The present invention relates to a hot-rolled and coated steel sheet for hot-stamping, having a thickness comprised between 1 .8 mm and 5 mm, with an excellent coating adhesion after hot-stamping, and to a hot-stamped coated steel part, at least one portion of which has a thickness comprised between 1 .8 mm and 5 mm, with an excellent coating adhesion. The present invention also relates to a method for manufacturing a hot-rolled and coated steel sheet for hot-stamping having a thickness comprised between 1 .8 mm and 5 mm, and a method for manufacturing a hot-stamped coated steel part.

As the use of high strength steels in automotive applications increases, there is a growing demand for steels having both an increased strength and a good formability. Growing demands for weight saving and safety requirement motivate intensive elaboration of new concepts of automotive steels that can achieve higher ductility and strength.

Thus, several families of steels offering various strength levels have been proposed.

In recent years, the use of coated steels in hot-stamping processes for the shaping of parts has become important, especially in the automotive field.

The steel sheets from which these parts are produced by hot-stamping, having a thickness generally comprised between 0.7 and 2 mm, are obtained through hot-rolling and further cold-rolling.

Furthermore, there is an increasing need for steel sheets for hot-stamping having a thickness higher than 1 .8 mm, and even higher than 3 mm, up to 5 mm. Such steel sheets are for example desired to produce chassis parts or suspension arms, which have been, until now, produced by cold pressing, or to produce parts obtained by hot-stamping tailor rolled blanks (TRB).

However, coated steel sheets for hot-stamping having a thickness higher than 3 mm cannot be produced by cold-rolling. Indeed, the existing cold-rolling lines are not adapted to produce such cold-rolled steel sheets. Moreover, producing cold-rolled coated steel sheets having a thickness comprised between 1 .8 mm and 5 mm involves the use of a low cold-rolling reduction ratio, which is incompatible with the recrystallization which is needed in the annealing step after cold-rolling. Thus, cold-rolled coated steel sheets having a thickness comprised between 1 .8 mm and 5 mm would have an insufficient flatness, resulting for example in misalignment defects during tailored welded blank production.

It has therefore been proposed to produce steel sheets with a high thickness by hot- rolling. For example, JP 2010-43323 discloses a process for manufacturing hot-rolled steel sheets for hot-stamping, having a thickness higher than 1 .6 mm. However, the inventors have discovered that, when producing coated steel sheets by hot-rolling, the adhesion of the coating on the surface of the steel part further to hot- stamping is unsatisfactory, which leads to poor adhesion of the painting on the hot- stamped part. The adhesion of the painting is for example assessed through a wet painting adhesion test.

Furthermore, in some particular cases, the thickness of the coating, before and after hot-stamping, cannot be tightly controlled, so that the thickness of the coating obtained is not within the targeted thickness range. This targeted thickness range is generally comprised between 10 μηι and 33 μηι, for example the range 10 - 20 μηι, the range 15 - 33 μηι or the range 20 - 33 μηι. This uncontrolled coating thickness leads to a poor weldability.

Moreover, as explained in further details hereinbelow, the inventors have discovered that the coating adhesion can be improved under certain circumstances, without however improving the control of the thickness of the coating. Rather, under these circumstances, the control of the thickness of the coating, and therefore the weldability, is even worsened.

Therefore, the invention aims at providing a hot-rolled and coated steel sheet having a thickness comprised between 1 .8 mm and 5 mm and a method for manufacturing the same, allowing to achieve an improved coating adhesion after hot-stamping, whilst allowing the control of the thickness of the coating of the hot-rolled and coated steel sheet to the targeted range, especially in the range comprised between 10 and 33 μηι.

The invention also aims at providing a hot-stamped coated steel part, at least one portion of which has a thickness comprised between 1 .8 mm and 5 mm, having an improved coating adhesion, and a method for manufacturing the same.

For this purpose, the invention relates to a method for manufacturing a hot-rolled and coated steel sheet having a thickness comprised between 1 .8 mm and 5 mm, said method comprising:

- providing a steel semi-product having a composition comprising, by weight percent:

0.04%≤ C≤ 0.38%

0.40%≤ Mn≤ 3%

0.005%≤ Si≤ 0.70%

0.005%≤AI ≤0.1 %

0.001 %≤Cr ≤2%

0.001 %≤ Ni≤ 2%

0.001 %≤Ti≤ 0.2%

Nb≤0.1 %

B≤ 0.010% 0.0005%≤N≤0.010%

0.0001 %≤S≤ 0.05%

0.0001 %≤P≤0.1 %

Mo≤ 0.65 %

W≤ 0.30%

Ca≤ 0.006%

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting,

- hot-rolling the semi-product with a final rolling temperature FRT, so as to obtain a hot-rolled steel product having a thickness comprised between 1 .8 mm and 5 mm, then

- cooling the hot-rolled steel product down to a coiling temperature T _coi | and coiling the hot-rolled steel product at said coiling temperature T _coi | to obtain a hot-rolled steel substrate, the coiling temperature T _coi | satisfying:

450°C≤ T _coi |≤ T _coi | _maXi

wherein T _coi | _max is a maximal expressed as:

coiimax being expressed in degrees Celsius and fy designating the austenite fraction in the hot-rolled steel product just before the coiling,

- pickling the hot-rolled steel substrate,

- coating the hot-rolled steel substrate with Al or an Al alloy by continuous hot- dipping in a bath, to obtain a hot-rolled and coated steel sheet comprising a hot-rolled steel sheet and an Al or an Al alloy coating, having a thickness comprised between 10 and 33 μηι, on each side of the hot-rolled steel sheet.

According to an embodiment, the composition is such that 0.075%≤ C≤ 0.38%.

According to a particular embodiment, the steel has the following chemical composition, by weight percent:

0.040%≤C≤ 0.100%

0.80%≤ Mn≤ 2.0%

0.005%≤ Si≤ 0.30%

0.010%≤AI≤ 0.070%

0.001 %≤Cr≤ 0.10%

0.001 % < Ni < 0.10%

0.03%≤ Ti≤ 0.08%

0.015%≤Nb≤ 0.1 %

0.0005%≤ N≤ 0.009%

0.0001 %≤S≤ 0.005% 0.0001 %≤P≤0.030%

Mo≤ 0.10%

Ca≤ 0.006%

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

According to another particular embodiment, the steel has the following chemical composition, by weight percent:

0.062%≤ C≤ 0.095%

1 .4%≤ Mn≤ 1 .9%

0.2%≤ Si≤ 0.5%

0.020%≤ Al≤ 0.070%

0.02%≤Cr≤ 0.1 %

wherein 1 .5%≤ (C + Mn +Si + Cr)≤ 2.7%

3.4 x N≤ Ti≤ 8 x N

0.04%≤ Nb≤ 0.06%

wherein 0.044%≤ (Nb+Ti)≤ 0.09%

0.0005%≤ B≤ 0.004%

0.001 %≤N≤0.009%

0.0005%≤ S≤ 0.003%

0.001 %≤P≤0.020%

and optionally 0.0001 %≤ Ca≤ 0.006%,

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

According to another particular embodiment, the steel has the following chemical composition, by weight percent:

0.15%≤C≤ 0.38%

0.5%≤ Mn≤ 3%

0.10%≤ Si≤ 0.5%

0.005%≤AI≤ 0.1 %

0.01 %≤Cr≤1 %

0.001 % < Ti <0.2%

0.0005%≤ B≤ 0.08%

0.0005%≤N≤0.010%

0.0001 %≤S≤ 0.05%

0.0001 %≤P≤0.1 % the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

According to another particular embodiment, the steel has the following chemical composition, by weight percent:

0.24%≤ C≤ 0.38%

0.40%≤ Mn≤ 3%

0.10%≤ Si≤ 0.70%

0.015%≤ Al ≤ 0.070%

0.001 %≤Cr ≤2%

0.25%≤ Ni≤ 2%

0.015% < Ti < 0.1 %

0%≤ Nb≤ 0.06%

0.0005%≤ B≤ 0.0040%

0.003%≤N≤0.010%

0.0001 %≤S≤0.005%

0.0001 %≤P≤0.025%,

the titanium and nitrogen contents satisfying the following relationship:

Ti/N > 3.42,

the carbon, manganese, chromium and silicon contents satisfying the following relationship:

2.6C ₊ ^ ₊ ^ ₊ ^ > 1 .1 % ,

5.3 13 15

the chemical composition optionally comprising one of several of the following elements:

0.05%≤ Mo≤ 0.65%

0.001 %≤W≤0.30%

0.0005%≤ Ca≤ 0.005%,

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

Preferably, after pickling and before coating, the surface percentage of voids in the surface region of the hot-rolled steel substrate is lower than 30%, the surface region being defined as the region extending from the upper point of the surface of the hot-rolled steel substrate to a depth, from this upper point, of 15 μηι.

Preferably, the hot-rolled steel sheet has a depth of intergranular oxidation lower than 4 μηι. According to an embodiment, the bath contains, by weight percent, from 8% to 1 1 % of silicon and from 2% to 4% of iron, the remainder being aluminum or aluminum alloy and impurities inherent to the processing.

According to another embodiment, the bath contains, by weight percent, from 0.1 % to 10% of magnesium, from 0.1 % to 20% of aluminum, the remainder being Zn or Zn- alloy, optional additional elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and/or Bi, and impurities inherent to the processing.

According to another embodiment, the bath contains, by weight percent, from 2.0% to 24.0% of zinc, from 7.1 % to 12.0% of silicon, optionally from 1 .1 % to 8.0% of magnesium, and optionally additional elements chosen from Pb, Ni, Zr or Hf, the content of each additional element being inferior to 0.3%, the balance being aluminum and unavoidable impurities and residual elements, the ratio Al/Zn being above 2.9.

According to another embodiment, the bath contains, by weight percent, from 4.0% to 20.0% of zinc, from 1 % to 3.5% of silicon, optionally from 1 .0% to 4.0% of magnesium, and optionally additional elements chosen from Pb, Ni, Zr or Hf, the content of each additional element being inferior to 0.3%, the balance being aluminum and unavoidable impurities and residual elements, the ratio Zn/Si being comprised between 3.2 and 8.0.

According to another embodiment, the bath contains, by weight percent, from 2.0% to 24.0% of zinc, from 1 .1 % to 7.0% of silicon, optionally from 1 .1 % to 8.0% of magnesium when the amount of silicon is between 1 .1 and 4.0%, and optionally additional elements chosen from Pb, Ni, Zr or Hf, the content of each additional element being inferior to 0.3%, the balance being aluminum and unavoidable impurities and residual elements, the ratio Al/Zn being above 2.9.

According to an embodiment, the method further comprises, after coating the hot- rolled steel sheet with Al or an Al alloy, a step of depositing a Zn coating on the Al or Al- alloy coating through cementation, through electrodeposition or through sonic jet vapor deposition, the Zn coating having a thickness lower than or equal to 1 .1 μηι.

Preferably, the pickling is performed in an HCI bath during a time comprised between 15 and 65 s.

In an embodiment, the hot-rolled steel sheet has a ferrite-pearlitic structure.

The invention also relates to a hot-rolled and coated steel sheet, comprising:

- a hot-rolled steel sheet having a thickness comprised between 1 .8 mm and 5 mm, the composition of which comprises, by weight percent:

0.04%≤ C≤ 0.38%

0.40%≤ Mn≤ 3%

0.005%≤ Si≤ 0.70% 0.005%≤AI ≤0.1 %

0.001 %≤Cr ≤2%

0.001 %≤ Ni≤ 2%

0.001 %≤Ti≤ 0.2%

Nb≤0.1 %

B≤ 0.010%

0.0005%≤N≤0.010%

0.0001 %≤S≤ 0.05%

0.0001 %≤P≤0.1 %

Mo≤ 0.65 %

W≤ 0.30%

Ca≤ 0.006%,

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting,

said hot-rolled steel sheet having a depth of intergranular oxidation of less than 4 μηι, an Al or an Al alloy coating, having a thickness comprised between 10 and 33 μηι, on each side of the hot-rolled steel sheet.

According to an embodiment, the composition is such that 0.075%≤ C≤ 0.38%. According to a particular embodiment, the steel has the following chemical composition, by weight percent:

0.040%≤C≤ 0.100%

0.80%≤ Mn≤ 2.0%

0.005%≤ Si≤ 0.30%

0.010%≤AI≤ 0.070%

0.0001 %≤Cr≤ 0.10%

0.0001 %≤Ni≤ 0.10%

0.03%≤ Ti≤ 0.08%

0.015%≤Nb≤ 0.1 %

0.0005%≤ N≤ 0.009%

0.0001 %≤S≤0.005%

0.0001 %≤P≤0.030%

Mo≤ 0.10%

Ca≤ 0.006%

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting. According to another particular embodiment, the steel has the following chemical composition, by weight percent:

0.062%≤ C≤ 0.095%

1 .4%≤ Mn≤ 1 .9%

0.2%≤ Si≤ 0.5%

0.020%≤ Al≤ 0.070%

0.02%≤Cr≤ 0.1 %

wherein 1 .5%≤ (C + Mn +Si + Cr)≤ 2.7%

3.4 x N≤ Ti≤ 8 x N

0.04%≤ Nb≤ 0.06%

wherein 0.044%≤ (Nb+Ti)≤ 0.09%

0.0005%≤ B≤ 0.004%

0.001 %≤N≤0.009%

0.0005%≤ S≤ 0.003%

0.001 %≤P≤0.020%

and optionally 0.0001 %≤ Ca≤ 0.006%,

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

According to another particular embodiment, the steel has the following chemical composition, by weight percent:

0.15%≤C≤ 0.38%

0.5%≤ Mn≤ 3%

0.10%≤ Si≤ 0.5%

0.005%≤AI≤ 0.1 %

0.01 %≤Cr≤1 %

0.001 % < Ti <0.2%

0.0005%≤ B≤ 0.08%

0.0005%≤N≤0.010%

0.0001 %≤S≤ 0.05%

0.0001 %≤P≤0.1 %

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

According to another particular embodiment, the steel has the following chemical composition, by weight percent:

0.24%≤ C≤ 0.38%

0.40%≤ Mn≤ 3% 0.10%≤ Si≤ 0.70%

0.015%≤ Al ≤ 0.070%

0.001 %≤Cr ≤2%

0.25%≤ Ni≤ 2%

0.015% < Ti < 0.1 %

0%≤ Nb≤ 0.06%

0.0005%≤ B≤ 0.0040%

0.003%≤N≤0.010%

0.0001 %≤S≤0.005%

0.0001 %≤P≤0.025%,

the titanium and nitrogen contents satisfying the following relationship:

Ti/N > 3.42,

the carbon, manganese, chromium and silicon contents satisfying the following relationship:

the chemical composition optionally comprising one of several of the following elements:

0.05%≤ Mo≤ 0.65%

0.001 %≤W≤0.30%

0.0005%≤ Ca≤ 0.005%,

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

Preferably, the coating comprises an intermetallic layer having a thickness of at most 15 μηι.

According to an embodiment, the hot-rolled and coated steel sheet further comprises, on each side, a Zn coating having a thickness lower than or equal to 1 .1 μηι.

In an embodiment, the hot-rolled steel sheet has a ferrito-pearlitic structure.

The invention also relates to a method for manufacturing a hot-stamped coated steel part, comprising the steps of:

- providing a hot-rolled and coated steel sheet according to the invention or produced by a method according to the invention,

- cutting the hot-rolled and coated steel sheet to obtain a blank,

- heating the blank in a furnace to a temperature Tc to obtain a heated blank,

- transferring the heated blank to a die and hot-stamping the heated blank in the die, to thereby obtain a hot-stamped blank, - cooling the hot-stamped blank to a temperature less than 400°C to obtain a hot- stamped coated steel part.

According to an embodiment, after the cutting of the hot-rolled and coated steel sheet to obtain the blank and before the blank is heated to the temperature Tc, the blank is welded to an other blank made of a steel having a composition comprising, by weight percent:

0.04%≤ C≤ 0.38%

0.40%≤ Mn≤ 3%

0.005%≤ Si≤ 0.70%

0.005%≤AI ≤0.1 %

0.001 %≤Cr ≤2%

0.001 %≤ Ni≤ 2%

0.001 %≤Ti≤ 0.2%

Nb≤0.1 %

B≤ 0.010%

0.0005%≤N≤0.010%

0.0001 %≤S≤ 0.05%

0.0001 %≤P≤0.1 %

Mo≤ 0.65 %

W≤ 0.30%

Ca≤ 0.006 %

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

The invention also relates to a hot-stamped coated steel part, comprising at least one portion having a thickness comprised between 1 .8 mm and 5 mm, said hot-stamped coated steel part comprising an Al or Al-alloy coating, the coating having a surface percentage of porosities of less than or equal to 3%.

The invention also relates to the use of a hot-stamped coated steel part according to the invention or produced by a method according to the invention for the manufacture of chassis or body-in-white parts or suspension arms for automobile vehicles

The invention will now be described in details and illustrated by examples without introducing limitations, with reference to the appended figures among which:

- Figure 1 is a cross-section of a hot-rolled coated steel part, illustrating the assessment of the coating adhesion after hot-stamping, - Figure 2 is a cross-section of a hot-rolled steel substrate, prior to coating and hot- stamping, illustrating the determination of the surface percentage of voids at the surface of the hot-rolled steel substrate.

By hot-rolled steel product, substrate, sheet or part, it must be understood that the product, substrate, sheet or part is hot-rolled but not cold-rolled.

The present invention refers to a hot-rolled steel sheet which has not been further cold-rolled.

Hot-rolled sheets or substrates differ from cold-rolled sheets or substrates with respect to the following features: in general, the hot and cold-rolling steps create some damage around the second-phase particles due to the differences in rheological behavior between the matrix and the second phase particles (oxides, sulfides, nitrides, carbides...). In the case of cold-rolling, voids can nucleate and grow around cementite, carbides or pearlite. Furthermore, the particles can be fragmented. This damage can be observed on sheets which are cut and prepared by Ion Beam Polishing. This technique avoids artifacts due to the metal flow in mechanical polishing which can fill partly or totally the eventual voids. Further observation of the presence of eventual voids is performed through Scanning Electron Microscopy. As compared to a hot-rolled steel sheet rolled in the austenitic range, the local damage observed around or within cementite particles can be specifically attributed to cold rolling since these particles are not present at the hot rolling step. Thus, the damage observed within or around cementite, carbides or pearlite, in a rolled steel sheet, is an indication that the steel sheet has been cold-rolled.

Besides, in the following, a hot-rolled steel substrate will designate the hot-rolled steel product which is produced when performing the manufacturing method before any coating step, and a hot-rolled and coated steel sheet will designate the product resulting from the manufacturing method, including the coating step. The hot-rolled and coated steel sheet therefore results from the coating of the hot-rolled steel substrate, and comprises a steel product and a coating on each side of the steel product.

To distinguish the steel product of the hot-rolled and coated steel sheet (i.e. excluding the coating) from the hot-rolled steel substrate prior to the coating, the steel product of the hot-rolled and coated steel sheet will be designated hereinafter by "hot- rolled steel sheet".

Hot-rolled steel substrates are generally produced from a steel semi-product which is heated, hot-rolled to the targeted thickness, cooled to a coiling temperature T _coil coiled at the coiling temperature Τ _∞Η , and pickled so as to eliminate the scale. The hot-rolled steel substrates may then be coated to create hot-rolled and coated steel sheets, which are destined to be cut, heated in a furnace, hot-stamped and cooled to the room temperature to obtain the desired structure.

The inventors have investigated the problem of lack of adhesion of the coating further to hot-stamping, and discovered that this lack of adhesion mostly occurs at parts of the sheets that were located in the core and longitudinal axis region of the coil during the coiling.

The inventors have further investigated this phenomenon and discovered that the lack of adhesion of the coating after hot stamping is caused by intergranular oxidation occurring during the coiling.

Especially, just before the coiling, the steel comprises austenite. After coiling, part of this austenite transforms into ferrite and pearlite, generating heat. The heat which is generated leads to an increase in temperature in the coiled steel substrate, especially in the core and axis region of the coil.

The core of the coil is defined as the portion of the substrate (or sheet) which extends, along the longitudinal direction of the substrate, from a first end located at 30% of the overall length of the substrate, to a second end located at 70% of the overall length of the substrate. Besides, the axis region is defined as the region centered on the longitudinal middle axis of the substrate, having a width equal to 60% of the overall width of the substrate.

In the core and axis region, during the coiling, the windings are contiguous, and the partial pressure in oxygen is such that only the elements more easily oxidized than iron, especially silicon, manganese or chrome, are oxidized.

The iron-oxygen phase diagram at 1 atmosphere shows that the iron oxide which is formed at high temperatures, namely wustite (FeO), is not stable at temperatures lower than 570°C, and transforms, at the thermodynamic equilibrium, into two other phases: hematite (Fe ₂ 0 ₃ ) and magnetite (Fe ₃ 0 ₄ ). Conversely, if the increase in temperature at some parts of the coil during the coiling, especially in the core and axis region of the coil, is such that the temperature exceeds 570°C, hematite and magnetite transform into wustite, one of the products of this decomposition being oxygen.

The oxygen resulting from this reaction combines with elements more easily oxidized than iron, especially silicon, manganese, chrome and aluminum, which are present at the surface of the steel substrate.

These oxides naturally form at the grain boundaries, rather than diffusing homogeneously in the matrix. As a result, the oxidation is more pronounced at the grain boundaries. This oxidation will be referred to herein after as intergranular oxidation. Thus, at the end of the coiling, the coil comprises intergranular oxidation, at the surface and down to a certain depth, which can be as high as 17 micrometers.

The inventors have discovered that an important intergranular oxidation in the hot- rolled steel substrate, and consequently in the hot-rolled steel sheet, results in a poor adhesion of the coating after hot-stamping. Indeed, after coating, when the sheet heated to be hot-stamped, carbon diffuses towards the coating and meets the intergranular oxides, in particular manganese and silicon oxides. This diffusion of carbon results in a reaction between Si0 ₂ and C, between MnO and C, and between Mn ₂ Si0 ₄ and C, to form carbon oxides. These carbon oxides migrate and are dissolved until final coating solidification, when they gather to form pockets, resulting in porosities in the coating, and thus to a poor coating adhesion.

The impact of the intergranular oxidation on the coating adhesion is specific to hot- rolled steel sheets, which are not subjected to cold-rolling further to the coiling, by contrast with cold-rolled steel sheets. Indeed, during the production of such cold-rolled sheets, the intergranular oxidation that may be present at the surface of the substrate, prior to cold- rolling, is subjected during the cold-rolling, as the whole sheet, to a thickness reduction. Consequently, the depth of intergranular oxidation of the cold-rolled sheet, before hot- stamping, is reduced to a large extent in comparison with the depth of intergranular oxidation of a hot-rolled steel sheet.

The intergranular oxidation can be reduced or even removed, prior to coating, by intensively pickling the steel substrate, for example in an HCI bath during a time of 375 s.

However, intensive pickling requires a very low line speed, which is not compatible with industrial processing.

Furthermore, this intensive pickling results in a very important developed surface, at the surface of the steel substrate. The developed surface designates the total area of the surface of the steel substrate, which is in contact with the bath during the coating.

This important developed surface results in a more intense iron dissolution from steel surface during the hot-dip coating in the bath, resulting in a growth of the intermetallic layer, which is finally not limited to a single limited region of the coating adjacent to the steel sheet, but reaches the surface of the coating. As a consequence, the thickness of the coating cannot be controlled in the targeted thickness range.

The inventors have therefore found that suppressing or limiting the intergranular oxidation during the coiling allows manufacturing a hot-rolled and coated steel sheet with a thickness comprised between 1 .8 mm and 5 mm having an improved coating adhesion after hot-stamping whilst allowing the control of the thickness of the coating to the targeted range, especially between 10 and 33 μηι. The composition of the steel is such that the steel can be hot-stamped to create a part having a tensile strength higher than or equal to 500 MPa, or higher than or equal to 1000 MPa, or higher than or equal to 1350 MPa, or higher than or equal to 1680 MPa.

As regards the chemical composition of the steel, carbon plays an important role in the hardenability and the tensile strength obtained after hot-stamping, thanks to its effect on the hardness of the martensite.

Below a content of 0.04%, it is not possible to obtain a tensile strength above 500 MPa after stamping under any cooling conditions. Above 0.38%, the adhesion of the coating after hot stamping is not satisfactory. Without being bound by a theory, a C content higher than 0.38% results in an important formation of carbon oxides during the heating of the sheet prior to hot-stamping, aggravating the negative impact of the intergranular oxidation on the coating adhesion. Furthermore, above 0.38%, the resistance to delayed cracking and the toughness of the steel decrease.

The C content depends on the desired tensile strength TS of the hot-stamped part, produced by hot-stamping the steel sheet. Especially, for carbon contents ranging from 0.06% to 0.38% by weight, the tensile strength TS of hot-stamped parts produced through total austenitization and stamping, followed by a martensitic quenching, depends practically only on the carbon content and is linked to the carbon content by the expression:

TS (MPa)=3220(C%)+908,

wherein C% designates the carbon content, by weight percent.

According to an embodiment, the C content is higher than or equal to 0.75%.

Apart from its deoxidizing role, manganese has an important effect on quenchability, in particular when its content is of at least 0.40%. Above 3%, the stabilization of austenite by Mn is too important, which leads to the formation of a too pronounced banded structure. According to an embodiment, the Mn content is lower than or equal to 2.0%.

Silicon is added in a content of at least 0.005% to help deoxidizing the liquid steel and to contribute to the hardening of the steel. Its content must however be limited in order to avoid excess formation of silicon oxides. Besides, the silicon content must be limited to avoid a too important stabilization of austenite. The silicon content is therefore lower than or equal to 0.70%. Preferably, the Si content is of at least 0.10%.

Aluminum may be added as a deoxidizer, the Al content being lower than or equal to 0.1 %, and higher than 0.005%, generally higher than or equal to 0.010%. Preferably, the Al content is lower than or equal to 0.070%.

Optionally, the steel composition comprises chromium, tungsten and/or boron, to increase the quenchability of the steel. Especially, Cr may be added to increase the quenchability of the steel and contributes to achieving the desired tensile strength TS after hot-stamping. When Cr is added, its content is higher than or equal to 0.01 %, up to 2%. If no voluntary addition of Cr is performed, the Cr content may be as low as 0.001 %.

W may be added to increase the quenchability and the hardenability of the steel by forming tungsten carbides. When W is added, its content is higher than or equal to 0.001 %, and lower than or equal to 0.30%.

When B is added, its content is higher than 0.0002%, and preferably higher than or equal to 0.0005%, up to 0.010%. The B content is preferably lower than or equal to 0.005%.

Up to 0.1 % of niobium and/or up to 0.2% of titanium are optionally added to provide precipitation hardening.

When Nb is added, its content is preferably of at least 0.01 %. In particular, when the

Nb content is comprised between 0.01 % and 0.1 %, fine hardening carbonitrides Nb(CN) precipitates form in the austenite or in the ferrite during hot-rolling. The Nb content is preferably lower than or equal to 0.06%. Still preferably, the Nb content is comprised between 0.03% and 0.05%.

When Ti is added, its content is preferably of at least 0.015%, up to 0.2%. When the

Ti content is comprised between 0.015% and 0.2%, precipitation at very high temperature occurs in the form of TiN and then, at lower temperature, in the austenite in the form of fine TiC, resulting in hardening. Furthermore, when titanium is added in addition to boron, titanium prevents combination of boron with nitrogen, the nitrogen being combined with titanium. Hence, the titanium content is preferably higher than 3.42N. However, the Ti content should remain lower than or equal to 0.2%, preferably lower than or equal to 0.1 %, to avoid precipitation of coarse TiN precipitates. If no voluntary addition of Ti is performed, Ti is present as an impurity in a content of at least 0.001 %.

Molybdenum may be added in a content of at most 0.65%. When Mo is added, its content is preferably of at least 0.05%. Mo is preferably added together with Nb and Ti, to form co-precipitates which are very stable at high temperatures, and limit the austenitic grain growth upon heating. An optimal effect is obtained when the Mo content is comprised between 0.15% and 0.25%.

Nickel is optionally added to increase the resistance to delayed fracture of the steel, in a content comprised between 0.25% and 2%. If not added, Ni is present as an impurity in a content which may be as low as 0.001 %.

Sulfur, phosphorus and nitrogen and generally present in the steel composition as impurities. The nitrogen content is of at least 0.0005%. The nitrogen content must be at most 0.010%, so as to prevent precipitation of coarse TiN precipitates.

When in excessive amounts, sulfur and phosphorus reduce the ductility. Therefore, their contents are limited to 0.05% and 0.1 % respectively.

Preferably, the S content is of at most 0.03%. Achieving a very low S content, i.e. lower than 0.0001 %, is very costly, and without any benefit. Therefore, the S content is generally higher than or equal to 0.0001 %.

Preferably, the phosphorus content is of at most 0.05%, still preferably of at most 0.025%. Achieving a very low P content, i.e. lower than 0.0001 %, is very costly. Therefore, the P content is generally higher than or equal to 0.0001 %.

The steel may undergo a treatment for globularization of sulfides performed with calcium, which has the effect of improving the bending angle, due to MnS globularization. Hence, the steel composition may comprise at least 0.0001 % of Ca, up to 0.006%.

The balance of the composition of the steel consists of iron and unavoidable impurities resulting from the smelting.

According to a first embodiment, the steel has the following chemical composition, by weight percent:

0.040%≤C≤0.100%

0.80%≤ Mn≤ 2.0%

0.005%≤ Si≤ 0.30%

0.010%≤AI≤ 0.070%

0.001 %≤Cr≤ 0.10%

0.001 %≤ Ni≤ 0.10%

0.03%≤ Ti≤ 0.08%

0.015%≤Nb≤ 0.1 %

0.0005%≤ N≤ 0.009%

0.0001 %≤S≤0.005%

0.0001 %≤P≤0.030%

Mo≤ 0.10%

Ca≤ 0.006%

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

With this composition, steel parts having, after hot-stamping, a tensile strength of at least 500 MPa, can be produced.

According to a second embodiment, the steel has the following chemical composition, by weight percent: 0.062%≤ C≤ 0.095%

1 .4%≤ Mn≤ 1 .9%

0.2%≤ Si≤ 0.5%

0.020%≤ Al≤ 0.070%

0.02%≤Cr≤ 0.1 %

wherein 1 .5%≤ (C + Mn +Si + Cr)≤ 2.7%

3.4 x N≤ Ti≤ 8 x N

0.04%≤ Nb≤ 0.06%

wherein 0.044%≤ (Nb+Ti)≤ 0.09%

0.0005%≤ B≤ 0.004%

0.001 %≤N≤0.009%

0.0001 %≤S≤0.003%

0.0001 %≤P≤0.020%

and optionally 0.0001 %≤ Ca≤ 0.006%,

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

With this composition, steel parts having, after hot-stamping, a tensile strength of at least 1000 MPa, can be produced.

According to a third embodiment, the steel has the following chemical composition, by weight percent:

0.15%≤C≤0.38%

0.5%≤ Mn≤ 3%

0.10%≤ Si≤ 0.5%

0.005%≤AI≤ 0.1 %

0.01 %≤Cr≤1 %

0.001 % < Ti <0.2%

0.0005%≤ B≤ 0.08%

0.0005%≤N≤0.010%

0.0001 %≤S≤ 0.05%

0.0001 %≤P≤0.1 %

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

With this composition, steel parts having, after hot-stamping, a tensile strength of at least 1350 MPa, can be produced. According to a fourth embodiment, for producing steel sheets destined to have, after hot-stamping, a tensile strength of at least 1680 MPa, the steel has the following chemical composition, by weight percent:

0.24%≤ C≤ 0.38%

0.40%≤ Mn≤ 3%

0.10%≤ Si≤ 0.70%

0.015%≤ Al ≤ 0.070%

0.001 %≤Cr ≤2%

0.25%≤ Ni≤ 2%

0.015% < Ti≤ 0.1 %

0%≤ Nb≤ 0.06%

0.0005%≤ B≤ 0.0040%

0.003%≤N≤0.010%

0.0001 %≤S≤0.005%

0.0001 %≤P≤0.025%,

the titanium and nitrogen contents satisfying the following relationship:

Ti/N > 3.42,

the carbon, manganese, chromium and silicon contents satisfying the following relationship: 2.6C ₊ ^ ₊ ^ ₊ ^ > 1 .1 % ,

5.3 13 15

the chemical composition optionally comprising one of several of the following elements:

0.05%≤ Mo≤ 0.65%

0.001 %≤W≤0.30%

0.0005%≤ Ca≤ 0.005%,

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

As explained above, the inventors have discovered that the lack of adhesion of the coating of a steel part, produced by hot-stamping a hot-rolled and coated steel sheet, results from intergranular oxidation present on the surface of the hot-rolled and coated steel sheet, prior to hot-stamping, and through a certain thickness.

First, the inventors have sought a criterion that has to be satisfied by the hot stamped coated steel part to guarantee a satisfactory adhesion of the coating.

The inventors have found that the quality of the coating adhesion can be assessed by determining the surface percentage of porosities in the coating. The surface percentage of porosities in the coating is determined on the hot stamped coated steel part, i.e. after hot-stamping and cooling to the room temperature.

The surface percentage of porosities in the coating is determined by observing five different cross-sections from a sample under optical microscope, with a x1000 magnification. Each cross-section has a length l _ref , which is selected to characterize the coating in a representative manner. The length l _re , is chosen as 150 μηι.

As illustrated on Figure 1 , for each cross-section, an image analysis is performed, by means of an image analysis, for example Olympus Stream Essentials®, to determine the surface percentage of the porosities in the coating in this cross-section. To that end, the upper and lower boundaries B and B ₂ of the coating are identified. Especially, the upper boundary follows the contour of the coating, at the interface with the surrounding environment, and the lower boundary separates the steel material from the coating. Then, the total surface occupied by the coating, including the porosities P, between the lower and upper boundaries is determined, and the surface occupied by the porosities which are located between the lower and the upper boundaries is assessed (grey colored areas on Figure 1 ). The surface percentage of porosities in the coating of the cross-section under consideration is then computed as the ratio between the surface occupied by the porosities and the total surface occupied by the coating (multiplied by 100).

Finally, the surface percentage of porosities in the coating is determined as the average of the five values thus obtained.

The coating adhesion is considered as satisfactory if the surface percentage of porosities in the coating is lower than or equal to 3%. By contrast, if the surface percentage of porosities in the coating is higher than 3%, the coating adhesion is considered as unsatisfactory.

Furthermore, the inventors have identified two criteria that have to be satisfied, by the hot-rolled steel substrate and the hot-rolled steel sheet respectively, to ensure that the thickness of the coating can be controlled to be in the targeted range, especially in the range from 10 to 33 μηι, for example between 20 and 33 μηι or between 10 and 20 μηι, and that, after stamping, the adhesion of the coating will be satisfactory.

The first criterion is related to the surface state of the hot-rolled steel substrate, after pickling and before coating.

Especially, as explained above, the developed surface of the hot-rolled steel substrate just before coating must be controlled to avoid intense iron dissolution from steel surface and uncontrolled growth of the intermetallic layer during hot-dipping in the bath, which would result in the impossibility to control the coating thickness within the targeted range. Indeed, the intergranular oxidation of the hot-rolled steel substrate can be reduced by intensive pickling, which in turns allows reducing the intergranular oxidation of the hot- rolled steel sheet. However, owing to this intensive pickling, the substrate would have a surface state (i.e. a developed surface) incompatible with the control of the coating thickness.

The inventors have found that in order to ensure that the coating thickness will be comprised in the targeted range, i.e. comprised between 10 and 33 μηι, the thickness of the intermetallic layer formed during the coating must remain lower than 15 μηι, and that in order to obtain a thickness of the intermetallic layer lower than 15 μηι, the surface percentage of voids in the surface region of the hot-rolled steel substrate, after any pickling and before coating, must be lower than 30%. The thickness of the intermetallic layer here designates the thickness of the intermetallic layer of the coating of the hot- rolled and coated steel sheet.

The criterion on the surface percentage of voids must in particular be met in the region of the hot-rolled steel substrate that was located at the core and axis region of the coil during the coiling.

As illustrated on Figure 2, the surface region is defined as the region extending from the upper point of the surface of the hot-rolled steel substrate to a depth, from this upper point, of 15 μηι. The surface percentage of voids in the surface region is determined from five distinct cross-sections representative of the hot-rolled steel substrate, each cross- section having a length l _ref of 150 μηι. The cross-sections are preferably taken from a sample collected from the core and axis region of the coil. On each cross-section, a sample surface region is determined by means of an image analysis, for example Olympus Stream Essentials®, as a rectangular region whose upper side joins the two higher points Pt1 and Pt2 of the surface profile of the cross-section, and whose lower side is distant from the upper side of 15 μηι. Hence, each sample surface region has a length lre _f of 150 μηι and a depth of 15 μηι.

For each cross-section, the regions of the sample surface region which are not steel are identified, and the total surface of these regions is determined. The surface percentage of voids in the sample surface region is then determined as the ratio between the total surface of the regions which are not steel and the total surface of the sample surface region, multiplied by 100. Finally, the surface percentage of voids of the hot-rolled and pickled steel substrate is determined as the average of the five values thus obtained.

The second criterion is a maximal depth of intergranular oxidation of the hot-rolled steel sheet, i.e. of the steel product after the coating. Indeed, the inventors have discovered that in order to obtain a satisfactory coating adhesion after hot-stamping, the depth of intergranular oxidation of the hot-rolled steel sheet must be lower than 4 μηι.

This criterion must in particular be met in the region of the hot-rolled and coated steel sheet that was located at the core and axis region of the coil during the coiling.

The depth of intergranular oxidation is determined on the hot-rolled and coated steel sheet, i.e. after coating.

The depth of intergranular oxidation is defined as the thickness of the region of the hot-rolled steel sheet, from the surface of the hot-rolled steel sheet (i.e. from the interface between the coating and the hot-rolled steel sheet) towards the inside of the hot-rolled steel sheet, in a direction orthogonal to this surface, in which intergranular oxidation is observed.

Especially, the intergranular oxidation is observed with an optical microscope with a x1000 magnification, on five different cross-sections, each having a length l _ref of 150 μηι, from a sample collected from the core and the axis region of the coil. On each cross- section, the maximal depth of the intergranular oxidation is measured. Finally, the depth of intergranular oxidation is determined as the average of the five values thus obtained.

Hence, in order to ensure that, after coating, the coating thickness can be controlled to be in the targeted range and that, after hot-stamping, the coating adhesion will be satisfactory, i.e. the surface percentage of porosities in the coating will be lower than or equal to 3%, the two following conditions must be met:

- the surface percentage of voids in the surface region of the hot-rolled steel substrate, after pickling and before coating, must be lower than 30%, and

- the depth of intergranular oxidation of the hot-rolled steel sheet, after pickling and coating, must be lower than 4 μηι.

Hot rolled steel products can be produced by casting a steel having a composition as mentioned above so as to obtain a steel semi-product, reheating the steel semi-product at a temperature T _reh eat comprised between 1 150°C and 1300°C, and hot rolling the reheated steel semi-product, with a final rolling temperature FRT, to obtain a hot-rolled steel product.

The final rolling temperature FRT is generally comprised between 840°C and

1000°C.

The hot-rolling reduction is adapted so that the hot-rolled steel product has a thickness comprised between 1 .8 mm and 5 mm, for example comprised between 3 mm and 5 mm.

The hot-rolled steel product is then cooled on the run-out table to reach the coiling temperature Τ _∞Ν , and coiled to obtain a hot-rolled steel substrate. The coiling temperature T _coi | is selected so as to avoid or at least limit intergranular oxidation.

Especially, the coiling temperature T _coi | is selected so that the depth of intergranular oxidation of the hot-rolled steel substrate is lower than 5 μηι. Indeed, if the depth of intergranular oxidation of the hot-rolled steel substrate is lower than 5 μηι, the depth of intergranular oxidation of the hot-rolled steel sheet, after the coating, will remain lower than 4 μηι. Still preferably, the coiling temperature T _coi | is selected so that no intergranular oxidation occurs.

The inventors have found that to obtain a depth of intergranular oxidation of the hot- rolled steel sheet of less than 4 μηι, the coiling temperature T _coi | must be lower than a maximum coiling temperature T _coi | _max , which depends on the austenite fraction just before the coiling, denoted fy.

Indeed, a high austenite fraction fy just before the coiling will result in a substantial transformation of the austenite during the coiling, hence to an important increase in the temperature, especially in the coil and axis region of the sheet during the coiling. By contrast, if the austenite fraction fy just before the coiling is low, no or little transformation of the austenite will occur during the coiling, so that the increase in the temperature of the sheet will be reduced.

As a consequence, the maximal coiling temperature T _coi | _max is a decreasing function of the austenite fraction fy just before the coiling.

The inventors have discovered that, in order to obtain a depth of intergranular oxidation in the hot-rolled steel sheet of less than 4 μηι, the maximal coiling temperature Tcoiimax is expressed as:

T _C oilmax=650-140xfY

wherein T _coi | _max is expressed in degrees Celsius, and fy designates the austenite fraction in the steel just before the coiling, comprised between 0 (corresponding to 0% of austenite) and 1 (corresponding to 100% of austenite). The maximal coiling temperature T _coi | _max is therefore comprised between 510°C and 650°C.

Thus, the coiling temperature T _coi | must satisfy:

T _coi | < 650-140χίγ

The austenite fraction fy in the steel just before the coiling can be determined through an electromagnetic (EM) non-contact non-destructive technique, by using a device for detecting magnetic properties of the steel sheet.

The principle of this technique, which is for example described in the document Online electromagnetic monitoring of austenite transformation in hot strip rolling and its application to process optimization", A.V. Marmulev et al., Revue de Metallurgie 1 10, pp.205-213 (2013), is based on the difference between the magnetic properties of the austenite, which is paramagnetic, and the magnetic properties of ferrite, pearlite, bainite and martensite, which are ferromagnetic phases.

A device for determining the austenite fraction fy is for example disclosed in US

2003/0038630 A1 .

The austenite fraction fy just before the coiling depends on the steel composition, especially on the C content, on the final rolling temperature FRT, and on the cooling process between the final rolling temperature FRT and the coiling temperature Τ _∞Η .

In particular, the higher the C content of the steel, the higher the austenite fraction fy in the steel sheet just before the coiling. Hence, all other parameters being equal, the higher the C content, the lower the maximal coiling temperature T _coi | _max . Especially, if the C content of the steel is higher than or equal to 0.075%, the austenite fraction in the substrate remains higher than 0.5, so that the coiling temperature T _coi | _max is lower than 580°C.

The maximal coiling temperature T _coi | _max can be determined, for a steel having given composition and thickness, on a given line, the final rolling temperature FRT being fixed, by determining the austenite fraction in the steel product during the cooling from the final rolling temperature FRT, and by comparing, during the cooling, the temperature T of the substrate to the value 650 - 140 ίγ(Τ), fy(T) being the austenite fraction of the substrate at the temperature T during the cooling.

The maximal coiling temperature T _coi | _max is the temperature at which T = 650 -140 fY(T).

Generally, the coiling temperature is preferably lower than 580°C, still preferably lower than 570°C.

However, the coiling temperature should remain higher than 450°C, in order to avoid an undesired increase in the mechanical properties of the steel that would result from a low coiling temperature.

Under these conditions, the intergranular oxidation in the hot-rolled steel substrate is limited, so that the depth of intergranular oxidation of the hot-rolled steel sheet after coating will be lower than 4 μηι.

After coiling, the hot-rolled steel substrate is pickled. Since the depth of intergranular oxidation is limited, the pickling conditions do not have an influence on the adhesion of the coating after hot-stamping or on the thickness of the coating.

Especially, even if a light pickling is performed, owing to the low depth of intergranular oxidation before pickling, the depth of intergranular oxidation in the hot-rolled steel sheet after pickling and coating will in any case be lower than 4 μηι so that little or no carbon oxides will be formed during the heating prior to hot-forming, and that the coating adhesion after hot-stamping will not be impaired.

Besides, even if intensive pickling is performed, owing to the low depth of intergranular oxidation before pickling, the surface percentage of voids in the surface region of the hot-rolled steel substrate after pickling will remain lower than 30%. Hence, no intense iron dissolution from steel surface and no uncontrolled growth of the intermetallic layer will occur during the hot-dip coating of the steel sheet in the bath, and the thickness of the coating can be controlled to the targeted thickness.

The pickling is for example performed in a HCI bath, for a time comprised between

15 and 65s.

The hot-rolled steel substrate, which is pickled, thus obtained therefore satisfies the first criterion defined hereinabove, i.e. has a surface percentage of voids in the surface region lower than 30%. Besides, the hot-rolled and pickled steel sheet has no or little intergranular oxidation, which allows satisfying the second criterion defined above, i.e. obtaining a depth of intergranular oxidation lower than 4 μηι in the hot-rolled steel sheet after coating.

After pickling, the hot-rolled and pickled steel substrate may be oiled or may be applied an organic film, for example Easyfilm ^® HPE, to temporarily protect the surface of the sheet.

The hot-rolled and pickled steel substrate is then continuously hot-dip coated in a bath, with either Al or an Al-alloy, so as to obtain a hot-rolled and coated steel sheet.

For example, the coating may be an Al-Si coating. A typical bath for an Al-Si coating generally contains in its basic composition, by weight percent, from 8% to 1 1 % of silicon, from 2% to 4% of iron, the remainder being aluminum or aluminum alloy, and impurities inherent to the processing. Alloying elements present with aluminum include strontium and/or calcium, between 15 and 30 ppm each.

As another example, the coating may be a Zn-AI-Mg coating. A typical bath for a Zn-AI-Mg coating contains, by weight percent, between 0.1 % and 10% of magnesium, between 0.1 % and 20% of aluminum, the remainder being Zn or Zn-alloy, optional additional elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and/or Bi, and impurities inherent to the processing.

For example, the bath contains between 0.5% to 8% of aluminum, between 0.3% and 3.3% of magnesium, the remainder being Zn or Zn-alloy, optional additional elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and/or Bi, and impurities inherent to the processing. As another example, the coating is an Al-Zn-Si-Mg coating.

A first example of bath for a Al-Zn-Si-Mg coating contains, by weight percent, from 2.0% to 24.0% of zinc, from 7.1 % to 12.0% of silicon, optionally from 1 .1 % to 8.0% of magnesium, and optionally additional elements chosen from Pb, Ni, Zr or Hf, the content of each additional element being inferior to 0.3%, the balance being aluminum and unavoidable impurities and residual elements, the ratio Al/Zn being above 2.9.

A second example of bath for a Al-Zn-Si-Mg coating contains, by weight percent, from 4.0% to 20.0% of zinc, from 1 % to 3.5% of silicon, optionally from 1 .0% to 4.0% of magnesium, and optionally additional elements chosen from Pb, Ni, Zr or Hf, the content of each additional element being inferior to 0.3%, the balance being aluminum and unavoidable impurities and residual elements, the ratio Zn/Si being comprised between 3.2 and 8.0.

A third example of bath for a Al-Zn-Si-Mg coating contains, by weight percent, from 2.0% to 24.0% of zinc, from 1 .1 % to 7.0% of silicon, optionally from 1 .1 % to 8.0% of magnesium when the amount of silicon is between 1 .1 and 4.0%, and optionally additional elements chosen from Pb, Ni, Zr or Hf, the content of each additional element being inferior to 0.3%, the balance being aluminum and unavoidable impurities and residual elements, the ratio Al/Zn being above 2.9.

After the deposition of the coating by hot-dipping, the coated steel sheet is usually wiped with nozzles ejecting gas on both sides of the coated steel sheet, and the coated steel sheet is then cooled.

The hot-rolled and coated steel sheet thus obtained comprises a hot-rolled steel sheet and, on each side of the hot-rolled steel sheet, an Al or an Al alloy coating.

The hot-rolled steel sheet generally has a ferrito-pearlitic structure

The thickness of the Al or Al alloy coating, on each side of the hot-rolled steel sheet, is comprised between 10 μηι and 33 μηι.

According to a first embodiment, the thickness of the coating is controlled to be comprised in the range between 20 μηι and 33 μηι.

According to a first embodiment, the thickness of the coating is controlled to be comprised in the range between 10 μηι and 20 μηι.

According to a third embodiment, the thickness coating is controlled to be in the range between 15 μηι and 25 μηι.

After the coating, the depth of intergranular oxidation in the hot-rolled steel sheet remains lower than 4 μηι, generally lower than 3 μηι owing to the pickling. This depth extends from the surface of the hot-rolled steel sheet (i.e. the surface which separates the hot-rolled steel sheet from the coating) towards the inside of the steel sheet. Moreover, owing to the low surface percentage of voids in the surface region of the hot-rolled steel substrate before coating, even after pickling, the thickness of the coating is comprised within the targeted thickness range, especially betweenI O μηι and 33 μηι, on each side of the hot-rolled and coated steel sheet, and at every location on each side of the hot-rolled and coated steel sheet.

The hot-rolled and coated steel sheet is destined to be hot-stamped.

To that end, the hot-rolled and coated steel sheet is cut to obtain a blank. Optionally, this blank may be welded to a second blank, to thereby obtain a tailor welded blank (TWB) comprising a first blank cut from a hot-rolled and coated steel sheet according to the invention and a second blank. The second blank may also be obtained from a hot-rolled and coated steel sheet according to the invention, or may be a blank cut from a cold-rolled and coated steel sheet. Especially, the first blank, having a thickness comprised between 1 .8 mm and 5 mm, may be welded to a second blank having a different thickness and/or made from a steel having a different composition. The second blank is preferably made of a steel having a composition comprising, by weight percent:

0.04%≤ C≤ 0.38%

0.40%≤ Mn≤ 3%

0.005%≤ Si≤ 0.70%

0.005%≤AI ≤0.1 %

0.001 %≤Cr ≤2%

0.001 %≤ Ni≤ 2%

0.001 %≤Ti≤ 0.2%

Nb≤0.1 %

B≤ 0.010%

0.0005%≤N≤0.010%

0.0001 %≤S≤ 0.05%

0.0001 %≤P≤0.1 %

Mo≤ 0.65 %

W≤ 0.30%

Ca≤ 0.006 %

the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting.

For sake of simplification, the term "blank" will be used hereinafter to designate a blank obtained from a hot-rolled and coated steel sheet according to the invention, or a tailor welded blank including this blank. The blank is then submitted to a heat treatment in a furnace prior to hot-stamping, and hot-stamped to obtain a hot-stamped coated steel part.

Especially, the blank is heated in a furnace to a temperature Tc which is generally comprised between 880°C and 950°C, to obtain a heated blank.

The heated blank is then removed from the furnace and transferred from the furnace to a die, where it undergoes a hot deformation (hot-stamping), for the purpose of obtaining the desired geometry of the part to obtain a hot-stamped blank. The hot- stamped blank is cooled down to 400°C at a cooling rate Vr which is preferably of more than 10°C/s, still preferably of more than 30°C/s, thereby obtaining a hot-stamped coated steel part.

The hot-stamped coated steel part which is thus obtained has a very satisfactory coating adhesion.

Especially, the surface percentage of porosities in the coating of the hot-stamped coated steel part is lower than or equal to 3%.

In addition, after painting, for example by spraying, the painting adhesion is very satisfactory. The painting adhesion can in particular be assessed by performing a wet painting adhesion test according to the standard ISO 2409:2007. The painting adhesion is considered as good if the result of the wet painting adhesion test is lower than or equal to 2, and poor if the result of the wet painting adhesion test is higher than 2.

EXAMPLES

Hot-rolled and coated steel sheets were produced by casting semi-products having the compositions disclosed in Table 1 , by weight percent:

Table 1

The semi-products were hot-rolled down to a thickness th, with a final rolling temperature FRT. The hot-rolled steel products were cooled to a coiling temperature T _coi | and coiled at the coiling temperature T _coi |, to obtain hot-rolled steel substrates.

The hot-rolled steel substrates were then pickled in an HCI bath, for a time tp _ick iing. After pickling, samples were taken from the core and axis region of the hot-rolled steel substrates, and for each sample, the surface percentage of voids in the surface region was determined according to the procedure described hereinabove.

The hot-rolled steel substrates were then hot-dip coated in a bath comprising, by weight percent, 9% of silicon and 3% of iron, the remainder being aluminum and unavoidable impurities, to obtain hot-rolled and coated steel sheets. A coating thickness comprised between 20 and 33 μηι on each side of the sheet was targeted.

After coating, samples were taken from the core and axis region of the sheets, and for each sample, the depth of intergranular oxidation was determined according to the procedure described hereinabove. In addition, the thickness of the coating and the thickness of the intermetallic layer were determined.

The hot-rolled and coated steel sheets thus obtained were cut to obtain blanks. The blanks cut from the core and axis region of the hot-rolled and coated steel sheets were heated in a furnace to a temperature of 920°C for a time t _c . This time t _c includes the heating phase to the targeted temperature and the holding phase at this temperature. The heated blanks were then transferred to a die, hot-stamped and cooled down to the room temperature.

From each hot-stamped coated part, a sample was taken, and the coating adhesion was assessed by determining the surface percentage of porosities in the coating according to the procedure described above. Furthermore, the coating thickness was measured.

Finally, a electro-deposited painting of 20μιτ\ΜΆ5 applied on one side of each part, and the adhesion of the painting on the parts was assessed by a wet painting adhesion test according to the standard ISO 2409:2007. The painting adhesion was considered as good if the result of this test was lower than or equal to 2, or poor if the result of this test was higher than 2.

In all these examples, the width of the sheets was equal to 1 m.

The manufacturing conditions (steel composition, thickness th after hot-rolling, final rolling temperature FRT, austenite fraction just before the coiling fy and maximal coiling temperature T _coi | _max , coiling temperature T _coi |, pickling time t _piCk iing and heating time t _c ) for each part are indicated in Table 2.

Table 2 th FRT

Sample Steel Tcoil max Tcoil tpickling tc

fy

(mm) (°C) (°C) (°C) (s) (s)

1 A 3.3 875 0.65 559 585 25 600

2 A 3.3 875 0.65 559 655 45 600

3 A 3.3 875 0.65 559 585 45 600

4 A 3.3 875 0.65 559 585 375 600

5 A 3.3 850 0.61 565 540 375 600

6 A 3.3 850 0.59 567 515 16 600

7 A 3.3 850 0.59 567 515 21 600

8 A 3.3 885 0.87 528 520 28 600

9 A 3.3 885 0.87 528 520 35 600

10 A 3.3 905 0.88 527 510 26 600

1 1 A 3.3 905 0.88 527 510 23 600

12 A 3.3 865 0.61 565 533 63 600

13 A 3.3 905 0.87 528 519 22 600

14 A 3.3 904 0.87 528 515 15 600

15 A 3.3 867 0.64 560 554 52 600

16 A 3.3 861 0.64 560 548 24 600

17 A 3.3 851 0.85 531 476 45 600

18 A 3.3 857 0.83 534 504 60 600

19 B 2.6 845 0.1 636 655 41 520

20 B 2.6 905 0.1 636 555 25 520

21 B 2.6 845 0.1 636 555 60 520

22 C 3.2 905 0.8 538 655 21 600

23 D 3.2 875 0.9 524 531 28 600

24 D 3.2 872 0.9 524 495 38 600

25 D 3.2 874 0.9 524 581 20 600

In this table, the underlined values are not according to the invention.

The properties measured on each hot-rolled steel substrate, sheet or part (surface percentage of voids SV _S s in the surface region of the hot-rolled steel substrate, depth of intergranular oxidation D _!0 of the hot-rolled steel sheet, coating thickness C _t, thickness IM, of the intermetallic layer, and surface percentage of porosities in the coating of the hot- stamped part SPcoating, and quality of the painting adhesion - good or poor) are indicated in Table 3.

Table 3

Sample SVss D,o c _t IM, SPcoating Painting

(%) (μιτι) (μιη) (μιη) < 3%? adhesion < 30 < 4 20-33 < 15

1 18.1 5 27.5 1 1 .4 NO Poor

2 17.1 10 30.52 8.6 NO Poor

3 17.5 4 27.9 1 1 .2 NO Poor

4 37.1 NA 37.6 37.6 YES Good

5 5.7 0 31 .8 10.9 YES Good

6 18.2 0 31 .2 12.8 YES Good

7 1 1 0 29 1 1 YES Good

8 15.3 0 29.6 12 YES Good

9 19.9 0 24.3 10.4 YES Good

10 nd 2 23 10.4 YES Good

1 1 1 1 .5 2 21 .3 1 1 .7 YES Good

12 10.8 0 21 .9 10.3 YES Good

13 14.2 2 26.9 12.6 YES Good

14 14.4 0 28.4 10.5 YES Good

15 20.2 0 23.5 10.2 YES Good

16 13.9 0 22.7 10.9 YES Good

17 13 0 26.5 9.9 YES Good

18 16 0 27.2 1 1 .1 YES Good

19 nd 9 28.2 8.7 NO Poor

20 nd 0 22.6 1 1 .8 YES Good

21 nd 0 26.8 10.3 YES Good

22 nd 12 30 10 NO Poor

23 nd 8 27.6 10.1 NO Poor

24 nd 0 24.9 9.9 YES Good

25 nd 9 28.4 1 1 .5 NO Poor

In Table 3, nd means "non determined", and NA means "not applicable".

Samples 1 -4, 19, 22, 23 and 25 were produced with coiling temperatures not in accordance with the invention. Especially, samples 1 -4, 19, 22, 23 and 25 were coiled at a temperature higher than the maximal coiling temperature T _coi | _max, leading to a high depth of intergranular oxidation before pickling.

Samples 1 -3, 19, 22, 23 and 25 were pickled under normal conditions, i.e. during a time comprised between 15 and 65 s. As a consequence, the depth of intergranular oxidation of the steel sheet (measured after coating) for samples 1 -3 19, 22, 23 and 25 is higher than or equal to 4 μηι, i.e. higher than the maximal depth of oxidation admissible.

Thus, after hot-stamping, the coating adhesion is poor, the surface percentage of porosities in the coating being higher than 3%, and the painting adhesion is poor. Sample 4 was intensively pickled, during a time of 375 s. As a consequence, even if the hot-rolled steel sheet does not comprise intergranular oxidation after coating, the surface percentage of voids in the surface region of the steel substrate before coating was very high (37.1 %). As a result, an uncontrolled growth of the intermetallic layer occurred during the hot-dip coating, so that the coating thickness could not be controlled in the range 20-33 μηι, the coating thickness for sample 4 being 37.6 μηι.

By contrast, Sample 5 was intensively pickled, during the same time as Sample 4, but, unlike Sample 4, was produced with a coiling temperature in accordance with the invention. Hence, before pickling, the hot-rolled steel substrate comprised no or little intergranular oxidation, so that, after pickling, the surface percentage of voids in the surface region of the steel substrate was low (5%), contrary to Sample 4. As a result, the coating thickness could be controlled in the range 20-33 μηι. The comparison of Samples 4 and 5 thus illustrates that the manufacturing conditions according to the invention allow achieving an improved coating adhesion after hot-stamping, and an excellent painting adhesion whilst allowing the control of the coating thickness.

Besides, the comparison of Samples 5 and 6, which are pickled either intensely (Sample 5) or slightly (Sample 6) shows that, under the condition that the coiling temperature is selected according to the invention, the intensity of the pickling has no influence on the coating adhesion and does not affect the control of the coating thickness.

Samples 5 to 18, 20, 21 and 24 show that when the hot-rolled and coated steel sheet is produced by a method according to the invention, the hot-rolled steel sheet comprises no or little intergranular oxidation, so that the surface percentage of porosities in the coating of the hot-stamped part SP _coatin g is low, and the painting adhesion is good. In addition, the depth of intergranular oxidation before pickling is low, so that the surface percentage of voids in the surface region of the steel substrate before coating is low. As a consequence, the coating thickness can be controlled in the range 20-33 μηι.