RU2799379C1 - Assembly manufacturing method - Google Patents

Abstract

EFFECT: steel treatment.

SUBSTANCE: group of inventions is related to a pre-coated steel substrate, a method for manufacturing said substrate, a method for manufacturing an assembly from at least two said substrates, an assembly obtained by said method, and the use of said assembly for the manufacture of vehicle parts. The pre-coated steel substrate comprises the following layers: a first pre-coating of titanium, optionally an intermediate pre-coating layer containing at least 8 wt.% nickel and at least 10 wt.% chromium, the rest being iron, or optionally an intermediate pre-coating layer, containing Fe, Ni, Cr and Ti, in which the content of Ti is greater than or equal to 5 wt.%, and the following equation is satisfied: 8 wt.% < Cr + Ti < 40 wt.%, Fe and Ni - the rest; second pre-coating based on zinc. The specified steel substrate contains more than 0.05 wt.% Si.

EFFECT: assembly that includes at least a steel substrate that is resistant to cracks resulting from liquid metal embrittlement is provided.

19 cl, 2 dwg, 5 tbl, 2 ex

Description

The present invention relates to a pre-coated steel substrate, a method for manufacturing a coated steel substrate; assembly manufacturing method and assembly. It is particularly suitable for the construction and automotive industries.

Zinc-based coatings are commonly used because they provide corrosion protection through barrier protection and cathodic protection. The barrier effect is achieved by applying a metallic or non-metallic coating to the steel surface. Thus, the coating prevents the steel from coming into contact with an aggressive atmosphere. The barrier effect does not depend on the nature of the coating and substrate. In contrast, sacrificial cathodic protection is based on the fact that zinc is an active metal compared to steel according to the EMF series. Thus, if corrosion occurs, zinc is consumed rather than steel. Cathodic protection is important in areas where the steel is directly exposed to aggressive atmospheres, such as cut edges where the surrounding zinc is consumed before the steel.

However, when heating steps are performed on such zinc-coated steel sheets, such as during hot pressing or resistance spot welding, cracks are observed in the steel that occur at the steel/plating interface. Indeed, sometimes there is a decrease in mechanical properties due to the presence of cracks in the coated steel sheet after the above operation. These cracks appear under the following conditions: high temperature above the melting point of the coating materials; contact between a liquid metal having a low melting point (for example, zinc) and a substrate in combination with the presence of critical stresses; diffusion and wetting by molten metal in the grains and grain boundaries of the steel substrate. The designation for this phenomenon is known as liquid metal embrittlement (LME) and is also called liquid metal embrittlement (LMAC).

Thus, it is an object of the invention to propose an assembly including at least a steel backing which does not have problems with LME. In particular, it aims to provide an easy-to-implement process for producing this assembly without LME problems after hot pressing and/or welding.

To solve this problem, an invention is proposed, which relates to a steel substrate with a pre-coated according to any one of paragraphs 1 to 13.

The invention also relates to a method for manufacturing this pre-coated steel substrate according to any one of claims 14 to 16.

The invention also relates to a method for manufacturing an assembly according to any one of claims 17 or 18.

The invention relates to the assembly according to any one of claims 19 to 23.

Finally, the invention relates to the use of the assembly of claim 24.

The invention will now be illustrated by way of illustrative examples, given for informational purposes only and without limitation, with reference to the accompanying drawings, in which:

- Fig. 1 schematically represents a pre-coated steel substrate in accordance with the invention, and

- Fig. 2 represents the collection according to the present invention.

The designation "steel" or "steel sheet" means a steel sheet, coil, plate, the composition of which allows the part 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, preferably 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. Said containing pre-coated steel substrate contains:

the first pre-coating containing titanium, said first coating having a thickness of 40-1200 nm,

optional intermediate layer pre-coat containing at least 8% of the mass. Nickel and at least 10% of the mass. chromium, the rest is iron, or optionally an intermediate pre-coating layer containing Fe, Ni, Cr and Ti, in which the content of Ti is greater than or equal to 5% wt., and the following equation is fulfilled: 8% wt. < Cr + Ti < 40 wt. %, the rest is Fe and Ni, and the intermediate pre-coating layer has a thickness of 2 - 30 nm,

the second pre-coating, which is a zinc-based coating,

while the specified steel substrate contains more than 0.05% of the mass. Si.

Indeed, without wishing to be bound by any 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 a first pre-coating containing Ti, there is a critical embrittlement phenomenon after this first rapid dissolution due to preferential diffusion of Zn along the grain boundaries of the steel, especially if the steel contains Si, which leads to a significant decrease in their cohesive strength. When a first pre-coating including titanium is present, precipitates enriched in Fe, Ti and Si are formed in the molten Zn, so that the saturation of the coating with iron is greatly slowed down, and the dissolution can proceed longer and deeper, thus protecting the substrate from LME.

If the thickness of the first pre-coating containing titanium is less than 40 nm, there is a risk that the titanium content will not be sufficient to form precipitates in the molten coating during the entire duration of the critical welding operation to prevent LME. Adding more than 1200 nm brings no additional benefit.

Preferably, the first precoat consists of titanium, i.e. the titanium content is greater than or equal to 99 wt%.

In a preferred embodiment, the first precoat has a thickness of 40-80 nm. In another preferred embodiment, the first precoat has a thickness of 80-150 nm. In another preferred embodiment, the first precoat has a thickness of 150-250 nm. In another preferred embodiment, the first precoat has a thickness of 250-450 nm. In another preferred embodiment, the first precoat has a thickness of 450-600 nm. In another preferred embodiment, the first precoat has a thickness of 600-850 nm. In another preferred embodiment, the first precoat has a thickness of 850-1200 nm. Indeed, without wishing to be bound by any theory, it is believed that such a thickness further improves LME resistance.

Preferably, an intermediate precoat is present between the steel substrate and the first precoat, such intermediate layer comprising iron, nickel, chromium and optionally titanium. Without wishing to be bound by any theory, it is believed that the intermediate coating layer further enhances the adhesion of the second precoat to the first precoat.

In the preferred implementation of the intermediate layer includes at least 8% of the mass. Nickel and at least 10% of the mass. chromium, the rest is iron. For example, the metal plating layer is 316L stainless steel, including 16-18% of the mass. Cr and 10 - 14% of the mass. Ni, the rest is Fe.

In another preferred implementation, the intermediate layer includes Fe, Ni, Cr and Ti, where the content of Ti is greater than or equal to 5% of the mass. and thus the following equation is fulfilled: 8% of the mass. <Cr + Ti <40 mass%, the rest is Fe and Ni, and such an intermediate coating layer is directly covered with a coating layer, which is an anti-corrosion coating.

The thickness of the intermediate precoat, if present, is 2-30 nm. Indeed, without wishing to be bound by any theory, it is believed that this range of thickness allows for improved adhesion of the second precoat.

In another preferred embodiment, the zinc-based coating comprises 0.01-8.0% Al, optionally 0.2-8.0% Mg, and the balance is Zn. For example, based on zinc includes 1.2% of the mass. Al and 1.2% wt. Mg or 3.7% wt. Al and 3% wt. mg. More preferably, the zinc-based coating includes 0.10 - 0.40% of the mass. Al, the rest is Zn.

Preferably, the steel substrate has the following weight percent chemical composition:

0.05 ≤ C ≤ 0.4%,

0.5 ≤ Mn ≤ 30.0%,

0.05 ≤ Si ≤ 3.0%,

and optionally 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 rest of the composition consists of iron and inevitable impurities that arise during the production process. More preferably, the Mn content in the steel substrate is less than or equal to 10% by mass, preferably less than or equal to 6% by mass. or even preferably less than 3.5% of the mass.

Fig. 1 shows a pre-coated steel substrate in accordance with the present invention. In this example, steel sheet 1 containing more than 0.05% of the mass. Si, and the surface of the steel is coated with the first pre-coating of titanium 2, having a thickness of 40 - 1200 nm, and the second pre-coating of zinc 3.

The invention also relates to a method for manufacturing a coated steel substrate in accordance with the present invention, comprising the following successive steps:

a. Preparation of steel substrate,

B. Optional preparation of the surface of the steel substrate,

B. Applying the first precoat,

D. Optional intermediate pre-coating,

E. Applying a second precoat.

Preferably in step B), the surface preparation is carried out by etching or pickling. This step appears to allow the steel substrate to be cleaned, resulting in improved adhesion of the first precoat.

Preferably, in steps C) and D), the application of the first and intermediate pre-coating is independently carried out by physical vacuum deposition. More preferably, the application of the first and intermediate precoats independently of each other is carried out using a magnetron cathode sputtering process or a jet vapor deposition process.

Preferably, in step E), the application of the second precoat is carried out by hot dip, electrodeposition or vacuum deposition.

The invention also relates to a method for manufacturing an assembly, which includes the following successive stages:

- Preparation of at least two metal substrates, wherein at least one metal substrate is a pre-coated steel substrate in accordance with the present invention and

- Welding of at least two metal 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 metal substrates welded together by a welded joint, in which at least one metal substrate is a steel substrate coated with a coating comprising iron, a Fe ₂ TiSi compound , the rest is zinc, and the specified coating is covered with a layer including titanium oxides. At least one metal substrate is obtained from a pre-coated steel substrate according to the present invention.

Without wishing to be bound by any theory, it appears that the Fe ₂ TiS compounds precipitate out in the liquid Zn coating during welding, promoting intense dissolution of the steel, which prevents the zinc from penetrating into the steel grain boundaries. Moreover, it appears that part of the first pre-coating layer comprising titanium migrates over the zinc-based coating and oxidizes during welding. Thus, the assembly according to the present invention has a high LME resistance.

Fig. 2 shows a welded joint of an assembly of two metal substrates, where one metal substrate is a steel sheet 11 coated with a first coating including iron, Fe ₂ TiSiz 12 compounds, where z is 0.01 - 0.8 and represents the atomic ratio, and the rest is zinc 13, and a second coating comprising titanium oxides 14. In this example, the second metal substrate 15 is an uncoated steel sheet.

In one implementation, the steel substrate does not include internal oxides of steel alloying elements.

In another preferred embodiment, the steel substrate includes internal oxides of the alloying elements of the steel. Preferably, the steel substrate includes internal oxides of alloying elements, including silicon oxides, manganese oxides, chromium oxides, aluminum oxides, or mixtures thereof.

Preferably, the second metal substrate is a steel substrate or an aluminum substrate. Preferably, the second metal substrate is a pre-coated steel substrate in accordance with the present invention.

Advantageously, the assembly includes a third metal substrate. Preferably, the third metal substrate is a steel substrate or an aluminum substrate. Preferably, the third metal substrate is a pre-coated steel substrate in accordance with the present invention.

Finally, the assemblies produced by the method of the present invention are used to manufacture vehicle parts.

To emphasize the improved performance obtained by using the assemblies according to the invention, some specific embodiments will be described in detail in comparison with prior art assemblies, substrates.

Examples

For testing, two steel sheets with the chemical composition in mass percent indicated in Table 1 are used.

Table 1.

steel sheets C Mn Si Al S P 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 Rest 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 Rest 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 Rest

Example 1: Critical elongation LME

For test 1, a first titanium precoat having a thickness of 900 nm was magnetron sputtered onto a steel sheet having composition 1. Then, an intermediate precoat layer of 316L stainless steel was applied to the titanium. The thickness of the intermediate layer is 10 nm. Finally, the second layer of pre-coating, which is a zinc coating, is applied by spray vapor deposition. The thickness of the second precoat layer is 7 µm. Test 4 is carried out according to the same procedure on the steel sheet of composition 3.

For test 2, a zinc coating 7 µm thick is applied to steel sheet 1 by electroplating. Test 5 is carried out in the same manner on the steel sheet of composition 3.

Test 3 uses uncoated steel sheet 1.

Table 2.

Trial Steel Coating 1 Intermediate Coating Coating 2 1* 1 Ti FeNiCr
(Stainless steel 316L) Zn 2 1 Zn - - 3 1 - - - 4* 3 Ti FeNiCr
(Stainless steel 316L) Zn 5 3 Zn - -

*: according to the present invention

Then in the test steel 1 - 3 is heated from ambient temperature to 800°C, 850°C and 900°C at a heating rate of 1000°C per second using a Gleeble device. Tensile strain to failure is applied to each tensile test piece. The strain rate is 3 mm per second. Tensile forces and displacement are recorded and elongation at break can be determined from these stress-strain curves. This elongation at break is the so-called critical elongation LME. The higher the critical strain LME, the more resistant the specimen to LME. The methodology is also explained in a publication entitled "Critical LME Elongation: Un essai Gleeble pour èvaluer la sensibilité au LME d'un acier revêtu soudé par points (Gleeble test for evaluating LME sensitivity in spot welding of coated steel)", Journées Annuelles SF2M 2017 , October 23-25, 2017, JA0104, ArcelorMittal-Research Maizières-lès-Metz.

The results are presented in the following table 3.

Table 3

Trial Temperature(°C) Critical elongation LME (%) 1* 800 46 850 38 900 38 2 800 7 850 13 900 5 3 800 48 850 49 900 40

*: according to the present invention

The results show that Sample 1 has improved LME resistance compared to Sample 2. Sample 1 and Sample 3 have the same LME resistance.

Example 2: three sheets superimposed one on top of the other

The LME sensitivity of various assemblies was evaluated by resistance spot welding. For this purpose, for each test, three steel sheets are welded by resistance spot welding.

Test 6 represents the assembly of sample 1 with two galvanized steel sheets of composition 2.

Test 7 is an assembly of sample 2 with two galvanized steel sheets of composition 2.

Test 8 is an assembly of sample 4 with two galvanized steel sheets of composition 2.

Test 9 is an assembly of sample 5 with two galvanized steel sheets of composition 2.

Type of welding electrode F1 with an end diameter of 6 mm; the clamping force of the electrode is 450 daN (decaNewton). The welding cycle is presented in table 4.

Table 4

Welding period Current (Hz) Welding period (ms) Time between current pulses (ms) Cyclical 50 380 260

Each test is repeated 10 times to make 10 spot welds at a current strength defined as the upper limit of the welding range: Imax, Imax is 0.9 - 1.1 * I _exp , I _exp represents the intensity above which during welding, splash, I _exp is determined in accordance with ISO 18278-2.

The longest crack length in the spot weld joint is then evaluated after the cross section of the surface crack and using an optical microscope, as indicated in the following Table 5. The LME crack resistance is evaluated for 10 spot welds (representing a total of 100%).

Table 5

Trial No cracks 0 < Crack
< 100 µm 100 µm < Crack
< 200 µm Crack > 0.5* assembly thickness (assembly thickness =1.0 mm) Sample 6* 70% 20% 10% - Sample 7 thirty% 10% thirty% thirty% Sample 8* 20% 50% 20% 10% Sample 9 - thirty% thirty% 40%

*: according to the present invention.

Samples 6 and 8 according to the present invention show a corresponding resistance to LME compared to samples 7 and 9.

Claims (47)

1. Pre-coated steel substrate containing the first pre-coating consisting of titanium and having a thickness of 40-1200 nm, optional intermediate layer pre-coat containing at least 8 wt. % Nickel and at least 10 wt. % chromium, iron - the rest, or optionally an intermediate precoat layer containing Fe, Ni, Cr and Ti, in which the Ti content is greater than or equal to 5 wt.%, and the following equation is fulfilled: 8 wt.% < Cr + Ti < 40 wt.%, Fe and Ni - the rest, and the intermediate layer of the preliminary coating has a thickness of 2-30 nm, the second pre-coating, which is a zinc-based coating, while the specified steel substrate contains more than 0.05 wt.% Si. 2. Steel substrate according to claim. 1, in which the thickness of the first preliminary coating is 40-80 nm. 3. Steel substrate according to claim 1, wherein the thickness of the first precoat is 80-150 nm. 4. Steel substrate according to claim 1, wherein the thickness of the first precoat is 150-250 nm. 5. Steel substrate according to claim 1, wherein the thickness of the first precoat is 250-450 nm. 6. Steel substrate according to claim 1, wherein the thickness of the first precoat is 450-600 nm. 7. Steel substrate according to claim. 1, in which the thickness of the first preliminary coating is 600-850 nm. 8. Steel substrate according to claim. 1, in which the thickness of the first preliminary coating is 850-1200 nm. 9. Steel substrate according to any one of paragraphs. 1-8, in which the intermediate pre-coating layer consists of stainless steel containing 10-13 wt.% nickel, 16-18 wt.% chromium, iron - the rest. 10. Steel substrate according to any one of paragraphs. 1-9, in which the second pre-coating, which is a coating based on zinc, contains 1.2 wt.% Al, 1.2 wt.% Mg, Zn - the rest or 3.7 wt.% Al, 3 wt.% Mg , Zn - rest. 11. Steel substrate according to any one of paragraphs. 1-9, in which the second pre-coating, which is a zinc-based coating, contains optionally 0.10-0.40 wt.% Al, the rest is zinc. 12. Steel substrate according to any one of paragraphs. 1-11, in which the steel substrate has the following chemical composition, wt.%: 0.05 ≤ C ≤ 0.4, 0.5 ≤ Mn ≤ 30.0, 0.05 ≤ Si ≤ 3.0 and optionally 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, iron and inevitable impurities - the rest. 13. A method of manufacturing a pre-coated steel substrate according to any one of paragraphs. 1-12, including the following successive stages: a) providing steel backing, B) optional surface preparation of said steel substrate, C) applying a layer of the first pre-coating, consisting of titanium and having a thickness of 40-1200 nm, D) optionally applying an intermediate layer of pre-coating containing at least 8 wt. % Nickel and at least 10 wt. % chromium, iron - the rest, or an intermediate precoat layer containing Fe, Ni, Cr and Ti, in which the content of Ti is greater than or equal to 5 wt.%, and the following equation is fulfilled: 8 wt.% <Cr + Ti < 40 wt. .%, Fe and Ni - the rest, and the said intermediate pre-coating layer has a thickness of 2-30 nm, E) applying a layer of a second pre-coating based on zinc. 14. The method according to claim 13, 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 vacuum physical deposition. 15. The method according to claim 14, wherein in steps C) and D) the application of the first precoat layer and the intermediate precoat layer is carried out independently of each other by means of magnetron cathode sputtering or inkjet vapor deposition. 16. A method for manufacturing an assembly from at least two pre-coated steel substrates, comprising the following successive steps: providing at least two steel substrates, each of which is a pre-coated steel substrate according to any one of paragraphs. 1-12 or obtained by the method according to any one of paragraphs. 13-15, and welding of said steel substrates. 17. The method of claim 16 wherein the welding is performed by spot welding or arc welding. 18. Assembly of at least two pre-coated steel substrates according to any one of paragraphs. 1-12 connected by welding. 19. The assembly of claim. 18, in which the steel substrate contains internal oxides of the alloying elements of the steel. 20. The assembly of claim. 19, in which the internal oxides of the alloying elements of the steel substrate include the following oxides: silicon oxides, manganese oxides, chromium oxides, aluminum oxides, or mixtures thereof. 21. Application of the assembly manufacturing method according to claim 16 or 17 for the manufacture of vehicle parts. 22. The use of an assembly of at least two pre-coated steel substrates according to any one of paragraphs. 18-20, connected by welding, for the manufacture of vehicle parts.

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