EP3631033A1 - Method for producing high-strength steel parts with improved ductility, and parts obtained by said method - Google Patents

Abstract

The invention relates to a rolled steel sheet for press hardening, the chemical composition of which comprises, in weight percentages: 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%, the titanium and nitrogen contents satisfying: Ti/N > 3.42, and the carbon, manganese, chromium and silicon contents satisfying: 2.6C + + + ≥ 1.1%, 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 unavoidable impurities resulting from the preparation, the sheet having a nickel Nisurf content at any point of the steel in the vicinity of the surface of said sheet over a depth Δ, such that: Nisurf > Ninom, Ninom being the nominal nickel content of the steel, and such that, Nimax being the maximum nickel content in Δ: (Nimax + Ninom) x (Δ) ≥ 0.6, and such that: (Nimax − Ninom) ≥ 0.01 2 ∆, and the surface density of all of the particles Di and the surface density of the particles larger than 2 micrometres D(>2µm) satisfy, at least over a depth of 100 micrometres in the vicinity of the surface of said sheet: Di + 6,75 D(>2µm) < 270, Di and D(>2µm) being expressed as a number of particles per square millimetre, and said particles being all of the oxides, sulfides, and nitrides, either pure or mixed such as oxysulfides and carbonitrides, present in the steel matrix.

Description

 PROCESS FOR MANUFACTURING STEEL PARTS HAVING HIGH MECHANICAL RESISTANCE AND IMPROVED DUCTILITY, AND PARTS OBTAINED THEREBY

The invention is in the field of steel sheets intended to obtain very high mechanical strength parts after curing in press. Pressurized hardening is known to heat steel blanks at a temperature sufficient to obtain austenitic transformation, and then to hot stamp the blanks by holding them within the blank. tooling 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.

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 resistance would be even higher, that is to say, whose resistance Rm would be greater than 1800 MPa. 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 can indeed be likely to crack or break after a certain delay.

 Publication WO2016016707 discloses a method of manufacturing parts and a rolled steel sheet for press hardening which simultaneously makes it possible to obtain a very high mechanical strength Rm greater than or equal to 1800 MPa, a high resistance to delayed cracking after curing under press, and to have a wide range of thickness in cold-rolled sheet. To do this, the nickel content of the chemical composition of the sheet is between 0.25% and 2% and is concentrated on the surface of the sheet or the workpiece in a specific form. Such enrichment of nickel forms a barrier effect to the penetration of hydrogen and thus slows the diffusion of hydrogen.

 More specifically, the steel sheet of the publication WO2016016707 has a chemical composition which comprises, the contents being expressed 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 levels of titanium and nitrogen satisfy: Ti / N> 3.42, and that carbon contents,

 _, _ Mn Cr Si

Manganese, chromium and silicon satisfy: 2.6C + - + - + -> 1.1%, the chemical composition optionally including 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 elaboration, the sheet containing a nickel content Ni _on f at any point in the in the vicinity of the surface of said sheet to a depth Δ, such that: Ni _surf > Ni _n0 m, Ni _n0 m denoting the nominal nickel content of the steel, and such that, Ni _max denoting the maximum content of nickel within (A)> 0.6, and such that: ^{(ma; t ~} " ^om ^ ≥ 0.01, the depth Δ being expressed in micrometers, the Ni _max and Ni _n0 m contents being expressed in percentages by weight.

 In addition, the publication WO2016016707 discloses a method of manufacturing a hot-rolled steel sheet which notably provides a step of reheating the slabs at a temperature of between 1250 and

1300 ° C for a holding time of between 20 and 45 minutes.

This specific range of temperature and slab reheating time ensures the diffusion of nickel to the interface between the formed oxide layer and the steel substrate, causing the nickel-enriched layer to appear.

 The steel parts obtained with the chemical composition and process disclosed in the publication WO2016016707 are particularly adapted, by their very high strength, for the manufacture of anti-intrusion parts of motor vehicles.

 Certain parts or parts of parts of the structural elements of motor vehicles must have a preferential functionality relating to their ability to absorb energy, especially during an impact.

This is particularly the case of the longitudinal members and the lower parts of the middle foot reinforcements.

 Publication WO2017006159 discloses a steel sheet and an associated manufacturing method which ensure that the steel sheet has a very good ductility characterized by a bending angle greater than 80 °.

 The resulting parts are suitable for forming structural members, or part of a motor vehicle body member, which are particularly resistant to impact. But the strength of the steel sheet of the publication WO2017006159 is significantly less than 1800 MPa, which does not meet the highest requirements in terms of anti-intrusion properties.

Therefore, certain elements of motor vehicle structures which have both a part whose preferential functionality is the mechanical strength and another part whose preferential functionality is the energy absorption, can be realized for example by welding a part obtained according to publication WO2016016707 and a part obtained according to the publication WO2017006159.

 However, the welding requires the realization of an additional operation of manufacturing parts, which increases the costs and the manufacturing time. In addition, it must be ensured that this welding does not reduce the resistance of the final part around the weld, which requires precise control of the welding parameters. There is therefore a need to realize in one piece the structural elements that combine the features of high mechanical strength and high energy absorption capacity.

 There is also the need to have hot stamped parts with satisfactory ductility, that is to say having a bending angle greater than or equal to 50 °.

 Therefore, the main objective of the invention is the production of a steel sheet having both a high mechanical strength characterized by a tensile strength Rm greater than 1800 MPa, and an improved ductility. These two characteristics are a priori difficult to reconcile since it is well known that an increase in mechanical strength generally leads to a decrease in ductility.

 Another property sought for safety parts and structural elements of motor vehicles is the decrease of the sensitivity to different forms of damage by hydrogen, including stress corrosion in aqueous medium as in saline.

 This is why the invention also aims at producing a steel sheet having improved resistance to stress corrosion.

For this purpose, the rolled steel sheet of the invention intended to be cured in press, is essentially characterized in that its chemical composition comprises, the contents being expressed by weight: that is 0.24% <C <0.38% and 0.40% <Mn <3%,

 either 0.38% <C <0.43% and 0.05% <Mn <0.4%

 0, 10% <Si≤ 1, 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 contents of titanium and nitrogen satisfy:

 Ti / N> 3.42,

 and that the carbon, manganese, chromium and silicon contents

 satisfy:

_, _ Mn Cr Si ^ _{Λ ηί}

 2.6C + + - + -> 1.1%

 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 _surf at any point of the steel in the vicinity of the surface of said sheet to a depth Δ, such that:

 Nisurf> Nlnom,

 Ninom designating the nominal nickel content of steel,

and such that, Ni _max denoting the maximum nickel content within Δ and such that:

the depth Δ being expressed in micrometers,

the contents Ni _max and Ni _n0 m being expressed in percentages by weight, and such that the surface density of all the particles D, and the surface density of the particles larger than 2 microns 0 (> 2μΓπ) satisfy, at least to a depth of 100 micrometers in the vicinity of the surface of said sheet, to:

D + 6.75 D ₍ m m) <270

 Di and D (m m) being expressed in number of particles per square millimeter, and said particles denoting all the oxides, sulphides, nitrides, pure or mixed, such as oxysulphides and carbonitrides, present in the matrix of the steel.

The rolled steel sheet of the invention may also include the following optional characteristics considered in isolation or in any possible technical combination:

 the composition comprises, by weight:

0.39% <C <0.43%

 0.09% <Mn≤ 0, 1 1%

 the composition comprises, by weight:

 0.95% <Cr <1.05%

 the composition comprises, by weight:

 0.48% <Ni <0.52%.

 the composition comprises by weight:

 1, 4% <If <1, 70%

- The microstructure of the steel sheet is ferrito-pearlitic. - The steel sheet is a hot rolled sheet.

 - the steel sheet is a cold rolled sheet and annealed.

 - The steel sheet is pre-coated with a metal layer of aluminum or aluminum alloy or aluminum-based.

 - The steel sheet is pre-coated with a metal layer of zinc or zinc alloy or zinc-based.

the steel sheet is pre-coated with one or more layers of intermetallic alloys containing aluminum and iron, and optionally silicon, the pre-coating not containing free aluminum, of phase r ₅ of the type Fe ₃ Si2Al ₂ , and r ₆ of the type

Fe ₂ Si ₂ AI ₉ .

The invention also relates to a part obtained by press hardening of a steel sheet of composition according to any one of the above modes of martensitic or martensite-bainitic structure, whose mechanical strength Rm is greater than or equal to at 1,800 MPa, and for which the surface density of all the particles D, and the surface density of particles larger than 2 micrometers D _{( m m} ) satisfy, at least over a depth of 100 microns in the vicinity of the surface of said part, to:

D + 6.75 D ₍ m m) <270

Di and D (m m) being expressed in number of particles per mm ²

 The part of the invention may also include the following optional features considered in isolation or in any possible technical combination:

 - The piece has at least in the rolling direction a bending angle greater than 50 °.

 the contents of manganese, phosphorus, chromium, molybdenum and silicon of the part satisfy:

[455Exp (-0.5 [Mn + 25P]) + [390Cr + 50Mo] + 7Exp (1.3Si)] [6 - 1.22xl0 ^"9 σ _ν ³ ] [C _SCC] > 750 a _y being the yield strength which is between 1,300 and 1,600 MPa, and Cscc being equal to 1 for an uncoated sheet, and equal to 0.7 for a coated sheet.

 the contents of manganese, phosphorus, chromium, molybdenum and silicon satisfy:

[455Exp (-0.5 [Mn + 25P]) + [390Cr + 50Mo] + 7Exp (1.3Si)] [6 - 1.22x10 ^"9 σ _ν ³ ] [C _SC c _] ≥ 1100 the part contains a nominal nickel content Ni _n0 m, 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 designating the maximum nickel content within of Δ:

(^ ^' max ^~ * ^~ Ni _name ) ^> 0 6 and in that

 (Neither nor

 AT

 the depth Δ being expressed in micrometers,

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

the part is coated with an aluminum alloy or an aluminum alloy, or a zinc alloy or a zinc alloy resulting from the diffusion between the steel substrate and the pre-coating, during the treatment thermal curing in press

 The invention also relates to a method of manufacturing a hot-rolled steel sheet comprising the successive steps according to which:

 a liquid steel is produced in which manganese, silicon, niobium and chromium are added, the additions being carried out in a vacuum chamber, then

 - Desulfurization of the liquid metal is carried out without increasing its nitrogen content, then

 titanium is added, said additions being carried out so as to obtain a liquid metal of chemical composition as defined above, and then

- we pour a half-product, 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 an end-of-rolling temperature TFL 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. The invention also relates to a method for manufacturing a hot-rolled steel sheet, then a cold-rolled and annealed sheet, comprising the successive stages in which:

 a hot-rolled, pickled and pickled sheet is supplied, manufactured by the method previously stated, then

 said cold rolled and pickled sheet is rolled cold to obtain a cold-rolled sheet, and then

 said cold-rolled sheet is annealed at a temperature of between 740 and 820 ° C. in order to obtain a cold-rolled and annealed sheet.

 The invention also relates to a method of manufacturing a pre-coated sheet, according to which a laminated sheet produced according to one of the two previously defined processes is supplied, then a continuous pre-coating is carried out by dipping, said pre-coating being aluminum or an aluminum alloy or aluminum-based, or zinc or a zinc alloy or zinc-based.

 The invention also relates to a method for manufacturing a pre-coated and pre-alloyed sheet metal, according to which:

 a laminated sheet is supplied according to one of the two processes previously defined, 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 so that the pre-coating does not contain any more free aluminum, of phase r ₅ of the Fe ₃ Si ₂ Ah _{2 type} , and r ₆ of the type Fe ₂ Si ₂ Al _g The invention furthermore relates to a method for manufacturing a press-hardened part as defined above, comprising the successive steps according to which:

 a sheet manufactured by a process such as those previously defined is supplied, then

 said sheet is cut to obtain a blank, and then

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

 said blank is heated to a temperature of between 810 and 950 ° C. in order to obtain a totally austenitic structure in steel and then

the flan is transferred into a press and then

 - The said blank is hot stamped to obtain a room, then

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

 Finally, the invention relates to the use of a press-hardened part as previously stated, or manufactured according to the method of manufacturing a cured part as defined above, for the manufacture of structural parts or reinforcement for vehicles. automobiles.

 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 shows the surface density of all the particles as a function of the surface density of particles of average size greater than 2 micrometers of hot stamped parts, with a breaking strength greater than 1800 MPa for five test conditions,

- Figure 2 shows the bending angle of hot stamped parts, breaking strength greater than 1800MPa, as a function of a parameter quantifying the density of the particles present in the hot stamped parts. This parameter depends on the surface density of all the particles as well as the density of the particles. particles of average size greater than 2 micrometers; these were evaluated for the same five test conditions, and

 FIG. 3 shows the surface density of the particles as a function of the size of these particles for the five test conditions.

 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% when the manganese content is between 0.4% and 3%. Carbon plays a major role in the quenchability and mechanical strength obtained after 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 for a manganese content of between 0.4% and 3%, the risk of delayed cracking is increased, and the ductile / brittle transition temperature, measured from Charpy-type notch bending tests can become 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. Spot welding ability is particularly good when the carbon content is between 0.24 and 0.38%.

an increased carbon content of between 0.38% and 0.43% when the manganese content is lowered between 0.05% and 0.4% to obtain a piece of steel having a increased resistance to stress corrosion. Preferably, the carbon content is between 0.39% and 0.43% for a manganese content of between 0.09% and 0.1%. The lowering of the manganese content is thus compensated by the increase in the carbon content while giving the steel part resistance to stress corrosion.

 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 deoxidizing role, manganese plays a role in quenchability. it is thus expected, when the carbon content is between 0.24 and 0.38%, that the manganese content must be greater than 0.40% by weight to obtain a start-of-transformation temperature (austenite → martensite). when cooling in press, low enough, which increases the resistance Rm. The limitation of the manganese content to 3% provides 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 can be formed during reheating of the slabs, so that the nickel does not have time to diffuse sufficiently for lie beneath this layer of oxides of iron and manganese.

 An alternatively lowered manganese content of between 0.05% and 0.4% is provided in conjunction with an increased carbon content of 0.38% to 0.43%. The lowering of the manganese content makes it possible to obtain a sheet and a piece of resistance to pitting corrosion and thus improved resistance to corrosion under stress. Maintaining high mechanical strength is achieved by substantially increasing the carbon content.

 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 content of in chromium between 0.05% and 1, 20%, allow simultaneously obtain excellent resistance to delayed cracking through the presence of a particularly effective nickel-enriched surface layer, and a very good aptitude for mechanical cutting of sheets. The Mn content is ideally between

0.50% and 0.70% to reconcile the achievement of high mechanical strength and resistance to delayed cracking.

 when the carbon content is between 0.24% and 0.38%, in combination with a manganese content of between 1.5% and 3%, the spot welding ability is particularly good.

 when the carbon content is between 0.38% and 0.43% in combination with a manganese content of between 0.05% and 0.4% and more preferably between 0.09% and 0.1%. %, resistance to stress corrosion is greatly increased, as will be seen later.

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 1.70% 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. The silicon content can be increased to 1.70% while avoiding the presence of excessive surface oxides which could adversely affect the deposition of the coating. This increase in the silicon content, however, necessitates stripping operations of the hot-rolled coil and subjecting the sheet to an annealing treatment atmosphere in a manner adapted to limit the formation of oxides.

For a carbon content of between 0.24% and 0.38%, the silicon content is preferably greater than 0.50% in order to avoid a softening of the fresh martensite, which can occur when the part is held in place. tools of the press after martensitic transformation. For a carbon content of between 0.38% and 0.43% and a manganese content of between 0.05% and 0.4%, the silicon content is preferably between 0.10% and 1.70%. to reduce the rate of pitting by corrosion, which makes it possible to increase the resistance to corrosion under stress.

 The silicon content can be increased up to 1.70% provided that the other alloying elements present in the steel make it possible to reach a conversion temperature at heating Ac3 (ferrite + perlite → austenite) of less than 880 ° C. C, so as to be compatible with the usual industrial practices of austenitization preceding the hot stamping step

 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 the tensile strength 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. For a carbon content of between 0.24% and 0.38%, a chromium addition of between 0.30 and 0.50% is preferred, which 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, 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.

 Alternatively, for a carbon content of between 0.38% and

0.43%, an increased chromium content of greater than 0.5% and more preferably of between 0.950% and 1.050% is preferred in order to increase the resistance to pitting corrosion as well as, consequently, the resistance to stress corrosion.

 In addition to the conditions on each of the elements C, Mn, Cr, Si defined above, these elements are jointly specified according to the

"Mn Cr Si

parameter P _{1 =} 2.6C + - ₊ - ₊ - As explained in the publication WO2016016707, 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. When a resistance value Rm in traction greater than or equal to 1800 MPa is sought, it is demonstrated that the parameter Pi must be such that: Pi> 1 .1

 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 increases very strongly hardenability. 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 an amount 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 between 0.25% and 2% in weight, very significantly reduces the sensitivity to delayed fracture when it is concentrated on the surface of the sheet or the workpiece in a specific form.

In addition and as disclosed in the publication WO2016016707, the steel piece is enriched in nickel in the vicinity of its surface up to a maximum Ni _max according to two parameters to obtain an effective resistance to delayed cracking,

A first parameter P2 is defined according to:

Δ being the enriched nickel depth of the steel part and Ni _n0 m being the nominal nickel content of the steel.

 This first parameter characterizes the overall nickel content in the enriched layer Δ

The second parameter P3 is defined by: Γ3 ₌

 Δ

 This second parameter characterizes the average nickel concentration gradient, that is to say the intensity of the enrichment within the Δ layer.

By satisfying these two parameters, the steel piece has a very significant resistance to delayed cracking.

The method for producing a steel sheet of the invention will now be described: A semi-finished product, in the form of liquid steel, of the composition mentioned above is cast. Unlike a conventional method where the addition of elements is carried out during the ladle casting from the converter, the inventors have demonstrated that it was necessary to perform this addition without the presence of air which leads to an increase in the nitrogen content of the liquid metal. In the process according to the invention, the additions of elements such as manganese, silicon, niobium, chromium are carried out in an enclosure where a vacuum atmosphere prevails. After this vacuum treatment, the liquid metal is desulphurized by stirring between the metal and the slag which is carried out under conditions that do not increase the nitrogen content. After control of the nitrogen content in the liquid metal, titanium is added, for example in the form of ferro-titanium. Titanium is thus added at the end of the secondary metallurgy step. Thus, during the addition operation, the amount of nitrogen introduced is reduced and the formation of particles that may adversely affect the ductility of the steel part is limited. By thus achieving the introduction of the additive elements, the amount of precipitated particles is reduced at the end of the solidification and thus the sheet and the resulting steel piece have improved ductility as will be detailed later.

 The half-product obtained after casting may be in the form of a slab of thickness typically between 200 and 250 mm, or thin 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 increase occurs, however, 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 750 ° C.

 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, a cold rolling is carried out with a suitable reduction ratio, for example between 30 and 70%, and 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.

As explained in the publication WO2016016707, the step which has a predominant influence on the characteristics of the nickel-enriched layer on the final sheet is the step of reheating the slabs, in a specific range of temperature and holding time. Conversely, the annealing cycle of the cold rolled sheet, with or without 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 nickel enrichment of this layer are practically identical on a hot rolled sheet and a sheet which has also undergone 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 7-1 5% 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, 3-1 0% 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 pre-treatment is carried out at a temperature θι during a holding time ti, so that the pre-coating does not contain any more free aluminum, of phase rs of the Fe3Si2Ah ₂ type, and τ e of the Fe2Si2Al _g type. . 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 7-1 2% after heat treatment of alliation achieved parade immediately after the galvanizing bath.

 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 piece stamped and hardened 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 microstructure preferentially ferrito-pearlitic, or ferrito-pearlitic.

 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.

The inventors have discovered that in order to obtain a piece of steel having improved ductility, in addition to the advantageous properties of mechanical strength and delayed crack resistance previously explained, the density of the particles present in the vicinity of the surface of the sheet should satisfy on special conditions. In the context of the invention, these particles denote all the oxides, sulphides, nitrides, pure or mixed, such as oxysulfides and carbonitrides, present in the matrix of steel. It has indeed been shown that certain particles were sites of early damage that reduced the ability to bend. In the context of the invention, the vicinity of the surface designates the area between the surface of the sheets and 100 micrometers below this surface.

 In particular the density of the particles and in particular that of the particles of average size greater than 2 micrometers had to meet certain criteria.

 Tables 1 and 2 below and Figures 1 and 2 are used to describe the tests and measurements leading to the establishment of a parameter for particle densities.

 Five steel sheets A, B, C, D, E whose respective chemical compositions are given in Table 1, were made. The compositions are expressed in percent by weight, the remainder of the composition consisting of iron and impurities resulting from the preparation. These sheets were obtained from steel prepared in the liquid state according to different processes: for test A (reference test), the addition elements (manganese, silicon, chromium, niobium) were added under air when pouring in the pocket from the converter.

 For tests B, C, D, E, carried out under the conditions of the invention, these addition elements were added during a treatment RH (Ruhrstahl Heraeus) in the tank RH kept under vacuum. The subsequent desulphurization treatment was carried out without taking up nitrogen in the liquid steel. The addition of titanium was carried out as ferro-titanium at the end of the secondary metallurgy process.

 After casting in the form of semi-finished products, slabs of these different steels were heated to a temperature of 1275 ° C. and held at this temperature for 45 minutes. They were then rolled with a rolling end temperature of 950 ° C, and wound at a temperature of 650 ° C. After stripping, the sheets were cold-rolled to a thickness of 1.5 mm. The sheets were then annealed at a temperature of 760 ° C, then continuously aluminized by dipping in a bath containing 9% by weight of silicon 3% by weight of iron, the balance being aluminum and aluminum. unavoidable impurities.

The cut sheets were hot stamped after reheating to a temperature of 900 ° C and a total holding time in the oven of 6'30.

After curing in press, measurements were made on three samples by scanning electron microscopy, considering particles larger than 0.5 micrometers on a surface of 6 mm 2 and on a depth of 100 micrometers near the surface of the part.

A first type of measurement consists in evaluating the density D 1 of all the particles, namely oxides, sulphides, nitrides, pure or mixed, such as oxysulphides and carbonitrides, present in the matrix of the steel. A second type of measurement consists in evaluating the density D ^ _m ) of these same particles whose size is greater than 2 micrometers. In Table 2 below, the test references D1, D2, E1 and E2 respectively correspond to steel sheets of composition D and E as presented in Table 1 below and which result from two coils of different steel.

The bending angle was determined on the 60x60mm ² hardened parts supported by two rollers, according to the VDA-238 bending standard. The bending force is exerted by a 0.4 mm radius punch. The spacing between the rolls and the punch is equal to the thickness of the pieces tested, a set of 0.5 mm being added. The appearance of the crack is detected since it coincides with a decrease in the load in the displacement curve of the load. The tests are interrupted when the load decreases more than 30 N of its maximum value. The bending angle of each test reference is measured at maximum load. The results presented in Table 2 below correspond to the seven samples taken in the rolling direction. An average value of the bending angle is then obtained.

Table 2: Particle density (D) and density of particles of average size greater than 2 micrometers (D ^ _m)) to a depth of 100 micrometers near the surface of the sheet, and corresponding bend angle. Underlined values: not in accordance with the invention

In order to meet the industrial requirements in terms of ductility in the event of impact, satisfactory parts with regard to breaking stress are those with a bending angle greater than 50 °. The hot stamped part under the conditions of test A, where the additions of elements were made according to a conventional method, has a folding angle of less than 50 °.

Figure 3 illustrates the distribution of particles according to their average size as a function of their density for the seven test references of Table 2. It can be seen that the test reference A has a distribution of the density of the particles according to their size. substantially different from that of the other test references. Mainly, the density of particles of average size less than 2 micrometers of reference A is significantly lower than that of the other test references. The conditions of elaboration according to the invention make it possible to obtain a significant decrease in all the particles, and in particular particles larger than 2 microns in size. This favorable distribution is found on the sheet as well as the hot stamped part from this sheet.

 FIG. 1 and for each test reference of FIG.

Table 2 the density D (^ _m ) relative to the particles of average size greater than 2 micrometers, and the density Di relative to all the particles. Considering that only the reference A does not satisfy the desired criterion of a bending angle greater than 50 °, a relationship arises between the density Di and the density D ^ _m ) which is obtained on the basis of the straight line D d equation:

 Y = - 6.75 (X-40)

 Considering that the parts likely to have a folding angle greater than 50 ° are located under the straight line D in the shaded area F, it follows that the criterion for satisfying a good ductility at folding is as follows:

D + 6.75 D ₍ > z _m) <270

D, and 0 (> ₂ μΓτι) being both expressed in number of particles per mm ² .

 This criterion highlights the important influence of particles of average size greater than 2 micrometers on the ductility of hot stamped parts.

In Table 3 below and in FIG. 2, the defined criterion D, + 6.75 D _{( m m} ) and the bending angle obtained for the seven test conditions A, B, C, have been reported. D1, D2, E1 and E2. The gray area G in FIG. 2 defines the zone according to the invention for which the part has a folding angle greater than 50 ° and in which the criterion is less than 270. In this zone G, the part has a ductility improved and mechanical strength Rm greater than 1800 MPa. Reference Criterion Folding angle (°)

Dj + 6.75 D ₍ > _2um)

 A 577

 B 181 50.85

 C 143 52

 Dl 220 51

 D2 180 51

 El 144 55

 E2 246 55

Table 3: Criterion D, + 6.75 D ₍ > _2Mm) and corresponding bending angle

 Underlined values: not in accordance with the invention

The inventors have also discovered that the reduction in the manganese content adjusted by a substantial increase in the carbon content makes it possible to substantially increase the resistance of the steel part to stress corrosion while preserving a high mechanical strength greater than 1800 MPa.

 It is known to measure the sensitivity to stress corrosion by methods using a four-point constant load bending test and:

 - either immersion of the steel piece thus constrained in a saline solution at room temperature for 30 days,

 or spraying at 35 ° C. for 4 hours with a saline solution on the piece of steel under stress, this operation being repeated for 20 days.

 But these methods do not sufficiently reproduce the environmental conditions in which the steel pieces are likely to be.

This is why another so-called cyclic method provides alternating saline phase, wet phase and dry phase. The salt phase is applied for 2% of the test period for a weight percent NaCl in the atmosphere of 1% at pH4. The next wet phase is applied for 28% of the test period at a relative humidity percentage of 90% at a temperature of 35 ° C. The last dry phase is applied during 70% of the test period, to a percentage of humidity relative humidity of 55% and at a temperature of 35 ° C. This cyclic test is applied for 42 days.

 However, this cyclic method is not severe enough to ensure that the steel part has satisfactory stress corrosion resistance for the intended applications. A new cyclic method called VDA (Verband der Automobilindustrie) has therefore been applied in which the stressed steel part is subjected to more severe corrosion conditions. A test period, or cycle, is one week.

 In this VDA method, the salt phase is applied for 5% of the test period (instead of 2% for the cyclic method) for a weight percentage of NaCl in the atmosphere of 1% at pH7. The next wet phase is applied for 25% of the test period, at a relative humidity percentage of 95% (instead of 90% for the cyclic method) at a temperature of 35 ° C. The last dry phase is applied for 65% of the test period, at a relative humidity percentage of 70% (instead of 55% for the cyclic method) and at a temperature of 35 ° C. The VDA method is applied for 6 cycles, 6 weeks or 42 days.

 It is considered according to the invention that a piece of steel satisfies the criterion of stress corrosion if no rupture of the material occurs for at least 42 days.

 Four test conditions H, I, J and K were considered, the chemical compositions of which are given in Table 4 below. The compositions are expressed in percent by weight, the remainder of the composition consisting of iron and impurities resulting from the preparation.

The four test conditions H, I, J and K satisfy the previously defined criteria for particle density and surface enrichment of nickel. Reference

 c Mn S P Si Cr N b

 trial

 H 0.35 0.60 0.0003 0.012 0.53 0.33 0.045

1 0.35 0.62 0.0003 0.013 0.57 0.51 0.039

J 0.40 0.10 0.0001 0.012 0.21 1.00 0.041

K 0.33 0.48 0.0001 0.012 1.53 0.96 0.047

Reference

 Al Ti N i Mo B (ppm) N

 trial

 H 0.045 0.017 0.38 0.17 32 0.004

 1 0.030 0.020 0.40 0.20 24 0.005

 J 0.023 0.015 0.50 0.24 19 0.003

 K 0.016 0.020 0.39 0.19 33 0.004

 Table 4: Composition of steel for four test conditions H, I, J and K

The sheet manufactured in condition H has an Ac3 temperature of 829 ° C. This temperature is evaluated by the formula of Andrews, known in itself. The sheet manufactured in condition I has a temperature Ac3 calculated by the Andrews formula of 820 ° C, the sheet produced in test condition J has a temperature Ac3 calculated by the Andrews formula of 807 ° C, and the sheet made in test condition K has a temperature Ac3 calculated by the Andrews formula of 871 ° C.

 The test reference J thus has a temperature of austenization particularly favorable to its industrial development.

 The temperatures Ms (martensitic transformation start temperature at cooling) calculated from the Andrews formula, are 362 ° C., 345 ° C., 353 ° C., 348 ° C. for the plates produced respectively under the conditions H, I, J and K.

 The steel sheets of references H, I, J and K were made under the following conditions:

 - reheating at a temperature of 1275 ° C for 30 minutes

- Hot rolling to an end of rolling temperature TFL of 900 ° C. - winding at 540 ° C for reference H, 550 ° C for references I and J, and 580 ° C for reference K,

 - cold rolling with a reduction rate of 58%,

 annealing at a temperature of 760 ° C. so as to obtain a recrystallization of the hardened metal, and

 - cooling.

 In test H, the sheet is coated with a dipping alloy of AISi as mentioned above, the sheets manufactured under conditions I, J and K are not coated.

A steel sheet with a thickness of 1.5 millimeters is obtained for conditions H, I and K and 1.3 millimeters for condition J.

 After having cut the sheet to obtain a blank, it is heated in an oven at 900 ° C. for 6 minutes and 30 seconds (total time of maintenance in the oven), so that a total austenitic transformation takes place. in the steel, then the blank is transferred quickly into a device simulating hot stamping. The transfer is carried out in less than 10 seconds, so that no transformation of the austenite occurs during this step. The pressure exerted by the tools of the press is 5000 MPa. The part is held in the press to obtain a hardening by martensitic transformation of the austenitic structure. A heat treatment of 170 ° C. is then applied to the sheet for 20 minutes, corresponding to a firing cycle of a paint applied to the hot-stamped part.

The mechanical tensile characteristics (elastic limit σ _γ and resistance Rm) measured on the stamped parts H, I, J and K are presented in Table 5 below. Reference σ _γ

 Rm (MPa)

 test (MPa)

 H 1482 1845

 1 1587 1996

 J 1599 1923

K 1497 1824

 Table 5. Mechanical tensile characteristics measured under the four test conditions H, I, J and K

Three test pieces taken from hot-stamped parts for each of the test references H, I, J and K were subjected to the VDA stress corrosion test previously described. The bending stress applied to the specimen on the outer surface between the two rollers is 750 MPa.

 The results are shown in Table 6 below.

Table 6: Results of stress corrosion tests by the method

VDA HK test conditions be seen that for Test Condition H, two pieces are broken during the ^2nd cycle, and the third piece has broken during the 3 ^rd cycle.

For the test I reference, a first piece is broken during the 3 ^rd cycle, and the other two parts are broken during the 4 ^th cycle.

For test references J and K, no parts are broken at the end the 6 ^th cycle. The test reference J with a low manganese content and the reference K with a high silicon content thus have excellent resistance to stress corrosion.

 Without being bound by a theory, the inventors have defined the expression of a criterion making it possible to ensure, for a hot-stamped part having a yield strength of between 1300 and 1600 MPa, a resistance to corrosion under sufficient stress. to satisfy the VDA test.

 This criterion depends on three parameters: a parameter P1 depending on the composition of the part, a parameter P2 depending on the stress applied and a parameter P3 depending on the possible presence of a coating on the hot-stamped part.

 Parameter P1 is expressed as follows depending on the contents of manganese, phosphorus, chromium, molybdenum and silicon:

Pl = 455Exp (-0.5 [Mn + 25P]) + [390Cr + 50Mo] + TExp (1.3Si), the contents being expressed as percentages by weight.

 Parameter P2 is expressed as follows:

P2 = [6 - 1.22x10 ⁹ ay ³ \

where _y denotes the elastic limit, expressed in MPa, and is between 1300 and 1600 MPa.

 The parameter P3 is quantized by a parameter Cscc whose value is equal to 1 if the part is not coated and is naked, and equal to 0.7 if the part is coated.

 The stress corrosion failure threshold Xo is then defined as being: Xo = P1 x P2 x P3

The Xo stress corrosion failure thresholds thus determined for the stampings H, I, J and K are shown in Table 7 below. Reference

 Xo

 trial

 H 627

 1,570

 J 793

K 1417

 Table 7: Xo stress corrosion failure thresholds for the four test references H, I, J and K

The inventors have thus demonstrated that if Xo is greater than or equal to 750, and preferably greater than or equal to 790, the corresponding sheet or part satisfies the VDA test for resistance to stress corrosion.

 The following criterion is then defined which, if satisfied, ensures a good resistance to stress corrosion of the sheet and steel part:

[455Exp (-0.5 [Mn + 25P]) + [390O + 50 o] + 7Exp (l 3Si)] [ό - 1.22x1 (Γ ⁹ σ) \ c _scc]> 750

Preferentially, the value of Xo is greater than or equal to 790, and very preferably greater than 1 100 to obtain a very high resistance to stress corrosion.

 In addition to demonstrating that the reduction in the Mn content makes it possible to increase the resistance to stress corrosion, it is found that the increase in the chromium content (0.33% for the test reference H, 0 , 51% for reference I and of the order of 1% for references J and K) also improves the resistance to stress corrosion of the workpiece. The tests on the reference K also show that a silicon content of 1.53% makes it possible to obtain a high resistance to stress corrosion.

Thus, the invention allows the manufacture of press hardened parts, simultaneously offering high mechanical tensile properties, good toughness and high resistance to stress corrosion. 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:

 either 0.24% <C <0.38% and 0.40% <Mn <3%, ie 0.38% <C <0.43% and 0.05% <Mn <0.4%

 0.10% <Si≤ 1, 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 contents of titanium and nitrogen satisfy:

 Ti / N> 3.42,

and that the carbon, manganese, chromium and silicon contents satisfy:

 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 _surf at any point 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 _ma x denoting the maximum nickel content within Δ

and such that:

Δ

the depth Δ being expressed in micrometers,

the contents Ni _ma x and Ni _n0 m being expressed in percentages by weight, and such that the surface density of the set of particles D, and the surface density of particles larger than 2 microns 0 (> 2μΓπ) satisfy, at less to a depth of 100 micrometers in the vicinity of the surface of said sheet, to:

D + 6.75 D (m m) <270

 D 1 and D 1 m) being expressed in number of particles per square millimeter, and said particles denoting all the oxides, sulphides, nitrides, pure or mixed, such as oxysulphides and carbonitrides, present in the matrix of the steel.

Sheet steel according to Claim 1, characterized in that its composition comprises, by weight:

0.39% <C <0.43%

0.09% <Mn≤ 0, 1 1%

3. Sheet steel according to any one of claims 1 and 2, characterized in that its composition comprises, by weight:

 0.95% <Cr <1, 05% 4. Steel sheet according to any one of 2 and 3, characterized in that its composition comprises, by weight:

 0.48% <Ni <0.52%.

5. Sheet steel according to any one of claims 2 to 4, characterized in that its composition comprises by weight:

 1, 4% <Si≤ 1, 70%

6. Sheet steel according to any one of the preceding claims, characterized in that its microstructure is ferrito-pearlitic.

7. Sheet steel according to any one of the preceding claims, characterized in that said sheet is a hot rolled sheet.

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

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

10. Sheet steel according to any one of the preceding claims, characterized in that it is pre-coated with a metal layer of zinc or zinc alloy or zinc-based. 1 1. Steel sheet according to any one of the preceding claims, 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 Fe3Si2Ah type ₂ , and ze type Fe ₂ Si ₂ AI ₉ .

2. Part obtained by hardening in press of a steel sheet of composition according to any one of claims 1 to 5, characterized in that it has a martensitic or martensitic-bainitic structure, in that its mechanical strength Rm is greater than or equal to 1,800 MPa, and in that the surface density of all D particles and the surface density of particles larger than 2 micrometers D _{( m m} ) satisfy, at least over a depth of 100 micrometers in the vicinity of the surface of said part, to:

D + 6.75 D ₍ > z _m) <270

Di and Dm) being expressed in number of particles per mm ²

Press-hardened part according to claim 1, characterized in that it has at least in the rolling direction a bending angle greater than 50 °.

Press-hardened part according to any one of claims 1 2 and 1 3, characterized in that the contents of manganese, phosphorus, chromium, molybdenum and silicon satisfy: [455Exp (-0.5 [Mn + 25P]) + [390Cr + 50Mo] + 7Exp (1.3Si)] [6 - 1.22xl0 ^"9 σ _ν ³ ] [C _SCC] ≥ 750 _y being the yield strength which is between 1300 and 1600 MPa, and Cscc being equal to 1 for an uncoated sheet, and equal to 0.7 for a coated sheet.

Press-hardened part according to claim 14, characterized in that the contents of manganese, phosphorus, chromium, molybdenum and silicon satisfy: [455Exp (-0.5 [Mn + 25P]) + [390Cr + 50Mo] + 7Exp (1.3Si)] [6 - 1.22xl0 ^"9 σ _ν ³ ] [ _CSC] > 1100

Press-hardened part according to any one of claims 12 to 15, containing a nominal nickel content Ni _n0 m, characterized in that the nickel content Ni _surf in the steel in the vicinity of the surface is greater than Ni _n0 m on a depth Δ, and in that, Ni _max denoting the maximum nickel content within Δ:

 (Mm + Μηοη, - Χ (Δ)> 0.6, and in that

(Ni _m3K Ni _name )> QQ - |

 Δ

 the depth Δ being expressed in micrometers,

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

Press-hardened part according to any one of claims 12 to 16, characterized in that it is coated with an aluminum alloy or aluminum-based alloy, or a zinc alloy or based on zinc resulting from the diffusion between the steel substrate and the pre-coating, during the press hardening heat treatment.

18. A method of manufacturing a hot-rolled steel sheet, comprising the successive steps according to which:

 a liquid steel is produced in which manganese, silicon, niobium and chromium are added, the additions being carried out in a vacuum chamber, then

 - Desulfurization of the liquid metal is carried out without increasing its nitrogen content, then

 titanium is added, said additions being carried out so as to obtain a liquid metal of chemical composition according to any one of claims 1 to 5, and then

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

 said half-product is hot rolled to an end-of-rolling temperature TFL 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 in the preceding steps is etched.

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 18 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 ° C to obtain a cold-rolled and annealed sheet.

A method of manufacturing a pre-coated sheet, according to which a laminated sheet manufactured according to the method 18 or 19 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 laminated sheet is supplied according to the method 19 or 20, 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 in such a way that the pre-coating no longer contains free aluminum, of phase r ₅ of the Fe ₃ Si ₂ Ah _{2 type} , and r ₆ of the type Fe ₂ Si ₂ Al _g

A method of manufacturing a press-cured part according to any one of claims 12 to 17, comprising the successive steps according to which:

 a manufactured sheet is supplied by a process according to any one of claims 18 to 21, 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 12 to 17, or manufactured according to the method of claim 22, for the manufacture of structural or reinforcement parts for motor vehicles.