due to the preferential diffusion of Zn at the steel grain boundaries, especially if the steel contains Si, leading to a significant decrease in its cohesive strength. When a first precoat containing titanium is present, precipitates enriched with Fe, Ti, and Si form in the molten Zn, so that the saturation of the coating with iron is significantly delayed and the dissolution can proceed deeper and deeper, thereby protecting the substrate from LME (Liquid Metal Emission). If the thickness of the first titanium precoat is less than 40 nm, there is a risk that the amount of titanium will not be sufficient to form precipitates in the molten coating throughout the critical welding operation to prevent LME (Low Molten Effect). Adding more than 1200 nm provides no additional benefit. Preferably, the first pre-coating consists of titanium, i.e., the amount of titanium is greater than or equal to 99% by weight. In a preferred embodiment, the first precoating has a thickness between 40 nm and 80 nm. In another preferred embodiment, the first precoating has a thickness between 80 nm and 150 nm. In another preferred embodiment, the first precoating has a thickness between 150 nm and 250 nm. In another preferred embodiment, the first precoating has a thickness between 250 nm and 450 nm. In another preferred embodiment, the first precoating has a thickness between 600 nm and 600 nm. In another preferred embodiment, the first precoating has a thickness between 850 nm and 1200 nm. In fact, without wishing to be limited by any theory, it is believed that these thicknesses further improve LME resistance. Preferably, an intermediate pre-coating is present between the steel substrate and the first pre-coating, such intermediate layer comprising iron, nickel, chromium and optionally titanium. Without wanting to be subject to any theory, it seems that the intermediate coating layer further improves the adhesion of the second pre-coating onto a first pre-coating. In a preferred embodiment, the intermediate layer comprises at least 8 wt% nickel and at least 10 wt% chromium, the remainder being iron. For example, the metallic coating layer is 316L stainless steel containing 16 to 18 wt% Cr and 10 to 14 wt% Ni, the remainder being Fe. In another preferred embodiment, the intermediate layer comprises Fe, Ni, Cr and Ti where the amount of Ti is greater than or equal to 5% by weight and where the following equation is satisfied: 8% by weight < Cr + Ti < 40% by weight, the remainder being Fe and Ni, such an intermediate coating layer is directly coated by a coating layer that is a metallic anti-corrosion coating. The thickness of the intermediate precoat, when present, is from 2 nm to 30 nm. In fact, without wanting to be limited by any theory, it is believed that this thickness range allows for improved adhesion of the second precoat. In another preferred embodiment, the zinc-based coating comprises from 0.01% to 8.0% Al, optionally from 0.2% to 8.0% Mg, the remainder being Zn. For example, the zinc-based coating comprises 1.2 wt% Al and 1.2 wt% Mg or 3.7 wt% Al and 3 wt% Mg. More preferably, the zinc-based coating comprises from 0.10% to 0.40 wt% Al, the remainder being Zn. Preferably, the steel substrate has the following chemical composition in percentage by weight: 0.05% < C < 0.4% 0.5% < Mn < 30.0% 0.05% < Si < 3.0%, and on a purely optional basis, one or more elements such as: Al < 2.0%, P<0.1% Nb < 0.5%, B < 0.005%, Cr < 2.0%, Mo < 0.50% Ni < 1.0% V < 0.50% Ti < 0.5%, the remainder of the composition is made up of iron and unavoidable impurities resulting from manufacturing. More preferably, the amount of Mn in the steel substrate is less than or equal to 10% by weight, advantageously less than or equal to 6% by weight, or even better, below 3.5% by weight. Figure 1 illustrates a pre-coated steel substrate according to the present invention. In this example, a steel sheet 1, containing more than 0.05 wt% Si, has its steel surface covered by a first titanium pre-coating 2 having a thickness of 40 nm at 1200 nm and a second zinc pre-coating 3. The invention also relates to a method for manufacturing the coated steel substrate according to the present invention, comprising the following successive steps: A. The provision of a steel substrate, B. Optionally, surface preparation of the steel substrate, C. The deposition of the first pre-coating, D. Optionally, the deposition of the intermediate precoat, E. The deposition of the second pre-coating. Preferably, in step B), surface preparation is carried out by etching or pickling. This step appears to clean the steel substrate, leading to improved adhesion of the first coating. Preferably, in steps C) and D), the primer and intermediate precoat are deposited independently of each other by physical vacuum deposition. More preferably, the primer and intermediate precoat are deposited independently of each other by a magnetron cathode sputtering process or a vapor jet deposition process. Advantageously, in step E), the deposition of the second precoat is carried out by hot-dip coating, by an electrodeposition process, or by vacuum deposition. The invention also relates to a method for manufacturing an assembly comprising the following successive steps: I. The provision of at least two metallic substrates wherein at least one metallic substrate is the pre-coated steel substrate according to the present invention and II. Welding of at least two metallic substrates. Preferably, in step II), welding is performed by spot welding, arc welding or laser welding. Using the method according to the present invention, it is possible to obtain an assembly of at least two metallic substrates welded together by a welded joint, wherein the at least one metallic substrate is such that the steel substrate is covered by a coating comprising iron compounds, FezTiSi, the remainder being zinc, and the coating is covered by a layer comprising titanium oxides. The at least one metallic substrate originates from the pre-coated steel substrate according to the present invention. Without wishing to be limited by any particular theory, it is believed that Fe2TiSi compounds precipitate in the liquid Zn of the coating during welding, promoting intense steel dissolution that prevents zinc from penetrating the steel grain boundaries. Furthermore, it appears that a portion of the first pre-coating layer, comprising titanium, migrates onto the surface of the zinc-based coating and oxidizes during welding. The assembly according to the present invention therefore exhibits high resistance to liquid metal erosion (LME). Figure 2 illustrates a welded joint of an assembly of two metallic substrates where one metallic substrate is a steel sheet 11, covered by a first coating comprising iron, Fe2TiSiz compounds 12, z is from 0.01 to 0.8 and is expressed in atomic ratio, the remainder being zinc 13, and a second coating comprising titanium oxides 14. In this example, the second metallic substrate 15 is a bare steel sheet. In one embodiment, the steel substrate does not comprise internal oxides of steel alloying elements. In another preferred embodiment, the steel substrate comprises internal oxides of steel alloying elements. Preferably, the steel substrate comprises internal oxides of alloying elements comprising silicon oxides, manganese oxides, chromium oxides, aluminum oxides, or a mixture thereof. Preferably, the second metallic substrate is a steel substrate or an aluminum substrate. Preferably, the second metallic substrate is a pre-coated steel substrate according to the present invention. Advantageously, the assembly comprises a third metallic substrate. Preferably, the third metallic substrate is a steel substrate or an aluminum substrate. Preferably, the third metallic substrate is a steel substrate pre-coated according to the present invention. Finally, the use of an assembly that can be obtained from the method according to the present invention for the manufacture of vehicle parts. In order to highlight the improved performance obtained by using the assemblies according to the invention, some specific examples of modalities will be detailed in comparison with assemblies based on the prior art. Examples For the tests, two steel sheets were used that have the chemical composition in percentage by weight described in Table 1: Steel sheet C Mn Si Al SP Cr Nb Cu Ni Ti B Fe 1 0.21 1.65 1.65 0.042 0.001 0.013 0.026 0.001 0.008 0.011 0.008 0.006 Remainder 2 <0.002 0.11 0.007 0.050 0.008 0.010 0.020 <0.002 0.018 0.021 0.054 <0.0003 Remainder 3 0.19 2.50 1.70 0.048 0.002 0.011 0.024 0.001 0.009 0.012 0.009 0.005 Remainder Example 1: Critical elongation of LME For test 1, a first titanium precoat with a thickness of 900 nm was deposited by magnetron sputtering onto a steel sheet of composition 1. Then, an intermediate precoat layer of 316L stainless steel was deposited onto the titanium. The thickness of this intermediate layer was 10 nm. Finally, a second precoat layer of zinc was deposited by vapor jetting. The thickness of this second precoat layer was 7 nm. Test 4 was performed according to the same procedure on a steel sheet of composition 3. For test 2, a zinc coating 7 µm thick was deposited on steel sheet 1 by electrodeposition. Test 5 was performed according to the same procedure on a steel sheet having composition 3. ML / a / zuz i / u 14» i □ Test 3 is a bare steel sheet 1. Tests Steel 1st coating Intermediate coating 2nd coating Γ 1 Ti FeNiCr (316L stainless steel) Zn 2 1 Zn - - 3 1 - - - 4* 3 Ti FeNiCr (316L stainless steel) Zn 5 3 Zn - - *: according to the present invention Tests 1 to 3 were then heated from room temperature to 800 °C, 850 °C, and 900 °C at a heating rate of 1000 °C per second using a Gleeble device. A tensile displacement was applied to each tensile specimen until fracture. The strain rate was 3 mm per second. Tensile forces and displacement were recorded, and the elongation at fracture could be determined from these stress-strain curves. This elongation at fracture represents the so-called critical LME elongation. The higher the critical LME strain, the more resistant the LME test specimen is. The methodology is also explained in the publication entitled «Critical LME Elongation: Un essai Gleeble pour évaluer la sensibilité au LME d'un acier revétu soudé par points », Journées Annuelles SF2M 2017, 23 to 25 October 2017, JA0104, ArcelorMittal Research Maiziéres-lés-Metz. The results are compiled in Table 1 below. Tests Temperature (°C) Critical elongation of LME (%) Γ 800 46 850 38 900 38 2 800 7 850 13 900 5 3 800 48 Tests Temperature (°C) Critical elongation of LME (%) 850 49 900 40 *: according to the present invention iviA / a / ¿u¿ ί / u 14» i □ The results showed that trial 1 has improved LME resistance compared to trial 2. Trials 1 and 3 have the same LME resistance. Example 2: Three stacked sheets The sensitivity to LME of different assemblies was evaluated by the resistance spot welding method. For this purpose, for each test, the three steel sheets were welded together by resistance spot welding. Test 6 was an assembly of test 1 with two galvanized steel sheets having composition 2. Test 7 was an assembly of test 2 with two galvanized steel sheets having composition 2. Test 8 was an assembly of test 4 with two galvanized steel sheets having composition 2. Test 9 was an assembly of test 5 with two galvanized steel sheets having composition 2. The welding electrode type was F1 with a face diameter of 6 mm; the electrode clamping force was 450 daN. The welding cycle was reported in Table 2: Welding time Current (Hz) Welding time (ms) Cooling time (ms) Cycle 50 380 260 Each test was reproduced 10 times in order to produce 10 spot welds at a current level defined as the upper welding limit of the current range: Imax, Imax comprised between 0.9 and 1.1* lexp, with lexPla being the intensity beyond which expulsion appears during welding, lexp was determined in accordance with ISO 18278-2. The highest crack length in the spot-welded joint was then evaluated after cross-section through the surface crack and using an optical microscope as reported in Table 3 below. The crack strength behavior of LME was evaluated with respect to the 10 spot welds (representing 100% in total). Crack-free tests 0 < Crack < 100 pm 100 pm < Crack < 200 pm Crack > 0.5*assembly thickness (assembly thickness = 1.0 mm) Test 6* 70% 20% 10% - Test 7 30% 10% 30% 30% Trial 8* 20% 50% 20% 10% Trial 9 - 30% 30% 40% * according to the present invention. Tests 6 and 8 according to the present invention show excellent resistance to LME compared to tests 7 and 9.

Claims

CLAIMS 1. A pre-coated steel substrate, coated with: - a first pre-coating comprising titanium, this first coating having a thickness of 40 nm to 1200 nm, - optionally, an intermediate pre-coating layer comprising at least 8 wt% nickel and at least 10 wt% chromium, the remainder being iron or an intermediate pre-coating layer comprising Fe, Ni, Cr and Ti wherein the amount of Ti is greater than or equal to 5 wt% and wherein the following equation is satisfied: 8 wt% < Cr + Ti < 40 wt%, the remainder being Fe and Ni, the intermediate pre-coating layer having a thickness between 2 nm and 30 nm - a second pre-coating which is a zinc-based coating and - the steel substrate comprising more than 0.05 wt% Si.

2. A pre-coated steel substrate according to the first pre-coating consists of titanium.

3. A pre-coated steel substrate according to the thickness of the first pre-coating is between 40 nm and 80 nm.

4. A pre-coated steel substrate according to the thickness of the first pre-coating is between 80 nm and 150 nm.

5. A pre-coated steel substrate according to which the thickness of the first pre-coating is between 150 nm and 250 nm. with claim 1, wherein claim 2, wherein the 6. A coated steel substrate according to claim 1 of the first pre-coating is between 250 nm and 450 nm.

7. A coated steel substrate according to claim 1 of the first pre-coating is between 450 nm and 600 nm.

8. A coated steel substrate according to claim 1 of the first pre-coating is between 600 nm and 850 nm.

9. A coated steel substrate according to claim 1 or 2, wherein the thickness of the first pre-coating is between 850 nm and 1200 nm.

10. A coated steel substrate according to any of claims 1 to 9, wherein the intermediate pre-coating layer(s) include stainless steel containing between 10% and 13% by weight of nickel, between 16% and 18% by weight of chromium, the remainder being iron.

11. A coated steel substrate according to any of claims 1 to 10, wherein the second pre-coating is a zinc-based coating comprising 0.01% to 8.0% Al, optionally 0.2% to 8.0% Mg, the remainder being Zn.

12. A pre-coated steel substrate according to any of claims 1 to 10, wherein the second pre-coating is a zinc-based coating optionally comprising between 0.10% and 0.40% by weight of Al, the remainder being zinc.

13. A pre-coated steel substrate according to any of claims 1 to 12, wherein the steel substrate has the following chemical composition in weight percent: 0.05% < C < 0.4%, 0.5% < Mn < 30.0%, 0.05% < Si < 3.0%, and on a purely optional basis, one or more elements such as: Al < 2.0%, P < 0.1%, Nb < 0.5%, B < 0.005%, Cr < 2.0%, Mo < 0.50%, Ni < 1.0%, V < 0.50%, Ti < 0.5%, the remainder of the composition being made of iron and unavoidable impurities resulting from manufacturing.

14. A method for manufacturing the coated steel substrate according to any of claims 1 to 13 comprising the following steps: A. supplying a steel substrate according to any of claims 1 to 13, B. optionally, preparing the surface of the steel substrate, C. depositing the first pre-coating layer according to any of claims 1 to 9, D. optionally, depositing an intermediate pre-coating layer according to any of claims 1 or 10, E. depositing a second pre-coating layer according to any of claims 1, 11 or 12.

15. A method according to claim 14, wherein in steps C) and D), the deposition of the first precoat layer and the intermediate precoat layer is carried out independently of each other by physical vacuum deposition.

16. A method according to claim 15, wherein in steps C) and D), the deposition of the first precoat and the intermediate precoat is carried out independently of each other by a magnetron cathode sputtering process or a vapor jet deposition process.

17. A method for manufacturing an assembly of at least two metallic substrates comprising the following successive steps: I. Providing at least two metallic substrates wherein at least one metallic substrate is the pre-coated steel substrate according to any of claims 1 to 13 or obtainable by the method according to any of claims 14 to 16 and II. Welding at least two metallic substrates.

18. A method according to claim 17, wherein in step II), the welding is performed by spot welding or arc welding.

19. An assembly obtainable from the method according to claim 17 or 18, of at least two metallic substrates welded together through a welded joint wherein the at least one metallic substrate is such that the steel substrate is covered by a coating comprising iron compounds, Fe2TiSiz, z being from 0.01 to 0.8 and expressed in atomic ratio, the remainder being zinc, the coating being covered by a layer comprising titanium oxides.

20. An assembly according to claim 19, wherein the steel substrate comprises internal oxides of steel alloying elements.

21. An assembly according to claim 20, wherein the steel substrate comprises alloying element oxides comprising silicon oxides, manganese oxides, chromium oxides, aluminum oxides or a mixture thereof.

22. An assembly according to any of claims 19 to 21, wherein the second sheet of metallic substrate is a steel substrate or an aluminum substrate.

23. An assembly according to claims 19 to 22, wherein the second metallic substrate is a pre-coated steel substrate according to any of claims 1 to 13 or obtainable by the method according to any of claims 14 to 16.

24. Use of an assembly obtainable from the method according to any of claims 17 to 18 or according to claims 19 to 23 for the manufacture of vehicle parts.