(SAW)

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, shipbuilding and offshore industries.

It is known to use steel parts in construction and equipments in the energy sector. 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 a bottleneck in production since the steel substrates are thick. There is not a deep weld penetration and several welding steps are needed to fully weld the steel substrates.

Sometimes, steel parts are welded by Submerged arc welding (SAW) which is a common arc welding process. SAW requires a continuously fed consumable solid or tubular (metal cored) electrode. The molten weld and the arc zone are protected from atmospheric contamination by being "submerged" under a blanket of granular fusible flux. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. SAW is normally operated in the automatic or mechanized mode, however, semi-automatic (hand-held) SAW guns with pressurized or gravity flux feed delivery are available.

The patent application US3393102 discloses a Submerged Arc Welding flux comprising silicon dioxide, manganese dioxide and certain emitter oxides. The emitter oxides are the oxides of calcium, magnesium, aluminum, and titanium. All of the flux ingredients are ground to a fine powder, thoroughly mixed and then bound together into granules of a preferred size to pass through a 16 mesh, corresponding to 1 190pm, and remain on a 100 mesh screen, 100 mesh corresponding to 149pm.

Nevertheless, this flux is protecting the arc and, in combination with the consumable wire, reacting with the melt pool to produce the adequate chemical composition and mechanical properties, but not productivity improvement or higher penetration is revealed. 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 SAW welding with an increase in the deposition rate and productivity, 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 22.

Finally, the invention relates to the use of the assembly according to claim 23.

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. Indeed, 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, arc constriction, the reverse Marangoni effect and an improvement of arc stability.

Indeed, the titanate mixed with nanoparticles allows for a keyhole effect 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, caused by its vaporization, which allows the energy beam to penetrate even more deeply resulting in a deeper penetration and an increase in the deposition rate. Energy is delivered very efficiently into the join, which maximized weld depth to width ratio, which in turn limits part distortion. Moreover, the pre-coating modifies the Marangoni flow, which is the mass transfer along an interface between 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 flow towards the center of the weld pool which in this case results in more penetration depth and more material deposition leading to an increase in productivity.

Preferably, the pre-coating comprises at least one titanate chosen from among: IS^TbOz, 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 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 from TiC>2, 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 modify the melt pool physics allowing a deeper resulting penetration of the weld metal. Moreover, without willing to be bound by any theory, it is believed that the nanoparticles diameter further improves the homogeneous distribution of the coating.

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 diameter 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, the arc constriction and the reverse Marangoni effect.

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.

Preferably, the pre-coated steel substrate comprises a shielding flux. It seems that this flux protects the steel substrate against oxidation during the welding process.

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 submerged arc welding (SAW) welding.

Preferably, in step II), the electric current average is between 1 and 1000A.

Preferably, in step II), the voltage of the welding machine is between 1 and 100 V.

Preferably, in step II), there is a consumable electrode (so-called wire). For example, the consumable electrode is made of Fe, Si, C, Mn, Mo and/or Ni.

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

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

- a welded zone comprises the dissolved and/or precipitated pre-coating comprising at least one titanate and at least one nanoparticle.

Preferably, the nanoparticle is chosen from among: T1O2, S1O2, Yttria- stabilized zirconia (YSZ), AI2O3, M0O3, CrC>3, CeC>2 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 SAW, at least a part of titanate and nanoparticles are present in the steel substrate.

Preferably, when the Al amount of the steel substrate is above 50ppm, the steel substrate comprises Al precipitates, along with component of the coating.

Finally, the invention relates to the use of an assembly according to the present invention for the manufacture of pressure vessels, offshore components. 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:

Example 1 :

Trial 1 was not coated with a pre-coating.

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 20mm thick.

Trials 1 and 2 were then coated with a shielding flux is an agglomerated basic mild steel low alloy. Finally, Trial 1 and 2 were joined with a steel substrate having the above composition by SAW welding. The welding parameters are in the following Table 2:

The composition of the consumable electrode used in both Trials 1 and 2 is in the following Table 3:

After the SAW welding, the welding penetration into the steel substrates and the steel microstructure were analyzed by Scanning Electron Microscopy (SEM). Trials were bended until 180° according to the norm ISO 15614-7. The hardness of both T rials 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). The residual stress of the welded area was determined by simulations. Results are in the following Table 4:

^* : according to the present invention

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

Example 2

Different coatings were tested by Finite Element Method (FEM) simulations on the steel substrates. In the simulations, the pre-coating comprises optionally MgTiC>3 (diameter: 2pm) and nanoparticles having a diameter of 10-50 nm nm. The thickness of the coating was of 40pm. Arc welding was simulated with each pre coating. results of the Arc welding by simulations are in the following Table 5:

Trials Coating composition (wt.%) Results

Results shown that Trials according to the present invention improve the arc welding.