The present invention relates to a pre-coated steel substrate wherein the coating comprising at least one titanate and at least one nanoparticle, a method for the manufacture of an assembly; a method for the manufacture of a coated metallic substrate and a coated metallic substrate. It is particularly well suited for construction and automotive industries.

It is known to use steel parts to produce vehicles. Usually, the steel parts can be made of high strength steel sheets to achieve lighter weight vehicle bodies and improve crash safety. The manufacture of steel parts is generally followed by the welding of at least two metallic substrates comprising the steel part with another metallic substrate. The welding of at least two metallic substrates can be difficult to realize since there is not a deep weld penetration in steel substrates, requiring several welding passes and compromising productivity.

Sometimes, steel parts are welded by Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding. TIG is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area and electrode are protected from oxidation or other atmospheric contamination by an inert shielding gas (argon or helium), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.

The patent application WO00/16940 discloses that penetration gas tungsten arc welds are achieved using titanates such as Na2Tb07 or K2Ti03. Titanate is applied to the weld zone in a carrier fluid paste or as part of a wire filler to afford deep penetration welds in carbon, chrome-molybdenum, and stainless steels as well as nickel-based alloys. To control arc wander, bead consistency, and slag and surface appearance of the weldments, various additional components may be optionally added to the titanate flux including transition metal oxides such as TiO, T1O2, Cr203, and Fe203, silicon dioxide, manganese silicides, fluorides and chlorides. In addition, it is disclosed that a flux of titanium oxides, Fe203 and Cr203 affords weld penetration in carbon steels and nickel-based alloys but with some heat-to-heat variation.

The patent application discloses that the titanate compounds typically are used in the form of high-purity powders of about 325 mesh or finer, 325 mesh corresponding to 44pm. The requisite amount of titanate in a particular composition should be sufficient to afford a thin open or closed coating of a 325 mesh titanate when all other components are removed. The compounds of the flux have all micrometers dimensions.

Although the penetration is improved with the flux discloses in WO00/16940, the penetration is not optimum for steel substrates.

Thus, there is a need to improve the weld penetration in steel substrates and therefore the mechanical properties of a welded steel substrates. There is also a need to obtain an assembly of at least two metallic substrates welded together by TIG welding, said assembly comprising a steel substrate.

To this end, the invention relates to a pre-coated metallic substrate according to anyone of claims 1 to 7.

The invention relates to a method for the manufacture of this pre-coated metallic substrate according to anyone of claims 8 to 1 1.

The invention also relates to a method for the manufacture of an assembly according to claims 12 to 14.

The invention relates to an assembly according to claims 15 to 18.

Finally, the invention relates to the use of the assembly according to claim 19.

The following term is defined:

- Nanoparticles are particles between 1 and 100 nanometers (nm) in size.

The invention relates to a pre-coated steel substrate coated with:

- optionally, an anticorrosion coating and

- a flux comprising at least one titanate and at least one nanoparticle chosen from: T1O2, S1O2, Yttria-stabilized zirconia (YSZ), AI2O3, M0O3, CrC>3, CeC>2 or a mixture thereof, the flux thickness being between 30 and 95pm.

Indeed, without willing to be bound by any theory, it is believed that the flux mainly modifies the melt pool physics of the steel substrate allowing a deeper melt penetration. On contrary to the patent application WO00/16940 wherein the titanate compound is the essential component to control in the formulations to improve the deep penetration welds, it seems that in the present invention, not only the nature of the particles, but also the size of the particles being equal or below 100nm improve the penetration thanks to the keyhole effect caused by the depression of the surface of the melt pool, the reverse Marangoni effect, the arc constriction and an improvement of arc stability.

Indeed, the titanate mixed with the specific nanoparticles allows for a keyhole mode due to the combined effects of the constriction of the arc by electrical insulation, resulting in higher current density and an increase in weld penetration. The keyhole effect refers to a literal hole, a depression in the surface of the melt pool, which allows the energy beam to penetrate even more deeply. Energy is delivered very efficiently into the join, which maximized weld depth and increases weld depth to width ratio, which in turn limits part distortion.

Moreover, the flux reverses the Marangoni flow, which is the mass transfer between two fluids due to the surface tension gradient, which is modified by the components of the flux. This modification of surface tension results in an inversion of the fluid flow towards the center of the weld pool, which in this case results in more welded depth.

Preferably, the percentage in weight of the nanoparticles is below or equal to 80% and preferably between 2 and 40%.

Preferably, the titanate has a particle size distribution between 1 and 40pm, more preferably between 1 and 20pm and advantageously between 1 and 10pm. Indeed, without willing to be bound by any theory, it is believed that this titanate diameter further improves the the depression of the surface of the melt pool, the arc constriction and the reverse Marangoni effect.

Preferably, the flux comprises at least titanate chosen from among: Na2TbC>7, K2T1O3, K2T12O5, MgTiCb, SrTiCb, BaTiCb, and CaTiCb, FeTiCb and ZnTiCb or a mixture thereof. More preferably, the titanate is MgTiCb. Indeed, without willing to be bound by any theory, it is believed that these titanates further increase penetration depth based on the effect of the reverse Marangoni flow. Preferably, the percentage in weight of at least one titanate is above or equal to 45% and for example of 50 or of 70%.

Advantageously, the flux further comprises an organic solvent. Indeed, without willing to be bound by any theory, it is believed that the organic solvent allows for a well dispersed coating. Preferably, the organic solvent is volatile at ambient temperature. For example, the organic solvent is chosen from among: acetone, methanol and ethanol.

Preferably, the anti-corrosion coating layer(s) include a metal selected from among the group comprising zinc, aluminum, copper, silicon, iron, magnesium, titanium, nickel, chromium, manganese and their alloys.

In a preferred embodiment, the anti-corrosion coating is an aluminum-based coating comprising 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. in another preferred embodiment, the anti-corrosion coating is a zinc-based coating comprising 0.01 - 8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn.

The invention also relates to a method for the manufacture of the pre-coated metallic substrate, comprising the successive following steps:

A. The provision of a steel substrate according to the present invention,

B. The deposition of the flux according to the present invention,

C. Optionally, the drying of the coated metallic substrate obtained in step B).

Preferably, in step B), the deposition of the flux is performed by spin coating, spray coating, dip coating or brush coating.

Advantageously, in step B), the flux comprises from 1 to 200 g/L of nanoparticles, more preferably between 5 and 75 g.L ¹ .

Preferably, in step B), the flux comprises from 100 to 500 g/L of titanate, more preferably between 175 and 250 g.L ¹ .

When a drying step C) is performed, the drying is performed by blowing air or inert gases at ambient or hot temperature.

Preferably, the drying step C) is not performed when the organic solvent is volatile at ambient temperature. Indeed, it is believed that after the deposition of the coating, the organic solvent evaporates leading to a dried flux on the metallic substrate.

The invention also relates to a method for the manufacture of 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. The welding by tungsten inert gas (TIG) welding of the at least two metallic substrates

Preferably, in step II), the welding is performed with a shield gas being an inert gas. For example, the inert gas is chosen from helium, neon, argon, krypton, xenon or a mixture thereof. Advantageously, the inert gas comprises at least argon.

Preferably, in step II), the electric current during welding is between 10 and

200A.

With the method according to the present invention, an assembly of at least two metallic substrates at least partially welded together through tungsten inert gas (TIG) welding is obtained, said assembly comprising:

- at least one steel substrate coated with optionally an anticorrosion coating,

- a welded zone comprises the dissolved and/or precipitated flux comprising at least one titanate and at least one nanoparticle chosen from: T1O2, S1O2, Yttria-stabilized zirconia (YSZ), AI2O3, M0O3, Cr03, Ce02 or a mixture thereof.

Preferably, the second metallic substrate is a steel substrate or an aluminum substrate. More preferably, the second steel substrate is a pre-coated steel substrate according to the present invention.

Preferably, the at least two metallic substrates comprises dissolved and/or precipitated titanate and nanoparticles.

Advantageously, the steel substrate comprises dissolved and/or precipitated titanate and nanoparticles. Indeed, it seems that during TIG welding, at least a part of titanate and nanoparticles is present in the steel substrate.

Preferably, when the Al amount of the steel substrate is above 50ppm, the steel substrate comprises Al precipitates. Finally, the invention relates to the use of the coated metallic substrate according to the present invention for the manufacture of piping elements and parts of structures.

With a view to highlight the enhanced performance obtained through using the assemblies according to the invention, some concrete examples of embodiments will be detailed in comparison with assemblies based on the prior art.

Examples

For the Trials, the steel substrates having the chemical composition in weight percent disclosed in Table 1 were used:

Example 1 :

For Trials 1 to 3, an acetone solution comprising MgTiC>3 (diameter: 2pm), S1O2 (diameter: 10nm) and T1O2 (diameter: 50nm) was prepared by mixing acetone with said elements. In the acetone solution, the concentration of MgTi03 was of 175 g.L ¹ . The concentration of S1O2 was of 25g.L ¹ . The concentration of T1O2 was of 50 g.L ¹ . Then, Trials 1 to 3 were coated with different thicknesses of the acetone solution by spraying. The acetone evaporated. The percentage of MgTiC>3 in the coating was of 70wt.%, the percentage of S1O2 was of 10wt.% and the percentage of T1O2 was of 20wt.%.

Trial 4 was coated with an acetone solution comprising microparticles of MgTiC>3 (diameter: 2pm), S1O2 (diameter: 2pm) and T1O2 (diameter: 2pm).

Trial 5 was not coated.

Then, the TIG welding was applied on each Trial. The welding parameters are in the following Table 2:

After the IG welding, the aspect of the coating was analyzed by naked eyes and by Field Emission Gun-Scanning Electron Microscopy (FEG-SEM). Thermal images of the welding arc on the coatings were taken. The penetration of the coatings into the steel substrates was analyzed by Scanning Electron Microscope (SEM). Trials were bended until 180° according to the norm ISO 15614-7. The hardness of both Trials was determined in the center of the welded area using a microhardness tester. The composition of the welded area was analyzed by Energy Dispersive X-ray Spectroscopy and inductively coupled plasma emission spectroscopy (ICP-OES). Results are in the following Table 3:

^* : according to the present invention

Results shown that Trial 2 improves the TIG welding compared to comparative Trials.

Example 2 Different coatings were tested by Finite Element Method (FEM) simulations on the steel substrates. In the simulations, the flux comprises optionally MgTiC>3 (diameter: 2pm) and nanoparticles having a diameter of 10-50 nm. The thickness of the coating was of 40pm. Arc welding was simulated with each flux results of the Arc welding by simulations are in the following Table 4:

Results shown that Trial according to the present invention improve the TIG welding compared to comparative Trials.