The present invention deals with a method to manufacture press-hardened steel parts with a variable thickness and having a minimum risk of delayed fracture. 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. To this end, steel sheets having improved mechanical properties are usually used, such steel sheets being formed by austenitization and subsequent press-hardening.

The sensitivity to delayed cracking increases with the mechanical strength, after press-hardening 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, meaning that cracking 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.

Press hardening is known as critical for hydrogen absorption, increasing the sensitivity to delayed fracture. Absorption may occur at the austenitization heat treatment, which is the heating step prior to the press forming itself. The saturation of hydrogen into steel is indeed dependent from the metallurgic phase. Furthermore, at high temperature the water in the furnace dissociates at the surface of the steel sheet into hydrogen and oxygen.

It is also known in the automotive field to design parts with variable thickness, so the mechanical resistance is present only in the region where it is needed, without adding weight where it is not necessary. Weight reduction of automotive vehicles is mandatory for reasons of energy consumption and exhaust emissions.

Parts with variable thickness are usually produced by continuous flexible rolling, a process where the sheet thickness obtained after rolling is variable in the rolling direction. This occurs in relationship with the load which has been applied through the rollers to the sheet during the rolling process as described in the patent EP1074317. Flexible rolling is characterized in that the roll gap is deliberately changed during the rolling operation. The object of flexible rolling is to produce a rolled sheet with a load- and weight-optimized cross section. The thickness of such a steel sheet with variable thickness is inherited from the rolling. Here and in the following, the rolling ratio is defined by the following formula: thickness before rolling — thickness after rolling rolling ratio = - — — - - — - — - thickness before rolling

A blank cut from a strip with variable thickness is commonly known as a tailor rolled blank. Tailor rolled blanks have one portion rolled at a rolling ratio and at least another portion rolled at a different rolling ratio. Because of flexible rolling, tailor rolled blanks have a portion with a thickness ti and at least another portion with a different thickness t2. Usually, such rolling ratios have values from 1 to 60%.

The resulting parts with variable thickness are known to absorb more hydrogen during the austenitization heat treatment than standard parts with uniform thickness.

The patent application EP3489386 discloses a coated steel sheet which is subject to particularly low hydrogen absorption during the press-hardening process and whose surface enables simple and good further processing. The solution proposed is a coated steel substrate for hot working, comprising: a first coating containing at least 85% by weight aluminum and a second coating overlying the first coating; wherein the second coating is a copper-containing coating. The method according to this application is particularly suitable for flexibly rolled strip material, since the thinner rolled substrate sections also have increased resistance to the absorption of hydrogen after the application of the nanocrystalline zinc-copper coating. However, this solution requires a second coating on top of the aluminum- based coating. This induces an additional process step and corresponding costs and complexity.

Regarding hydrogen intake during the heating step, not only the rolling ratio increases the hydrogen amount, but the parameters used to heat the blanks as well. The higher the dwelling time or the dew point in the furnace, the higher the hydrogen amount in the press-hardened part will be.

Thus, the object of the invention is to provide a steel sheet with variable thickness suitable for press hardening, that can be used to manufacture a part with variable thickness with a limited hydrogen absorption, independently of the heating time used for press hardening. It particularly aims to make available a part having excellent resistance to delayed fracture.

This object is achieved by the steel sheet of claims 1 to 3.

Another object of the invention is to provide a manufacturing method according to claim 4.

The object of the invention is also achieved by the providing a part according to claims 5 to 9.

A final object of the invention is the use of such a part according to claim 10.

The invention relates to a steel sheet coated with a metallic coating comprising zinc, silicon, magnesium, up to 3.0% of iron, optional elements chosen from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, the content by weight of each element being less than 0.3%, optionally up to 100 ppm of Calcium, and unavoidable impurities up to 0.02 %, the balance being aluminum.

Preferably, the coating comprises, by weight, from 1.0 to 11.0 of zinc, from 1.0 to 7.0 % of silicon, from 1.0 to 8.0 % of magnesium, up to 3.0% of iron, and unavoidable impurities up to 0.02 %, the balance being aluminum.

Advantageously, the coating comprises, by weight, from 6.0 to 10.0 % of zinc, from 1 .0 to 4.0 % of silicon, from 1 .0 to 4.0 % of magnesium, up to 3.0% of iron, and unavoidable impurities up to 0.01 %, the balance being aluminum. In another embodiment, the coating comprises, in weight percent, from 7.5 to 9.0 % of zinc, from 2.0 to 4.0 % of silicon and from 1 .5 to 2.5 % of magnesium, the balance being aluminum.

The steel sheet according to the invention can be manufactured by hot dip galvanizing in a bath, the temperature of which is set from 600 to 700°C, preferably from 620 to 650°C.

The coating weight controlled by the wiping process can be from 50 to 500 g/m ² , possibly from 80 to 150 g/m ² and preferably from 100 and 120 g/m ² for the sum of both sides of the steel sheet.

Before being coated, the steel sheet according to the invention can be obtained by hot rolling and optionally cold rolling depending on the desired thickness, which can be for example between 1 .0 and 4.0 mm.

The substrates to be coated can have any composition, depending on the mechanical properties required. When steel is used for press-hardening, its composition is preferably as described below.

After being coated, the coated steel sheet is submitted to a flexible rolling operation, after which the sheet has a variable thickness in rolling direction.

The flexible rolling is preferably a cold rolling operation. The rolling ratio is from 1 to 60%, preferably from 5 to 50%. The re-rolled material obtained is then a tailor rolled steel sheet. Said tailor rolled steel sheet is then is cut to obtain a tailor rolled blank. The flexible rolling operation usually occurs on a single stand, reversible rolling mill in one stage. The coating is then also reduced in thickness. After rolling, the sheet may have a thickness down to 0.8 or even 0.6 mm.

The method according to the invention comprises the following steps:

A. the provision of a coated steel sheet with variable thickness according to the invention,

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

C. the heat treatment of the tailor rolled blank to obtain a fully austenitic microstructure in the steel, D. the transfer of the tailor rolled blank into a press tool,

E. the press-hardening of the tailor rolled blank to obtain a part having a variable thickness,

F. the cooling of the part having a variable thickness obtained at step E) to obtain a press-hardened part with variable thickness.

In step A, any steel can be advantageously used in the frame of the invention. However, in case steel having high mechanical strength is needed, for parts of structure of automotive vehicle, steel having a tensile resistance superior to 500MPa, advantageously between 500 and 2000MPa before or after heattreatment, 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 remainder being iron and unavoidable impurities from the manufacture of steel.

For example, the steel sheet is 22MnB5 with the following weight composition: 0.20% < C < 0.25%; 0.15% < Si < 0.35%; 1.10% < Mn < 1.40%; 0% < Cr < 0.30%; 0.020% < Ti < 0.060%; 0.020% < Al < 0.060%; 0.002% < B < 0.004%, the remainder being iron and unavoidable impurities from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.24% < C < 0.38%; 0.40% < Mn < 3%; 0.10% < Si < 0.70%; 0.015% < Al < 0.070%; Cr < 2%; 0.25% < Ni < 2%; 0.015% < Ti < 0.10%; Nb < 0.060%; 0.0005% < B < 0.0040%; the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

Alternatively, the steel sheet can have the following weight composition: 0.30% < C < 0.40%; 0.5% < Mn < 1.0%; 0.40% < Si < 0.80%; 0.1 % < Cr < 0.4%; 0.1 % < Mo < 0.5%; 0.01 % < Nb < 0.1 %; 0.01 % < Al < 0.1 %; 0.008% < Ti < 0.003%; 0.0005% < B < 0.003%; 0.0% < P < 0.02%; 0.0% < Ca < 0.001 %; 0.0% < S < 0.004 %; 0.0% < N < 0.005 %, the remainder being iron and unavoidable impurities resulting from the manufacture of steel. In another embodiment, the steel sheet has the following weight 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%, the balance being iron and unavoidable impurities from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.06% < C < 0.1 %, 1 % < Mn < 2%, Si < 0.5%, Al <0.1 %, 0.02% < Cr < 0.1 %, 0.02%

< Nb < 0.1 %, 0.0003% < B < 0.01 %, N < 0.01 %, S < 0.003%, P < 0.020% less than 0,1 % of Cu, Ni and Mo, the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.015% < C < 0.25%; 0.5% < Mn < 1.8%; 0.1 % < Si < 1.25%; 0.01 % < Al < 0.1 %; 0.1 % < Cr < 1 .0%; 0.01 % < Ti < 0.1 %; 0% < S < 0.01 %; 0.001 % < B < 0.004%; 0%

< P < 0.020%; 0% < N < 0.01 %; the balance being iron and unavoidable impurities from the manufacture of steel.

Alternatively, the steel sheet has the following weight composition: 0.2% < C

< 0.34%; 0.5% < Mn < 1 .24%; 0.5% < Si < 2.0%; 0% < S < 0.01 %; 0% < P < 0.020%; 0% < N < 0.01 %, the balance being iron and unavoidable impurities from the manufacture of steel.

In step C, a heat treatment of the blank is performed at a temperature from 800 to 970°C, preferably from 840 to 950°C. Said blank is maintained during a dwell time from 1 to 15 minutes to have a full austenitic structure. During the heat treatment, the pre-coating forms an alloy layer having a high resistance to corrosion and abrasion. The atmosphere of the furnace has an influence on the amount of hydrogen absorbed into the steel sheet during heat treatment. For instance, with a dew point of 20°C, it is known that the hydrogen absorption may be significant, whereas a heat treatment under dry atmosphere is much less risky. In step D, after the heat treatment, the blank is then transferred to a presshardening tool.

In step E, the press-hardening takes place preferably at a temperature from 600 to 830°C.

In step F, the part is cooled in the press-hardening 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 press hardening is consistent with the targeted mechanical properties. After press hardening, the part can be additionally tempered to reach the targeted microstructure and mechanical properties.

In a preferred embodiment, the steel microstructure comprises, in terms of volume fraction, at least 95% of martensite.

In another embodiment, the steel microstructure comprises after press hardening, in terms of volume fraction, at least 50% of martensite and less than 40 % of bainite.

In another embodiment, the steel microstructure comprises after press hardening, in terms of volume fraction, from 5 to 20 % of martensite, up to 10 % of bainite and at least 75 % of equiaxed ferrite.

A coated part according to the invention is thus obtained by press hardening but is also achievable by any suitable combination of cold-stamping and press hardening.

The part obtained in step F is topped by a superficial oxide layer on its outer surface. This oxide layer comprises aluminum, zinc and magnesium from the coating and iron from the steel substrate. Iron has diffused through the coating during heat treatment.

After heat treatment under an atmosphere with a dew point of 20°C, a hydrogen content of 0.6 ppm or less is considered satisfying. On the contrary, a hydrogen content of more than 0.6 ppm may induce risks of later delayed fracture. The inventors have found that the composition of the metallic coating has an influence on hydrogen absorption of rolled material. The coating composition according to the invention allows to keep the hydrogen content in the press- hardened part below 0.6 ppm, whatever the rolling ratio.

It is believed the superficial oxide layer can act as a barrier to hydrogen, notably if said oxide layer contains zinc, magnesium and if it has a minimum thickness.

The oxide layer contains elements coming from the coating. According to the invention, the oxide layer contains zinc and magnesium from the aluminum-based coating; and the oxide layer has a minimum thickness of 0.4 pm. Preferably, the oxide layer has a minimum thickness of 0.5 pm, advantageously 0.6 pm.

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 by weight: C = 0.22%; Mn = 1 .2%; Si = 0.25%; Cr = 0.2%; Al = 0.041 %; Ti = 0.04%; B = 0.003%.

All coatings were deposited by hot dipping in a single molten bath.

After coating deposition, some samples were left unrolled, some other ones were rolled to 50%. i. e. their thickness became half of the thickness before rolling.

Following the rolling step, the samples were heated for 5 and 12 minutes in a furnace at 900°C with a dew point of + 20°C.

After press-hardening, two different measurements were performed: the hydrogen amount and the thickness of the outer oxide layer

The hydrogen content absorbed by the steel sheet during the heat treatment was measured by thermic desorption using a Thermal Desorption Analyzer or TDA. To this end, each sample was placed in a quartz room and heated slowly in an infrared 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. The oxide layer was measured by observation of cross-section with a microscope. The minimum value along the cross-section is reported.

Results are shown in Table 1 : Table 1 :

Jnderlined values are not according to the invention.

Trials 4 to 6, which composition of coating is not according to the invention, contain more than 0.60 ppm hydrogen, which induces risks of delayed fracture. Their oxide layer thickness is too small. Trials 1 to 3 contain at most 0.40 ppm of hydrogen and a thick oxide layer. They show that a part coated with a coating according to the invention, with variable thickness obtained by rolling a portion of it at a rolling ratio of 30% and another portion of it at a rolling ratio of 50% is solving the problem of fracture risks.