The present invention relates to a press hardening method comprising the provision of a steel sheet coated with a 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.

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.

To overcome this problem, it is usually known to pre-coat a steel sheet with a barrier pre-coating comprising nickel and chromium wherein the weight ratio Ni/Cr is between 1 .5 and 9 to prevent the adsorption of hydrogen into the steel during the austenitization thermal treatment.

For example, the patent application WO2017/187255 discloses a press hardening method comprises the following steps:

A. the provision of a steel sheet coated with a barrier pre-coating comprising nickel and chromium wherein the weight ratio Ni/Cr is between 1 .5 and 9,

B. the cutting of the coated steel sheet to obtain a blank,

C. the thermal treatment of the blank, D. the transfer of the blank into a press tool,

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

F. the cooling of the part obtained at step E) in order to obtain a microstructure in steel being martensitic or martensito-bainitic or made of at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 0%.

In this patent application, in step C), the thermal treatment can be performed in an inert atmosphere or an atmosphere comprising air. All the Examples are performed in an atmosphere consisting of nitrogen.

The patent application PCT/IB2018/057719 discloses a press hardening method comprises the following steps:

A. the provision of a steel sheet coated with a barrier pre-coating comprising nickel and chromium wherein the weight ratio Ni/Cr is between 1.5 and 9,

B. the cutting of the coated steel sheet to obtain a blank,

C. the thermal treatment of the blank in an atmosphere having an oxidizing power equal or higher than that of an atmosphere consisting of 1% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 50% by volume of oxygen, such atmosphere having a dew point between -30 and +30°C,

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

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

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

In both patent applications, although the hydrogen absorption during the austenitization treatment is improved, it is not enough to obtain a part having an excellent resistance to delayed cracking. Indeed, even if the pre-coating barrier decreases the hydrogen absorption, few hydrogen molecules are still absorbed by the steel sheet.

Thus, the object of the invention is to provide a press hardening method wherein the hydrogen adsorption into the steel sheet 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 according to claim 1 . The method can also comprise characteristics of claims 2 to 20.

The invention also covers a part according to claim 21. The part can also comprise characteristics of claim 22 to 25.

Finally, the invention covers the use of such part for the manufacture of an automotive vehicle according to claim 26.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

The following terms will be defined:

- all percentage “%” are defined by weight and

- “steel sheet” means a steel sheet having less than 10.5% by weight of chromium. For example, stainless steel is not included in the definition of a steel sheet.

Any steel can be advantageously used in the frame of the invention. Flowever, in case steel having high mechanical strength is needed, in particular for parts of structure of automotive vehicle, steel having a tensile resistance superior to 500MPa, advantageously between 500 and 2000MPa before or after heat- treatment, can be used. 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: Mn Cr Si

² ' ^{6C +} W ⁺ i3 ⁺ iS ^{³ U%} 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.

The invention relates to a press hardening method comprising the following steps:

A. the provision of a steel sheet coated with a barrier pre-coating comprising chromium and not comprising nickel,

B. the cutting of the coated steel sheet to obtain a blank,

C. the thermal treatment of the blank in an atmosphere having an oxidizing power equal or higher than that of an atmosphere consisting of 1% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 50% by volume of oxygen, such atmosphere having a dew point between -30 and +30°C,

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

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

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

Indeed, without willing to be bound by any theory, the inventors have surprisingly found that when the steel sheet is pre-coated with a barrier pre-coating comprising chromium and not comprising nickel and when the thermal treatment is performed in the above atmosphere, this barrier effect of the pre-coating is further improved preventing even more the adsorption of hydrogen into the steel sheet. Indeed, on the contrary to an atmosphere consisting of nitrogen with which a thinner layer of selective oxides is formed on the surface of the barrier pre-coating during the thermal treatment, in particular the austenitization treatment, it is believed that thermodynamically stable oxides are formed on the surface of the barrier pre coating with a low kinetic.

Finally, it is believed that on the contrary to the results disclosed in the patent application WO2017/187255, the barrier pre-coating comprising chromium and not comprising nickel in the specific above atmosphere allows for a high reduction of the H2 absorption compared to a pre-coating comprising nickel and chromium. Indeed, it is believed that the chromium forms an oxide layer thicker than the pre coating comprising nickel and chromium.

One of the essential characteristics of the method according to the invention consists in choosing the atmosphere having an oxidizing power equal or higher than that of an atmosphere consisting of 1% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 50% by volume of oxygen. The atmosphere may in particular be made of N2 or Ar or mixtures of nitrogen or argon and gas oxidants such as, for example, oxygen, mixtures of CO and CO2 or mixtures of H2 and H2O. it is also possible to use mixtures of CO and CO2 or mixtures of H2 and H2 without addition of inert gas.

Preferably, in step C), the atmosphere has an oxidizing power equal or higher than that of an atmosphere consisting of 10% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 30% by volume of oxygen. For example, the atmosphere is air, i.e. consisting of about 78% of N2, about 21% of O2 and other gas such as rare gases, carbon dioxide and methane.

Preferably, in step C), the dew point is between -20 and +20°C and advantageously between -15°C and +15°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 adsorption during the thermal treatment. Optionally, in step A), the barrier pre-coating comprises impurities 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.

In a preferred embodiment, in step A), the barrier pre-coating does not comprise at least one of the elements chosen from Al, Fe, Si, Zn, and N. Indeed, without willing to be bound by any theory, there is a risk that the presence of at least one of these elements decreases the barrier effect of the coating.

Preferably, in step A), the barrier pre-coating consists of Cr, i.e. the barrier coating comprises only Cr and optional impurities.

Preferably, in step A), no further pre-coating is deposited on the barrier pre coating before steps B to F).

Preferably, in step A), the barrier pre-coating has a thickness between 10 and 550 nm and more preferably between 10 and 90 or between 150 and 250 nm. For example, the thickness of the barrier coating is of 50, 200 or 400 nm.

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

In step A), the steel sheet can be directly topped by an anticorrosion pre coating, this anticorrosion pre-coating layer being directly topped by the barrier pre coating. For example, the anticorrosion pre-coating comprises at least one of the metal selected from the group comprising zinc, aluminum, copper, magnesium, titanium, nickel, chromium, manganese and their alloys. Preferably, the anticorrosion coating is based on aluminum or based on zinc.

In a preferred embodiment, the anticorrosion pre-coating based on aluminum 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 anticorrosion coating is AluSi®.

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

The pre-coatings can be deposited by any methods known to the man skilled in the art, for example hot-dip galvanization process, roll coating, electrogalvanization process, physical vapor deposition such as jet vapor deposition, magnetron sputtering or electron beam induced deposition. Preferably, the barrier pre-coating is deposited by electron beam induced deposition or roll coating. After the deposition of the pre-coatings, a skin-pass can be realized and allows work hardening the coated 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.

After the provision of the steel sheet pre-coated with the metallic coating according to the present invention, the coated steel sheet is cut to obtain a blank. A thermal treatment is applied to the blank in a furnace. Preferably, the thermal treatment is performed under non- protective atmosphere or under protective atmosphere 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 coating forms 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 barrier coating comprising chromium and not comprising nickel can prevent water dissociation at the barrier coating surface and also prevent the hydrogen diffusion through the coating. With an atmosphere having an oxidizing power equal or higher than that of an atmosphere consisting of 1% volume percent oxygen and equal or smaller than that of an atmosphere consisting of 50% by volume of oxygen, it is believed that the oxides being thermodynamically stable further inhibit the water dissociation. 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 a steel sheet coated with a barrier coating comprising chromium and not comprising nickel and an oxide layer comprising thermodynamically stable iron, chromium oxides and not comprising nickel oxides, such barrier coating being alloyed through diffusion with the steel sheet.

More preferably, a part the steel sheet directly topped by an anticorrosion coating, this anticorrosion coating layer being directly topped by the barrier coating comprising chromium and not comprising nickel and an oxide layer comprising thermodynamically stable iron, chromium oxides and not comprising nickel oxides, such barrier coating being alloyed through diffusion with the anticorrosion coating, the anticorrosion coating being alloyed with the steel sheet. Indeed, without willing to be bound by any theory, it seems that iron from steel diffuses to the surface of the barrier pre-coating during the thermal treatment. With the atmosphere of step C), it is believed that iron and chromium slowly oxidize forming thermodynamically stable oxides preventing H2 adsorption into the steel sheet.

Preferably, the thermodynamically stable chromium and iron oxides can comprise respectively ( 203; FeO, Fe2C>3 and/or Fe3C>4.

If an anti-corrosion coating based on zinc is present, the oxides can also comprise ZnO. If an anti-corrosion coating based on aluminum is present, the oxides can also comprise AI2O3

Preferably, the thickness of the oxide layer is between 10 and 550nm. 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 25pm, preferably inferior or equal to 20pm. 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%.

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

Some steel sheets are coated with a 2 ^nd coating deposited by magnetron sputtering.

Example 1 : hydrogen test:

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

Trials are steel sheets coated with a 1 ^st coating being AluSi® (25pm) and a 2 ^nd coating comprising 80% of Ni and 20% of Cr or consisting of Cr.

After the deposition of the coated steel sheets, coated trials were cut in order to obtain a blank. Blanks were then heated at a temperature of 900°C during a dwell time varying between 5 and 10 minutes. The atmosphere during the thermal treatment was air or nitrogen with a dew point between -15°C and +15°C. Blanks were transferred into a press tool and hot-stamped in order to obtain parts having an omega shape. 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 :

*: examples according to the invention.

Trial 4 according to the present invention release a very low amount of hydrogen compared to comparative examples.