EP3175006A1 - Process for manufacturing steel sheets for press hardening, and parts obtained by means of this process - Google Patents

Abstract

The invention relates to a rolled steel sheet, for press hardening, the chemical composition of which comprises, the contents being expressed by weight: 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%, 0.015% ≤ 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 titanium and nitrogen contents satisfy: Ti/N > 3.42, and that the carbon, manganese, chromium and silicon contents satisfy: formula (I), the chemical composition optionally comprising one or more of the following elements: 0.05% ≤ Mo ≤ 0.65%, 0,001% ≤ W ≤ 0.30%, 0.0005 % ≤ Ca ≤ 0.005%, the remainder consisting of iron and of unavoidable impurities originating from the production, the sheet containing a nickel content Ni_surf at any point of the steel in the region of the surface of said sheet over a depth Δ, such that: Ni_surf > Ni_nom, Ni_nom denoting the nominal nickel content of the steel, and such that, Ni_max denoting the maximum nickel content within Δ: formula (II) and such that: formula (III) the depth Δ being expressed in micrometres, the Ni_max and Ni_nom contents being expressed as weight percentages.

Description

 PROCESS FOR PRODUCING STEEL SHEETS

 FOR PRESSURE CURING, AND PIECES OBTAINED THEREBY

The invention relates to a method of manufacturing steel sheets intended to obtain parts with very high mechanical strength after curing in press. Pressurized hardening is known to heat steel flasks at a temperature sufficient to achieve austenitic transformation, and then to hot stamp the blanks by holding them within the tooling. of the press so as to obtain quenching microstructures. According to a variant of the process, a cold pre-cold-drawing can be carried out beforehand on the blanks before heating and curing in press. These blanks may be pre-coated, for example aluminum alloy or zinc. In this case, during the heating in the oven, the pre-coating diffuses with the steel substrate to form a compound providing protection of the surface of the workpiece against decarburization and scale formation. This compound is suitable for hot forming.

 The parts thus obtained are used in particular as structural elements in motor vehicles to provide anti-intrusion or energy absorption functions. Thus, for example, the application of the bumper rails, door reinforcements or foot support or the longitudinal members. Such press hardened parts can also be used for example for the manufacture of tools or parts of agricultural machines.

Depending on the composition of the steel and the cooling rate obtained in the press, the mechanical strength may reach a higher or lower level. Thus, publication EP 2 137 327 discloses a steel composition containing: 0.040% <C <0.100%, 0.80% <Mn <2.00%, Si <0.30%, S <0.005%, P <0.030 %, 0.010% <AI <0.070%, 0.015% <Nb <0, 100%, 0.030% <Ti <0.080%, N <0.009%, Cu, Ni, Mo <0.100%, Ca <0.006%, which allows get a tensile strength Rm after press curing greater than 500 MPa.

 Obtaining higher levels of resistance is disclosed by the publication FR2780984: a steel sheet containing 0.15% <C <0.5%, 0.5% <Mn <3%, 0.1% <Si <0.5%, 0.01% <Cr <1%, Ti <0.2%, Al and P <0.1%, S <0.05%, 0.0005% <B <0.08% , makes it possible to obtain a resistance Rm greater than 1000, or even 1500 MPa.

Such levels of resistance are satisfactory for many applications. However, the requirements for reducing the energy consumption of motor vehicles lead to the search for even greater vehicle lightening thanks to the use of parts whose level of mechanical strength is even higher, that is to say of which the resistance R _m would be greater than 1800 MPa. As some parts are painted and undergo a paint bake cycle, this value should be achieved with or without heat treatment.

 However, such a level of resistance is generally associated with a microstructure totally or very predominantly martensitic. It is known that this type of microstructure has a lower resistance to delayed cracking: after curing the press, the manufactured parts may indeed be able to crack or break after a certain time, under the conjunction of three factors:

 a predominantly martensitic microstructure

 a sufficient quantity of diffusible hydrogen. This can be introduced during the heating of the blanks in the oven before the hot stamping and press curing step: in fact, the water vapor present in the oven can decompose and be adsorbed on the surface of the oven. blank, the presence of constraints, applied or residual, of a sufficient level.

In order to solve the problem of delayed cracking, it has been proposed to rigorously control the atmosphere of the reheating furnaces and the blank cutting conditions so as to minimize the level of stress. It has also been proposed to carry out thermal post-treatments on the hot-stamped parts, so as to achieve a degassing of hydrogen. However, these operations are binding for the industry, which wants to have a material to avoid this risk and to overcome these constraints and additional costs. It has also been proposed to deposit on the surface of the steel sheet specific coatings to reduce the adsorption of hydrogen. However, a simpler method is sought to provide equivalent delayed crack resistance.

 We are therefore looking for a part manufacturing process that would simultaneously obtain a very high mechanical strength Rm, and a high resistance to delayed cracking after curing in press, objectives a priori difficult to reconcile.

 On the other hand, it is known that steel compositions richer in hardening and / or hardening elements (C, Mn, Cr, Mo, etc.) lead to the production of hot-rolled sheets with a higher hardness. However, this increase in hardness is a hindrance to obtaining cold-rolled sheets over a wide thickness range, given the limited power of certain cold rolling mills. An excessively high level of resistance at the stage of the hot-rolled sheet therefore does not make it possible to obtain cold-rolled sheets of very thin thickness. A method is therefore sought to provide a wide range of cold rolled sheet thickness.

 Moreover, the presence of hardening and / or hardening elements in greater quantity may have consequences during the thermomechanical manufacturing process since a possible variation of certain parameters (end of rolling temperature, winding temperature, speed variation cooling in the width direction of the rolled strip) can lead to a variation of the mechanical properties within the sheet. A steel composition that is insensitive to a variation of certain manufacturing parameters is therefore sought so as to produce a sheet having a good homogeneity of mechanical properties.

It also seeks a steel composition that can be coated easily, especially by soaking, so that the sheet can be available in different forms: uncoated, or coated with aluminum alloy or zinc alloy, according to the wish of the end user. A method is also sought that makes it possible to have a metal sheet which exhibits good aptitude for mechanical cutting during the step of obtaining blanks intended for hardening in press, that is to say of which the mechanical strength is not sufficient. would not be too high at this stage, in order to avoid a degradation of cutting tools or punching.

 The present invention aims to solve all of the problems mentioned above by means of an economical manufacturing process.

Surprisingly, the inventors have demonstrated that these problems were solved by supplying a sheet of the composition detailed below, this sheet further having the characteristic of having a specific nickel enrichment in the vicinity of its surface.

For this purpose, the subject of the invention is a rolled steel sheet, for press hardening, the chemical composition of which comprises the contents being by weight: 0.24% <C <0.38%, 0.40% % <Mn <3%, 0.10% <Si <0.70%, 0.015% <AI <0.070%, 0% <Cr <2%, 0.25% <Ni <2%, 0.015% <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 of nitrogen satisfy: Ti / N> 3.42, and that the carbon contents,

 _, _ Mn Cr Si

manganese, chromium and silicon satisfy: 2.6C H 1 1-≥1,1%, the

 5.3 13 15

chemical 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 remainder being iron and unavoidable impurities from the production, the sheet containing a nickel content Ni _SU r _f at any point of the steel in the vicinity of the surface of said sheet to a depth Δ, such that: Ni _surf > Ni _n0 m, Ni _n0 m designating the nominal nickel content of steel, and such that, Ni _ma x denoting the maximum nickel content within

Δ: ^{(max +} "° '" ⁾ x (A)> 0.6, and such that: ^{(max ~ Νι} "° ^{» »} ≥ 0.01, | _a depth

 2 Δ

Δ being expressed in micrometers, the contents Ni _max and Ni _nom being expressed in percentages by weight.

According to a first mode, the composition of the sheet comprises, by weight: 0.32% <C <0.36%, 0.40% <Mn <0.80%, 0.05% <Cr <1, 20% . According to a second mode, the composition of the sheet comprises, by weight: 0.24%

<C <0.28%, 1, 50% <Mn <3%.

 The silicon content of the sheet is preferably such that: 0.50% ≤ Si <0.60%.

According to one particular embodiment, the composition comprises, by weight: 0.30% <Cr <0.50%.

 As a preference, the composition of the sheet comprises, by weight: 0.30% <Ni

<1, 20%, and very preferably: 0.30% ≤ Ni <0.50%.

 The titanium content is preferably such that: 0.020% <Ti.

The composition of the sheet advantageously comprises: 0.020% <Ti <0.040%.

 According to a preferred embodiment, the composition comprises, by weight: 0.15% <Mo <0.25%.

 The composition comprises, by weight, preferentially: 0.010% <Nb <0.060%, and very preferably: 0.030% <Nb <0.050%.

 In a particular embodiment, the composition comprises, by weight: 0.50% <Mn <0.70%.

 Advantageously, the microstructure of the steel sheet is ferrito-pearlitic. In a preferred embodiment, the steel sheet is a hot-rolled sheet.

By way of preference, the sheet is a cold-rolled and annealed sheet.

 In a particular embodiment, the steel sheet is pre-coated with a metal layer of aluminum or aluminum alloy or aluminum-based.

In another particular embodiment, the steel sheet is pre-coated with a metal layer of zinc or zinc alloy or zinc-based.

In another embodiment, the steel sheet is pre-coated with one or more layers of intermetallic alloys containing aluminum and iron, and optionally silicon, the pre-coating containing no aluminum free, phase τ ₅ of the type Fe ₃ Si ₂ Ali ₂ , and 7- ₆ of the type Fe ₂ Si ₂ Al ₉ . The invention also relates to a part obtained by hardening in press of a steel sheet of composition according to any of the above modes, martensitic structure or martensito-bainitic.

Preferably, the press-hardened part contains a nominal nickel content Ni _n0 m, and is characterized in that the nickel Ni content _on f in the steel in the vicinity of the surface is greater than Ni _n0 m over a depth Δ, and in that, Ni _max denoting the maximum nickel content within Δ

. (Ni _mia - Ni _mm )

 x (Δ)> 0.6, and in that: ≥ 0.01, the depth Δ

2 A

being expressed in micrometers, the contents Ni _max and Ni _n0 m being expressed in percentages by weight.

 The press-hardened part advantageously has a mechanical strength Rm greater than or equal to 1800 MPa.

 According to one preferred embodiment, the press-hardened part is coated with an aluminum alloy or aluminum-based alloy, or with a zinc alloy or zinc-based alloy resulting from the diffusion between the steel substrate and the pre-coating, during the press hardening heat treatment.

The subject of the invention is also a process for manufacturing a hot-rolled steel sheet, comprising the successive stages in which a semi-product of chemical composition is cast according to one of the modes presented above, and then it is heated to a temperature of between 1250 and 1300 ° C for a holding time at this temperature of between 20 and 45 minutes. The half-product is hot-rolled to a TFL end-of-flow temperature of between 825 and 950 ° C., to obtain a hot-rolled sheet, and then the hot-rolled sheet is rolled at a temperature of between 500 and 750. ° C, to obtain a hot rolled and wound, and then etch the oxide layer formed in the previous steps.

The subject of the invention is also a process for manufacturing a cold-rolled and annealed sheet, characterized in that it comprises the successive steps according to which a hot-rolled, wound and pickled sheet, manufactured by the method described, is supplied. above and then cold rolled this hot rolled sheet, wound and stripped, to obtain a cold rolled sheet. This cold-rolled sheet is then annealed at a temperature between 740 and 820 ° C. to obtain a cold-rolled and annealed sheet. According to an advantageous embodiment, a rolled sheet manufactured according to one of the above processes is supplied, then a pre-coating is carried out continuously by dipping, the pre-coating being aluminum or an alloy aluminum or aluminum-based, or zinc or a zinc alloy or zinc-based.

Advantageously, the subject of the invention is also a process for manufacturing a pre-coated and pre-alloyed sheet, according to which a rolled sheet is supplied according to one of the above processes, and then continuous pre-coating is carried out at quenched with an aluminum alloy or aluminum-based, and then a pre-heat treatment of the pre-coated sheet is carried out at a temperature θι of between 620 and 680 ° C. for a holding period of between 6 and 15 hours , so that the pre-coating no longer contains free aluminum, of phase τ ₅ of the Fe ₃ Si ₂ Ali ₂ type, and τ ₅ of the Fe ₂ Si ₂ Al _{9 type} , and so as not to provoke austenitic transformation in the steel substrate, the pre-treatment being carried out in an oven under an atmosphere of hydrogen and nitrogen.

The subject of the invention is also a manufacturing method, of a press hardened part, comprising the successive steps according to which a sheet made by a method according to any one of the above modes is supplied, then said sheet is cut to obtain a blank, then optionally performs a deformation step by cold stamping the blank. The blank is heated to a temperature of between 810 and 950 ° C. to obtain a totally austenitic structure in the steel and then the blank is transferred into a press. The blank is hot stamped to obtain a part, then it is held in the press to obtain a hardening by martensitic transformation of the austenitic structure.

The invention also relates to the use of a press-hardened part having the characteristics described above, or manufactured according to the method described above, for the manufacture of structural parts or reinforcement for motor vehicles. Other features and advantages of the invention will become apparent from the following description given by way of example and with reference to the following appended figures: FIG. 1 schematically shows the variation of the nickel content in the vicinity of the surface of press-hardened sheets or parts, and illustrates certain parameters defining the invention: Ni _max , Ni _SU ref, Ni _n0 m, Δ.

FIG. 2 shows the mechanical strength of hot stamped and press-hardened parts, as a function of a parameter combining the contents of C, Mn, Cr and Si, sheets.

 FIG. 3 shows the diffusible hydrogen content, measured on hot stamped pieces and hardened in press, as a function of a parameter expressing the overall nickel content in the vicinity of the surface of the sheets. FIG. 4 shows the diffusible hydrogen content measured on hot-stamped and press-hardened parts, as a function of the nickel enrichment intensity in the surface layer of the sheets.

Figure 5 shows the variation of the nickel content in the vicinity of the sheet surface of different compositions.

Figure 6 shows the variation of the nickel content in the vicinity of the surface of sheets of identical composition, having undergone two modes of preparation of the surface before curing in press.

FIG. 7 shows the variation of the diffusible hydrogen content as a function of the enrichment intensity of nickel in the surface layer, for sheets having undergone two modes of preparation of the surface before curing in press.

 Figures 8 and 9 show the structures of hot-rolled sheet according to the invention.

 The thickness of the steel sheet used in the process according to the invention is preferably between 0.5 and 4 mm, thickness range used in particular in the manufacture of structural parts or reinforcement for the automotive industry. . This can be obtained by hot rolling or subsequent cold rolling and annealing. This thickness range is suitable for industrial press hardening tools, especially hot stamping presses.

Advantageously, the steel contains the following elements, the composition being expressed by weight: a carbon content of between 0.24 and 0.38%. This element plays a major role in the quenchability and the mechanical strength obtained after the cooling following the austenitization treatment. Below a content of 0.24% by weight, the mechanical strength level of 1800 MPa can not be reached after hardening by press-hardening, without additional addition of expensive elements. Beyond a content of 0.38% by weight, the risk of delayed cracking is increased, and the ductile / brittle transition temperature, measured from tests of Charpy-type notched bending, becomes greater than -40. ° C, which reflects an excessive decrease in toughness.

 A carbon content of between 0.32% and 0.36% by weight makes it possible to obtain the properties in question in a stable manner, maintaining weldability at a satisfactory level and limiting the production costs.

 The spot welding ability is particularly good when the carbon content is between 0.24 and 0.28%.

 As will be seen below, the carbon content must also be defined in conjunction with the manganese, chromium and silicon contents.

- In addition to its role as deoxidizer, manganese plays a role on the quenchability: its content must be greater than 0.40% by weight to obtain a temperature Ms of beginning of transformation (austenite → martensite) during cooling in press, sufficiently low This increases the resistance Rm. The limitation of the manganese content to 3% makes it possible to obtain an increased resistance to delayed cracking. Indeed, manganese segregates at austenitic grain boundaries and increases the risk of intergranular rupture in the presence of hydrogen. On the other hand, as will be explained below, the resistance to delayed cracking comes in particular from the presence of a surface layer enriched in nickel. Without wishing to be bound by theory, it is thought that when the manganese content is excessive, a thick layer of oxides is formed during the reheating of the slabs, so that the nickel does not have time to diffuse sufficiently for locate under this layer of oxides of iron and manganese.

The manganese content is preferably defined together with the carbon content, optionally in chromium: when the carbon content is between 0.32 and 0.36% by weight, a Mn content of between 0.40 and 0.80% and a chromium content of between 0.05 and 1.20%, allow simultaneous excellent resistance to delayed cracking thanks to the presence of a particularly effective nickel-enriched surface layer, and a very good aptitude for mechanical cutting of the sheets. The content of Mn is ideally between 0.50 and 0.70% to reconcile the achievement of a high mechanical strength and a resistance to delayed cracking,

 when the carbon content is carbon is between 0.24 and 0.28%, in combination with a manganese content of between 1.50 and 3%, the spot welding ability is particularly good.

These ranges of composition make it possible to obtain a temperature M _s from beginning of transformation to cooling (austenite → martensite) of between 320 and 370 ° C., which makes it possible to guarantee that the hot-cured parts have a sufficiently high resistance.

 the silicon content of the steel must be between 0.10 and 0.70% by weight: a silicon content greater than 0.10% makes it possible to obtain additional hardening and contributes to the deoxidation of the steel liquid. Its content must however be limited to 0.70% to avoid the excessive formation of surface oxides during the reheating and / or annealing steps, and not to damage the coating by dipping.

The silicon content is preferably greater than 0.50% in order to avoid a softening of the fresh martensite, which can occur when the workpiece is held in the press tooling after the martensitic transformation. The silicon content is preferably less than 0.60% so that the transformation temperature at heating Ac3 (ferrite + perlite → austenite) is not too high. In the opposite case, this makes it necessary to heat the blanks before hot stamping at a higher temperature, which is detrimental to the productivity of the process.

in an amount greater than or equal to 0.015%, aluminum is an element promoting deoxidation in the liquid metal during the preparation, and the precipitation of nitrogen. When its content is greater than 0.070%, coarse aluminates may be formed during processing which tend to reduce ductility. Optimally, its content is between 0.020 and 0.060%. the chromium increases the quenchability and contributes to obtaining Rm at the desired level after curing in press. Beyond a content equal to 2% by weight, the effect of chromium on the homogeneity of the mechanical properties in the press-hardened part is saturated. In an amount preferably between 0.05 and 1, 20%, this element contributes to increasing the resistance. Preferably, a chromium addition of between 0.30 and 0.50% makes it possible to obtain the desired effects on mechanical strength and delayed cracking, by limiting the costs of addition. When the manganese content is sufficient, it is that is to say between 1, 50% and 3% Mn, it is considered that the addition of chromium is optional, the quenchability obtained with manganese being considered sufficient.

 In addition to the conditions on each of the elements C, Mn, Cr and Si defined above, the inventors have demonstrated that these elements should be specified jointly: indeed, FIG. 2 illustrates the mechanical strength of hardened blanks in press. for different steel compositions with varying contents of carbon (between 0.22 and 0.36%), manganese (between 0.4 and 2.6%) and chromium (between 0 and 1.3%) and silicon (between 0.1 and

"^ Mn Cr Si

 0.72%), depending on the parameter P = 2.6C + + - + -

' ^K 5.3 13 15

The data illustrated in Figure 2 relate to blanks heated in the austenitic range at a temperature of 850 or 900 ° C maintained at this temperature for 150s, then hot stamped and quenched by holding in the tool. In all cases, the structure of the parts obtained after hot stamping, is entirely martensitic. The line 1 designates the lower envelope of the mechanical strength results. Despite the dispersion due to the variety of the compositions studied, it appears that a minimum value of 1800 MPa is obtained when the parameter Pi is greater than 1.1%. When this condition is fulfilled, the transformation temperature Ms during press cooling is less than 365 ° C. Under these conditions, the fraction of martensite autorevenue, under the effect of the maintenance in the press tooling, is extremely limited, so that the very high amount of unreturned martensite makes it possible to obtain a high value of mechanical strength.

Titanium has a high affinity for nitrogen. Given the nitrogen content of the steels of the invention, the titanium content must be greater than or equal to 0.015% so as to obtain effective precipitation. In an amount greater than 0.020% by weight, the titanium protects the boron so that this element is in free form to play its full effect on the quenchability. Its content must be greater than 3.42N, this quantity being defined by the stoichiometry of the TiN precipitation, so as to avoid the presence of free nitrogen. Above 0.10%, however, there is a risk of forming in the liquid steel, coarse titanium nitrides which play a detrimental role on toughness. The titanium content is preferably between 0.020 and 0.040%, so as to form fine nitrides which limit the growth of the austenitic grains during the heating of the blanks before hot stamping.

 in an amount greater than 0.010% by weight, the niobium forms niobium carbonitrides which are also likely to limit the growth of the austenitic grains during the heating of the blanks. Its content must, however, be limited to 0.060% because of its ability to limit recrystallization during hot rolling, which increases the rolling forces and the difficulty of manufacture. The optimal effects are obtained when the niobium content is between 0.030 and 0.050%.

 in an amount greater than 0.0005% by weight, boron greatly increases the quenchability. By diffusing at the austenitic grain boundaries, it exerts a favorable influence in preventing the intergranular segregation of phosphorus. Above 0.0040%, this effect is saturated.

 a nitrogen content greater than 0.003% makes it possible to obtain a precipitation of TiN, Nb (CN) or of (Ti, Nb) (CN) mentioned above in order to limit the growth of the austenitic grain. The content should however be limited to 0.010% so as to avoid the formation of coarse precipitates.

- optionally, the sheet may contain molybdenum in quantity between 0.05 and 0.65% by weight: this element forms a co-precipitation with niobium and titanium. These precipitates are very thermally stable, reinforcing the limitation of austenitic grain growth during heating. An optimal effect is obtained for a molybdenum content of between 0.15 and 0.25%.

 - Optionally, the steel may also comprise tungsten in an amount between 0.001 and 0.30% by weight. In the amounts indicated, this element increases the quenchability and curing ability through carbide formation.

- As an option, the steel can also contain calcium in a quantity between 0.0005 and 0.005%: by combining with oxygen and sulfur, calcium makes it possible to avoid the formation of large inclusions which are harmful to the ductility of the sheets or parts thus manufactured.

 In excessive amounts, sulfur and phosphorus lead to increased brittleness. This is why the weight content of sulfur is limited to

0.005% so as to avoid excessive formation of sulphides. An extremely low sulfur content, that is to say less than 0.001% is however unnecessarily expensive to achieve insofar as it does not bring additional benefit.

For similar reasons, the phosphorus content is between 0.001 and 0.025% by weight. In excessive content, this element segregates at the austenitic grain boundaries and increases the risk of delayed cracking by intergranular rupture.

 nickel is an important element of the invention: in fact, the inventors have demonstrated that this element, in an amount of between 0.25% and 2% by weight, very significantly reduces the sensitivity to delayed fracture when it is concentrated on the surface of the sheet or part in a specific form:

For this purpose, reference is made to FIG. 1 which diagrammatically illustrates certain characteristic parameters of the invention: the variation of the nickel content of a steel in the vicinity of the surface of the sheet, for which a surface enrichment has been noted. For reasons of convenience, only one of the surfaces of the sheet has been shown, it is understood that the The following description also applies to the other surfaces of this sheet. The steel has a nominal nickel content Ni _n0 m- Thanks to the manufacturing process to be described later, the steel sheet is enriched with nickel in the vicinity of its surface, up to a maximum Ni _max . This maximum Ni _max may be on the surface of the sheet, as shown in Figure 1, or slightly below this surface, a few tens or hundreds of nanometers below it, without this changing the following description and the results of the invention. Similarly, the variation of the nickel content may not be linear as shown schematically in FIG. 1, but adopt a characteristic profile resulting from diffusion phenomena. However, the definition of characteristic parameters that follows, is also valid for this type of profile. The nickel-enriched surface zone is therefore characterized by the fact that at any point the local nickel content Ni _SU r _f of the steel is such that: Ni _surf > Ni _n0 m. This enriched zone has a depth Δ.

Surprisingly, the inventors have demonstrated that resistance to delayed cracking is obtained by considering two parameters P ₂ and P ₃ characteristic of the enriched surface area, these having to satisfy critical conditions. We define in the first place:

 This first parameter characterizes the overall nickel content in the enriched layer Δ and corresponds to the hatched area shown in FIG.

The second parameter P ₃ is defined by:

P _{3 =} This second parameter characterizes the average nickel concentration gradient, that is to say the intensity of the enrichment within the Δ layer.

The inventors have sought the conditions which make it possible to avoid the delayed cracking of parts with very high mechanical strength hardened under press. It will be recalled that this process is characterized by the fact that blanks of steel, bare or pre-coated with a metal coating (aluminum or aluminum alloy, zinc or zinc alloy), are heated. here being then transferred to a hot stamping press. During the heating step, the water vapor possibly present in a smaller quantity in the oven is adsorbed on the surface of the blank. Hydrogen from the dissociation of water can be dissolved in the austenitic steel substrate at high temperature. The introduction of hydrogen is thus facilitated by an oven atmosphere with a high dew point, a high austenitization temperature and a long holding time. During cooling, the solubility of hydrogen decreases very strongly. After returning to ambient temperature, the alloying coating between the optional metal pre-coating and the steel substrate forms a substantially water-proof barrier to hydrogen desorption. A significant diffusible hydrogen content will therefore increase the risk of delayed cracking for a martensitic steel substrate. The inventors have therefore sought means for lowering the diffusible hydrogen content hot stamped part at a very low level, that is to say less than or equal to 0.16ppm. This level makes it possible to guarantee the absence of cracking on a part subjected to bending stress under a stress equal to that of the elastic limit of the material, for a duration of 150 hours.

They have shown that this result is achieved when the surface of the hot stamped part, or that of the sheet or the blank before hot stamping, has the following specific characteristics:

FIG. 3, established for parts cured in a resistance press Rm of between 1800 and 2140 MPa, indicates that the content of diffusible hydrogen depends on the parameter P ₂ above. A diffusible hydrogen content of less than 0.16 ppm is obtained when ^ ^{max + Ni} " ^om ^ χ (Δ)> 0.6, the depth Δ being expressed in micrometres, the contents Ni _max and Ni _n0 m being expressed in percentages in weight.

in FIG. 4, relative to the same parts cured in press, the inventors have also demonstrated that a diffusible hydrogen content of less than 0.16 ppm was reached when the nickel enrichment in the Δ layer reached a value critical with respect to the nominal content Ni _n0 m , that is, when the parameter P ₃ satisfies: --- o, 0, the

 Δ

units being the same as for parameter P ₂ . In FIG. 4, curve 2 corresponding to the lower envelope of the results is shown.

Without wishing to be bound by theory, it is thought that these characteristics translate a barrier effect to the penetration of hydrogen into the sheet at high temperature, in particular by nickel enrichment at the old austenitic grain boundaries, which slows down the diffusion of the 'hydrogen.

The rest of the composition of the steel consists of iron and unavoidable impurities resulting from the elaboration.

The process according to the invention will now be described: a half-product of composition mentioned above is cast. This semi-finished product can be in the form of a slab of thickness typically between 200 and 250 mm, or a slab whose typical thickness is of the order of a few tens of millimeters, or in any other suitable form. This is brought to a temperature between 1250 and 1300 ° C and maintained in this temperature range for a period of between 20 and 45 minutes. By reaction with oxygen in the furnace atmosphere, an oxide layer substantially rich in iron and manganese is formed for the composition of the steel of the invention, in which the solubility of the nickel is very high. low, the nickel remains in metallic form. In parallel with the growth of this oxide layer, nickel is diffused towards the interface between the oxide and the steel substrate thus causing the appearance of a layer enriched in nickel in the steel. At this stage, the thickness of this layer depends in particular on the nominal nickel content of the steel, and the temperature and maintenance conditions defined above. During the subsequent manufacturing cycle, this enriched initial layer simultaneously undergoes:

 a decrease in thickness, due to the reduction rates conferred by the successive stages of rolling,

- An increase in thickness due to the stay of the sheet at high temperature during the successive stages of manufacture. This augmentation However, it intervenes in lesser proportions than during the slab reheating stage.

 A production cycle of a hot-rolled sheet typically comprises:

- hot rolling steps (roughing, finishing) in a temperature range of 1250 to 825 ° C,

 a winding step in a temperature range of 500 to 750X.

 The inventors have demonstrated that a variation of the parameters of hot rolling and winding, in the ranges defined by the invention, did not modify the mechanical characteristics significantly, so that the process was tolerant to a certain variation. within these ranges, without any significant impact on the resulting products.

 At this stage, the hot-rolled sheet, the thickness of which can typically be 1.5-4.5 mm, is etched by a method known per se, which only removes the oxide layer, so that the The nickel-enriched layer is located near the surface of the sheet.

 when it is desired to obtain a sheet of thinner thickness, cold rolling is carried out with a suitable reduction ratio, for example between 30 and 70%, then annealing at a temperature typically between 740 and 820 ° C. so as to obtain a recrystallization of the hardened metal. After this heat treatment, the sheet may be cooled so as to obtain an uncoated sheet, or continuously coated by passing through a dip bath, according to methods known per se, and finally cooled.

The inventors have demonstrated that, among the manufacturing steps detailed above, the step which had a predominant influence on the characteristics of the nickel-enriched layer on the final sheet, was the step of heating the slabs, in a specific range of temperature and hold time. In particular, they have demonstrated that the annealing cycle of the cold rolled sheet, whether or not a coating step, has only a secondary influence on the characteristics of the nickel-enriched surface layer. In other words, with the exception of the cold rolling reduction ratio which decreases the thickness of the nickel-enriched layer by a homothetic amount, the characteristics of the enrichment nickel of this layer are substantially identical on a hot-rolled sheet and on a sheet which has further undergone a cold rolling and annealing, whether or not it includes a pre-coating step dipping.

This pre-coating may be aluminum, an aluminum alloy (comprising more than 50% aluminum) or an aluminum-based alloy (of which aluminum is the majority constituent). This pre-coating is advantageously an aluminum-silicon alloy comprising by weight 7-15% of silicon, 2 to 4% of iron, optionally between 15 and 30 ppm of calcium, the remainder being aluminum and unavoidable impurities resulting from the preparation.

 The pre-coating may also be an aluminum alloy containing 40-45% Zn, 3-10% Fe, 1 -3% Si, the balance being aluminum and unavoidable impurities resulting from the elaboration.

According to one variant, the pre-coating may be an aluminum alloy coating, which is in the form of intermetallic compounds comprising iron. This type of pre-coating is obtained by performing a heat pre-treatment of the sheet pre-coated with aluminum or aluminum alloy. This thermal pretreatment is carried out at a temperature θ- ₁ during a holding period ti, so that the pre-coating no longer contains free aluminum, of phase τ 5 of the Fe ₃ Si ₂ Ali ₂ type, and τ Q of the type Fe ₂ Si ₂ Al 9, and so as not to cause austenitic transformation in the steel substrate. As a preference, the temperature ΘΪ is between 620 and 680 ° C., the holding time t | is between 6 and 15 hours. This results in a diffusion of iron from the steel sheet to aluminum or aluminum alloy. This type of pre-coating then makes it possible to heat the blanks, before the hot-stamping step, with a much faster speed, which makes it possible to minimize the holding time at high temperature during the heating of the blanks. that is to say to reduce the amount of hydrogen adsorbed during this blank heating step.

Alternatively, the pre-coating may be galvanized, or galvanized-alloy, that is to say having an amount of iron of between 7-12% after heat treatment of alloying realized with the parade immediately after the bath of galvanization.

 The pre-coating may also be composed of a superposition of deposited layers in successive steps, at least one of the layers may be aluminum or an aluminum alloy.

 After the manufacture described above, the sheets are cut or punched by methods known per se, so as to obtain blanks whose geometry is related to the final geometry of the stamped part and cured in press. As explained above, the cutting of sheets comprising in particular between 0.32 and 0.36% C, between 0.40 and 0.80% Mn, between 0.05 and 1, 20% Cr, is particularly easy because of the low mechanical resistance at this stage, associated with a ferrito-pearlitic microstructure.

 These blanks are heated to a temperature between 810 and 950 ° C so as to completely austenitize the steel substrate, hot-stamped, and then held in the press tool so as to obtain a martensitic transformation. The degree of deformation applied during the hot stamping step may be greater or lesser depending on whether a cold deformation step (stamping) was carried out before or after the austenitization treatment. The inventors have demonstrated that the thermal heating cycles for press curing, which consist of heating the blanks in the vicinity of the transformation temperature Ac3, and then keeping them at this temperature for a few minutes, did not cause any problems. substantial modification of the nickel-enriched layer.

 In other words, the characteristics of the nickel-enriched surface layer are similar on the sheet before curing in press, and on the part after curing in press, obtained from this sheet.

Thanks to the compositions of the invention which have a lower Ac3 transformation temperature than conventional steel compositions, it is possible to austenitize the blanks with reduced holding-time temperatures, which makes it possible to reduce the possible adsorption. hydrogen in the heating furnaces. By way of nonlimiting examples, the following embodiments will illustrate advantages conferred by the invention.

 Example 1

 Steel semi-finished products have been supplied, the composition of which is

Table 1 below.

 Table 1 Steel compositions (% by weight)

 Underlined values: not in accordance with the invention

 These semi-finished products were brought to 1275X and maintained at this temperature for 45 minutes, then hot rolled with a TFL end temperature of 950 ° C, a winding temperature of 650 ° C. The hot-rolled sheets were then etched in an inhibitor-acid bath so as to remove only the oxide layer created during the previous manufacturing steps, and then cold-rolled to a thickness of 1.5 mm. The sheets thus obtained were cut into blanks. The mechanical cutting ability was evaluated by the effort required to perform this operation. This characteristic is particularly related to the strength and hardness of the sheet at this stage. The blanks were brought to the temperature indicated in Table 2, and maintained 150 s. at this temperature before being hot-stamped and cooled by pressing in the press. The cooling rate, measured between 750 ° C and 400 ° C is between 180 and 210 ° C / s. The tensile strength Rm was measured on the resulting martensitic structural parts using ISO 12.5 x 50 tensile test pieces.

In addition, some blanks were heated at a temperature between 5,850 and 950 ° C for 5 minutes in an oven under an atmosphere with dew point -5 ° C. These blanks were then hot stamped under conditions identical to those presented above. The values of diffusible hydrogen on the parts thus obtained were then measured by a thermodisorption ("TDA") method, known per se: in this method, a test sample is heated up to 900 ° C. an infrared heating oven under a stream of nitrogen. The hydrogen content from desorption is measured as a function of temperature. The diffusible hydrogen is quantified by all of the desorbed hydrogen between room temperature and 360 ° C. The variation of the nickel content in steel in the vicinity of the surface was also measured on hot-stamped plates by Glow Discharge Optical Emission ("LDS") or "GDOES" spectroscopy. Spectrometry, technique known per se) This made it possible to define the values of the parameters Ni _max , Ni _surf , Ni _n0 m and Δ.

 The results of these tests have been reported in Table 2.

Table 2 Blank heating conditions and properties obtained after curing in press. Underlined values: not in accordance with the invention o = sheet more particularly suitable for cutting In particular, AD sheets have good cutting properties because of their ferritic-pearlitic structure. The plates and the hardened pieces in the AF press have characteristics in terms of composition and nickel-enriched surface layer, corresponding to the invention.

Examples AD show that a composition containing in particular a C content of between 0.32 and 0.36%, an Mn content of between 0.40 and 0.80% Mn, a chromium content of between 0.05 and and 1, 20%, in combination with a nominal Ni content of 0.30-1, 20% and a specific enriched layer in this element, make it possible to obtain a resistance Rm greater than 1950 MPa and a diffusible hydrogen content at a value less than or equal to 0.16ppm.

 The example of test A shows that the Ni content can be lowered between 0.30 and 0.50%, which makes it possible to obtain satisfactory results in terms of mechanical strength and resistance to delayed cracking, in economic conditions of manufacture.

 Examples E-F show that satisfactory results can be obtained with a composition containing in particular a carbon content of between 0.24 and 0.28% and a manganese content of between

1, 50 and 3%. The high value of the parameter ^ ^N ' ^{max + Nimm} ^ χ (Δ) is associated with a particularly low diffusible hydrogen content.

Conversely, the parts of Examples G-K have a diffusible hydrogen content greater than 0.25 ppm, because the steels do not have a nickel-enriched surface layer. In addition, Examples JK correspond to steel compositions whose parameter P _t is less than 1.1%, so that a resistance Rm of 1800 MPa is not obtained after curing in press.

For the steel compositions AD and H, that is to say the carbon content of which is between 0.32 and 0.35% C, the variation of the nickel content in FIG. function of the depth measured with respect to the surface of the sheet, measured by SDL technique. In this figure, the marks next to each curve correspond to the reference of the steel. In comparison with a sheet having no nickel (reference H), there is that the sheets according to the invention have an enrichment in the surface layer. With a given nominal nickel content (0.79%), it can be seen from Examples B and C that a variation in the chromium content of 0.51 to 1.05% makes it possible to preserve an enrichment in the superficial layer. satisfying the conditions of the invention.

Example 2

 Hot-rolled steel sheet was supplied with the composition corresponding to that of the steels E and F above, that is to say respectively containing a Ni content of 1% and 1.49%, manufactured in the conditions mentioned above.

 After rolling, the sheets have undergone two types of preparation:

 - X: an acid pickling with inhibitor so as to remove only the oxide layer

 - Y: a correction of 100 micrometers

 FIG. 6, illustrating the nickel content measured by Luminescent Discharge Spectroscopy from the surface for the sheet F, shows that in the preparation method X, a nickel-enriched surface layer is present (curve marked X), whereas the grinding removed the oxide layer and the nickel-enriched underlayer (curve marked Y)

After cold rolling to a thickness of 1.5 mm, thus prepared blanks were then heated in an oven at a rate of 10 ° C./s to 850 ° C., held at this temperature for 5 minutes, and then hot-stamped. . The diffusible hydrogen content measured on the stamped parts is as follows, in the two modes of preparation:

 Exhibit E- Part Content F- Content

Advance preparation

 hydrogen diffusible hydrogen of the sheet

 (ppm) diffusible (ppm)

Stripping retaining

 0.09 0.08 the Ni enriched layer

 Rectification eliminating

0.21 0.19 the Ni enriched layer FIG. 7 shows the diffusible hydrogen content as a function of the steel composition and the method of preparation. The reference EX is for example relative to the sheet and hot stamped part made from the steel composition E, with the method of preparation X.

 These results show that the presence of a surface layer enriched in nickel, that is to say having a sufficient nickel concentration gradient, is necessary in order to obtain a low content of diffusible hydrogen.

Example 3

Slabs of 235 mm thickness were produced with the following composition: Table 3 Composition of steel (% by weight)

These slabs were brought to the temperature of 1290 ° C. and held at this temperature for 30 minutes.

 They were then hot-rolled to a thickness of 3.2 mm, according to different end-of-rolling or winding temperatures. The mechanical tensile properties (elastic limit Re, resistance Rm, total elongation At) of these hot-rolled sheets have been reported in Table 4.

Conditions for producing hot-rolled sheet and mechanical properties obtained At almost identical winding temperature (tests T and U), it can be seen that a rolling end temperature of 70 ° C. has only a very slight influence on the mechanical properties. At a close end-of-rolling temperature (tests U and V), it can be seen that a decrease in the winding temperature from 650 to 580 ° C. has only a slight influence, especially on the resistance which varies by less than 5%. Thus, it is demonstrated that the steel sheet manufactured under the conditions of the invention is insensitive to manufacturing variations, which means that the rolled strips have good homogeneity.

 FIGS. 8 and 9 show the respective microstructures of the hot rolled sheets of the T and V tests. It can be seen that the ferrito-pearlitic microstructures are very similar for both conditions.

The hot-rolled sheets were continuously etched to remove only the oxide layer formed in the previous steps, leaving the nickel-enriched layer in place. The sheets were then rolled to a target thickness of 1.4mm. Whatever the hot rolling conditions, the desired thickness could be achieved, the rolling forces being similar for the different conditions.

These sheets were then annealed at a temperature of 760 ° C., which is immediately above the transformation temperature Ac 1, then cooled and aluminized continuously by dipping in a bath containing 9% by weight of silicon and 3% by weight of iron. the balance being aluminum and unavoidable impurities. This produces sheets with a coating of the order of 80 g / m ² per side, this coating having a very regular thickness, without defects.

Blanks obtained from the test conditions T in Table 4 above were then cut, heated under different conditions and then hot stamped. In all cases, the rapid cooling thus obtained imparts a martensitic structure to the steel substrate. Some parts have also undergone a thermal cycle of paint baking. Temperature

 Heating / Cycle

 Reference Re Rm At

 cooking time

 Test (MPa) (MPa)

 keeping in painting (%)

 oven

 T1 900 ° C-7 'Without 1337 1944 6.5

T2 900 ° C-7 '170 ° C-20' 1495 825 7.4

Τ3 930 ° C-10 'Without 1296 1915 7

Τ4 930 ° C-10 '170 ° C-20' 1471 1827 7.5

Table 4 Conditions of realization of hot-rolled sheet and mechanical properties obtained It is found that the resistance obtained exceeds 1800 MPa, regardless of the temperature and the holding time of the blank in the oven, with or without subsequent paint curing treatment.

Example 4

Cold-rolled and annealed steel sheets having a thickness of 1.4 mm were supplied with corresponding compositions to those of Steels A and J above, that is to say respectively containing a Ni content of 0.39% and 0%, manufactured under the conditions mentioned in Example 1. It was then carried out by dipping in a bath whose composition is described in Example 3. Thus, sheets were obtained with a precoated alloy coating. aluminum 30 μιη thick, in which blanks were cut.

These blanks were austenitized in an oven at a maximum temperature of 900 ° C, in an atmosphere with a controlled dew point of -10 ° C, the total blank holding time in the oven being 5 or 15 minutes. After austenitization, the blanks were rapidly transferred from the furnace to a hot stamping press and quenched by holding in the tooling. The test conditions reported in Table 5 are representative of an industrial hot stamping process for thin sheets.

 Table 5 Conditions for carrying out hot-melt testing on blanks with pre-coating of aluminum alloy

The mechanical tensile properties (Rm resistance and total elongation At) and the diffusible hydrogen content were measured on the cured press pieces and reported in Table 6.

 Table 6 Mechanical properties and diffusible hydrogen content obtained on press-hardened parts, with pre-coating of aluminum alloy It can be seen that the resistance obtained on parts A5-A6 exceeds 1800 MPa and that the hydrogen content diffusible is less than 0.16 ppm, while on parts J5-J6 the resistance is lower than 1800 MPa and the diffusible hydrogen content is greater than 0.16 ppm. Under the conditions of the invention, the characteristics of resistance and hydrogen content of the

15 pieces vary little according to the duration of maintenance in the oven, which ensures a very stable production.

 Thus, the invention allows the manufacture of hardened parts in press, simultaneously offering a very high mechanical strength and resistance to delayed cracking. These parts will be used profitably as structural parts or reinforcement in the field of automotive construction.

Claims

Rolled steel sheet for press hardening, the chemical composition of which comprises the contents being by weight:

 0.24% <C <0.38%

 0.40% <Mn <3%

 0.10% <If <0.70%

 0.015% <AI <0.070%

 0% <Cr <2%

 0.25% <Ni <2%

 0.015% <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 carbon, manganese, chromium and silicon contents satisfy:

" _^ Mn Cr Si _{1 in /}

 2.6C + - + - + -> l, l%

 5.3 13 15

 the chemical 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 rest being iron and unavoidable impurities from the elaboration, said sheet containing a nickel content Ni _SU r _f at all points of the steel in the vicinity of the surface of said sheet to a depth Δ, such that:

 Nisurf> Ninom,

Ninom designating the nominal nickel content of steel, and such that, Ni _max designating the maximum nickel content within Δ:

Χ (Δ)> 0.6, and such that

AT

 the depth Δ being expressed in micrometers,

the contents Ni _ma x and Ni _n0 m being expressed in percentages by weight.

2 steel sheet according to claim 1 characterized in that its composition comprises, by weight:

 0.32% <C <0.36%

 0.40% <Mn <0.80%

 0.05% <Cr <1, 20% 3 Steel sheet according to claim 1, characterized in that its composition comprises, by weight:

 0.24% <C <0.28%

 1, 50% <Mn≤3% 4 Steel sheet according to any one of Claims 1 to 3, characterized in that its composition comprises, by weight:

 0.50% <If <0.60%.

Steel sheet according to one of Claims 1 to 4, characterized in that its composition comprises, by weight:

0.30% <Cr <0.50%. Steel sheet according to any one of Claims 1 to 5, characterized in that its composition comprises, by weight:

 0.30% <Ni <1, 20%.

Steel sheet according to any one of Claims 1 to 6, characterized in that its composition comprises, by weight:

 0.30% <Ni <0.50%. Steel sheet according to one of the claims

 characterized in that its composition comprises, by weight:

 0.020% <Ti

9. Sheet steel according to any one of Claims 1 to 8, characterized in that its composition comprises, by weight:

 0.020% <Ti <0.040%.

Steel sheet according to one of Claims 1 to 9, characterized in that its composition comprises, by weight:

 0.15% <Mo <0.25%.

11 steel sheet according to any one of claims 1 to 10 characterized in that its composition comprises, by weight:

 0.010% <Nb <0.060%.

. Sheet steel according to any one of the preceding claims, characterized in that its composition comprises, by weight:

 0.030% <Nb <0.050%. Steel sheet according to Claim 2, characterized in that its composition comprises, by weight:

0.50% <Mn <0.70%. Steel sheet according to claim 2, characterized in that its microstructure is ferrito-pearlitic.

Steel sheet according to any one of claims 1 to 14, characterized in that said sheet is a hot-rolled sheet.

Sheet steel according to any one of claims 1 to 14, characterized in that said sheet is a cold rolled sheet and annealed.

Sheet steel according to any one of claims 1 to 16, characterized in that it is pre-coated with a metal layer of aluminum or aluminum alloy or aluminum-based.

Sheet steel according to any one of claims 1 to 16, characterized in that it is pre-coated with a metal layer of zinc or zinc alloy or zinc-based.

Steel sheet according to any one of claims 1 to 16, characterized in that it is pre-coated with one or more layers of intermetallic alloys containing aluminum and iron, and optionally silicon , the pre-coating containing no free aluminum, phase τ ₅ Fe3Si2Ali type ₂ , and τ ₆ of the Fe ₂ Si ₂ AI _{9 type} .

Part obtained by hardening in press of a steel sheet of composition according to any one of claims 1 to 13, of martensitic structure or martensito-bainitic.

Cured part in press according to claim 20, containing a nominal content of nickel Ni _name, characterized in that the nickel content of Ni _SU r _f in the steel in the vicinity of the surface is greater than Neither _name over a depth Δ, and in that, Ni _max denoting the maximum nickel content within Δ:

and in that :

(- ^ ^' max ^~ Ni _name ) _> QQ -j

 Δ

 the depth Δ being expressed in micrometers,

the contents Ni _max and Ni _nom being expressed in percentages by weight.

Press-hardened part according to claim 20 or 21, characterized in that its mechanical strength Rm is greater than or equal to 1800 MPa.

Press-hardened part according to any one of claims 20 to 22, characterized in that it is coated with an aluminum alloy or aluminum-based alloy, or a zinc alloy or zinc-based alloy resulting diffusion between the steel substrate and the precoat, during the press hardening heat treatment. A method of manufacturing a hot-rolled steel sheet, comprising the successive steps according to which:

 - A semi-product of chemical composition is cast according to any one of claims 1 to 13, then

 said half-product is heated to a temperature of between 1250 and 1300 ° C. for a holding period at this temperature of between 20 and 45 minutes, and then

 said half-product is hot-rolled to a TFL end-of-roll temperature of between 825 and 950 ° C., to obtain a hot-rolled sheet, and then

 said hot-rolled sheet is reeled at a temperature of between 500 and 750 ° C., to obtain a hot-rolled and wound roll, and then

the oxide layer formed during the preceding steps is etched off. A method of manufacturing a cold-rolled and annealed sheet, characterized in that it comprises the successive steps in which:

supplying a hot rolled, wound and pickled sheet manufactured by the method of claim 24 and then

these hot-rolled, wound and pickled sheets are cold-rolled to obtain a cold-rolled sheet, and then

said cold-rolled sheet is annealed at a temperature between 740 and 820 ° G to obtain a cold-rolled and annealed sheet.

A process for manufacturing a pre-coated sheet, according to which a laminated sheet manufactured according to process 24 or 25 is supplied, then a pre-coating is carried out continuously by dipping, said pre-coating being aluminum or an alloy of aluminum or aluminum-based, or zinc or a zinc alloy or zinc-based.

A method of manufacturing a pre-coated and pre-alloyed sheet, according to which:

a rolled sheet is supplied according to method 24 or 25, then a pre-coating is carried out continuously by dipping with an aluminum alloy or aluminum-based, and then

a pre-heat treatment of said pre-coated sheet is carried out at a temperature θ-ι of between 620 and 680 ° C. for a holding period ti of between 6 and 15 hours, so that the pre-coating does not contain more than free aluminum, of phase r ₅ of Fe ₃ Si ₂ Ali _{2 type} , and τ ₆ of Fe ₂ Si ₂ Al _{9 type} , and so as not to cause austenitic transformation in the steel substrate, said pretreatment being carried out in the oven under the atmosphere of hydrogen and nitrogen. A method of manufacturing a press-hardened part according to any one of claims 20 to 23, comprising the successive steps according to which:

a manufactured sheet is supplied by a process according to any one of claims 24 to 27, then

said sheet is cut to obtain a blank, then

an optional deformation step is carried out by cold stamping said blank, and then

said blank is heated to a temperature of between 810 and 950 ° C. to obtain a totally austenitic structure in the steel, then the blank is transferred into a press and then

said blank is hot stamped to obtain a part, then

said part is held in the press to obtain a hardening by martensitic transformation of said austenitic structure.

Use of a press-hardened part according to claim 20 to 23, or manufactured according to method 28, for the manufacture of structural or reinforcement parts for motor vehicles.