The present invention relates to a pre-coated steel substrate wherein the coating comprising at least one titanate and at least one nanoparticle, said steel substrate having a reflectance higher or equal to 60% at all wavelengths between 6.0 and 15.0pm; 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.

Sometimes, steel parts are welded by Laser Beam welding which is a common welding process. Laser beam welding (LBW) is a welding technique used to join pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications using automation, such as in the automotive industry. It is based on keyhole or penetration mode welding. LBW is a process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. Due to high cooling rates, cracking is a concern when welding high- carbon steels. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the workpieces. The high-power capability of gas lasers makes them especially suitable for high volume applications. LBW is particularly dominant in the automotive industry.

Nevertheless, especially for carbon steels, there is a need to improve the welding penetration and to reduce the risk of cracks of carbon steels.

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 Laser 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 10.

The invention relates to a method for the manufacture of this pre-coated metallic substrate according to anyone of claims 1 1 to 14.

The invention also relates to a method for the manufacture of an assembly according to claims 15 to 17.

The invention relates to an assembly according to claims 18 to 21 .

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

22.

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 pre-coating comprising at least one titanate and at least one nanoparticle,

- said bare steel substrate having a reflectance higher or equal to 60% at all wavelengths between 6.0 and 15.0pm.

Without willing to be bound by any theory, it is believed that the pre-coating mainly modifies the melt pool physics of the steel substrate allowing a deeper melt penetration. It seems that in the present invention, not only the nature of the compounds, but also the size of the particles being equal or below 100nm improve the penetration thanks to the keyhole effect, the Marangoni effect and an increase of absorbance.

Indeed, the titanate mixed with nanoparticles enhances the keyhole effect weld which causes a deep penetration. The keyhole refers to a literal hole in the steel substrate, caused by its vaporization, which allows the energy beam to penetrate even more deeply. Energy is delivered very efficiently into the join, which maximized weld depth and minimizes the heat affected zone, which in turn limits part distortion.

Moreover, the pre-coating improves 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 pre-coating. This modification of surface tension results in an inversion of the fluid flows towards the center of the weld pool which in this case results in more welded depth.

Finally, it seems that the chosen nanoparticles increase the absorbance of steel substrate leading to higher penetration.

Preferably, the pre-coating comprises at least one titanate chosen from among: Na2Tb07, K2T1O3, K2T12O5 MgTiCb, SrTiCb, BaTiCb, and CaTiC , FeTiCb and ZnTiCU or a mixture thereof. Indeed, without willing to be bound by any theory, it is believed that these titanates further increase the deposition of the metallic coating and increase the coating penetration depth based on the 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%.

Preferably, the pre-coating comprises at least nanoparticles is chosen fromPO2, S1O2, Yttria-stabilized zirconia (YSZ), AI2O3, M0O3, CrCb, CeC>2 or a mixture thereof. Indeed, without willing to be bound by any theory, it is believed that these nanoparticles further decrease the reflectance and modify the melt pool physics allowing a deeper penetration of the weld metal.

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

Advantageously, the pre-coating 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 pre-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 thickness of the coating is between 10 to 140 pm, more preferably between 30 to 100 pm.

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 keyhole effect and the Marangoni effect.

According to the present invention, the bare metallic substrate has a reflectance higher or equal to 60%, more preferably above or equal to 70%, at all wavelengths between 6.0 and 15.0pm, preferably between 8.0 and 13.0pm and for Example between 9.0 and 1 1 .0pm. Indeed, without willing to be bound by any theory, it is believed that the reflectance of the metallic substrate depends on the wavelengths of the laser source.

With the pre-coating according to the present invention, it is believed that the metallic substrate is reduced to be below 30%, preferably below 20% at all wavelengths between 6.0 and 15.0pm.

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 pre-coating 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 pre-coating is performed by spin coating, spray coating, dip coating or brush coating.

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

Preferably, in step B), the pre-coating 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 pre-coating 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 of at least two metallic substrates by Laser welding, the Laser welding machine having a laser having wavelengths between 6.0 and 15.0pm.

Preferably, in step II), the laser deposition is performed with a shielding gas being an inert gas and/or active gas. For example, the inert gas is chosen from helium, neon, argon, krypton, xenon or a mixture thereof. For example, the active gas is chosen from among: CO2, CO, and a mixture of thereof. For example, the shield gas comprises 60-85v.% of helium, 13-55v.% of nitrogen and 1 -9v.% of carbon dioxide.

Preferably, in step II), the laser power is between 1 and 20 kW, more preferably between 1 and 10kW.

According to the present invention, the laser source has wavelengths between 6.0 and 15.0pm, preferably between 8.0 and 13.0pm and for Example between 9.0 and 1 1 .Opm.

With the method according to the present invention, an assembly of at least two metallic substrates at least partially welded together through Laser welding is obtained, said assembly comprising:

- at least one bare 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,

- said bare steel substrate having a reflectance higher or equal to 60% at wavelengths between 6.0 and 15.0pm.

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 Laser 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 an assembly according to the present invention for the manufacture of a part for automotive or shipbuilding.

With a view to highlighting 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:

The reflectance of the steel substrates was of 90% at wavelengths of 10.6pm. These wavelengths are commonly used in laser sources of CO2 Laser welding.

Example 1 :

Trial 1 was not coated.

For Trial 2, 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 MgTiC>3 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, Trial 2 was coated with 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.%. The coating thickness was of 40pm. The steel substrate was 4mm thick. Then, Trial 1 and 2 were joined with a steel substrate having the above composition by Laser welding. The welding parameters are in the following Table 2:

After the Laser welding, the welding penetration into the steel substrates and the steel microstructure were analyzed by Scanning Electron Microscopy (SEM). The composition of the welded area was analyzed by Energy-Dispersive X-ray Spectroscopy (EDS). The reflectance and the residual stress of the welded area was determined by simulations. Results are in the following Table 3:

^* : according to the present invention

Results shown that Trial 2 improves the Laser welding compared to comparative Trial 1 .