A press hardening method

The present invention relates to a press hardening method comprising the provision of a carbon steel sheet coated with a barrier pre-coating which inhibits hydrogen adsorption 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 Carbon 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 know to modify the composition of the steel to prevent the adsorption of hydrogen into the steel.

For example, the patent application US2008035249 discloses a TWIP steel comprising at least one metal element chosen from vanadium, titanium, niobium, chromium and molybdenum, where 0.050%<V<0.50%; 0.040%<Ti<0.50%; 0.070%<Nb<0.50%; 0.070%<Cr<2%; 0.14%<Mo<2% and, optionally, one or more elements chosen from 0.0005%<B<0.003%; Ni<1 % Cu<5%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the amounts of metal elements in the form of precipitated carbides, nitrides or carbonitrides being: 0.030%<Vp<0.150%; 0.030% Tip<0.130%; 0.040%<Nbp<0.220%; 0.070%≤Crp<0.6%; 0.14%<Mop<0.44%. Indeed, the inventors firstly demonstrated that precipitated vanadium, titanium or niobium carbides, nitrides or carbonitrides are very effective as hydrogen traps. Chromium or molybdenum carbides may also fulfill this role.

Nevertheless, when hot-forming is performed, such modifications are not sufficient. Indeed, when a carbon steel sheet has to be hardened by press- hardening process, there is a high risk that the steel adsorbs hydrogen originating from the dissociation of H ₂ O in the furnace during the austenitization treatment.

DE102010030465 discloses a method for the production of a sheet metal formed part which is provided with a corrosion protection coating and is formed from a high tensile steel sheet material. This method comprises the following steps:

- transforming a provided output sheet material into a sheet metal blank;

- formation of the anti-corrosive coating by electrolytic application of a zinc-nickel coating (C) to the sheet metal part, whereby at the beginning of the coating process a thin nickel layer is first deposited, which also prevents hydrogen embrittlement of the steel sheet material.

DE102010030465 further relates to a hot-formed and, in particular, press- hardened sheet-metal part (P) made of a high-tensile steel sheet material with an electrolytically applied zinc-nickel coating (C). It is mentioned that the heat treatment serves to expel the hydrogen contained in the starting sheet material and the hydrogen (which is to be substantially prevented by the initially deposited thin nickel layer) which is possibly introduced during the application of the zinc- nickel coating in the steel sheet material. The heat treatment causes the hydrogen atoms embedded in the structure of the steel sheet material to be expelled by effusion. The hydrogen embrittlement of the steel sheet material is thus counteracted.

However, there is a risk that the nickel coating layer deposited onto the steel substrate is not enough efficient regarding the prevention of hydrogen absorption into the steel.

Thus, the object of the invention is to provide a press hardening method wherein the hydrogen adsorption into the carbon 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 steel sheet can also comprise characteristics of claims 2 to 24.

The invention also covers a part according to claim 25. The part can also comprise characteristics of claim 26 or 27.

Finally, the invention covers the use of such part for the manufacture of an automotive vehicle according to claim 28.

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

- "carbon 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 carbon steel sheet.

Any steel can be advantageously used in the frame of the invention. However, 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 carbon 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 carbon 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 carbon 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 ⁺ 13 ⁺ 15 ^{≥ 1} ' ^1%

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 Carbon 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.

Carbon 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 comprises the following steps:

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

1.5 and 9, preferably between 2.3 and 9 and more preferably between 3 and 5.6,

B. the cutting of the coated carbon 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 10%. Indeed, without willing to be bound by any theory, the inventors have surprisingly found that when a pre-coating comprising nickel and chromium, the ratio Ni/Cr being in the above specific range, is deposited on a carbon steel sheet, this coating acts like a barrier that prevents the adsorption of hydrogen into the carbon steel sheet. Indeed, it is believed that specifics complexes oxides are formed on the surface of the coating having the specific ratio Ni/Cr and act like a barrier by inhibiting the H ₂ adsorption during the thermal treatment, in particular the austenitization 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.

Advantageously, in step A), the barrier pre-coating comprises from 55 to 90%, preferably from 70 to 90%, more preferably from 75 to 85% by weight of nickel.

Preferably, in step A), the barrier pre-coating comprises from 10 to 40%, preferably from 10 to 30% and advantageously from 15 to 25% of chromium.

In a preferred embodiment, in step A), the barrier pre-coating does not comprise at least one of the elements chosen from Zn, B, N, Al and Mo. 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 and Ni, i.e. the barrier coating comprises only Ni and Cr and optional impurities.

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

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 covers enough the carbon 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 carbon 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 up to 0.3% Al, 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 carbon 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 carbon steel sheet pre-coated with the metallic coating according to the present invention, the coated carbon 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 950°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 nickel and chromium can prevent water dissociation at the barrier coating surface and also prevent the hydrogen diffusion through the coating.

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 carbon steel sheet pre-coating with a barrier pre-coating comprising nickel and chromium, such barrier coating being alloyed through diffusion with the carbon steel sheet. More preferably, a part comprises the carbon steel sheet directly topped by an anticorrosion pre-coating, this anticorrosion pre-coating layer being directly topped by the barrier pre-coating, such barrier coating being alloyed through diffusion with the anticorrosion coating, the anticorrosion coating being alloyed with the carbon steel sheet.

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, carbon 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 carbon 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 carbon 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 1 , 3 and 5 are naked carbon steel sheets, i.e. no coating is applied on the carbon steel sheet.

Trials 2, 4 and 6 are carbon steel sheets coated with a coating comprising 80% of Ni and 20% of Cr.

Trial 7 is a carbon steel sheet coated only with an AluSi® coating.

Trial 8 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating being WN. Trial 9 is a carbon steel sheet coated with 1 coating being AluSi® and a 2 ^nd coating being CrN.

Trial 10 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating comprising 40% of Ni and 60% of Cr.

Trial 1 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a

2 ^nd coating being Si0 ₂ .

Trial 12 is a Carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating being Ti.

Trial 13 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating being Cr.

Trial 14 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating being Ag.

Trial 15 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating being Y.

Trial 16 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a

2 ^nd coating being Mo.

Trial 17 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating being Au.

Trial 18 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating being W.

Trial 19 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating being Inox 316L. Inox 316L comprises 65% of Fe, 0.03% of C, 12% of Ni, 17% of Cr, 2% of Mn, 1% of Si and 2.5% of Mo.

Trial 20 is a carbon steel sheet coated with 1 ^st coating being AluSi® and a 2 ^nd coating being Inconel 690. Inconel 690 comprises from 7 to 11 % by weight of Fe, 0.05% of C, from 57 to 65% of Ni, from 27 to 31 % of Cr, 0.05% of Mn and 0.5%Si.

Trials 21 , 22 are carbon steel sheets coated with a 1 ^st coating being AluSi® and a 2 ^nd coating comprising 80% of Ni and 20% of Cr.

Trials 7 to 22 have an AluSi® thickness of 25pm.

Trial 23 is a carbon steel sheet coated with a 1 ^st coating being AluSi®.

Trial 24 is a carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating being Ni. Trials 25 is carbon steel sheet coated with a 1 ^st coating being AluSi® and a 2 ^nd coating comprising 80% of Ni and 20% of Cr.

Trials 23 to 25 have an AluSi® thickness of 14pm.

After the deposition of the coated carbon 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. 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 :

Carbon steel H ₂ sheet 1 ^st Thickness 2"d Ratio Thickness amount

Trials

thickness coating (Mm) coating Ni/Cr (nm) (ppm by (mm) mass)

1 1 - - - - - 0.27

Ni/Cr

2* 1 - - 4 100 0.056

80/20

3 1.5 - - - - - 0.31

Ni/Cr

4* 1.5 - - 4 100 0.066

80/20

5 2 - - - - - 0.39

Ni/Cr

6* 2 - - 4 100 0.17

80/20

7 1.5 AluSi® 25 - - - 0.61

8 1.5 AluSi® 25 WN - 200 0.48

9 1.5 AluSi® 25 CrN - 200 0.44

Ni/Cr

10 1.5 AluSi® 25 0.67 200 0.34

40/60

11 1.5 AluSi® 25 Si0 ₂ - 200 0.51

12 1.5 AluSi® 25 Ti - 200 0.85

13 1.5 AluSi® 25 Cr - 200 0.40

14 1.5 AluSi® 25 Ag - 200 0.49

15 1.5 AluSi® 25 Y - 200 0.80

16 1.5 AluSi® 25 Mo - 200 0.48

17 1.5 AluSi® 25 Au - 200 0.65

18 1.5 AluSi® 25 W - 200 1.00

Inox

19 1.5 AluSi® 25 0.7 200 0.5

316L

Inconel 2.1 to

20 ^* 1.5 AluSi® 25 200 0.3

690 2.097

Ni/Cr

21 * 1.5 AluSi® 25 4 200 0.27

80/20

Ni/Cr

22* 1.5 AluSi® 25 4 500 0.27

80/20

23 1.5 AluSi® 14 - - - 0.73

24 1.5 AluSi® 14 Ni - 200 0.54

Ni/Cr

25 ^* 1.5 AluSi® 14 4 200 0.34

80/20 examples according to the invention. Firstly, we can see that trials 2, 4 and 6 comprising a barrier coating according to the present invention release less hydrogen amount with respect to the trials 1 , 3 and 5 without any barrier coating.

Secondly, we can see that Trials 8 to 19 having a 2 ^nd coating different from the one of the present invention and Trial 7 having no barrier coating release more hydrogen than Trials 20 to 22 according to the present invention.

We can also see the importance of the ratio Ni/Cr in the 2 ^nd coating in Trial 10 and 21. Indeed, Trial 10 having a ratio Ni/Cr outside the invention range releases more hydrogen than Trial 21 according to the present invention.

Moreover, we can see with Trials 21 and 22 that the thickness of the 2 ^nd coating Ni/Cr 80/20 show excellent results with two different thicknesses.

Finally, we can see that trial 25 having a barrier coating according to the present invention releases less hydrogen than Trials 23 and 24, even when the thickness of AluSi® changes.