The present invention relates to a press hardening method comprising the provision of a steel sheet coated with a pre-coating for anti-corrosion purpose, being directly topped by a hydrogen barrier pre-coating which better inhibits hydrogen absorption and a part having excellent resistance to delayed cracking. The invention is particularly well suited for the manufacture of automotive vehicles.

Coated steel sheet for press hardening are sometimes termed "pre-coated," this prefix indicating that a transformation of the nature of the pre-coating will take place during heat treatment before stamping. There can be more than one pre coating. This invention discloses two pre-coatings.

It is known that certain applications, especially in the automotive field, require metal structures to be further lightened and strengthened in the event of an impact, and also good drawability. To this end, steels having improved mechanical properties are usually used, such steel being formed by cold and hot-stamping.

However, it is known that the sensitivity to delayed cracking increases with the mechanical strength, in particular after certain cold-forming or hot-forming operations since high residual stresses are liable to remain after deformation. In combination with atomic hydrogen possibly present in the Steel sheet, these stresses are liable to result in delayed cracking, that is to say cracking that occurs a certain time after the deformation itself. Hydrogen may progressively build up by diffusion into the crystal lattice defects, such as the matrix/inclusion interfaces, twin boundaries and grain boundaries. It is in the latter defects that hydrogen may become harmful when it reaches a critical concentration after a certain time. This delay results from the residual stress distribution field and from the kinetics of hydrogen diffusion, the hydrogen diffusion coefficient at room temperature being low. In addition, hydrogen localized at the grain boundaries weakens their cohesion and favors the appearance of delayed intergranular cracks.

Some parts are produced by pre-alloying an aluminum based coated steel sheet and then by hot-forming the pre-alloyed coated steel sheet. Usually, these parts have really bad behavior concerning the hydrogen absorption during the batch annealing and during the hot stamping. Indeed, since the batch annealing is performed during hours, a high amount of hydrogen can be absorbed specially during the batch annealing.

The patent application EP3396010 discloses a method of manufacturing an Al-Fe alloy coated steel sheet for hot forming, the Al-Fe alloy coated steel sheet having high resistance to hydrogen delayed fracture and coating layer separation and high weldability, the method comprising:

- forming an Al-Si coating layer on a surface of a base steel sheet,

- heating the Al-Si coated base steel sheet to a heat treatment maximum temperature ranging from 450°C to 750°C at a heating rate of 1 °C/hr to 500°C/hr in a heating furnace in which an atmosphere having a dew point of less than -10°C is present; and

- forming an Al-Fe alloy coating layer on the surface of the base steel sheet by maintaining the Al-Si coated base steel sheet at the heat treatment maximum temperature for 1 to 100 hours.

The atmosphere of the batch annealing process and the heat treatment conditions are adjusted to obtain a specific microstructure and characteristics of Al- Fe for preventing hydrogen delayed fracture.

Indeed, this patent application discloses an aluminum-iron (Al-Fe) alloy coated steel sheet for hot forming, having high resistance to hydrogen delayed fracture and coating layer separation and high weldability, the Al-Fe alloy coated steel sheet comprising a base steel sheet and an alloy coating layer formed between the base steel sheet and an oxide layer, wherein the alloy coating layer comprises: an Al-Fe alloy layer I formed on the base steel sheet and having a Vickers hardness of 200 Hv to 800 Hv; an Al-Fe alloy layer III formed on the Al-Fe alloy layer I and having a Vickers hardness of 700 Flv to 1200 Hv; and an Al-Fe alloy layer II formed in the Al-Fe alloy layer III continuously or discontinuously in a length direction of the steel sheet, and having a Vickers hardness of 400 Hv to 900 Hv, wherein an average oxygen content at a depth of 0.1 pm from a surface of the oxide layer is 20% or less by weight.

However, in practice, the aluminum-iron alloy coated steel sheet having the specific microstructure and characteristics is very difficult to obtain. Indeed, a broad range of dew point and heating speed is disclosed. Thus, there is a risk that the specific Al-Fe alloy coating is not obtained in the whole range resulting in important research efforts to find the right parameters.

The patent application EP2312005 discloses a method of production of aluminum plated steel sheet for rapid heating hot-stamping characterized by annealing aluminum plated steel sheet having an aluminum plating deposition amount per side of 30 to 100 g/m ² in a box annealing furnace as is in a coil state during which annealing by a combination of a retention time and annealing temperature in an inside region including the sides of a pentagon having five points of coordinates (600°C, 5 hours), (600°C, 200 hours), (630°C, 1 hour), (750°C, 1 hour), and (750°C, 4 hours) as vertices in an XY plane having the retention time and annealing temperature as its X-axis and Y-axis and with the X-axis expressed logarithmically. This patent application also discloses the aluminum plated steel sheet for rapid heating hot-stamping obtained by the above method.

The patent recommends conditions to perform a batch annealing at 600 to 750°C in an air atmosphere to lower the hydrogen in the steel. However, the amount of hydrogen absorbed during the batch annealing is still high.

Thus, the object of the invention is to provide an easy-to-implement press hardening method wherein the hydrogen absorption into the pre-alloyed aluminum- based steel sheet and therefore into the press hardened part is prevented. It aims to make available a part having excellent resistance to delayed cracking obtainable by said press-hardening method including hot-forming.

This object is achieved by providing a press hardening method comprising the following steps:

A. the provision of a steel sheet for heat treatment, precoated with a zinc- or aluminum-based pre-coating for anti-corrosion purpose,

B. the deposition of a hydrogen barrier pre-coating over a thickness from 10 to 550 nm,

C. the batch annealing of the precoated steel sheet in an inert atmosphere to obtain a pre-alloyed steel sheet,

D. the cutting of the pre-alloyed steel sheet to obtain blank, E. the thermal treatment of the blank to obtain a fully austenitic microstructure in the steel,

F. the transfer of the blank into a press tool,

G. the hot-forming of the blank to obtain a part,

H. the cooling of the part obtained at step G) in order to obtain a microstructure in steel being martensitic or martensito-bainitic or made of at least 75 % in terms of volume fraction of equiaxed ferrite, from 5 to 20 % in volume of martensite and bainite in amount less than or equal to 10 % in volume.

Indeed, without willing to be bound by any theory, the inventors have surprisingly found that when the steel sheet is precoated with a hydrogen barrier pre-coating and when the batch annealing is performed in an inert atmosphere, the absorption of hydrogen into the steel sheet is reduced. Indeed, it is believed that thanks to the hydrogen barrier pre-coating, thermodynamically stable oxides are formed on the surface of the hydrogen barrier pre-coating with a low diffusion kinetic. These thermodynamically stable oxides reduce H2 absorption. Moreover, it seems that when the atmosphere of the batch annealing is not oxidizing, it allows further preventing the absorption of hydrogen because the pre-coating diffuses and oxidizes at the surface of the precoated steel sheet. Thus, the zinc- or aluminum- based and the hydrogen barrier pre-coatings oxidize at the surface of the precoated steel sheet, both acting like barriers to hydrogen.

In step A), the steel sheet used is made of steel for heat treatment as described in the European Standard EN 10083. It can have a tensile resistance superior to 500MPa, advantageously between 500 and 2000MPa before or after heat-treatment.

The weight composition of steel sheet is preferably as follows: 0.03% < C < 0.50% ; 0.3% < Mn < 3.0% ; 0.05% < Si < 0.8% ; 0.015% < Ti < 0.2% ; 0.005% < Al < 0.1% ; 0% < Cr < 2.50% ; 0% < S < 0.05% ; 0% < P< 0.1% ; 0% < B < 0.010% ; 0% < Ni < 2.5% ; 0% < Mo < 0.7% ; 0% < Nb < 0.15% ; 0% < N < 0.015% ; 0% < Cu < 0.15% ; 0% < Ca < 0.01% ; 0% < W < 0.35%, the balance being iron and unavoidable impurities from the manufacture of steel. For example, the steel sheet is 22MnB5 with the following composition: 0.20% < C < 0.25%; 0.15% < Si < 0.35%; 1.10% < Mn < 1.40%; 0% < Cr < 0.30%; 0% < Mo < 0.35%; 0% < P < 0.025%; 0% < S < 0.005%; 0.020% < Ti < 0.060%; 0.020% < Al < 0.060%; 0.002% < B < 0.004%, the balance being iron and unavoidable impurities from the manufacture of steel.

The steel sheet can be Usibor®2000 with the following composition: 0.24%

< C < 0.38%; 0.40% < Mn < 3%; 0.10% < Si < 0.70%; 0.015% < Al < 0.070%; 0 % < Cr < 2%; 0.25% < Ni < 2%; 0.020% < Ti < 0.10%; 0% < Nb < 0.060%; 0.0005% < B

< 0.0040%; 0.003% < N < 0.010%; 0.0001 % < S < 0.005%; 0.0001% < P < 0.025%; it being understood that the contents of titanium and nitrogen satisfy Ti/N > 3.42; and that the contents of carbon, manganese, chromium and silicon satisfy: the composition optionally comprising one or more of the following: 0.05% < Mo < 0.65%; 0.001 % < W < 0.30%; 0.0005% < Ca < 0.005%, the balance being iron and unavoidable impurities from the manufacture of steel.

For example, the Steel sheet is Ductibor®500 with the following composition: 0.040% < C < 0.100%; 0.80% < Mn < 2.00%; 0% < Si < 0.30%; 0% < S < 0.005%; 0% < P < 0.030%; 0.010% < Al < 0.070%; 0.015% < Nb < 0.100%; 0.030% < Ti < 0.080%; 0% < N < 0.009%; 0% < Cu < 0.100%; 0% < Ni < 0.100%; 0% < Cr <

0.100%; 0% < Mo < 0.100%; 0% < Ca < 0.006%, the balance being iron and unavoidable impurities from the manufacture of steel.

Steel sheet can be obtained by hot rolling and optionally cold rolling depending on the desired thickness, which can be for example between 0.7 and 3.0mm.

Optionally, in step A), the hydrogen barrier pre-coating comprises optional elements 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.

Preferably in step A), the hydrogen barrier pre-coating comprises at least one element chosen from among: nickel, chromium, aluminum, magnesium and yttrium.

Preferably in step A), the hydrogen barrier pre-coating consists of nickel and chromium, i.e. the barrier pre-coating comprises nickel, chromium and unavoidable impurities. Advantageously, the weight ratio Ni/Cr is between 1 .5 and 9. Indeed, without willing to be bound by any theory, it is believed that this specific ratio further decreases the hydrogen absorption during the austenitization treatment.

In another preferred embodiment, the hydrogen barrier pre-coating consists of nickel and aluminum, i.e. the hydrogen barrier pre-coating comprises Ni, Al and unavoidable impurities.

In another preferred embodiment, the hydrogen barrier pre-coating consists of chromium at 50% or 75% or 90% by weight. More preferably it consists of chromium, i.e. the hydrogen barrier pre-coating comprises only Cr and unavoidable impurities.

In another preferred embodiment, the hydrogen barrier pre-coating consists of magnesium at 50% or 75% or 90% by weight. More preferably it consists of magnesium, i.e. the hydrogen barrier pre-coating comprises only Mg and unavoidable impurities.

In another preferred embodiment, the hydrogen barrier pre-coating consists of nickel, aluminum and yttrium, i.e. the hydrogen barrier pre-coating comprises Ni, Al and Y and unavoidable impurities.

Preferably, in step A), the hydrogen barrier pre-coating has a thickness between 10 and 90 nm or between 150 and 250 nm. For example, the thickness of the hydrogen barrier pre-coating is of 50, 200 or 400 nm.

Without willing to be bound by any theory, it seems that when the hydrogen barrier pre-coating is below 10 nm, there is a risk that hydrogen absorbs into steel because the hydrogen barrier pre-coating does not cover enough the steel sheet. When the hydrogen barrier pre-coating is above 550 nm, it seems that there is a risk that the hydrogen barrier pre-coating becomes more brittle and that the hydrogen absorption begins due to the barrier coating brittleness.

In a preferred embodiment, the zinc or aluminum-based pre-coating is based on aluminum and comprises 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. For example, the zinc or aluminum-based pre-coating is AluSi®.

In another preferred embodiment, the zinc or aluminum pre-coating is based on zinc and comprises less than 6.0% Al, less than 6.0% of Mg, the remainder being Zn. For example, the zinc or aluminum-based pre-coating is a zinc coating so to obtain the following product: Usibor® Gl. The zinc or aluminum-based pre-coating can also comprise impurities and residual elements such iron with a content up to 5.0%, preferably 3.0%, by weight.

Preferably, the pre-coatings of step A) are deposited by physical vapor deposition, by electro-galvanization, hot-dip galvanization or roll-coating. Preferably, the hydrogen barrier pre-coating is deposited by electron beam induced deposition or roll coating. Preferably, the zinc or aluminum-based precoating is deposited by hot-dip galvanization.

Optionally, after the deposition of the pre-coatings, a skin-pass can be realized and allows work hardening the precoated steel sheet and giving it a roughness facilitating the subsequent shaping. A degreasing and a surface treatment can be applied in order to improve for example adhesive bonding or corrosion resistance.

Preferably, in step C), the batch annealing is performed at a temperature between 450 and 750°C, preferably between 550 and 750°C.

Preferably, in step C), the inert gas is chosen from helium (He), neon (Ne), argon (Ar), nitrogen, hydrogen or a mixture thereof.

Advantageously, in step C), the heating rate of the batch annealing is above or equal to 5000°C.lr ¹ , more preferably between 10000 and 15000°C.lr ¹ or between 20000 and 35000°C.lr ¹ .

Preferably, in step C), the cooling speed is below or equal to 100°C.lr ¹ . Preferably the cooling speed has three cooling rates varying from 1 °C.lr ¹ to 100°C.h ¹ .

Preferably, in step C), the batch annealing is performed during 1 to 100 hours.

After, the pre-alloyed steel sheet is cut to obtain a blank.

A thermal treatment is applied to the blank in a furnace with an inert atmosphere.

Preferably, in steps C) and/or E), the dew point is below or equal to -10°C, more preferably between -30 and -60°C. Indeed, without willing to be bound by any theory, it is believed that when the dew point is in the above range, the layer of thermodynamically stable oxides reduce even more the H2 absorption during the thermal treatment. Preferably, the thermal treatment is performed at a temperature between 800 and 970°C. More preferably, the thermal treatment is performed at an austenitization temperature Tm usually between 840 and 950°C, preferably 880 to 930°C. Advantageously, said blank is maintained during a dwell time tm between 1 to 12 minutes, preferably between 3 to 9 minutes. During the thermal treatment before the hot-forming, the pre-coatings form an alloy layer having a high resistance to corrosion, abrasion, wear and fatigue.

At ambient temperature, the mechanism of absorption of hydrogen into steel is different from high temperature, in particular the austenitization treatment. Indeed, usually at high temperature, the water in the furnace dissociates at the surface of the steel sheet into hydrogen and oxygen. Without willing to be bound by any theory, it is believed that the hydrogen barrier pre-coating and the inert atmosphere of the batch annealing can prevent water dissociation at the hydrogen barrier pre-coating surface and, can prevent the hydrogen diffusion through both pre-coatings.

After the thermal treatment, the blank is then transferred to a hot-forming tool and hot-formed at a temperature between 600 and 830°C. The hot-forming can be the hot-stamping or the roll-forming. Preferably, the blank is hot-stamped. The part is then cooled in the hot-forming tool or after the transfer to a specific cooling tool.

The cooling rate is controlled depending on the steel composition, in such a way that the final microstructure after the hot-forming comprises mostly martensite, preferably contains martensite, or martensite and bainite, or is made of at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.

A hardened part having excellent resistance to delayed cracking according to the invention is thus obtained by hot forming.

Preferably, the part comprises the steel sheet precoated with a zinc or aluminum-based pre-coating, this 1 ^st pre-coating layer being directly topped by the hydrogen barrier coating and an oxide layer comprising thermodynamically stable oxides, such hydrogen barrier coating being alloyed through diffusion with the zinc or aluminum-based pre-coating, the zinc or aluminum-based pre-coating being alloyed with the steel sheet. Indeed, without willing to be bound by any theory, it seems that iron from the steel sheet diffuses to the surface of the hydrogen barrier pre-coating during the thermal treatment. Preferably, the thermodynamically stable oxides can comprise respectively Cr2C>3; FeO; NiO; Fe2C>3; Fe3C>4, MgO, Y2O3 or a mixture thereof.

If the zinc or aluminum-based pre-coating is based on zinc, the oxides can also comprise ZnO. If the zinc or aluminum-based pre-coating is based on aluminum, the oxides can also comprise AI2O3 and/or MgAl2C>4.

Preferably, the thickness of the oxide layer is between 10 and 550 nm.

Preferably, the part is a front rail, a seat cross member, a side sill member, a dash panel cross member, a front floor reinforcement, a rear floor cross member, a rear rail, a B-pillar, a door ring or a shotgun.

For automotive application, after phosphating step, the part is dipped in an e- coating bath. Usually, the thickness of the phosphate layer is between 1 and 2 pm and the thickness of the e-coating layer is between 15 and 25 pm, preferably inferior or equal to 20 pm. The cataphoresis layer ensures an additional protection against corrosion. After the e-coating step, other paint layers can be deposited, for example, a primer coat of paint, a basecoat layer and a top coat layer.

Before applying the e-coating on the part, the part is previously degreased and phosphated so as to ensure the adhesion of the cataphoresis.

The invention will now be explained in trials carried out for information only. They are not limiting.

Examples

For all samples, steel sheets used are 22MnB5. The composition of the steel is as follows: C = 0.2252% ; Mn = 1.1735% ; P = 0.0126%, S = 0.0009% ; N = 0.0037% ; Si = 0.2534% ; Cu = 0.0187% ; Ni = 0.0197% ; Cr = 0.180% ; Sn = 0.004% ; Al = 0.0371% ; Nb = 0.008% ; Ti = 0.0382% ; B = 0.0028 % ; Mo = 0.0017% ; As = 0.0023% et V = 0.0284%.

All the steel sheets are precoated with a 1 ^st pre-coating for anti-corrosion prupose called hereinafter “AluSi®”. This pre-coating comprises 9% by weight of Silicon, 3% by weight of iron, the balance being aluminum. It is deposited by hot-dip galvanization.

Then, two Trials were precoated with a 2 ^nd pre-coating comprising 80% of Ni and 20% of Cr deposited by magnetron sputtering. Example 1 : hydrogen test:

This test is used to determine the quantity of hydrogen absorbed during the austenitization thermal treatment of a press hardening method.

Trial 1 is a steel sheet precoated with a 1 ^st pre-coating being AluSi® (25pm). Then, a batch annealing at a temperature of 650°C was performed during 5 hours. The heating rate was of 10800°C.lr ¹ . The atmosphere of the batch annealing was nitrogen. The cooling after the batch annealing was performed at a speed of 85°C.lr ¹ during 2 hours 20 minutes, 19°C.lr ¹ during 17 hours and 2.5°C.lr ¹ during 8 hours.

Trial 2 is a steel sheet precoated with a 1 ^st pre-coating being AluSi® (25pm) and a 2 ^nd pre-coating comprising 80% of Ni and 20% of Cr. Then, a batch annealing at a temperature of 650°C was performed during 5 hours. The heating rate was of 10800°C.h T The atmosphere of the batch annealing was nitrogen. The cooling after the batch annealing was performed at a speed of 85° C.lr ¹ during 2 h 20 minutes, 19°C.h ¹ during 17 hours and 2.5°C.h ¹ during 8 h.

Trial 3 is a steel sheet precoated with a 1 ^st pre-coating being AluSi® (25pm). Then, a batch annealing at a temperature of 650°C was performed during 5 hours. The heating rate was of 10800°C.lr ¹ . The atmosphere of the batch annealing was air The cooling after the batch annealing was performed at a speed of 85° C.lr ¹ during 2 hours 20 minutes, 19°C.lr ¹ during 17 hours and 2.5°C.lr ¹ during 8 hours.

Trial 4 is a steel sheet precoated with a 1 ^st pre-coating being AluSi® (25pm) and a 2 ^nd pre-coating comprising 80% of Ni and 20% of Cr. Then, a batch annealing at a temperature of 650°C was performed during 5 hours. The heating rate was of 10800°C.lr ¹ . The atmosphere of the batch annealing was air. The cooling after the batch annealing was performed at a speed of 85° C.lr ¹ during 2 hours 20 minutes, 19°C.lr ¹ during 17 hours and 2.5°C.lr ¹ during 8 hours.

After, all Trials were cut and heated at a temperature of 900°C during a dwell time of 3 minutes. The atmosphere during the thermal treatment was air. Blanks were transferred into a press tool and hot-stamped in order to obtain parts having a variable thickness. Then, parts were cooled by dipping trials into warm water to obtain a hardening by martensitic transformation. Finally, the hydrogen amount adsorbed by the trials during the heat treatment was measured by thermic desorption using a TDA or Thermal Desorption Analyser. To this end, each trial was placed in a quartz room and heated slowly in an infra-red furnace under a nitrogen flow. The released mixture hydrogen/nitrogen was picked up by a leak detector and the hydrogen concentration was measured by a mass spectrometer. Results are shown in the following Table 1 :

*: example according to the invention.

Trial 2 according to the present invention release a significantly low amount of hydrogen compared to comparative examples.