HIGH-STRENGTH STEEL AND MANUFACTURING PROCESS

Abstract

The invention relates to a cold-rolled, annealed steel sheet having a thickness of between 0,7 mm and 2mm, a mechanical strength of between 1180 MPa and 1320 MPa, a hole expansion ratio (Ac%) greater than 20%, a bending angle greater than or equal to 40°, and a chemical composition comprising (concentrations expressed by weight): 0.09 < C < 0.11 %, 2.6 ≤ Mn < 2.8%, 0.20 ≤ Si ≤ 0.55%, 0.25 ≤ Cr < 0.5 %, 0.025 < Ti ≤ 0.040 %, 0.0015 ≤ B < 0.0025 %, 0.005 ≤ Al < 0.18 %, 0.08 ≤ Mo < 0.15 %, 0.020 < Nb ≤ 0.040%, 0.002 ≤ N < 0.007 %, 0.0005% ≤ S ≤ 0.005%, 0.001 % ≤ P ≤ 0.020%, Ca ≤ 0.003%, the remainder comprising iron and inevitable impurities resulting from production. The sheet has a microstructure comprising martensite and / or lower bainite, said martensite comprising fresh martensite and / or self-tempered martensite, the sum of the surface area percentages of martensite and lower bainite being between 40 and 70 %, with between 15 and 45 surface area-% low-carbide-content bainite, between 5 and less than 20 surface area-% ferrite, the fraction of non-recrystallised ferrite in relation to the total ferrite fraction being less than 15%, and less than 5 surface area-% residual austenite in the form of islets, the fraction of former austenite grains with a size of less than one micrometre representing between 40 and 60% of the total population of said former austenite grains.

Description

High-strength steel and manufacturing process The present invention relates to a cold-rolled and annealed steel sheet having very high mechanical strength and deformation capacity for the manufacture of parts by forming, in particular in the automotive industry, for the manufacture of structural elements of automobile bodies, and the manufacture of such a sheet. We have developed steels with a very favorable yield strength / strength ratio during forming operations. Their high strength allows for even distribution of deformation in the event of a collision and results in a significantly higher yield strength after forming. This enables the production of parts as complex as those made with conventional steels, but with superior mechanical properties, allowing for a reduction in thickness while maintaining the same functional specifications. As such, these steels provide an effective solution to the weight reduction and safety requirements of vehicles. In particular, steels whose structure includes martensite, possibly bainite, within a ferritic matrix, have seen great development because they combine high strength with significant deformation possibilities. Recent requirements for weight reduction and reduced energy consumption have led to increased demand for very high strength steels, with a mechanical strength Rm greater than 1180 MPa. In addition to this level of strength, these steels must exhibit good ductility, good weldability and good coating suitability, in particular good suitability for continuous hot-dip galvanizing. These steels must also exhibit high yield strength and elongation at break, as well as good formability. Indeed, some automotive parts are manufactured using forming operations that combine different deformation modes. Certain microstructural characteristics of steel may be well-suited to one deformation mode but less so to another. Some parts must exhibit high tensile strength and / or good bendability and / or good edge forming ability, particularly during forming with raised edges. This edge forming ability is evaluated by determining a hole expansion ratio, denoted Ac%. This ratio measures the steel's ability to expand during cold drawing and thus provides an assessment of its formability in this deformation mode. The hole expansion ratio can be evaluated as follows: after a hole is cut in a sheet of metal, a conical tool is used to create expansion at the edges of the hole. It is during this operation that early damage can be observed near the edges of the hole during expansion, this damage initiating on second-phase particles or at the interfaces between the different microstructural constituents in the steel. According to US documents 2012 / 0312433 A1 and 2012 / 132327 A1, steels with a tensile strength (Rm) exceeding 1180 MPa are known. However, this tensile strength is achieved at the expense of formability and weldability. Furthermore, according to documents US 2013 / 0209833 A1, US 2011 / 0048589 A1, US 2011 / 01683000 A1 and WO 2013 / 144376 A1, steels with high mechanical strength exceeding 1000 MPa are known, but which do not simultaneously exhibit satisfactory formability and weldability. Under these conditions, one object of the invention is to make available a steel sheet exhibiting high mechanical resistance, in particular between 1180 and 1320 MPa, together with a high yield strength, in particular between 750 and 970 MPa, this value being determined before any skin-pass operation on the sheet, good formability, in particular a hole expansion ratio Ac% greater than or equal to 20%, a bending angle, for a sheet with a thickness between 0.7 mm and 2 mm, greater than or equal to 40°, and an elongation at break greater than 7%. To this end, the invention relates to a cold-rolled and annealed steel sheet with a thickness between 0.7 mm and 2 mm, a tensile strength between 1180 MPa and 1320 MPa, a hole expansion ratio (Ac%) greater than 20%, and a bend angle greater than or equal to 40°, the chemical composition of which comprises, with contents expressed by weight: 0.09 < C ≤ 0.11%, 2.6 ≤ Mn < 2.8%, 0.20 < Si < 0.55%, 0.25 < Cr < 0.5%, 0.025 ≤ Ti < 0.040%, 0.0015 < B ≤ 0.0025%, 0.005 < Al < 0.18%, 0.08 < Mo < 0.15%, 0.020 < Nb < 0.040%, 0.002 ≤ N < 0.007%, 0.0005% < S ≤ 0.005%, 0.001% < P < 0.020%, Ca < 0.003%, the remainder being made up of iron and unavoidable impurities from the manufacturing process, the sheet having a microstructure consisting of martensite and / or lower bainite, said martensite comprising fresh martensite and / or self-tempered martensite, the sum of the surface proportions of martensite and lower bainite being between 40 and 70%,from 15 to 45% as a surface proportion of bainite with a low quantity of carbides, from 5 to less than 20% as a surface proportion of ferrite, the fraction of non-recrystallized ferrite relative to the total ferrite fraction being less than 15%, and less than 5% as a surface proportion of residual austenite in the form of islands, the fraction of old austenitic grains whose size is less than one micrometer representing 40 to 60% of the total population of said old austenitic grains. In some embodiments, the sheet metal according to the invention further comprises one or more of the following characteristics: the microstructure comprises, on a surface basis, 15 to 45% fresh martensite. the microstructure comprises, in surface proportion, 5 to 50% of a sum of self-tempered martensite and lower bainite. - self-tempered martensite and the aforementioned lower bainite contain carbides in the form of rods oriented in the directions <111> martensitic and bainitic laths. Low carbide bainite contains less than 100 carbides per unit area of ​​100 square micrometers. - the steel sheet contains precipitates of the type (Ti, Nb, Mo)(C,N) of size less than 5 nanometers, present in quantities less than 10,000 precipitates / pm3 the chemical composition includes, the content being expressed by weight: 2.6 < Mn≤ 2.7%. the chemical composition preferentially includes, the content being expressed by weight: 0.30 < Si < 0.5%. according to a preferred mode, the chemical composition comprises, the content being expressed by weight: 0.005≤ Al < 0.030%. the sheet metal has a coating of zinc or zinc alloy, obtained by dipping, according to a particular method, the zinc or zinc alloy coating is a galvanized-alloy coating, the zinc or zinc alloy coating comprising 7 to 12% by weight of iron. The steel sheet has a coating of zinc or zinc alloy, obtained by vacuum deposition. The invention also relates to a process for manufacturing a cold-rolled and annealed sheet coated with zinc or a zinc alloy, according to any one of the above characteristics, comprising the following successive steps: - a semi-finished product is supplied, the chemical composition of which comprises, the contents being expressed by weight: 0.09 < C < 0.11%, 2.6 < Mn < 2.8%, 0.20 ≤ Si ≤ 0.55%, 0.25 ≤ Cr < 0.5%, 0.025 < Ti < 0.040%, 0.0015 < B < 0.0025%, 0.005 < Al ≤ 0.18%, 0.08 < Mo < 0.15%, 0.020 < Nb < 0.040%, 0.002 ≤ N < 0.007%, 0.0005 < S ≤ 0.005%, 0.001 ≤ P < 0.020%, Ca < 0.003%, the remainder being made up of iron and unavoidable impurities from the manufacturing process, then - the semi-product is heated to a temperature Tr greater than or equal to 1250°C, - then the semi-finished product is hot-rolled, the rolling end temperature being higher than the Ar3 temperature at the start of austenite transformation upon cooling, to obtain a hot-rolled sheet, then - the hot-rolled sheet is cooled at a rate greater than 30°C / s to prevent the formation of ferrite and pearlite, then The hot-rolled sheet metal is coiled at a temperature between 580° and 500°G, then - the hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet, then - the cold-rolled sheet is heated to between 600°C and Ac1, where Ac1 is the temperature at which austenitic transformation begins upon heating, with a heating rate Vc between 1 and 20°C / s, - the cold-rolled sheet is heated to a temperature Tm between 780°C and (Ac3-25°C), and the cold-rolled sheet is held at temperature Tm for a duration Dm between 30 and 150 seconds, it being understood that Ac3 designates the temperature at the end of the austenitic transformation upon heating, then - the sheet metal is cooled at a rate VR1 between 10 and 150°C / s until a temperature Te between 400 and 490°C is reached, then - the sheet metal is maintained at temperature Te for a duration De between 5 and 150 seconds, then - the sheet metal is coated by continuous immersion in a bath of zinc or zinc alloy at a temperature TZn between 450°C and 480°C, said temperatures Te and TZn being such that 0°C ≤ (Te-TZn) < 10°C, so as to obtain a coated sheet metal, then - Optionally, the coated sheet metal is heated to a temperature TG between 490°C and 550°C for a duration tG between 10 s and 40 s. The invention also relates to a method for manufacturing a cold-rolled and annealed sheet metal, comprising the following successive steps: - We supply a semi-finished product whose chemical composition includes, with contents expressed by weight: 0.09 < C < 0.1%, 2.6 ≤ Mn ≤ 2.8%, 0.20 < Si ≤ 0.55%, 0.25 < Cr < 0.5%, 0.025 < Ti < 0.040%, 0.0015 ≤ B < 0.0025%, 0.005 < Al ≤ 0.18%, 0.08 < Mo < 0.15%, 0.020 < Nb < 0.040%, 0.002 < N < 0.007%, 0.0005 ≤ S < 0.005%, 0.001 < P ≤ 0.020%, Ca < 0.003%, the remainder being composed of iron and inevitable impurities from the manufacturing process, then - the semi-product is heated to a temperature Tr greater than or equal to 1250°C, then - the semi-finished product is hot-rolled, with the final rolling temperature being above Ar3, to obtain a hot-rolled sheet, then - the hot-rolled sheet is cooled at a rate greater than 30°C / s to prevent the formation of ferrite and pearlite, then - the hot-rolled sheet metal is coiled at a temperature between 580 and 500°C, then - the hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet, then - the cold-rolled sheet is heated with a heating rate VR, between 600°C and Ac1, where Ac1 designates the temperature at which austenitic transformation begins during heating, between 1 and 20°C / s, then - The cold-rolled sheet is heated to a temperature Tm between 780 °C and (Ac3-25 °C), and the cold-rolled sheet is held at temperature Tm for a duration Dm between 30 and 150 seconds, it being understood that Ac3 designates the temperature at the end of the austenitic transformation upon heating, then - the sheet metal is cooled at a rate VR2 between 10 and 100°C / s until a temperature Te between 400°C and 490°C is reached, then - the sheet metal is maintained at temperature Te for a duration De of between 5 and 150 seconds, then - The sheet metal is cooled to room temperature. In particular embodiments, this latter process further comprises one or more of the following characteristics: In addition, a coating of zinc or zinc alloy is applied by vacuum deposition after the cooling stage to ambient temperature. - vacuum deposition is carried out by physical vapor deposition (PVD). Vacuum deposition is performed by Jet Vapor Deposition (JVD). Other features and advantages of the invention will become apparent during the description below, given by way of example and with reference to the attached figures, among which: - Figure 1 shows the microstructure of a steel sheet according to the invention, revealed by a first type of metallographic etch. - Figure 2 shows the microstructure of the steel sheet of Figure 1, revealed by a second type of metallographic etch. - Figure 3 shows an example of precipitation of (Ti, Nb, Mo)(C, N) carbonitrides in a sheet according to the invention, observed by transmission electron microscopy. - Figure 4 shows an example of (Ti, Nb, Mo)(C,N) carbonitride precipitation not corresponding to the invention, observed by transmission electron microscopy. We will also denote by Ac1 the temperature at the start of allotropic transformation when heating steel. We will denote by Ac3 the temperature at the end of the austenitic transformation under heating. Ar3 will denote the temperature at which the transformation of austenite begins upon cooling. Ms will denote the temperature at the start of the martensitic transformation. The microstructure of the sheet according to the invention comprises martensite. This is produced by a diffusion-free transformation of γ-austenite below the martensitic transformation onset temperature Ms upon cooling. Martensite occurs as thin, elongated laths oriented in one direction within each initial austenite grain. The term martensite encompasses both fresh and self-tempered martensite. A distinction will subsequently be made between self-tempered martensite and fresh martensite, that is, martensite that is neither tempered nor self-tempered. In particular, self-tempered martensite occurs as thin laths containing iron carbides dispersed within these laths, in the form of rods oriented along the directions <111> of the mesh a' of the slats. This self-tempered martensite is formed by the precipitation of iron carbides below the martensitic transformation temperature Ms when the cooling is not slow enough to produce fresh martensite. Conversely, fresh martensite does not contain carbides. Bainite, formed during cooling from the austenitic domain above the martensitic transformation onset temperature (Ms), occurs as an aggregate of ferrite laths and cementite particles. Its formation involves short-range diffusion. We will subsequently distinguish between lower bainite and bainite with a low quantity of carbides. Lower bainite is formed during cooling in a temperature range immediately above the martensitic transformation temperature (Ms). It occurs as thin laths and contains carbides dispersed within these laths. Furthermore, bainite containing fewer than 100 carbides per 100 micrometers of surface area will be called low-carbide bainite. Low-carbide bainite is formed during cooling between 550°C and 450°C. In contrast to low-carbide bainite, lower-carbide bainite always contains more than 100 carbides per 100 micrometers of surface area. In the chemical composition of the steel of the invention, carbon plays a role in the formation of the microstructure and in the mechanical properties. The carbon content by weight is between 0.09% and 0.11%. This carbon content range contributes to achieving a tensile strength exceeding 1180 MPa, an elongation at break greater than 7%, and a satisfactory hole expansion ratio (Ac%) of 20% or higher. Notably, a carbon content below 0.09% does not allow for sufficient tensile strength. For higher carbon contents, above 0.11%, weldability tends to decrease and the weld temperature (Ms) is lowered, such that the fraction of fresh martensite in the microstructure tends to increase, thus degrading the hole expansion ratio. The manganese content by weight is between 2.6% and 2.8%. Manganese is a gamma-stabilizing element that lowers the Ac3 temperature and the Ms temperature at which martensite formation begins. The low carbon content of the steel could lead to a high Ac3 temperature, above 860°C. A manganese content above 2.6% allows, by lowering the Ac3 temperature, complete austenization of the steel between approximately 840°C and 855°C, after holding at this temperature for at least 30 seconds. Manganese also allows the formation of self-tempered martensite and therefore contributes to achieving a hole expansion ratio (Ac%) greater than or equal to 20%. The manganese content by weight is limited to 2.8% to limit the formation of banded structures, and preferably between 2.6% and 2.7%. Silicon is an element involved in solid-solution hardening, with a weight content in steel of between 0.20% and 0.55%, preferably between 0.30% and 0.5%. A content of at least 0.30% ensures sufficient hardening of the ferrite and / or bainite. The silicon weight content is limited to 0.55% to guarantee a hole expansion ratio (Ac%) greater than or equal to 20%, thus limiting the formation of upper bainite. Furthermore, an increased silicon content degrades the steel's coating properties by promoting the formation of oxides that adhere to the sheet surface. A silicon content below 0.55% also contributes to ensuring good weldability. Silicon is an alpha-stabilizing element and contributes to raising the Ac3 temperature and promoting the formation of bainite with low carbide content. A silicon content below 0.55% therefore helps to prevent the formation of excessive amounts of low-carbide bainite. The steel sheet composition also includes chromium in an amount greater than or equal to 0.25% by weight, in order to improve the hardenability of the steel and increase its hardness and mechanical strength. The chromium content must be less than 0.5%, in order to maintain a satisfactory elongation at break and to limit manufacturing costs. Titanium is present in steel in quantities ranging from 0.025% to 0.040% by weight. At this concentration, titanium combines primarily with nitrogen and carbon to precipitate as nitrides and / or carbonitrides. Below 0.025%, there is a risk that the required tensile strength of 1180 MPa will not be achieved. Above a titanium content of 0.040%, there is a risk of the formation of coarse titanium nitrides that precipitate in the liquid state. These nitrides tend to reduce ductility and lead to premature damage during hole expansion. Indeed, when nitrides larger than 6 micrometers are present, they are found to be the primary cause of decohesion with the die during the cutting and stamping stages. Titanium also ensures that nitrogen is completely bound as a hardening precipitation, so boron remains in its free form and can play an effective role in hardenability. Titanium is present in a superstoichiometric quantity relative to nitrogen, resulting in a Ti / N ratio greater than 3.42. The boron content by weight is between 0.0015% and 0.0025%. By limiting carbon activity, boron effectively controls and limits diffusive phase transformations (ferritic or pearlitic transformation during cooling) and forms hardening phases (bainite or martensite) necessary for achieving high mechanical strength characteristics. The addition of boron also reduces the need for hardening elements such as manganese, molybdenum, and chromium, thus lowering the analytical cost of the steel grade. According to the invention, the minimum boron content required for effective hardenability is 0.0015%. Above 0.0025%, the effect on hardenability is saturated, and a detrimental effect on coating and ductility is observed. The steel sheet composition also includes molybdenum, in an amount between 0.08% and 0.15% by weight.Molybdenum, like chromium, plays an effective role in hardenability, in a content greater than 0.08%. A molybdenum content exceeding 0.15% delays the recrystallization of ferrite. The mechanical strength Rm is then too high, exceeding 1320 MPa, which also results in a decrease in ductility. When the annealing temperature is below (Ac3-25°C), an addition under the conditions of the invention makes it possible to obtain, in combination with titanium and niobium, a nanometric precipitation of molybdenum, titanium and niobium carbonitrides (Ti, Nb, Mo)(C,N) which contributes to hardening and makes it possible to obtain a resistance Rm greater than or equal to 1180 MPa after annealing. But too dense a precipitation of small compounds leads to excessive hardening: when the density of precipitates smaller than 5 nanometers exceeds 10,000 precipitates / μηη3, the strength Rm can exceed 1320 MPa and the ability to deform at cold becomes insufficient. The addition of molybdenum in the quantity specified by the invention makes it possible to obtain a steel sheet less sensitive to potential fluctuations in the steps following annealing at temperature Tm. Indeed, although the cooling rate VR and the temperature Te are continuously controlled under industrial conditions according to defined setpoints, a slight fluctuation in these parameters can sometimes occur transiently. It is important that these fluctuations do not affect the characteristics of the final product. Within the range defined by the invention, moderate fluctuations in VR or Te, for example of 7%, lead to a variation in mechanical strength Rm of less than 7%. The chemical composition of hot-rolled steel sheet includes niobium, with a weight content between 0.020 and 0.040%. Niobium above 0.020% increases the tensile strength (Rm). Above 0.040%, the recrystallization of austenite is delayed. The structure then contains a significant proportion of elongated grains, making it impossible to achieve the desired hole expansion ratio (Ac%). Furthermore, the nitrogen content by weight is between 0.002% and 0.007%. To form a sufficient quantity of nitrides and carbonitrides, the nitrogen content must be greater than 0.002%. The nitrogen content must be less than 0.007% to prevent the precipitation of boron nitrides, which would reduce the amount of free boron. An aluminum content by weight between 0.005% and 0.18% ensures the deoxidation of steel during its manufacture. The aluminum content must be less than 0.18%, or even less than 0.030%, to prevent a rise in Ac3 temperature and avoid the formation of ferrite during cooling. The sulfur content must be less than 0.005%. Above this level, ductility is reduced due to the excessive presence of sulfides such as MnS, which decrease formability, particularly the hole expansion ratio (Ac%). However, achieving extremely low sulfur content, below 0.0005%, is very costly, without any significant advantage in terms of manufacturing costs. Therefore, from a practical standpoint, a sulfur content of at least 0.0005% can be used. The phosphorus content must be less than 0.020%. Indeed, phosphorus is an element which confers solid solution hardening but which reduces spot weldability and hot ductility, particularly due to its ability to segregate at grain boundaries or to co-segregate with manganese. Obtaining extremely low phosphorus content, below 0.001%, is, however, very costly, without any significant advantage in terms of manufacturing costs. From a practical standpoint, a phosphorus content of at least 0.001% can therefore be considered acceptable. In quantities less than 0.003%, calcium helps to avoid the presence of elongated inclusions, particularly sulfides, which adversely influence ductility. In the microstructure of the steel sheet according to the invention, the sum of the surface proportions of martensite and lower bainite is between 40 and 70%. The microstructure also contains, by surface proportion, 15 to 45% bainite with a low amount of carbides, 5 to less than 20% ferrite, and less than 5% residual austenite in the form of islands. The surface fraction of unrecrystallized ferrite relative to the total ferrite fraction is less than 15%, which makes it possible to simultaneously obtain a tensile strength (Rm) between 1180 and 1320 MPa, an elongation greater than 7%, and a hole expansion ratio greater than or equal to 20%. As previously indicated, a distinction is made between self-tempered martensite and fresh martensite, that is, untempered and non-self-tempered. According to one embodiment, the martensite is formed in particular of self-tempered martensite, the surface percentage of the sum of the self-tempered martensite and the lower bainite representing at least 5% of the whole microstructure, this proportion being able to go up to 50%. Self-tempered martensite and lower bainite occur as thin laths and contain carbides dispersed within these laths. Specifically, self-tempered martensite and lower bainite contain Fe2C and Fe3C iron carbides in the form of rods oriented in the directions <111> of the lattice of martensitic and bainitic laths. The percentages of self-tempered martensite and lower bainite are specified jointly because self-tempered martensite and lower bainite have essentially the same effect on the service properties of the steel. Furthermore, these two constituents, present as thin laths, cannot be individually distinguished from one another by scanning electron microscopy. Only transmission electron microscopy examinations allow these two constituents to be differentiated. A surface percentage of the sum of self-tempered martensite and lower bainite between 40% and 70% promotes the formability of steel, particularly its bendability and its ability to form a cut edge. A self-tempered martensite and lower bainite percentage of at least 40% thus contributes to obtaining a satisfactory bend angle, specifically a bend angle of at least 40° for sheets with a thickness between 0.7 mm and 2 mm, and a hole expansion ratio (Ac%) greater than or equal to 20%. The percentage of the sum of self-tempered martensite and lower bainite in the microstructure must be less than 70% in order to retain a sufficient percentage of low-carbide bainite, allowing an elongation at break of at least 7%. The martensite may also include fresh martensite in a surface percentage of between 15 and 45% of the total microstructure. The surface percentage of fresh martensite must be less than 45%, particularly to avoid reducing the ductility of the steel and to ensure a good hole expansion ratio. The microstructure also includes, on a surface basis, 15 to 45% bainite with a low quantity of carbides. This bainite is formed during cooling after annealing at temperature Tm, and during holding between 550°C and 450°C. Its formation is favored in particular by the addition of silicon, which tends to delay the precipitation of carbides, in conjunction with a small quantity of hardening elements such as carbon or manganese. Low-carbide bainite increases elongation at break. Specifically, a low-carbide bainite surface percentage of at least 15% results in an elongation at break of at least 7%. The low-carbide bainite surface percentage should be limited to 45% to ensure a hole expansion ratio of 20% or higher and a tensile strength of 1180 MPa or higher. The microstructure also includes 5% to less than 20% ferrite by surface proportion. If the ferrite content is less than 5%, there is a risk of excessive self-tempered martensite combined with a low yield strength. If the ferrite content exceeds 20%, there is a risk that the tensile strength (Rm) will be less than 1320 MPa. The microstructure can also contain up to 5%, by surface proportion, of residual austenite in the form of islands, notably forming platelets between the laths of self-tempered martensite and lower bainite. Furthermore, the inventors also highlighted the importance of controlling the size of the austenitic grains created during the annealing of cold-rolled sheet metal, i.e., grains present at high temperature after the holding annealing process, before subsequent cooling. These austenitic grains are referred to as "old austenitic grains" because they are replaced by other constituents during allotropic transformations upon cooling. As will be explained, the size of these old austenitic grains can nevertheless be determined by various methods on the final product. According to the invention, the fraction of old austenitic grains smaller than one micrometer represents between 40 and 60% of the total population of these old austenitic grains. The fraction of old austenitic grains smaller than one micrometer is determined, for example, using a suitable reagent whose rate of attack depends on certain local segregations at the old seams, such as the Béchet-Beaujard reagent, which is known per se. For this purpose, a steel sample in its final state, i.e., after the manufacturing process according to the invention, is attacked with a suitable reagent, in particular a reagent composed of a saturated aqueous solution of picric acid with at least 0.5% sodium alkylsulfonate added, for a period ranging from a few minutes to one hour. Following this attack, a micrographic examination of the sample makes it possible to visualize the joints of the old austenitic grains, and to produce a histogram of the distribution of the size of these old austenitic grains, in particular to determine the fraction of the old austenitic grains whose size is less than one micrometer. Alternatively, the size of the old austenitic grains can be determined by interrupted quenching during cooling after annealing, by adopting initial cooling conditions so as to cause intergranular ferritic nucleation, and then interrupting this by quenching. The inventors demonstrated that the size of these old austenite grains influences the phase transformation kinetics during cooling following annealing. In particular, small austenitic grains, less than one micrometer in size, contribute to lowering the Ms temperature and thus increasing the formation of fresh martensite. Conversely, the presence of large austenitic grains reduces bainite formation at low carbide levels. A fraction of old austenitic grains with a size of less than one micrometer, between 40 and 60% of the total population of austenitic grains, helps to lower the martensitic transformation temperature Ms, to avoid the formation of too large a proportion of self-tempered martensite and lower bainite, which would decrease the elongation and the yield strength. The microstructural characteristics presented above are determined, for example, by observing the microstructure using Scanning Electron Microscopy with a field-effect scanning electron gun (FEG-SEM technique) at a magnification greater than 1200x, coupled with an EBSD (Electron Backscatter Diffraction) detector. The morphologies of the laths and grains are then determined by image analysis using well-known software, such as Aphelion®. The cold-rolled and annealed steel sheet according to the invention can be produced bare, without coating, but it can also be coated. For example, such a coating can be made of zinc or zinc alloy, in particular a galvanized-alloy coating comprising 7 to 12% by weight of iron. In particular, such a steel sheet is well-suited for the application of a metallic coating, especially by hot-dip galvanizing using standard processes. Specifically, the composition and mechanical properties of the steel are compatible with the stresses and thermal cycles of continuous hot-dip zinc coating processes. The coating process used depends on the intended application. In particular, the coating can be obtained by dipping, by a vacuum deposition technique such as JVD (Jet Vapor Deposition), or by cationic electrodeposition. The inventors have highlighted the fact that a steel sheet according to the invention has a mechanical strength of between 1180 and 1320 MPa, together with a yield strength of between 750 and 970 MPa, before any skin-pass operation, an elongation at break of at least 7%, in particular greater than 8%, and a hole expansion ratio Ac% greater than or equal to 20%. In particular, a yield strength between 800 and 970 MPa is achieved while maintaining a tensile strength below 1320 MPa. Furthermore, such a sheet exhibits a high bending angle. Specifically, when the sheet thickness is between 0.7 mm and 2 mm, the bending angle is at least 40°. The implementation of the manufacturing process for a rolled sheet according to the invention comprises the following successive steps: Steel of the composition according to the invention is supplied, and a semi-finished product is cast from this steel. This casting can be carried out in ingots or continuously in the form of slabs with a thickness of approximately 200 mm. The cast semi-finished products are first brought to a temperature TR above 1250 °C, in order to homogenize the steel and completely dissolve the precipitates. Then, the semi-finished product is hot-rolled at a temperature range where the steel structure is fully austenitic, that is, at a temperature TFi_ higher than the Ar3 temperature at which austenite transformation begins upon cooling. If the TFL temperature is lower than the Ar3 temperature, the ferrite grains are work-hardened by the rolling process, and ductility is reduced. Preferably, a rolling completion temperature above 875°C should be chosen. The hot-rolled sheet is cooled at a rate exceeding 30°C / s to prevent the formation of ferrite and pearlite, and then wound at a winding temperature (TBOb) between 500°C and 580°C. The winding temperature must be below 580°C to prevent oxidation during the winding process. A winding temperature that is too low, i.e., below 500°C, leads to an increase in the steel's hardness, which increases the stresses required during subsequent cold rolling. The winding temperature range also helps prevent pearlite formation. The sheet metal is then pickled, using one of the processes known in themselves. Next, a cold rolling process is carried out, with a reduction ratio, for example, between 40% and 70%, in order to introduce an amount of deformation allowing for subsequent recrystallization. The cold-rolled sheet is then heated, preferably in a continuous annealing installation, with an average heating rate Vc between 1 °C / s and 20°C / s between 600°C and the Ad temperature (temperature at which allotropic transformation into austenite begins upon heating). The temperature Ac1 can be measured by dilatometry, or evaluated using the following formula published in "Darstellung der Umwandlungen für technische Anwendungen und Môglichkeiten ihrer Beeinflussung”, HP Hougardy, Werkstoffkunde Stahl Band 1, 198-231, Verlag Stahleisen, Dusseldorf, 1984: Ad = 739 - 22*C - 7*Mn + 2*Si + 14*Cr + 13*Mo - 13*Ni. In this formula, the temperature Ac1 is expressed in degrees Celsius, and the elemental contents of the composition are expressed as percentages by weight. During the heating of steel between 600°C and Ac1, recrystallization begins and (TiNbMo)(CN) precipitates form, which allow control of the size distribution of the austenitic grains formed from Ac1. Surprisingly, the inventors highlighted the fact that controlling the average heating rate Vc between 600°C and Ac1, and thus the heating time between 600°C and Ac1, which corresponds to the time between the start of recrystallization and the start of phase transformation, is crucial for the kinetics of subsequent phase transformations, particularly during the subsequent holding stage at the annealing temperature Tm.The inventors have thus unexpectedly demonstrated that the choice of an average heating rate Vc between 600°C and Ac1 between 1°C / s and 20°C / s allows, at the end of the manufacturing process, to obtain a steel whose microstructure is made up, in surface proportion, of 40 to 70% in sum of martensite and lower bainite, 15 to 45% bainite with a low quantity of carbides, 5 to less than 20% ferrite, and less than 5% residual austenite in the form of islands. In particular, an average heating rate Vc of less than 1°C / s would lead to a heating time that is too long between 600°C and Ac1, therefore to an excessive formation of ferrite and a mechanical resistance that is too low. On the contrary, an average heating rate Vc greater than 20°C / s would lead to a heating time between 600°C and Ac1 that is too short, and to insufficient growth of ferritic grains during heating between 600°C and Ac1. However, the inventors demonstrated that the size of the ferritic grains obtained after heating between 600°C and Ac1 influences the size of the austenite grains after austenization. Insufficient growth of the ferritic grains leads to the formation of an excessive proportion of small austenite grains, and therefore insufficient formation of self-tempered martensite after annealing, i.e., less than 40%, due to the lowering of the Ms temperature. The cold-rolled sheet metal is then heated from temperature Ac1 to an annealing temperature Tm between 780°C and (Ac3-25°C) The temperature Ac3 can be measured by dilatometry or calculated according to the formula: Ac3= 912 - 370- 27.4 Mn +27.3Si -6.35 Cr -32.7 Ni +95.2 V +190T1 +72 Al +64.5 Nb +5.57 W +332 S +276 P+ 485 N -900B +16.2 C Mn +32.3 C Si +15.4 C Cr+ 48 C Ni +4.32 Si Cr - 17.3 Si Mo -18.6 Si Ni +4.8 Mn Ni +40.5 Mo V+ 174 C2+ 2.46 Mn2 -6.86 Si2 +0.322 Cr2+ 9.9 Mo2 +1.24 Ni2 -60.2 V2. In this formula, the temperature Ac3 is expressed in degrees Celsius, and the elemental contents of the composition are expressed as percentages by weight. When the temperature Tm is below 780°C, the density of precipitates of (Ti, Nb, Mo)(CN) is such that hardening can induce an increase in Rm beyond the value of 1320 MPa and a reduced formability. When the temperature Tm is above (Ac3-25°C), the austenitic grain size grows too much so that an excessive amount of lower bainite and self-tempered martensite is formed, at the expense of fresh martensite, and it is no longer possible to achieve a strength Rm of 1180 MPa. The cold-rolled sheet metal is maintained at temperature Tm for a duration Dm between 30 and 150 seconds. The holding time Dm is chosen such that the fraction of austenitic grains smaller than one micrometer represents 40 to 60% of the total austenitic grain population. A holding time Dm of less than 30 seconds would lead to the formation of an excessive proportion of ferrite at the end of the process. The size of the austenitic grains determines the phase transformation kinetics during cooling following annealing. In particular, small austenitic grains, smaller than one micrometer, help lower the temperature Ms and thus reduce the formation of self-tempered martensite. Heating the cold-rolled sheet at an average heating rate Vc between 1°C / s and 20°C / s between 600°C and temperature Ac1, followed by heating the cold-rolled sheet between Ac1 and Tm and maintaining the cold-rolled sheet at temperature Tm for a duration Dm between 50 and 100 seconds, thus allows control of the size of the austenite grains formed, and more particularly control of the fraction of these grains whose size is less than one micrometer. These heating parameters make it possible to obtain the microstructure according to the invention at the end of annealing and thus contribute to obtaining the desired mechanical characteristics. In the case of sheet metal intended for subsequent galvanizing, the steel sheet is then cooled at a rate VR1 of between 10 and 150°C / s to a temperature Te of between 400°C and 490°C. The cooling rate must be greater than 10°C / s in order to form less than 5% ferrite and to avoid the formation of excessive bainite with a low carbide content. In the case of manufacturing a bare sheet, the steel sheet is cooled at a rate VR2 of between 10 and 100°C / s to a temperature Te of between 400°C and 490°C. The cooling can be carried out from the temperature Tri' in a single or multiple stages and in the latter case may involve different cooling methods such as cold or boiling water baths, water jets or gas jets. The sheet metal is then maintained at temperature Te for a duration De of between 5 and 150 seconds. A partial transformation of austenite into bainite with a low amount of carbides occurs at this stage. The holding time at Te must be less than 150s in order to limit the surface proportion of bainite and thus obtain a sufficient proportion of martensite. The subsequent steps in the process differ depending on whether a continuously galvanized steel sheet, particularly galvanized-alloy, is being manufactured, or an uncoated one. According to a first embodiment, corresponding to the continuous production of a galvanized steel sheet, the sheet is coated by continuous immersion in a bath of zinc or zinc alloy at a temperature TZn between 450°C and 480°C for a few seconds. The temperatures Te and TZn are such that 0°C < (Te-TZn)≤10°C. The galvanized product is then cooled to room temperature, transforming a large fraction of the remaining austenite into fresh martensite and / or lower bainite. In the case of manufacturing a cold-rolled, annealed, and galvanized-alloy steel sheet, the galvanized product is heated immediately upon removal from the zinc or zinc alloy bath to a temperature TG between 490 and 550°C for a duration tG between 10 and 40 seconds. This causes the interdiffusion of iron and the thin layer of zinc or zinc alloy deposited during immersion, resulting in a galvanized-alloy sheet. The galvanized-alloy sheet metal is then cooled to room temperature, transforming a large fraction of the remaining austenite into fresh martensite and / or lower bainite. According to a second embodiment, corresponding to the manufacture of an uncoated steel sheet, the sheet is cooled from temperature Te to ambient temperature. According to a third embodiment, corresponding to the manufacture of a coated steel sheet by a vacuum process, as in the second embodiment, from temperature Te, the sheet is cooled to ambient temperature, then a coating of zinc or zinc alloy is deposited under vacuum, for example by physical vapor deposition (PVD) or by a "Jet Vapor Deposition" (JVD) type method. In all the embodiments described above, a cold-rolled and annealed steel sheet is obtained in this way containing, on a surface basis, a sum of 40 to 70% martensite and lower bainite, 15 to 45% bainite with a low quantity of carbides, 5 to less than 20% ferrite, and less than 5% residual austenite in the form of islands. The inventors have demonstrated that the implementation of this process makes it possible to obtain a steel sheet with a mechanical resistance of between 1180 and 1320 MPa, together with a yield strength of between 750 and 970 MPa (before any skin-pass operation), an elongation at break of at least 7%, or even 8%, whose hole expansion ratio Ac% is greater than or equal to 20%. Furthermore, the implementation of this process ensures that the sheet metal has a bending angle of at least 40° when the sheet metal has a thickness between 0.7 mm and 2 mm. Furthermore, the sheet metal obtained exhibits good weldability using conventional assembly processes such as resistance spot welding. By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention. We supplied semi-finished steel products whose compositions, expressed in weight percentages (%) are presented in Table 1 below. In addition to the steels 11 to 13 used in the manufacture of sheets according to the invention, the composition of steels R1 to R5 used in the manufacture of reference sheets has been indicated for comparison purposes. We measured by dilatometry, or calculated the temperature Ac3 using the formula mentioned above and reported these values ​​in Table 1. Table 1 - Steel compositions - Temperature Ac3 Underlined values: not in accordance with the invention. Cast semi-finished products corresponding to the above compositions were heated to a temperature TR above 1250°C, then hot rolled, the final rolling temperature being equal to 850°C, above Ar3 for all these steels. The hot-rolled sheets were cooled, avoiding the formation of ferrite and pearlite, and then coiled at a temperature of 545°C. The sheets were then cold-rolled to a thickness of 1.4 mm. In a first set of tests, sheets 11 were then heated with a heating rate Vc, between 600°C and Ac1, Ac1 designating the temperature of onset of austenitic transformation upon heating, then from Ac1 to a temperature Tm and maintained at temperature Tm for a duration Dm. The sheets were cooled at a rate VR1 to a temperature Te, then maintained at temperature Te for a duration De. In a particular test (11F), the sheet 11 was heated to 810°C, held for 120s at this temperature, then cooled by water quenching with a rate greater than 100°C / s to ambient temperature, i.e. without a holding step at temperature Te. These tests were carried out under different treatment conditions (A to H), indicated in Table 2. In this table, "na" means: not applicable. Indeed, under treatment F, no holding at a temperature Te is performed, so a relevant holding time De cannot be defined. According to a second set of tests, the R1-R5 grades were treated under conditions defined in Table 3. The sheets manufactured under all the above conditions were then coated by continuous dip coating in a zinc bath at 460°C and then immediately heated to 510°C to obtain a galvanized-alloy coating. Table 2: Test conditions carried out on grade 11 Underlined values: not in accordance with the invention. Table 3: Test conditions carried out on grades 12, 13 and R1 to R5 The mechanical characteristics of the resulting sheets, based on the steel composition and heat treatment, were determined. By convention, the sheets were identified by combining the chemical composition and heat treatment designations: thus, MA designates the sheet obtained from composition 11 to which the heat treatment conditions A were applied. Using tensile tests, the yield strength Re, tensile strength Rm, and total elongation At of the sheets obtained by these different manufacturing methods were determined. The bendability of these sheets was also determined by calculating the maximum angle before failure. This angle is determined by applying a punch to the sheet metal to bend it. The force required to bend the sheet increases until it breaks. Measuring the force applied during bending allows detection of the onset of breakage and measurement of the maximum bend angle at which this breakage occurs. The hole expansion ratio Ac% of each sheet was determined by cutting a 10 mm diameter hole into the sheet and then expanding the edges of this hole using a conical tool. As described in ISO 16630:2009, the initial diameter Di of the hole before stamping was measured, and then the final diameter Df of the hole after stamping, at the point where through-cracks were observed in the sheet thickness at the edges of the hole. The hole expansion capacity Ac% was determined according to the following formula: / \c% = 100* Pf ~ P . D, The characteristics of the microstructure of the steels were also determined. The surface fractions of martensite (including self-tempered and fresh martensite) and lower bainite (jointly), self-tempered martensite and lower bainite (jointly), and low-carbide bainite were quantified on polished sections etched with sodium bisulfite. The surface fraction of fresh martensite was quantified after etching with a NaOH-NaNO3 reagent. The surface fraction of ferrite was also determined through optical and scanning electron microscopy observations where the ferritic phase was identified. The nature, size and density of the precipitates on thin section were also observed using transmission electron microscopy. Details concerning the microstructures of the sheets are shown in Table 4 below. Density fraction of (Martensite Bainite to (Martensite grains (Ti,Nb,Mo)(C self-tempering Low martensite +Bainite Austenite Austenitic Ferrite N) of size<5 + fresh bainite quantity of lower) (%) (%) s of which nm < lower) (%) carbides (%) size is <1 <10000 / μ3? (%) (%) micron 50 5 45 43 0 7 50 Yes MY I1 B 40 10 30 5_Z 0 3 15 Yes I1C 60 45 15 25 0 15 nd Yes 1 D 22 2 20 38 0 40 10 Yes ME 25 5 20 35 0 40 10 Yes MF 93 78 15 0 2 5 50 Yes I1G 40 3 37 35 0 25 70 Yes 11:40 0 40 25 2 33 80 No I2J 40 5 35 45 0 15 60 Yes I3I 70 30 40 15 0 15 55 Yes R1 B 95 81 14 3 0 2 5 Yes R2B 60 30 30 40 0 ​​0 10 Yes R3B 7.5 45 30 25 0 0 10 Yes R3C 60 30 30 38 0 2 15 Yes R4J 25 0 25 40 0 ​​35 60 Yes R5B 50 25 25 50 0 0 5 Yes R5C 45 23 22 4Z 0 8 7 Yes Table 4 - Microstructural characteristics of the sheets obtained Underlined values: not in accordance with the invention. n / a: not determined The mechanical properties of the sheet metal are shown in Table 5 below. Angle of Re (MPa) Rm (MPa) At (%) Ac% folding (°) MA 793 1210 7.5 57 20 MB 803 1166 8 nd 25 I1C 803 1253 9.5 60 nd I1 D 621 1063 9.2 nd 25 ME 636 1069 9.2 nd 25 MF 1054 1370 6J nd 37 I1G 756 1237 10.1 nd 18 I1 H 856 1327 7.5 nd 5, I2J 763 1206 11 25 22 13 I 770 1270 8.1 44 21 R1 B 930 1353 7.3 56 43.9 R2B 800 1116 8 71 nd R3B 821 1131 7.6 56 nd R3C 798 1165 8.6 nd 37 R4J 669 1095 13.4 63 28 R5B 790 1151 6_Z 63 nd R5C 720 1137 10.4 nd 30 Table 5 - Mechanical properties of the obtained sheets nd: not determined. This highlights the relationships between the compositions of steels, their microstructure and their mechanical properties. The steel sheets according to the invention have a composition and microstructure which make it possible to obtain a mechanical strength, a yield strength, an elongation, a bending angle and a hole expansion ratio which satisfy the values ​​referred to. Figures 1 and 2 illustrate the microstructure of Example 11A. Figure 1 shows the result of etching the sheet metal with sodium bisulfite, while Figure 2 illustrates the result of etching the sheet metal with the NaOH-NaNO3 reagent. Figure 1 shows self-tempered martensite and lower bainite (M+BI), as well as low-carbide bainite (LCB). Figure 2 shows fresh martensite (FM) as darker areas. In the MA test, TEM observations (Figure 3) reveal the presence of carbonitrides (Ti, Nb, Mo)(CN) with an average size of 7 nm, in quantities less than 10,000 precipitates / Mm³, thus achieving all the desired mechanical properties. These observations also highlight the presence of carbides in the form of rods oriented in the directions <111> in the laths of self-tempered martensite and lower bainite. The low-carbide bainite contains less than 100 carbides per unit area of ​​100 square micrometers. In the MB test, the annealing temperature Tm is too close to AC3, resulting in a low carbon content in the austenite. Excessive bainite with low carbide content forms during cooling and holding at temperature Te. This results in insufficient mechanical strength. In tests 11D and 11E, the heating rate Vc is too low. This results in excessive ferrite grain growth. Consequently, too much ferrite remains in the steel, and there is insufficient martensite or lower bainite. Therefore, the required mechanical strength Rm is not achieved, even with holding times Dm of 60 seconds, as in example ME. In the MF test, the VR cooling rate is too high. As a result, too much martensite and lower bainite are formed, and not enough low-carbide bainite and fresh martensite. Consequently, the mechanical strength and yield strength are well above the target values, while the elongation is too low. The holding time Dm at temperature Tm of treatment G is too short, so the sheet produced under condition 11 G exhibits a ferrite recrystallization rate of less than 15%. This generates a banded structure which leads to a low hole expansion value. In test 11H, the excessively low annealing temperature led to an excessively high density of small precipitates: TEM observations (Figure 4) revealed an average size of 5 nm in a quantity exceeding 10,000 precipitates / pm³, resulting in a mechanical strength exceeding 1320 MPa. Furthermore, the low annealing temperature Tm prevented a ferrite recrystallization rate above 15%, leading to an excessively low hole expansion value. In tests 11G and 11H, the fraction of austenitic grains smaller than 1 μm is too high, resulting in an excessively low Ms temperature and, consequently, insufficient formation of lower bainite plus self-tempered martensite. This contributes to a reduced hole expansion, which is too low for both tests. The sheet metal according to example R1B has excessively high levels of C, Cr, Ti, and B, resulting in an excessively high tensile strength (Rm) despite an insufficient amount of Mo. Thus, although a satisfactory yield strength (Re) is achieved, this is obtained in conjunction with an excessively high tensile strength (Rm). The sheets according to tests R2B, R3B, and R3C contain insufficient levels of C, Mn, and Mo, and therefore do not exhibit satisfactory tensile strength despite very high levels of B and Cr. Example R5 has too low a Mn content, which leads to too much bainite formation with low amounts of carbides according to treatments B and C. The steel sheets according to the invention will be used profitably for the manufacture of structural or safety parts in the automotive industry.

Claims

DEMANDS 1. Cold-rolled and annealed steel sheet with a thickness between 0.7 mm and 2 mm, a mechanical strength between 1180 MPa and 1320 MPa, a hole expansion ratio Ac% greater than 20%, and a bend angle greater than or equal to 40°, the chemical composition of which includes, the contents being expressed by weight: 0.09 < C ≤ 0.11% 2.6 < Mn < 2.8% 0.20 < If < 0.55% 0.25 ≤ Cr < 0.5% 0.025 ≤ Ti ≤ 0.040% 0.0015 < B ≤ 0.0025% 0.005 < Al < 0.18% 0.08 < MB < 0.15% 0.020 ≤ Nb < 0.040% 0.002 < N < 0.007% 0.0005% ≤ S < 0.005% 0.001% < P < 0.020%, Ca < 0.003% the remainder being made up of iron and unavoidable impurities from the processing, the sheet having a microstructure made up of martensite and / or lower bainite, said martensite comprising fresh martensite and / or self-tempered martensite, the sum of the surface proportions of martensite and lower bainite being between 40 and 70%, of 15 to 45% in surface proportion of bainite with low quantity of carbides, of 5 to less than 20% in surface proportion of ferrite, the fraction of non-recrystallized ferrite relative to the total ferrite fraction being less than 15%, and less than 5% in surface proportion of residual austenite in the form of islands, the fraction of old austenitic grains whose size is less than one micrometer representing 40 to 60% of the total population of said old austenitic grains. 2 - Steel sheet according to claim 1, characterized in that said microstructure comprises in surface proportion of 15 to 45% of fresh martensite.

3. Steel sheet according to claim 1, characterized in that said microstructure comprises, in a surface proportion of 5 to 50%, a sum of self-tempered martensite and lower bainite.

4. Steel sheet according to claim 3, characterized in that said self-tempered martensite and said lower bainite contain carbides in the form of rods oriented in the directions <111> martensitic and bainitic laths.

5. Steel sheet according to any one of claims 1 to 4, characterized in that said low-carbide bainite contains fewer than 100 carbides per unit area of ​​100 square micrometers.

6. Steel sheet according to any one of characteristics 1 to 5, characterized in that it contains precipitates of the type (Ti, Nb, Mo)(C, N) with a size less than 5 nanometers, present in a quantity of less than 10,000 precipitates / Mm³.

7. - Steel sheet according to any one of claims 1 to 6, characterized in that the chemical composition comprises, the content being expressed by weight: 2.6 < Mn ≤ 2.7%.

8. - Steel sheet according to any one of claims 1 to 7, characterized in that the chemical composition comprises, the content being expressed by weight: 0.30 < Si≤ 0.5%.

9. - Steel sheet according to any one of claims 1 to 8, characterized in that the chemical composition comprises, the content being expressed by weight: 0.005 < Al < 0.030%.

10. - Steel sheet according to any one of claims 1 to 9, characterized in that said sheet comprises a coating of zinc or zinc alloy, obtained by quenching.

11. - Steel sheet according to claim 10, characterized in that said zinc or zinc alloy coating is a galvanized-alloy coating, said zinc or zinc alloy coating comprising 7 to 12% by weight of iron.

12. - Steel sheet according to any one of claims 1 to 9, characterized in that said sheet comprises a coating of zinc or zinc alloy, obtained by vacuum deposition.

13. A process for manufacturing a cold-rolled and annealed sheet according to any one of claims 1 to 11, comprising the following successive steps: - a semi-finished product is supplied, the chemical composition of which comprises, the contents being expressed by weight: 0.09 < C ≤ 0.11% 2.6 ≤ Mn < 2.8% 0.20 < Si ≤ 0.55% 0.25 ≤ Cr < 0.5% 0.025 < Ti < 0.040% 0.0015 ≤ B ≤ 0.0025% 0.005 ≤ Al ≤ 0.18% 0.08 ≤ Mo ≤ 0.15% 0.020 ≤ Nb < 0.040% 0.002 < N < 0.007% 0.0005% ≤ S < 0.005% 0.001% ≤ P < 0.020%, Ca ≤ 0.003% the remainder being made up of iron and unavoidable impurities from the manufacturing process, then - the said semi-product is heated to a temperature Tr greater than or equal to 1250°C, then - the said semi-finished product is hot-rolled, the rolling end temperature being higher than the Ar3 temperature at the start of austenite transformation upon cooling, to obtain a hot-rolled sheet, then - the hot-rolled sheet is cooled at a rate greater than 30°C / s to prevent the formation of ferrite and pearlite, then - the said hot-rolled sheet is coiled at a temperature between 580° and 500°C, then - the hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet, then - the cold-rolled sheet is heated to between 600°C and Ac1, where Ac1 is the temperature at which austenitic transformation begins upon heating, with a heating rate Vc between 1 and 20°C / s, then - The cold-rolled sheet is heated to a temperature Tm between 780°C and (Ac3-25°C), and held at said temperature Tm for a duration Dm between 30 and 150 seconds, it being understood that Ac3 designates the temperature at the end of the austenitic transformation upon heating, then - the sheet is cooled at a rate VR1 between 10 and 150°C / s to a temperature Te between 400 and 490°C, then - the sheet metal is maintained at temperature Te for a duration De of between 5 and 150 seconds, then - the sheet metal is coated by continuous immersion in a bath of zinc or zinc alloy at a temperature TZn between 450°C and 480°C, said temperatures Te and TZn being such that 0< (Te-TZn)≤10°C, so as to obtain a coated sheet metal, then - Optionally, the said coated sheet is heated to a temperature TG between 490°C and 550°C for a duration tG between 10 s and 40 s. 14.- A method for manufacturing a cold-rolled and annealed sheet according to any one of claims 1 to 9 and 12, comprising the following successive steps: - We supply a semi-finished product whose chemical composition includes, with the contents expressed by weight: 0.09 < C < 0.11% 2.6 < Mn ≤ 2.8% 0.20 ≤ Si ≤ 0.55% 0.25 < Cr < 0.5% 0.025 < Ti < 0.040% 0.0015 < B 0.0025% 0.005 < Al < 0.18% 0.08 < Mo≤ 0.15% 0.020 ≤ Nb < 0.040% 0.002 ≤ N ≤ 0.007% 0.0005% < S < 0.005% 0.001% ≤ P ≤ 0.020% Ca ≤ 0.003% the remainder being made up of iron and unavoidable impurities from the manufacturing process, then - the said semi-product is heated to a temperature Tr greater than or equal to 1250°C, then - the said semi-finished product is hot-rolled, the final rolling temperature being greater than Ar3, to obtain a hot-rolled sheet, then - The hot-rolled sheet is cooled at a rate exceeding 30°C / s to prevent the formation of ferrite and pearlite, then - the hot-rolled sheet is coiled at a temperature between 500 and 580°C, then The hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet, then - the said cold-rolled sheet is heated with a heating rate VR, between 600°C and Ac1, where Ac1 designates the temperature at which austenitic transformation begins upon heating, between 1 and 20°C / s, then - the cold-rolled sheet is heated to a temperature Tm between 780 °C and (Ac3-25 °C), and the cold-rolled sheet is maintained at said temperature Tm for a duration Dm between 30 and 150 seconds, it being understood that Ac3 designates the temperature at the end of the austenitic transformation upon heating, then - the sheet metal is cooled at a rate VR2 between 10 and 100°C / s until a temperature Te between 400°C and 490°C is reached, then - the said sheet metal is maintained at temperature Te for a duration De of between 5 and 150 seconds, then the sheet metal is cooled to room temperature.

15. - Method of manufacturing a cold-rolled, annealed and coated sheet according to claim 14, wherein a coating of zinc or zinc alloy is further carried out by vacuum deposition after said cooling step to ambient temperature.

16. - Method of manufacturing a sheet according to claim 14, characterized in that said vacuum deposition is carried out by physical vapor deposition (PVD).

17. - Method of manufacturing a sheet according to claim 14, characterized in that said vacuum deposition is carried out by Jet Vapor Deposition (JVD). 18 - Use of a steel sheet according to any one of claims 1 to 12, or manufactured by a process according to any one of claims 13 to 17, for the manufacture of structural or safety parts for motor vehicles