METAL SHEETS AND USE THEREOF

The present invention relates to a method for the manufacture of a coated steel sheet. The invention is particularly well suited for the manufacture of automotive vehicles.

Zinc based coatings are generally used because they allows for a protection against corrosion, thanks to barrier protection and cathodic protection. The barrier effect is obtained by the application of a metallic coating on steel surface. Thus, the metallic coating prevents the contact between steel and corrosive atmosphere. The barrier effect is independent from the nature of coating and substrate. On the contrary, sacrificial cathodic protection is based on the fact that zinc is a metal less noble that steel. Thus, if corrosion occurs, zinc is consumed preferentially to steel. Cathodic protection is essential in areas where steel is directly exposed to corrosive atmosphere, like cut edges where the surrounding zinc will be consumed before steel.

However, when heating steps are performed on such zinc coated steel sheets, for example hot press hardening or welding, cracks are observed in steel which spread from the steel/coating interface. Indeed, occasionally, there is a reduction of metal mechanical properties due to the presence of cracks in coated steel sheet after the above operation. These cracks appear with the following conditions: high temperature; contact with a liquid metal having a low melting point (such as zinc) in addition to presence of tensile stress; heterogeneous diffusion of molten metal with substrate grain and grain boundaries. The designation for such phenomenon known as liquid metal embrittlement (LME), and also called liquid metal assisted cracking (LMAC).

US2012100391 discloses a method for manufacturing a hot-dip galvanized steel sheet having good plating qualities, plating adhesion and spot weldability, the method comprising:

- coating a base steel sheet with Ni in a coating amount (CM) of 0.1 -1 .0 g/m ² ;

- heating the Ni-coated steel sheet in a reducing atmosphere;

- cooling the heated steel sheet to the temperature (Xs) at which the steel sheet is fed into a galvanizing bath; and - feeding and immersing the cooled steel sheet in the galvanizing bath having an effective Al concentration (CAI) of 0.1 1 -0.14 wt % and a temperature (T _p ) of 440- 460° C, wherein the temperature (Xs) at which the steel sheet is fed into the galvanizing bath satisfies the following relationship: CNi-(Xs ^_ Tp)/2CAi=5-100.

It also discloses a hot-dip galvanized steel sheet wherein the alloy phase is a Fe- Zn alloy phase accounting for 1 -20% of the cross-sectional area of the galvanized layer.

However, in the above method, galvanizing was carried out in a bath comprising from 0.1 1 to0.14wt.% Al bath and thus inhibition layer was very week and Fe-Zn intermetallic phases formed. In industrial scale, this method is difficult to apply since the spot weldability depends on controlling parameters, including the amount of Ni coated, the Al concentration of the galvanizing bath, and the difference between the temperature of the galvanizing bath and the temperature at which the steel sheet is fed into the galvanizing bath. Moreover, the spot weldability performed is evaluated based on the electrode life, i.e. the number of continuous welding spots at the time when the nugget diameter reached 4Vt (t: steel sheet thickness) was measured. There is no mention of a reduction of the presence of cracks in coated steel sheet after the spot welding.

Thus, the object of the invention is to provide a steel sheet coated with a metallic coating which does not have LME issues. It aims to make available, in particular, an easy to implement method in order to obtain a part which does not have LME issues after the forming and/or the welding.

This object is achieved by providing a method according to claim 1 . The method can also comprise any characteristics of claims 2 to 15.

Another object is achieved by providing a steel sheet according to claim 17.

The steel sheet can also comprise any characteristics of claims 16 to 22.

Another object is achieved by providing a spot welded joint according to claim 24. The spot welded joint can also comprise characteristics of claims 23 to 26.

Finally, another object is achieved by providing the use of the steel sheet or the assembly according to claim 27. Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

The designation "steel" or "steel sheet" means a steel sheet, a coil, a plate having a composition allowing the part to achieve a tensile strength up to 2500 MPa and more preferably up to 2000MPa. For example, the tensile strength is above or equal to 500 MPa, preferably above or equal to 980 MPa, advantageously above or equal to 1 180 MPa and even above or equal 1470 MPa.

The invention relates to method for the manufacture of a coated steel sheet comprising the following successive steps:

A. The provision of a annealed steel sheet having the following chemical composition in weight:

0.10 < C < 0.40%,

1 .5 < Mn < 3.0%,

0.7 < Si < 2.0%,

0.05 < Al < 1 .0%,

0.75 < (Si+AI) < 3.0 %,

and on a purely optional basis, one or more elements such as

Nb < 0.5 %,

B < 0.005%,

Cr≤ 1 .0%,

Mo < 0.50%,

Ni < 1 .0%,

Ti < 0.5%,

the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, such steel sheet being annealed at a temperature between 600 to 1200°C,

B the coating of the steel sheet obtained in step A) with a first coating comprising nickel, such first coating not comprising iron, having a thickness equal or above 0.5μιη and

C the coating of the steel sheet obtained in step B) with a second coating based on zinc. Without willing to be bound by any theory, it seems that Ni, being present at the interface between the steel having the above specific chemical composition and the overlying zinc coating, prevents liquid zinc penetration into steel during any heating steps being for example a welding. Thus, by applying the method according to the present invention, it is possible to obtain a barrier layer to LME.

Preferably, in step A), the steel sheet is annealed in a continuous annealing. For example, the continuous annealing comprises a heating, a soaking and a cooling step. It can further comprise a pre-heating step.

Advantageously, the thermal treatment is performed in an atmosphere comprising from 1 to 30% of H ₂ at a dew point between -10 and -60°C. For example, the atmosphere comprises from 1 to 10% of H ₂ at a dew point between -10°C and -60°C.

In step B), the first coating comprising nickel is deposited by any deposition method known by the man skilled in the art. It can be deposited by vacuum deposition or electro-plating method. Preferably, it is deposited by electro-plating method.

Preferably, in step B), the first coating comprises above 80%, more preferably above 90% by weight of nickel. Preferably, in step B), the first coating does not comprise phosphorus, nickel hydroxide or sulfur compounds such as sulfate salt.

In a preferred embodiment, the first coating consists of nickel. In this embodiment, the amount of nickel is >99% by weight and preferably is of 100%.

Preferably, in step A), the first coating has a thickness equal or above Ι .Ομιτι and advantageously equal or above 1 .6 μιη. More preferably, the first coating has a thickness between 1 .8 to 7.0μιη.

Advantageously, in step C), the second layer comprises above 50%, more preferably above 75% of zinc and advantageously above 90% of zinc. Preferably, the second layer does not comprise nickel. The second layer can be deposited by any deposition method known by the man skilled in the art. It can be by hot-dip coating, by vacuum deposition or by electro-galvanizing.

For example, the coating based on zinc comprises 0.01 -8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn. Preferably, the coating based on zinc is deposited by hot-dip galvanizing. In this embodiment, the molten bath can also comprise unavoidable impurities and residuals elements from feeding ingots or from the passage of the steel sheet in the molten bath. For example, the optionally impurities are chosen from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the content by weight of each additional element being inferior to 0.3% by weight. The residual elements from feeding ingots or from the passage of the steel sheet in the molten bath can be iron with a content up to 5.0%, preferably 3.0%, by weight.

In a preferred embodiment, the second layer consists of zinc. When the coating is deposited by hot-dip galvanizing, the percentage of Aluminum is comprised between 0.15 and 0.40% in the bath.

With the method according to the present invention, a steel sheet coated with a first coating comprising nickel and having a thickness equal or above Ο.δμιη, such coating being directly topped by a zinc based layer, is obtained. It is believed that the first coating acts like a barrier layer to LME and prevent liquid metal to penetrate inside into the steel.

Preferably, the steel sheet has a microstructure comprising from 1 to 50% of residual austenite, from 1 to 60% of martensite and optionally at least one element chosen from: bainite, ferrite, cementite and pearlite. In this case, the martensite can be tempered or untempered.

In a preferred embodiment, the steel sheet has a microstructure comprising from 5 to 25 % of residual austenite.

Preferably, the steel sheet has a microstructure comprising from 1 to 60% and more preferably between 10 to 60% of tempered martensite.

Advantageously, the steel sheet has a microstructure comprising from 10 to

40% of bainite, such bainite comprising from 10 to 20% of lower bainite, from 0 to 15% of upper bainite and from 0 to 5% of carbide free bainite.

Preferably, the steel sheet has a microstructure comprising from 1 to 25% of ferrite.

Preferably, the steel sheet has a microstructure comprising from 1 to 15% untempered martensite. After the manufacture of a steel sheet, in order to produce some parts of a vehicle, it is known to assembly by welding two metal sheets. Thus, a spot welded joint is formed during the welding of at least two metal sheets, said spot being the link between the at least two metal sheets.

To produce a spot welded joint according to the invention, the welding is performed with an effective intensity is between 3kA and 15kA and the force applied on the electrodes is between 150 and 850 daN with said electrode active face diameter being between 4 and 10mm.

Thus, a spot welded joint of at least two metal sheets, comprising the coated steel sheet according to the present invention, is obtained, such said joint containing less than 3 cracks having a size above Ι ΟΟμιτι and wherein the longest crack has a length below 300 μιη. Preferably, the second metal sheet is a steel sheet or an aluminum sheet. More preferably, the second metal sheet is a steel sheet according to the present invention.

In another embodiment, the spot welded joint comprises a third metal sheet being a steel sheet or an aluminum sheet. For example, the third metal sheet is a steel sheet according to the present invention.

The steel sheet or the spot welded joint according to the present invention can be used for the manufacture of parts for automotive vehicle.

The invention will now be explained in trials carried out for information only.

They are not limiting.

Example

For all samples, steel sheets used have the following composition in weight percent: C=0.37wt.%, Mn=1 .9wt.%, Si=1 .9wt.%, Cr=0.35wt.%, Al=0.05wt.% and Mo=0.1 wt.%.

Trials 1 to 4 were prepared by performing an annealing in a continuous annealing in an atmosphere comprising 5% of H ₂ and 95% of N ₂ at a dew point of -60°C. The steel sheets were heated at a temperature of 900°C. Then, Trials 1 to 4 were coated with a different nickel coating thicknesses deposited by electro- galvanizing method. Finally, a zinc coating was deposited by electro-galvanizing method. Trial 5 was prepared by deposited a zinc coating by electro-galvanizing method after the continuous annealing of the steel sheet under similar atmosphere.

The resistance to LME of above Trial samples were evaluated by resistance spot welding method. To this end, for each Trial, two coated steel sheets were welded together by resistance spot welding. The type of the electrode was ISO Type B with a diameter of 16mm; the force of the electrode was of 5kN and the flow rate of water of was 1 .5g/min. The details welding cycle has been reported in Table 1

Table 1. Welding schedule

The number of cracks above Ι ΟΟμιη was then evaluated using an optical microscope as well as SEM (Scanning Electron Microscopy) as shown in Table 2.

Table 2. LME crack details after spot welding (2 layer stack-up condition)

^* : according to the present invention.

Trials according to the present invention show an excellent resistance to LME as compared to Trial 5. Indeed, the number of cracks of Trials according to the present invention is very low, even nonexistent, compared to Trial 5. For each Trial, three coated steel sheets were also welded together by resistance spot welding. The number of cracks above Ι ΟΟμιη was then evaluated using an optical microscope as well as SEM (Scanning Electron Microscopy) as shown in Table 3.

Table 3. LME crack details after spot welding (3 layer stack-up condition)

^* : according to the present invention.

Trials according to the present invention show an excellent resistance to LME as compared to Trial 5.