Patent Description:
Zinc based coatings are generally used because they allow for protection against corrosion, thanks to barrier as well as cathodic protection. The barrier effect is obtained by the application of the 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 the coating and the substrate. On the contrary, sacrificial cathodic protection is based on the fact that zinc is a metal less noble than steel. Thus, if corrosion occurs, zinc is consumed preferentially as compared to steel. Cathodic protection is essential in areas where steel is directly exposed to corrosive atmosphere, like cut edges where surrounding zinc will be consumed before steel.

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

The patent application<CIT> discloses a surface-treated steel sheet obtained by subjecting both surfaces of a steel sheet to electroplating with any one of Ni, Cr, Zn, Zn-Ni alloy or Sn-Ni alloy, and heating in a non-oxidizing atmosphere to form a diffusion layer of the plating metal in the substrate steel, and subjecting one surface of the resulting plated steel sheet to hot-dip galvanizing process to form a galvanized layer. It is cited that the coating weight of galvanized layer may be reduced, which is extremely advantageous from the viewpoint of weldability and economic efficiency.

Indeed, above patent application shows the surface-treated steel sheet has an improved weldability only due to the decrease of the zinc coating weight. Moreover, there is no mention of LME resistance improvement, especially for high strength steels having alloying elements including Mn, Al and Si.

<CIT> discloses a method for manufacturing a high-strength alloying galvanized hot-dip steel sheet comprising the steps of: pre-plating a steel sheet having a steel composition containing, by mass%, <NUM> to <NUM>% Si, with a metal and/or an alloy containing one or more elements selected from [Z]: Fe, Ni, Co, Cu and Sn so as to satisfy the expression [Z]≥<NUM>×[Si], wherein [Z] represents the total coating mass of the element of Z, and [Si] represents Si quantity in the steel, before hot-dip galvanizing the steel sheet; annealing the pre-plated steel sheet; and hot-dip galvanizing the annealed steel sheet.

<CIT> discloses a method of manufacturing a hot-dip galvanized steel sheet including a base steel sheet, a composite layer formed on the base steel sheet and including transition metal, an inhibition layer formed on the composite layer and including a iron-aluminum (Fe-AI) based intermetallic compound, and a zinc (Zn)-plated layer formed on the inhibition layer, in which an average diameter of spangles in the zinc-plated layer is <NUM> or less.

<CIT> discloses a method wherein the surface of a steel sheet containing at least <NUM>% of Cr is coated with Ni of <NUM> to <NUM> thickness. Then, the steel sheet is subjected to heating treatment to a prescribed temp. in an atmosphere of an N2 or H2 gas or a mixed gas of N2-H2 having a dew point of <NUM> deg. C or less and is thereafter subjected to hot dip galvanizing.

Thus, the objective of the present invention is to provide a coated steel sheet which shows a high LME resistance behavior. It aims to make available, in particular, an easy to industrially implementable method in order to obtain an assembly which is susceptible to LME resistance especially after the hot press forming and/or the welding.

First objective is achieved by providing a method according to claim <NUM>. The method can also comprise any characteristics of claims <NUM> to <NUM>.

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 <NUM> MPa and more preferably up to 2000MPa. For example, the tensile strength is above or equal to <NUM> MPa, preferably above or equal to <NUM> MPa, advantageously above or equal to <NUM> MPa and even above or equal <NUM> MPa.

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

Without willing to be bound by any theory, it seems that in order to obtain a steel sheet having the specific above composition with high resistance to LME, it is an essential feature to deposit the first coating of nickel on the sheet steel before the recrystallization annealing. During recrystallization annealing Ni diffuses towards the substrate steel sheet allowing formation of a Fe-Ni alloy layer. Indeed, Ni rich layer concentrates in the surface and sub-surface area of the steel sheet and thus preventing liquid zinc penetration into the steel during any heating steps such as welding. Thus, by applying the above method according to the present invention, it is possible to obtain a barrier or buffer layer which prevents LME.

If the first coating consisting of nickel has a thickness below <NUM>, there is a possibility of significant decrease of LME resistance behavior of the specific above coated steel sheet. Indeed, it seems that there is not enough Ni present in the surface and sub-surface region of steel sheet which provides enough barrier against LME.

For above steel composition, if the first coating consisting of nickel having thickness above <NUM>, then after recrystallization annealing the amount of iron in the Fe-Ni alloy layer which is formed in the sub-surface and surface area is very low and is insufficient to form inhibition during subsequent hot dip galvanizing process. Due to presence of higher amount of Ni, a considerable amount of Ni diffuses in the steel substrate during recrystallization annealing and on the other hand, due to absence of inhibition layer, Ni also diffuses in the galvanized coating. Due to presence of higher amount of Ni in the coating, LME resistance behavior reduces. Moreover, the galvanized coating quality is poor due to absence of inhibition layer along with presence of higher amount of Ni in the coating.

The first coating consists of Nickel, i.e. Ni amount is > <NUM> wt. % and < <NUM>% is unavoidable impurities.

The first coating can be deposited by any deposition method known by the person skilled in the art. It can be deposited by vacuum deposition or electro-plating or roll coating method. Preferably, it is deposited by electro-plating method.

Preferably, in step B), the recrystallization annealing is a continuous annealing which comprises continuous pre-heating, heating, soaking and cooling step.

The recrystallization annealing is performed in an atmosphere comprising from <NUM> to <NUM>% of H<NUM> at a dew point between -<NUM> and +<NUM>. Indeed, without willing to be bound by any theory, it is believed that this dew point further improves the coatability of the steel sheet according to the present invention without considerable decrease of any mechanical property.

Advantageously, in step C), the second layer comprises above <NUM>%, more preferably above <NUM>% of zinc and advantageously above <NUM>% of zinc. 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 from <NUM> to <NUM>% Al, optionally <NUM>-<NUM>% Mg, the remainder being Zn.

Preferably, the coating based on zinc is deposited by hot-dip galvanizing method. 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 <NUM>% 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 <NUM>%, preferably <NUM>% by weight.

In a preferred embodiment, the second layer consists of zinc. When the coating is deposited by hot-dip galvanizing process, the percentage of Al is comprised between <NUM> and <NUM> wt. % in the bath. Moreover, the iron presents in the first coating after recrystallization annealing reacts with aluminum and forms the inhibition layer. Thus, it provides reactive wetting behavior during hot dip galvanizing.

With the method according to the present invention, a steel sheet coated with a diffused alloy layer comprising iron and nickel formed through diffusion of nickel into the steel, such layer being directly topped by a zinc based layer is obtained. It is believed that the diffused alloy layer acts like a barrier layer against LME.

Preferably, the steel sheet has a microstructure comprising from <NUM> to <NUM>% of residual austenite, from <NUM> to <NUM>% 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 <NUM> to <NUM> % of residual austenite.

Preferably, the steel sheet has a microstructure comprising from <NUM> to <NUM>% and more preferably between <NUM> to <NUM>% of tempered martensite.

Advantageously, the steel sheet has a microstructure comprising from <NUM> to <NUM>% of bainite, such bainite comprising from <NUM> to <NUM>% of lower bainite, from <NUM> to <NUM>% of upper bainite and from <NUM> to <NUM>% of carbide free bainite.

Preferably, the steel sheet has a microstructure comprising from <NUM> to <NUM>% of ferrite.

Preferably, the steel sheet has a microstructure comprising from <NUM> to <NUM>% untempered martensite.

Advantageously, the steel sheet has a decarburized layer having a depth maximum of <NUM>, preferably maximum of <NUM> and more preferably maximum of <NUM> on either side of the sub-surface area. The decarburization is defined in the norm ISO <NUM>:<NUM>. Indeed, without willing to be bound by any theory, it is believed that the decarburized layer further improves the LME resistance without considerably decreasing the mechanical properties of the steel sheet.

Preferably, an internal oxides layer having a thickness below or equal to <NUM> is present in the steel sheet. Without willing to be bound by any theory, it is believed that this layer leads to a good coatability of the zinc coating since the continuous inhibition layer Fe<NUM>Al<NUM> is formed which represents good reactive wetting.

After the manufacture of a steel sheet, in order to produce some parts of a vehicle, it is known to assembly by welding two or more 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, the welding is performed with an effective welding current between 3kA and 15kA and the force applied on the electrodes is between <NUM> and <NUM> daN with said electrode active face diameter being between <NUM> and <NUM>.

Thus, a spot-welded joint of at least two metal sheets comprising at least a steel sheet, comprising the coated steel sheet produced according to the present invention, is obtained. The above said joint contains less than <NUM> cracks having a size above <NUM> and wherein the longest crack has a length below <NUM>.

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 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.

For all samples, steel sheets used have the following composition in weight percent: C=<NUM>%, Mn=<NUM>%, Si=<NUM>%, Cr=<NUM>%, AI=<NUM>% and Mo=<NUM>%.

In Trial <NUM>, steel was annealed in an atmosphere comprising <NUM>% of H<NUM> and <NUM>% of N<NUM> at a dew point of -<NUM>. The annealing was carried out at <NUM> for 132seconds. After that steel was quenched at <NUM> followed by partitioned at <NUM> for 88seconds. Finally, steel sheet was cooled to room temperature. On annealed steel sheet Zinc coating was applied by electro-galvanizing method.

In Trials <NUM> to <NUM>, Ni was first deposited by electro-plating method having thickness of <NUM>, <NUM>, <NUM>, <NUM> and <NUM> respectively on full hard steel sheets before annealing. After that, the pre-coated steel sheets were annealed in an atmosphere comprising <NUM>% of H<NUM> and <NUM>% of N<NUM> at a dew point of -<NUM>. The annealing was carried out at <NUM> for 132seconds. At the end of the annealing, the steel sheets were cooled to quench temperature of <NUM> and again heated at partitioning temperature of <NUM>. Partitioning was carried out for <NUM> and then again heated up to galvanizing temperature of <NUM> and Zinc coating was applied by hot dip coating method using a liquid Zinc bath containing <NUM>. % Al maintained at <NUM>. The objective of above trials to determine the optimum Ni coating thickness which provides excellent LME resistance behavior. The susceptibility of LME of above coated steel was evaluated by resistance spot welding method. To this end, for each Trial, three coated steel sheets were welded together by resistance spot welding. The type of the electrode was ISO Type B with a face diameter of <NUM>; the force of the electrode was of 5kN and the flow rate of water of was <NUM>/min. the welding cycle was reported in Table <NUM>:.

The LME crack resistance behavior was evaluated using <NUM> layer stack-up conditions. The number of cracks having crack length of more than <NUM> was then evaluated using an optical microscope as reported in Table <NUM>.

Trials <NUM> and <NUM> according to the present invention show an excellent resistance to LME as compared to Trials <NUM>, <NUM>, <NUM> and <NUM>. Indeed, the number of cracks above <NUM> is below or equal to <NUM> and the longest crack has a length below <NUM>. It results in a reduction of the amount of heat input during spot welding and thus causes a significant reduction of number of cracks formation due to LME.

The LME crack resistance behavior was also evaluated using <NUM> layer stack-up conditions for Trials <NUM>, <NUM> and <NUM>. In this condition, two coated steel sheets were welded together by resistance spot welding. The number of cracks above <NUM> was then evaluated using an optical microscope as reported in Table <NUM>.

Trials <NUM> and <NUM> show an excellent resistance to LME as compared to Trial <NUM>. Indeed, the number of cracks above <NUM> is of <NUM> and the longest crack has a length of <NUM>. It results in a reduction of the amount of heat input during spot welding and thus causes a significant reduction of number of cracks formation due to LME.

From above trial, excellent LME resistance behavior was observed when Ni coating thickness was maintained between <NUM> to <NUM>. In order to enhance the LME resistance further, sub-surface area of the steel sheet was modified by formation of decarburized layer. Example <NUM> represents the combined effect of decarburized layer along with Ni coating having a specific thickness.

In order to prevent any decarburization, in Trial <NUM>, steel was annealed in an atmosphere comprising <NUM>% of H<NUM> and <NUM>% of N<NUM> at a dew point of -<NUM>. The annealing was carried out at <NUM> for 132seconds. After that steel was quenched at <NUM> followed by partitioned at <NUM> for 88seconds. Finally, steel sheet was cooled to room temperature. On annealed steel sheet Zinc coating was applied by electro-galvanizing method.

In Trials <NUM> and <NUM>, Ni was first deposited by electro-plating method having thickness of <NUM> on full hard steel sheets before annealing. After that, the pre-coated steel sheets were annealed in an atmosphere comprising <NUM>% of H<NUM> and <NUM>% of N<NUM> at a dew point of -<NUM>, for trial <NUM>, without any decarburized layer in the sub-surface area of the steel. For trial <NUM>, annealing dew point was maintained at -<NUM> with <NUM>% of H<NUM> and <NUM>% of N<NUM>. For trial <NUM> and <NUM>, the annealing was carried out at <NUM> for 132seconds. At the end of the annealing, the steel sheets were cooled to quench temperature of <NUM> and again heated at partitioning temperature of <NUM>. Partitioning was carried out for <NUM>. Finally, steel sheet was cooled to room temperature. On annealed steel sheet Zinc coating was applied by electro-galvanizing method.

Table <NUM> compares the decarburized layer thickness when steel was annealed at different dew point without and with Ni coating. Without compromising steel mechanical properties, decarburized layer thickness was restricted by controlling annealing dew point.

The LME susceptibility of above coated steels (Trials <NUM>, <NUM> and <NUM>) were evaluated by resistance spot welding method. For this purpose, for each Trial, three coated steel sheets were welded together by resistance spot welding. The type of the electrode was ISO Type B with a face diameter of <NUM>; the force of the electrode was of 5kN and the flow rate of water of was <NUM>/min. The welding cycle was reported in Table <NUM>:.

The LME crack resistance behavior was evaluated using <NUM> layer stack-up conditions for Trials <NUM>, <NUM> and <NUM>. In this condition, two coated steel sheets were welded together by resistance spot welding. The number of cracks above <NUM> was then evaluated using an optical microscope as reported in Table <NUM>.

Claim 1:
Method for the manufacture of a coated steel sheet comprising the following successive steps:
A. the coating of the steel sheet with a first coating consisting of nickel and having a thickness between <NUM> and <NUM>, the steel sheet having the following composition in weight: <MAT> <MAT> <MAT> <MAT> <MAT> and on a purely optional basis, one or more elements such as <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration,
B. the recrystallization annealing of said coated steel sheet at a temperature between <NUM> to <NUM>, performed in an atmosphere comprising from <NUM> to <NUM>% of H<NUM> at a dew point between -<NUM> and +<NUM>,
C. the coating of the steel sheet obtained in step B) with a second coating based on zinc not comprising nickel.