High strength and ductility steel sheet and a method for manufacturing it

Abstract

Cold rolled and heat treated steel sheet, having a composition including (in weight percent): 0.10%≤C≤0.25%, 3.5%≤Mn≤6.0%, 0.5%≤Si≤2.0%, 0.3%≤Al≤1.2%, with Si+Al≥0.8%, 0.10%≤Mo≤0.50%, S≤0.010%, P≤0.020%, N≤0.008%, said cold rolled steel sheet having a microstructure (in surface fraction) including: Between 10% and 45% ferrite, with an average grain size of maximum 1.3 m, the product of the surface fraction of ferrite and the average grain size of ferrite is maximum 35 m%, between 8% and 30% retained austenite, said retained austenite has a Mn content greater than 1.1*Mn%, Mn% indicates the Mn content of the steel, maximum 8% fresh martensite, maximum 2.5% cementite and partitioned martensite.

Description

High strength and ductility steel sheet and a method for manufacturing it The present invention relates to a method for manufacturing a high-strength steel sheet with high ductility and formability, and to the sheet obtained by this method. For the manufacture of various equipment such as structural body parts and body panels for automobiles, it is known to use sheets made from DP (dual phase) steels or TRIP (transformation induced plasticity) steels. To reduce the weight of automobiles in order to improve their fuel efficiency from an environmental point of view, it is desirable to have sheets with improved yield and tensile strengths. But such sheets must also have good ductility and formability, and very specifically good tensile flangeability. To solve this problem, the production of sheets by the quenching and partitioning process is known, in which the sheets are cooled from a annealing temperature to a quenching temperature below the Ms transformation point, and then heated to the partitioning temperature and held at this temperature for a given period of time. The resulting steel sheets have a structure consisting of martensite and retained austenite, and optionally bainite and / or ferrite. However, producing steel sheet or part with an improved combination of strength, ductility, and formability is desirable. In particular, it is desirable to produce a steel sheet having a yield strength YS between 1000 MPa and 1300 MPa, and a tensile strength TS between 1200 MPa and 1600 MPa, a uniform elongation UE of greater than or equal to 10%, a total elongation TE of greater than or equal to 14%, a hole expansion ratio HER of at least 20%, and the sum of the product of the yield strength times the uniform elongation UE (YS*UE), the product of the tensile strength TS times the total elongation TE (TS*TE), and the product of the tensile strength TS times the hole expansion ratio HER, (TS x HER), YS*UE + TS*TE + TS*HER, is at least 56,000 MPa%. The yield strength YS, tensile strength TS, uniform elongation UE and total elongation TE are measured according to ISO 6892-1, published in October 2009. It is emphasized that, due to differences in measurement methods, in particular due to differences in the specimen geometries used, the total elongation TE values ​​according to ISO are significantly different, and in particular smaller than the total elongation values ​​measured by JIS Z 2241 using a specimen according to Z 2201-05. The hole expansion ratio HER is measured according to ISO 16630:2009. Due to differences in measurement methods, the hole expansion ratio HER values ​​according to ISO 16630:2009 are very different and are not comparable with the hole expansion ratio λ values ​​according to JFS T 1001 (Japan Iron and Steel Federation Standard). Therefore, this invention relates to a cold-rolled and heat-treated steel sheet made of steel having the following composition in weight percent: 0.10% ≤ C ≤ 0.25% 3.5% ≤ Mn ≤ 6.0% 0.5% ≤ Si ≤ 2.0% 0.3% ≤ Al ≤ 1.2% with Si+Al ≥ 0.8% 0.10% ≤ Mo ≤ 0.50% S ≤ 0.010% P ≤ 0.020% N ≤ 0.008% and optionally one or more elements selected from Cr, Ti, Nb, V and B, such as: 0.01% ≤ Cr ≤ 1.0% 0.010% ≤ Ti ≤ 0.080% 0.010% ≤ Nb ≤ 0.080% 0.010% ≤ V ≤ 0.30% 0.0005% ≤ B ≤ 0.004%, The remainder is a combination of iron and inevitable impurities resulting from smelting, The aforementioned cold-rolled steel sheet has a microstructure consisting of (as a surface fraction): - Between 10% and 45% ferrite, with an average grain size of up to 1.3 m, the product of the surface fraction of ferrite and the average grain size of ferrite is up to 35 m%, - Between 8% and 30% retained austenite, said retained austenite has a Mn content greater than 1.1*Mn%, Mn% indicates the Mn content of the steel, - Maximum 8% fresh martensite, - Maximum 2.5% cementite and - Partitioned martensite. Preferably, the retained austenite has an average C content of at least 0.4%. In one example, the fresh, partitioned martensite contains carbides, the surface density of carbides having a surface area greater than 10x60nm2 is less than 0.05x106 / mm2. Preferably, the retained austenite is in the form of islands, the retained austenite islands having an average size of less than 500 nm. In one sample, the structure contains a maximum of 0.3% cementite, the cementite particles, if present, having an average size of less than 50 nm. In general, the cold-rolled and heat-treated steel sheet has a yield strength YS between 1000 MPa and 1300 MPa, a tensile strength TS between 1200 MPa and 1600 MPa, a uniform elongation US of at least 10%, a total elongation TE, measured according to ISO 6892-1, of at least 14%, a hole expansion ratio HER, measured according to ISO 16630:2009, of at least 20%, and the sum of the product of the yield strength YS by the uniform elongation UE (YS*UE), the product of the tensile strength TS by the total elongation TE (TS*TE), and the product of the tensile strength TS by the hole expansion ratio HER (TS x HER), YS*UE + TS*TE + TS*HER, is at least 56000 MPa%. In one example, cold-rolled and heat-treated steel sheet is plated with Zn or a Zn alloy. In another example, cold-rolled and heat-treated steel sheet is coated with Al or an Al alloy. Preferably, the steel has an equivalent carbon Ceq of less than 0.4%, the equivalent carbon being defined as Ceq = C+Si% / 55 + Cr% / 20 + Mn% / 19 – Al% / 18+ 2.2*P% – 3.24*B% - 0.133Mn% * Mo%. The invention further relates to a resistance spot welded joint of at least two steel sheets, wherein one of said two steel sheets is a cold rolled and heat treated steel sheet according to the invention. Preferably, the resistance spot welded joint has an alpha value, before any post-weld heat treatment, of at least 50 daN / mm2. Preferably, the resistance spot welded joint has an alpha value of at least 70 daN / mm2, especially after post-weld heat treatment. The invention also relates to a method for manufacturing a cold-rolled and heat-treated steel sheet, comprising the following sequential steps: - Molding the steel to obtain a slab, said steel having a composition including (in weight percentage): 0.10% ≤ C ≤ 0.25% 3.5% ≤ Mn ≤ 6.0% 0.5% ≤ Si ≤ 2.0% 0.3% ≤ Al ≤ 1.2% with Si+Al ≥ 0.8% 0.10% ≤ Mo ≤ 0.50% S ≤ 0.010% P ≤ 0.020% N ≤ 0.008% and optionally one or more elements selected from Cr, Ti, Nb, V and B, such as: 0.01% ≤ Cr ≤ 1.0% 0.010% ≤ Ti ≤ 0.080% 0.010% ≤ Nb ≤ 0.080% 0.010% ≤ V ≤ 0.30% 0.0005% ≤ B ≤ 0.004%, The remainder is a combination of iron and inevitable impurities resulting from smelting. - Reheating the slab to a Treheat temperature between 1150°C and 1300°C, - Hot rolling of heated slabs at temperatures higher than Ar3 to obtain a hot rolled steel sheet, - Coiling of hot rolled steel sheet at a coiling temperature Tcoil between 20°C and 600°C, - Hot-rolled steel sheet is subjected to an initial annealing temperature TA1 between 500°C and TA1max, TA1max being the temperature at which a maximum of 30% austenite is formed upon heating. The hot-rolled steel sheet is held at the initial annealing temperature TA1 for a period of between 3 seconds and 50,000 seconds, to obtain a hot-rolled and annealed steel sheet. - Cold rolling of hot-rolled and warped steel sheet to obtain a cold-rolled steel sheet, - Reheating the cold-rolled steel sheet to a secondary annealing temperature TA2 between Ae1 and Ae3 and holding the cold-rolled steel sheet at the secondary annealing temperature TA2 for a holding time tA between 30 seconds and 500 seconds, to obtain a structure of between 55 and 90% austenite and between 10% and 45% ferrite upon annealing. - Quenching of cold-rolled steel sheet at a cooling rate Vc between 1°C / s and 100°C / s, to a quenching temperature QT between 20°C and Ms-50°C, - reheating the cold-rolled steel sheet to a partitioning temperature TP between 350°C and 500°C, and maintaining the cold-rolled steel sheet at said partitioning temperature TP for a partitioning time tP between 3 seconds and 1000 seconds, - Cooling the cold-rolled steel sheet to room temperature to obtain a cold-rolled and heat-treated steel sheet. Preferably, the cold rolled and heat treated steel sheet has a structure comprising (as a surface fraction): - At least 67% ferrite, with an average grain size of less than 4 µm, - Maximum 30% retained austenite, - maximum 2% fresh martensite, and - Maximum 3% cementite. In one example, a cold-rolled, heat-treated steel sheet has a microstructure consisting of (as a surface fraction): - Between 10% and 45% ferrite, with an average grain size of up to 1.3 m, the product of the surface fraction of ferrite and the average grain size of ferrite is up to 35 m%, - Between 8% and 30%, retained austenite, said retained austenite has a Mn content greater than 1.1*Mn%, Mn% indicates the Mn content of the steel, - Maximum 8% fresh martensite, - Maximum 2.5% cementite and - Partitioned martensite. In general, retained austenite has an average C content of at least 0.4%. In the first example, the bending performed on the hot-rolled steel sheet is a batch bending, the initial bending temperature TA1 is between 500°C and 670°C, the hot-rolled steel sheet is held at said initial bending temperature TA1 for a period of between 1000 seconds and 50000 seconds. In this example, the hot-rolled and annealed steel sheet generally has a microstructure consisting of (as a surface fraction): - At least 75% ferrite with an average grain size of less than 4 µm, -Maximum 10% retained austenite -Maximum 2% fresh martensite, and - Maximum 3% cementite, The retained austenite has a Mn content greater than 1.5*Mn%, where Mn% represents the amount of Mn in the steel. In the second example, the rolling performed on the hot-rolled steel sheet is a continuous rolling, the first rolling temperature TA1 is between 650°C and the maximum continuous rolling temperature TICAmax, which is the temperature at which 30% of austenite is formed upon heating, the hot-rolled steel sheet is held at said first rolling temperature TA1 for a period of between 3 seconds and 500 seconds. In this example, hot-rolled and annealed steel sheet typically has a structure consisting of (as a surface fraction): - At least 67% ferrite, with an average grain size of less than 4 µm, - Maximum 30% austenite, - maximum 2% fresh martensite, and - Maximum 1% cementite, cementite particles, if present, have an average size of less than 150 nm. Cold rolled and heat treated steel sheet preferably has a microstructure consisting of (as a surface fraction): - Between 10% and 45% ferrite, with an average grain size of up to 1.3 m, the product of the surface fraction of ferrite and the average grain size of ferrite is up to 35 m%, - Between 8% and 30% retained austenite, said retained austenite has a Mn content greater than 1.1*Mn%, Mn% indicates the Mn content of the steel, - Maximum 8% fresh martensite, - maximum 0.3% cementite, cementite particles, if present, have an average size of less than 50 nm, and - Partitioned martensite. In one example, between holding at the partitioning temperature TP and cooling to room temperature, cold-rolled steel sheet is hot-dip coated in a bath. In another example, after holding the cold-rolled sheet at the partitioning temperature TP, the cold-rolled steel sheet is immediately cooled to room temperature. In this example, after the step of cooling the cold-rolled steel sheet to room temperature, the cold-rolled and heat-treated steel sheet may be plated by electrochemical method or through a vacuum plating process. In one example, cold-rolled and heat-treated steel sheet is plated with Zn or Zn alloy. In another example, cold-rolled and heat-treated steel sheet is coated with Al or Al alloy. Preferably, the steel has an equivalent carbon Ceq of less than 0.4%, the equivalent carbon being defined as Ceq = C+Si% / 55 + Cr% / 20 + Mn% / 19 – Al% / 18+ 2.2*P% – 3.24*B% - 0.133Mn% * Mo%. The invention also relates to a process for producing a spot welded joint of at least two steel sheets, comprising the following steps: - Providing cold-rolled and heat-treated steel sheet according to the invention or produced by the method according to the invention, - Preparing the second steel sheet, - Spot welding of a cold-rolled and heat-treated steel sheet to a second steel sheet. For example, the second steel sheet is a cold-rolled and heat-treated steel sheet according to the invention or produced by the method according to the invention. The invention will now be described in detail and illustrated by examples without limitation. In the following, Ae1 represents the equilibrium transformation temperature below which austenite is completely unstable, Ae3 represents the equilibrium transformation temperature above which austenite is stable throughout its entire length, Ar3 represents the temperature at which the transformation of austenite to ferrite begins upon cooling, Ms represents the martensite onset temperature, i.e., the temperature at which austenite begins to transform to martensite upon cooling, and Mf represents the martensite completion temperature, i.e., the temperature at which the transformation from austenite to martensite is complete upon cooling. For a given steel, a person of ordinary skill in the art knows how to determine these temperatures by dilatometric tests. The composition of the steel according to the invention includes (in weight percent): - 0.10% ≤ C ≤ 0.25% to ensure satisfactory strength and improve the stability of retained austenite, which is necessary to achieve sufficient elongation. Preferably, the carbon content is greater than or equal to 0.15%. If the carbon content is too high, the hot rolled sheet becomes too hard for cold rolling and has insufficient weldability. If the carbon content is below 0.10%, the tensile strength will not reach the target values. - 3.5% ≤ Mn ≤ 6.0% to ensure satisfactory strength and to obtain stabilization of at least part of the austenite, to obtain sufficient elongation. In particular, the minimum is defined to obtain a final structure comprising (as a surface fraction) between 8% and 30% retained austenite, which has a Mn content greater than 1.1*Mn%, Mn% representing the Mn content of the steel. Below 3.5%, the final structure will contain an insufficient amount of retained austenite, and an insufficient amount of Mn in the retained austenite, so that the desired combination of ductility and strength is not achieved. The maximum is to avoid having segregation phenomena detrimental to ductility. Preferably, the manganese content is greater than or equal to 3.7%. - 0.5% ≤ Si ≤ 2.0% and 0.3% ≤ Al ≤ 1.2%, the silicon and aluminum values ​​additionally satisfy the following relationship: Si+Al ≥ 0.8%. According to the invention, Si and Al together play an important role: silicon delays the precipitation of cementite upon cooling below the equilibrium transformation temperature Ae3. Si therefore helps to stabilize a sufficient amount of retained austenite. Si furthermore provides solid solution hardening and slows down the formation of carbides during the redistribution of carbon from martensite to austenite resulting from the reheating and holding step carried out after partial martensite transformation. At too high a level, silicon oxides are formed on the surface, which is detrimental to the coatability of the steel. Therefore, the Si content is less than or equal to 2.0%. Aluminum is an effective element for deoxidizing steel in the liquid phase during fabrication. In addition, Al is a gamma-generating element that increases the Ae1 and Ae3 temperatures of steel. Therefore, due to the addition of at least 0.3% Al, the range between the critical range (i.e. between Ae1 and Ae3) is in a favorable temperature range for Mn partitioning in austenite, as will be explained in detail below. The amount of Al is not more than 1.2% to avoid the occurrence of intrusion, to avoid oxidation problems and to ensure the hardenability of the material. In addition, similar to Si, Al stabilizes retained austenite. The effects of Si and Al on the stabilization of retained austenite are similar. When the amounts of Si and Al are such that Si+Al ≥ 0.8%, satisfactory stabilization of austenite is achieved, thereby enabling the achievement of desirable microstructures. - 0.10% ≤ Mo ≤ 0.50%. Molybdenum increases hardenability, stabilizes retained austenite and thus reduces decomposition of retained austenite during parting, and reduces the core segregation caused by high manganese content and detrimental to the hole expansion ratio. In addition, Mo helps to refine the structure. Above 0.50%, adding Mo is costly and ineffective from the point of view of the properties to be considered later. - Optionally 0.01% ≤ Cr ≤ 1.0% delays the decomposition of carbides and stabilizes retained austenite. A maximum of 1.0% Cr is allowed, above which its effect becomes saturated, and adding Cr is both useless and expensive. - Optionally 0.010% ≤ Nb ≤ 0.080%, in order to refine the austenite grains during hot rolling and provide precipitation hardening. Niobium content of 0.010% to 0.080% allows to obtain satisfactory yield strength, increase in elongation and hole expansion ratio. Above 0.080%, ductility and hole expansion ratio are not satisfactory. - Optionally 0.010% ≤ Ti ≤ 0.080%. In particular, titanium may be added in an amount between 0.010% and 0.080% in addition to the bromine to protect the bromine against BN formation. The amounts of Nb and Ti are not more than 0.080% each to limit the hardening of the steel at high temperatures provided by these elements, which makes it difficult to produce thin sheets due to the increased hot rolling forces. Optionally, 0.010% ≤ V ≤ 0.30% to provide precipitation hardening. If the vanadium content is above 0.30%, the vanadium consumes carbon by forming carbides and / or carbonitrides and this softens the martensite. In addition, the ductility of the steel according to the invention is reduced. Optionally 0.0005% ≤ B ≤ 0.004%, to increase the quenchability of the steel. The remainder is a combination of iron and impurities resulting from the smelting process. In this context, Ni, Cu, S, P and N are considered as at least residual elements, which are unavoidable impurities. Therefore, their values ​​are less than 0.05% for Ni, 0.03% for Cu, 0.010% for S, 0.020% for P and 0.008% for N. Preferably, the composition of the steel is such that the steel has an equivalent carbon Ceq of less than 0.4%, the equivalent carbon being defined as Ceq = C+Si% / 55 + Cr% / 20 + Mn% / 19 – Al% / 18+ 2.2*P% – 3.24*B% - 0.133Mn% * Mo%. With a carbon equivalent of less than 0.4%, the spot weldability of the steel sheet is very good. Furthermore, despite the small amounts of additional elements necessary to obtain a carbon equivalent of less than 0.4%, the steel sheet of the invention and its manufacturing method make it possible to obtain very high mechanical properties. Therefore, according to the invention, very high mechanical properties and very good spot weldability can be achieved. The microstructure of the cold-rolled and heat-treated steel sheet according to the invention will now be described. Cold rolled and heat treated steel sheet with a structure including (as a surface fraction): - Between 10% and 45% ferrite, with an average grain size of up to 1.3 m, the product of the surface fraction of ferrite and the average grain size of ferrite is up to 35 m%, - Between 8% and 30% retained austenite, said retained austenite has an average Mn content greater than 1.1*Mn%, Mn% represents the Mn content of the steel, - Maximum 8% fresh martensite, - Maximum 2.5% cementite and - Partitioned martensite. These surface fractions and grain sizes are determined by the following method: A sample of cold-rolled and heat-treated steel is cut, polished and etched with a known marker to determine the microstructure. This section is then examined by optical and scanning microscopy, for example with a scanning electron microscope with a field emission gun ("FEG-SEM") at a magnification greater than 5000x, coupled with electron backscatter diffraction ("EBSD") and a transmission electron microscope (TEM). The determination of the surface fraction of each constituent (partitioned martensite, fresh martensite, ferrite and austenite) is carried out by image analysis using known methods. The retained austenite fraction is determined, for example, by X-ray diffraction (XRD). Ferrite in structure is intercritical ferrite. If the ferrite fraction is less than 10%, the HER hole expansion ratio will not reach 20%. If the ferrite fraction is more than 45%, the tensile strength of at least 1200 MPa will not be achieved. Ferrite has an average size of up to 1.3 µm. Furthermore, the product of the ferrite surface fraction (expressed as a percentage) and the average ferrite grain size (expressed in m) is up to 35 µm%. An average grain size of maximum 1.3 µm and the product of the ferrite surface fraction and the average grain size of the ferrite grains of maximum 35 µm% make it possible to achieve a HER hole expansion ratio of at least 20%, a yield strength of at least 1000 MPa and a total of YS*UE + TS*TE + TS*HER of at least 56000 MPa%. The microstructure of cold-rolled and heat-treated steel sheet contains at least 8% austenite, which (at room temperature) is retained austenite. When present in the surface fraction of at least 8%, retained austenite contributes to increased ductility. Retained austenite is rich in manganese. Specifically, retained austenite has an average Mn content greater than or equal to 1.1*Mn%, where Mn represents the amount of Mn in the steel composition. Being rich in Mn makes retained austenite stable. Retained austenite is also generally rich in carbon. Specifically, retained austenite has an average C content of at least 0.4%, preferably between 0.4% and 1.0%. Being rich in C also stabilizes the austenite. The value of c in retained austenite is determined, for example, by determining the retained austenite fraction and the lattice parameters by X-ray diffraction (XRD) analysis, with a Rietveld refinement (Rietveld, H., “A profile refinement method for nuclear and magnetic structures”, Journal of applied Crystallography, 2(2), 65–71, 1969). The value of C in retained austenite is then determined using the Dyson and Holmes formula (J. Dyson, and B. Holmes: “Effect of alloying additions on the lattice parameter austenite”, Journal of the Iron and Steel Institute, 1970, 208, 469–474). Retained austenite is usually in the form of islands, with the average size of retained austenite islands being less than 500 nm. A surface fraction of at least 8% retained austenite, with a Mn content greater than 1.1*Mn%, allows for a composition with high ductility and high strength. In fact, the richness of retained austenite in Mn provides high austenite stabilization, so that when the steel sheet is subjected to deformation, the retained austenite deforms by slippage of individual deformations and mechanical coupling. The structure may comprise up to 2.5% cementite. In one example, the structure comprises a maximum of 1%, and preferably a maximum of 0.3% cementite. The average size of cementite particles in the final structure is usually less than 50 nm. Partitioned martensite exists as fine, increased laths arranged in the grains of prior austenite. Partitioned martensite has an average C value just below the nominal C value of the steel. Fresh martensite may be present in the structure, with a surface fraction of up to 8%. In fact, a fresh martensite fraction greater than 8% results in a HER hole expansion ratio according to ISO 16630:2009 of less than 20%. Partitioned martensite can be distinguished from fresh martensite on a polished and etched surface by a known marker, for example the Nital marker, by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Martensite, including partitioned martensite and fresh martensite, if any, contains a small amount of carbides. In particular, the surface density of carbides in martensite with a surface area greater than 10x60 nm2 is usually less than or equal to 0.05*106 / mm2. A method for producing a cold-rolled and heat-treated steel sheet as described above is now disclosed. Hot-rolled sheet with a thickness between 2 and 6 mm can be made by molding steel with the above composition to obtain a slab, heating the slab at a Treheat temperature between 1150°C and 1300°C, and hot-rolling the reheated slab (the final rolling temperature is higher than Ar3) to obtain a hot-rolled sheet. The final rolling temperature is preferably a maximum of 1000°C, to prevent austenitic grain coarsening. The hot-rolled steel is then cooled, for example, at a cooling rate between 1°C / s and 50°C / s, and coiled at a temperature between 20°C and 600°C. After coiling, the sheet has a structure that may include bainite, martensite, and retained austenite. After coiling, the steel is washed into pieces. The hot-rolled steel sheet is then annealed to improve the cold-rollability and toughness of the hot-rolled steel sheet, and to provide a hot-rolled and heat-treated steel sheet that has high mechanical properties, especially high strength and high ductility, for the production of heat-treated cold-rolled steel sheet. Specifically, the hot-rolled steel sheet is annealed at a pre-annealing temperature TA1 and a controlled pre-annealing time tA1 (until the end of this pre-annealing) to obtain a structure comprising (as a surface fraction) the following: - At least 67% ferrite, with an average size of less than 4 m, - Maximum 30% retained austenite, - maximum 2% fresh martensite, and - Maximum 3% cementite. Having at least 67% ferrite, with an average ferrite grain size of up to 4 m, makes it possible to produce cold-rolled and heat-treated steel sheet with a very fine structure, and therefore very high mechanical properties. A fresh martensite fraction of up to 2% allows for high strength of hot-rolled and heat-treated steel sheet. In addition, a cementite fraction of up to 3% indicates that the decomposition of cementite is facilitated during subsequent annealing of the cold-rolled steel sheet, thus improving the ductility and strength during further processing steps. The inventors have found that this structure is achieved if the hot-rolled steel sheet is annealed at an initial annealing temperature TA1 between 500°C and TA1max (TA1max is the temperature at which a maximum of 30% austenite is formed upon heating) and during an initial annealing time tA1 between 3 seconds and 50,000 seconds. The initial annealing time tA1 is the holding time at the initial annealing temperature TA1, and does not include the heating time to this initial annealing temperature TA1. If the initial annealing temperature TA1 is less than 500°C and / or the initial annealing time tA1 is less than 3 seconds, the softening through microstructure recovery is insufficient, so that the hardness of the hot-rolled and annealed steel sheet will be too high, resulting in poor cold rolling ability of the steel. If the initial annealing temperature TA1 is higher than TA1max, a very large austenite fraction will be generated during the initial annealing, so that the fresh martensite fraction in the hot-rolled and annealed steel sheet will be much greater than 2%, and the retained austenite fraction in the hot-rolled and annealed steel sheet may be greater than 30%. If the initial annealing time tA1 is greater than 50,000 seconds, the microstructure becomes coarse, so that the average ferrite size in the hot-rolled and annealed steel sheet is greater than 4 μm. The austenite that can be formed during the initial tempering is rich in Mn, especially with an average Mn content of at least 1.5*Mn%. This Mn richness is due to the partitioning of manganese into the austenite during holding at the initial tempering temperature TA1. This austenite is therefore stabilized, so that the structure of the hot-rolled and tempered steel sheet contains up to 30% retained austenite, which usually has an average Mn content of more than 1.5*Mn%, and contains up to 2% fresh martensite. In the prototype, the initial warp is a batch warp. In this example, primary bending is preferably performed to obtain a hot-rolled and bent steel sheet structure comprising (as a surface fraction) the following: - At least 75% ferrite, with an average size of less than 4 m, - Maximum 10% retained austenite, -Maximum 2% fresh martensite, and - Maximum 3% cementite, Retained austenite has an average Mn content of more than 1.5*Mn%, Mn% representing the amount of Mn in the steel. The inventors have found that such a structure is achieved if the initial annealing temperature TA1 is between 500°C and 670°C, and the initial annealing time tA1 is between 1000 seconds and 50000 seconds. The initial annealing temperature TA1 is less than 670°C in order to limit the coarseness of the structure. Above 670°C, an average ferrite grain size of more than 4 m will be obtained in the hot rolled and annealed steel sheet. In the prototype, the initial annealing time tA1 is at least 1000 seconds, to achieve sufficient softening. Below 1000 seconds, the hardness of the hot rolled and annealed steel sheet will be too high, resulting in poor cold rolling ability of the sheet. In the original sample, the austenite that can be formed during the initial tempering is rich in Mn. Specifically, it has an average Mn content of at least 1.5*Mn%. This Mn enrichment is due to the partitioning of manganese into the austenite during the holding at the initial tempering temperature TA1. Therefore, this austenite becomes stable, so that the structure of the hot-rolled and tempered steel sheet contains up to 10% retained austenite, which has an average Mn content of more than 1.5*Mn%, and contains up to 2% fresh martensite. In the secondary sample, the initial warp is a continuous warp. In this secondary example, the primary bending is preferably carried out in such a way that the structure of the hot-rolled and bent steel sheet includes (as a surface fraction) the following: - At least 67% ferrite, with an average size of less than 4 m, - Maximum 30% fresh austenite, - Maximum 2% fresh martensite, - Maximum 1% cementite, Cementite particles, if present, have an average size of less than 150 nm. In this second example, the initial annealing temperature TA1 is preferably between 650°C and the maximum continuous annealing temperature TICAmax, the temperature at which 30% of austenite is formed upon heating. In addition, the initial annealing time tA1 is between 3 seconds and 500 seconds. If the initial annealing temperature TA1 is lower than 650°C, the softening through the microstructure will be insufficient, so that the hardness of the hot-rolled and annealed steel sheet will be too high, resulting in poor cold rolling ability of the steel. If the initial tempering temperature TA1 is higher than TICAmax, a very high fraction of austenite will be generated during continuous tempering, which may lead to insufficient austenite stabilization, so that the fresh martensite fraction in the hot-rolled and tempered steel sheet will be more than 2%. In addition, in the second sample, if the initial annealing time tA1 is greater than 500 seconds, the microstructure becomes coarse, so that the average ferrite grain size in the hot-rolled and annealed steel sheet becomes greater than 4 μm. In this second example, the austenite that can be produced during initial tempering is also rich in Mn, specifically having a Mn content of at least 1.5*Mn%. Therefore, this austenite is strongly stabilized, so that up to 2% of fresh martensite will be formed upon cooling. The retained austenite in hot-rolled and tempered steel sheet therefore usually has an average Mn content of at least 1.5*Mn%. Hot-rolled and warped steel sheet is optionally section washed. The hot rolled and annealed steel sheet is then cold rolled to obtain a cold rolled steel sheet with a thickness between 0.7 mm and 3 mm, for example in the range of 0.8 mm and 2 mm. The reduction ratio of cold rolling is preferably between 20% and 80%. Below 20%, recrystallization during subsequent heat treatment is not desirable, which may reduce the ductility of the cold rolled and heat treated steel sheet. Above 80%, there is a risk of edge cracking during cold rolling. The cold-rolled steel sheet is then heat-treated on a continuous rolling line. Heat treatment includes the following steps: - Reheating the cold-rolled steel sheet to a secondary annealing temperature TA2 between Ae1 and Ae3 so that, upon annealing, a structure consisting of 55% and 90% austenite and between 10% and 45% ferrite is obtained, and holding the cold-rolled steel sheet at the secondary annealing temperature TA2 for a holding time tA2. A person of ordinary skill in the art knows how to determine Ae1 and Ae3 and the appropriate secondary annealing temperature TA2 to obtain the desired structure as soon as annealed for each steel composition from dilatometric tests. The reheating rate Vr to the secondary annealing temperature TA2 is preferably between 1°C / s and 200°C / s. During warping, any cementite that may be present in the structure dissolves. In particular, due to the amounts of Al in the steel composition, the annealing temperature to obtain a structure (as soon as annealing) consisting of between 55% and 90% austenite and between 10% and 45% ferrite is a favorable temperature range for the dissolution of carbides. This temperature range is also favorable for partitioning of Mn into austenite during storage at this temperature. After holding the steel at the secondary annealing temperature, the steel sheet structure therefore consists of 55% to 90% austenite, which is rich in Mn, and 10% to 45% ferrite. If the secondary annealing temperature is such that the structure obtained immediately after annealing contains less than 10% ferrite, the ferrite fraction in the final structure will be insufficient to achieve the desired elongation and hole expansion ratio. Furthermore, the austenite is insufficiently enriched in Mn to stabilize the retained austenite. - If the secondary annealing temperature is such that the structure obtained immediately after annealing contains more than 45% ferrite, the ferrite fraction in the final structure will be too high to obtain the desired tensile strength. In addition, the dissolution of carbides will be insufficient, which will lead to a coarsening of the final structure, in particular to an average grain size greater than 1.3 µm and a product of the surface fraction of ferrite and the average grain size of the ferrite grains greater than 35 µm%. The holding time tA2 at the secondary annealing temperature TA2 is between 30 seconds and 500 seconds. - If the holding time tA2 is less than 30 seconds, insufficient stabilization to austenite with Mn and insufficient carbide dissolution will be achieved. Holding time tA2 more than 500 seconds will lead to coarsening of the structure. In particular, holding time tA2 more than 500 seconds will lead to an average ferrite grain size of more than 1.3 µm and the product of the ferrite surface fraction and the average grain size of the ferrite grains of more than 35 µm%, so that the target characteristics, especially the target hole expansion ratio, yield strength and the sum of YS*UE + TS*TE + TS x HER, will not be achieved. - Quenching of cold-rolled steel sheet at a cooling rate Vc between 1°C / s and 100°C / s to avoid the formation of pearlite upon cooling, until the quenching temperature QT is higher than the Ms austenite transformation point. The quenching temperature QT is between 20°C and Ms-50°C. For any particular steel composition and any structure, the person of ordinary skill in the art knows how to determine the Ms austenite transformation point by dilatometry. The temperature 20°C is usually higher than Mf+20°C. During this quenching step, austenite partially transforms into martensite. The quenching temperature QT is preferably chosen to obtain (immediately after quenching) a structure consisting of between 8% and 38% austenite, between 10% and 45% ferrite, 12% to 82% martensite and possibly cementite. A person of ordinary skill in the art knows how to determine the quenching temperature adjusted to achieve the desired structure. If the QT quenching temperature is less than 20°C, the fraction of partitioned martensite in the final structure will be too high to stabilize a sufficient amount of retained austenite above 8%. Furthermore, if the QT quenching temperature is greater than Ms-50°C, the fraction of partitioned martensite in the final structure will be too low to achieve the desired tensile strength. - Optionally holding the quenched sheet at the quenching temperature QT for a holding time tQ comprised between 2 seconds and 200 seconds, preferably between 3 seconds and 7 seconds, to avoid the formation of epsilon carbides in the martensite, which would lead to a reduction in the elongation of the steel. - Reheating the cold-rolled steel sheet to a partitioning temperature TP between 350°C and 500°C, and maintaining the cold-rolled steel sheet at the partitioning temperature TP for a partitioning time tP between 3 seconds and 1000 seconds. During this partitioning step, carbon diffuses from martensite to austenite, thereby resulting in C enrichment in austenite. If the partitioning temperature TP is higher than 500°C or lower than 350°C, the elongation of the final product will not be satisfactory. - Optionally hot dip plating of the sheet in a bath at a temperature of, for example, less than or equal to 480°C. Any type of plating can be used, in particular zinc or zinc alloys, such as zinc-nickel, zinc-magnesium or zinc-magnesium-aluminum alloys, aluminum or aluminum alloys, for example aluminum-silicon. - Immediately after the partitioning step, or immediately after the hot dip coating step, if performed, cooling the cold rolled steel sheet to room temperature, to obtain a heat treated cold rolled steel sheet. The cooling rate is preferably greater than 1°C / s, for example between 2°C / s and 20°C / s. During the cooling step, some of the austenite may transform into fresh martensite. However, the surface fraction of fresh martensite remains less than or equal to 8%, due to the stabilization of the austenite by C and Mn. - Optionally, after cooling to room temperature, if the hot dip coating step is not performed, the sheet may be coated by electrochemical methods, for example electro galvanizing, or by any vacuum coating process, such as PVD or vapor jet deposition. Any type of coating may be used, in particular zinc or zinc alloys, such as zinc-nickel, zinc-magnesium or zinc-magnesium-aluminium alloys. Optionally, after coating by electro galvanizing, the sheet may be subjected to degassing. - Optionally, after cooling to room temperature and optional coating, the sheet may be subjected to an additional annealing treatment at a temperature between 150°C and 450°C, for a holding time between 1 and 20 minutes (the higher the temperature, the shorter the holding time). This annealing treatment is intended to improve the formability of the sheet. This fabrication method allows for a final structure (i.e., after partitioning and cooling to room temperature) that includes (as a surface fraction): - Between 10% and 45% ferrite, - 8% and 30% retained austenite, - Maximum 8% fresh martensite, - Partitioned martensite, and -Maximum 2.5% cementite. The ferrite is a ferrite between the critical range. The presence of intercritical ferrite in the structure, instead of ferrite that could be cooled during cooling below the secondary tempering temperature TA2 (Mn after transformation ferrite), allows for the Mn enrichment of the austenite during this secondary tempering. In fact, the presence of transformation ferrite instead of intercritical ferrite indicates that sufficient stabilization of the retained austenite with Mn could not occur during the secondary tempering, so that the desired properties, especially the desired elongation, were not achieved. Ferrite has an average grain size of up to 1.3 µm. Furthermore, the product of the ferrite surface fraction (expressed as a percentage) and the average ferrite grain size (expressed in m) is up to 35 µm%. This average grain size results from the microstructure of the hot-rolled and annealed steel sheet, and from subsequent heat treatment, specifically deformation of the sheet during cold rolling and subsequent recrystallization. The retained austenite is rich in Mn. This enrichment is mainly due to the partitioning of Mn from ferrite to austenite during the holding at the second tempering temperature TA2. In particular, this enrichment in Mn is due to the fact that tempering between the critical range at TA2 is carried out in the temperature range favorable for the diffusion of Mn from ferrite to austenite. Therefore, the Mn content in the retained austenite is greater than 1.1*Mn%. Retained austenite is usually rich in C, this enrichment resulting from the partitioning of carbon from martensite to austenite during storage at the partitioning temperature TP. The average C content in retained austenite is usually at least 0.4%, preferably between 0.4% and 1.0%. The retained austenite is in the form of islands, the average size of the retained austenite islands is less than 500 nm. Partitioned martensite is formed upon cooling below the transformation temperature Ms after secondary annealing, and subsequent heating and holding at the partition temperature TP between 350°C and 500°C. Partitioned martensite has an average C value just below the nominal C value of the steel. This low C value is due to the partitioning of carbon from martensite (which is formed upon quenching below the Ms temperature of the steel) to austenite during holding at the partitioning temperature TP. Fresh martensite may form upon cooling from the TP partitioning temperature to room temperature. However (due to the stabilization of retained austenite by Mn) the fraction of fresh martensite in the structure is less than or equal to 8%. Martensite, including partitioned martensite and fresh martensite, if any, contains a small amount of carbides. In particular, the surface density of carbides in martensite with a surface area greater than 10x60 nm2 is usually less than or equal to 0.05*106 / mm2. This low content of carbides in martensite is due to the partitioning of C from martensite to austenite during holding at the partitioning temperature TP. This low content of carbides contributes to the combination of high tensile strength and high overall elongation. In fact, the lower the carbide content, the lower the fraction of carbon and manganese in the form of carbides, and the higher the fraction of carbon and manganese in austenite. In comparison, the surface density of carbides in martensite, whose surface area exceeds 10x60 nm2, is greater than 0.05*106 / mm2, indicating that the austenite does not contain sufficient amounts of carbon and manganese and is not sufficiently stable. A small fraction of cementite may be formed upon cooling from the annealing temperature and during partitioning. However, the cementite fraction in the final structure will in any case remain less than 2.5%. Typically, the cementite fraction in the final structure is less than 1%, preferably less than 0.3%. Furthermore, the average size of cementite particles in the final structure is usually less than 150 nm. If the initial annealing is a continuous annealing, the final structure contains a maximum of 0.3% cementite, the cementite particles (if present) having an average size of less than 50 nm. In fact, continuous annealing leads to a very low cementite fraction, the hot rolled and annealed steel sheet having a cementite fraction of at most 1%. In fact, the cementite grains are very fine, the average particle size being at most 150 nm. Therefore, the cementite is completely or almost completely decomposed during the subsequent annealing at the secondary annealing temperature. Steel sheets with a yield strength YS between 1000 MPa and 1300 MPa, a tensile strength TS between 1200 MPa and 1600 MPa, a uniform elongation UE greater than or equal to 10%, a total elongation TE greater than or equal to 14%, a hole expansion ratio HER of at least 20%, and the sum of the product of the yield strength YS by the uniform elongation UE (YS*UE), the product of the tensile strength TS by the total elongation TE (TS*TE), and the product of the tensile strength TS by the hole expansion ratio HER (TS x HER), YS*UE + TS*TE + TS*HER is obtained to be at least 56000 MPa %. Typically, the product of the yield strength by the uniform elongation (YS x UE) is at least 11000 MPa% and the product of the tensile strength by the total elongation (TS x TE) is at least 19000 MPa%. Another object of the invention is a welded assembly comprising a primary steel sheet and a secondary steel sheet welded together by resistance spot welding. The primary steel sheet is a cold-rolled and heat-treated steel sheet according to the invention, and the secondary steel sheet may be according to the invention or have a different composition. In particular, the primary and secondary steel sheets may have the same composition or different compositions, and the same thickness or different thicknesses. Welded assemblies are manufactured by producing primary and secondary steel sheets, and resistance spot welding the primary and secondary steel sheets. Resistance spot welds connecting the primary steel sheet to the secondary steel sheet are characterized by high strength in the cross-tension test defined by an alpha value of at least 50 daN / mm2, and preferably at least 70 daN / mm2, especially after post-weld heat treatment. Here, the alpha value represents the maximum load in the cross test divided by the weld diameter and thickness. It is a normalized load for resistance spot welding expressed in daN / mm2. Examples: As examples and comparisons, sheets were made from steel compositions according to Table I, the values ​​being expressed as weight percentages. In this table, "residual" means that the element exists only as a residual, and no optional additions have been made to this element. The transformation temperatures of steels Ae1 and Ae3 were measured by dilatometry and are also reported in Table I. In this table, the underlined values ​​are not in accordance with the invention. Table I steel C (%) Mn (%) Si (%) Al (%) Si+Al (%) Mo(%) S (%) P(%) N(%) Cr (%) Ti (%) Nb (%) V (%) B (%) Ceq Ae1 (°C) Ae3 (°C) I1 0.146 3.78 1.46 0.79 2.25 0.187 0.001 0.009 0.004 res. res. 0.058 res. res. 0.25 650 900 I2 0.174 3.8 1.52 0.757 2.277 0.201 0.0015 0.013 0.008 res. res. 0.03 res. res. 0.29 650 900 I3 0.188 4.04 1.19 0.781 1.971 0.2 0.0012 0.013 0.0047 0.505 0.04 0.022 res. 0.0022 0.25 640 890 I4 0.184 3.72 1.2 0.79 1.99 0.2 0.001 0.013 0.0036 res. res 0.032 res. 0.0006 0.29 640 900 R1 0.152 3.65 1.58 0.003 1.583 res. 0.0008 0.009 0.004 res. 0.045 res. 0.106 res. 0.39 640 780 R2 0.157 3.52 1.52 0.028 1.548 res. 0.0008 0.01 0.002 res. res. 0.057 res. res. 0.39 640 780 R3 0.145 3.82 1.47 0.79 2.26 res. 0.001 0.009 0.005 res. res. 0.058 res. res. 0.35 650 900 R4 0.146 3.86 1.48 0.028 1.508 res. 0.001 0.009 0.004 res. res. 0.06 res. res. 0.39 640 780 R5 0.113 4.75 0.5 1.45 1.95 res. 0.001 0.012 0.004 res. res. 0.03 res. res. 0.32 600 950 The steels were cast to obtain ingots. The ingots were reheated at Treheat to 1250°C, heated at a temperature above Ar3 to obtain a hot rolled sheet, and sectioned. The hot rolled steels were then coiled at a temperature of Tcoil of 450°C with a temperature of 20°C, batch or continuous rolled at an initial rolling temperature of TA1 for a time of tA1, thereby obtaining hot rolled and rolled steel sheets. Microstructural analyses were performed on samples taken from hot-rolled and hot-rolled steel sheets. The hot-rolled and warped steel sheets were then section-cleaned and cold-rolled with a cold-rolling reduction ratio of 50% to obtain cold-rolled sheets with a thickness of 1.2 mm. The cold rolled sheets were reheated to the secondary annealing temperature TA2 at a heating rate of Vr=10°C / s and held at the secondary annealing temperature TA2 for a holding time of tA2. The cold rolled sheets were then quenched at a cooling rate of Vc=5°C / s to the quenching temperature QT and held at the QT temperature for 3 seconds. The sheets were then reheated to the partitioning temperature TP and held at the partitioning temperature TP for the partitioning time tP and immediately cooled to room temperature. The curing conditions are reported in Table II. In this table, the "Type of warp" column indicates whether the warp was a batch warp or a continuous warp. Table II Ex. فولاد Tcoil (°C) نوع تاب‌کاری TA1 (°C) tA1 (s) TA2 (°C) tA2 (s) Ms (°C) QT (°C) TP (°C) tP (s) I1A I1 450 batch 600 18000 840 100 255 130 400 220 I1B I1 450 batch 600 18000 840 100 255 110 400 220 I1C I1 450 batch 600 18000 870 100 283 140 400 220 I1D I1 450 batch 600 18000 870 100 283 120 400 220 I1E I1 450 batch 600 18000 900 200 337 215 400 220 I2A I2 450 batch 600 18000 810 200 230 110 400 220 I2B I2 450 batch 600 18000 840 100 285 140 400 220 I2C I2 450 batch 650 21600 870 100 300 180 400 220 I3A I3 450 batch 665 21600 840 120 285 150 450 220 I3B I3 450 batch 600 21600 840 120 290 150 425 220 I3C I3 20 continuous 700 600 770 120 200 40 450 220 I3D I3 20 continuous 700 600 830 120 280 130 450 220 I4A I4 450 batch 650 18000 840 100 290 140 400 220 I4B I4 450 batch 680 18000 840 100 290 140 400 220 R1A R1 450 batch 600 21600 775 200 265 140 400 230 R2A R2 450 batch 600 21600 775 200 270 120 400 230 R2B R2 450 batch 600 21600 775 200 270 140 400 230 R3A R3 450 batch 600 21600 840100 260 140 400 220 R3B R3 450 batch 600 21600 870 100 320 150 400 220 R4A R4 450 batch 600 21600 775 100 250 160 450 100 R4B R4 450 batch 600 21600 775 200 250 120 400 230 R5A R5 450 batch 600 21600 830 200 210 120 400 220 R5B R5 450 batch 600 21600 860 200 260 150 400 220 In Table II, the underlined values ​​are not in accordance with the invention. The microstructures of hot-rolled and annealed steel sheets are reported in Table III. Table III Steel Fα (%) Dα (m) F (%) I1A 98 1.5 <2 I1B 98 1.5 <2 I1C 98 1.5 <2 I1D 98 1.5 <2 I1E 98 1.5 <2 I2A 98 1.5 <2 I2B 98 1.5 <2 I2C 96 2.5 2 I3A 85 3.5 5 I3B 98 1.5 <2 I3C 92 0.4 7 I3D 92 0.1 7 I4A 94 3 5 I4B 90 6 10 R1A 98 1.5 <2 R2A 98 1.5 <2 R2B 98 1.5 <2 R3A 98 1.5 <2 R3B 98 1.5 <2 R4A 98 1.5 <2 R4B 98 1.5 <2 R5A 98 1.5 <2 R5B 98 1.5 <2 In Table III, F is the surface fraction of austenite (expressed as a percentage), Fα is the surface fraction of ferrite (expressed as a percentage), and Dα is the average ferrite grain size (in micrometers). The microstructures of cold-rolled and heat-treated steel sheets were analyzed by observation with a scanning electron microscope with a field emission gun ("FEG-SEM") at a magnification greater than 5000x, coupled with electron backscatter diffraction ("EBSD") and transmission electron microscopy (TEM). The microstructures of cold-rolled and heat-treated steel sheets are reported in Table IV. In this table: - F represents the surface fraction of retained austenite, expressed as a percentage, - D is the average size of retained austenite islands in nm, - Fα represents the surface fraction of ferrite in percent, - Dα is the average ferrite grain size in m, - Fα*Dα is the product of the ferrite surface fraction in percent, and the average ferrite size in m, - FM is the fresh martensite surface fraction in percent, -PM is the surface fraction of partitioned martensite in percent, - dc is the surface density of carbides in martensite with a surface area greater than 10x60nm2, expressed as 106 / mm2. Table IV Steel Fɣ (%) D (nm) Fα (%) Dα (m) Fα*Dα (m%) FM (%) PM (%) dC (x106 / mm2) I1A 15 400 33 0.8 26.4 6 46 0 I1B 12 380 33 0.8 26.4 3 52 0 I1C 10 350 25 1 25 6 59 0 I1D 9 350 25 1 25 4 62 0 I1E 8.2 450 0 NA NA 10 81.8 5 I2A 25 450 44 0.6 26.4 2 29 0 I2B 18 350 32 0.8 25.6 3 47 0 I2C 14 500 24 1.2 28.8 6 56 0 I3A 13 500 25 1.2 30 7 55 0 I3B 15 400 23 0.6 13.8 5 57 0 I3C 25 250 42 0.5 21 2 31 0 I3D 17 300 15 0.7 10.5 2 66 0 I4A 16 400 28 1.2 33.6 2 54 0 I4B 17 500 30 2 60 3 50 0 R1A 18 480 6 0.5 3 10 66 0.05 R2A 16 450 7 0.5 3.5 5 72 0.05 R2B 19 480 7 0.5 3.5 10 64 0.05 R3A 13 700 35 3 105 8 44 0 R3B 11 800 25 2.8 70 6 58 0 R4A 15 500 6 0.5 3 12 67 0.05 R4B 15 450 6 0.5 3 7 72 0.05 R5A 15 600 50 1 50 8 27 0 R5B 12 600 40 1.3 52 6 42 0 In this table, underlined values ​​are not in accordance with the invention. “nd” means “not determined” and “NA” means “not applicable.” In addition, the average Mn content in the retained austenite of examples I1A to I1D and I2A to I4A was determined. All of these examples have an average Mn content greater than 1.1*Mn%. The cementite fraction for these examples I1A to I1D and I2A to I4A is less than 2.5%. The mechanical properties of the cold-rolled and heat-treated steel sheets were also determined. In particular, the yield strength YS, tensile strength TS, uniform elongation UE, and total elongation TE were determined according to ISO standard ISO 6892-1, published in October 2009. The hole expansion ratio HER was measured according to ISO standard 16630:2009. The mechanical properties are reported in Table V. Table V YS (MPa) TS (MPa) UE (%) TE (%) HER (%) YS x UE (MPa%) TSxTE (MPa%) TSxHER (MPa%) YS*UE+ TS*TE+ TS*HER (MPa%) I1A 1015 1327 12.6 16.1 21 12789 21365 27867 62021 I1B 1082 1302 12.4 16.4 28 13417 21353 36456 71226 I1C 1154 1287 10.8 14.5 31.3 12463 18662 40283 71408 I1D 1181 1309 10.4 14.7 36 12282 19242 47124 78649 I1E 1126 1303 9.5 13.2 ndnd 10697 17199.6 ndnd I2A 1000 1286 11.8 14.9 25 11800 19161 32150 63111 I2B 1074 1320 11.6 15.9 24 12458 20988 31680 65126 I2C 1049 1292 10.3 14 22 10805 18088 28424 57317 I3A 1008 1329 11.8 15.2 23 11894 20201 30567 62662 I3B 1245 1369 10.7 14.4 28 13321.5 19713.6 38332 71367 I3C 1098 1290 12.3 15.4 nd 13505.4 19866 ndnd I3D 1246 1356 11.6 14.3 26 14453.6 19390.8 35256 69100 I4A 1039 1270 10.1 14.1 33.2 10493.9 17907 42164 70565 I4B 862 1213 11.5 15.6 23 9913 18922.8 27899 56735 R1A 940 1334 10.5 13.8 13.9 9870 18409 18543 46822 R2A 1028 1305 11.5 14.5 18.5 11822 18923 24143 54887 R2B 898 1313 11.4 14.5 13.6 10237 19039 17857 47133 R3A 734 1148 12.4 16.8 nd 9102 19286 ndnd R3B 956 1160 12 16.5 nd 11472 19140 ndnd R4A 758 1399 10.3 12.8 10.5 7807 17907 14690 40404 R4B 1012 1326 12.4 15.6 14 12549 20686 18564 51798 R5A 815 1130 8.8 11.7 14.2 7172 13221 16046 36439 R5B 960 1169 10.2 14 24 9792 16366 28056 54214. In this table, underlined values ​​are not in accordance with the invention. “nd” means “not determined.” Examples I1A-I1D and I2A-I4A are made of steels according to the invention, and are produced by the manufacturing method according to the invention. All of these examples have a yield strength YS between 1000 MPa and 1300 MPa, a tensile strength TS between 1200 MPa and 1600 MPa, a uniform elongation UE of at least 10%, a total elongation TE measured according to ISO standard ISO 6892-1 of at least 14%, a hole expansion ratio HER measured according to ISO standard ISO 16630:2009 of at least 20%, and a sum of YS*UE + TS*TE + TS*HER greater than 56000 MPa%. For comparison, Example I1E, although made of steel having the composition according to the invention, was subjected to secondary annealing at a secondary annealing temperature of Ae3. As a result, the final structure of the cold-rolled and heat-treated steel sheet does not contain ferrite, so that the desired uniform and overall elongation was not achieved. Example I4B is also made of steel having the composition according to the invention, but was batch-rolled at 680°C. As a result, the ferrite grain size of the hot-rolled steel sheet after batch-rolling is greater than 4 μm. Therefore, the cold-rolled and heat-treated steel sheet has a structure in which the average ferrite grain size is greater than 1.3 μm and the product Fα*Dα of the surface fraction of ferrite to the average ferrite size in nm is greater than 35 μm%. As a result, the yield strength of Example I4B is less than 1000 MPa. In addition, examples R1A, R2B and R4A were made from steels containing less than 0.3% Al and less than 0.1% Mo. As a result, the Ae3 temperature of these steels was too low to allow secondary annealing in the temperature range desired for enriching austenite with Mn, while the structure (as annealed) consisted of 10% to 45% ferrite and 55% to 90% austenite. Therefore, even if the steel was annealed at a secondary annealing temperature TA2 of 775°C, i.e. lower than the secondary annealing temperature of the examples according to the invention, the structure consisted of more than 90% austenite and less than 10% ferrite as annealed, such that the final structure contained less than 10% ferrite. Furthermore, the secondary annealing temperature TA2 of 775°C was not even high enough to allow sufficient diffusion of Mn into the austenite. Due to the high fraction of austenite formed immediately after annealing and due to the diffusion of Mn into the austenite, the austenite during secondary annealing was not sufficiently enriched in Mn. As a result, the final structures of Examples R1A, R2B and R4A contain more than 8% fresh martensite. Due to the very low ferrite fraction and the very high fresh martensite fraction, the YS yield strengths of Examples R1A, R2B, and R4A are less than 1000 MPa and the HER hole expansion ratios are less than 20%. In addition, the overall TE elongations of Examples R1A and R4A do not reach 14%. Example R2A was produced under the same conditions as Example R2B, except for the quenching temperature, which was lower for Example R2A (120°C) than for Example R2B (140°C). As a result, Example R2A contains more partitioned martensite and a reduction of 8% of the fresh martensite. However, the reduction of ferrite remains at 10%, resulting in a hole expansion ratio of HER of less than 20% and a sum of YS*UE + TS*TE + TS*HER of less than 56,000 MPa%. Examples R3A and R3B were made from steel containing no Mo. As a result, the ferrite grain size in the final structure is larger than 1.3 m, and the tensile strength does not reach 1200 MPa. Example R4B was made from steel containing less than 0.3% Al and less than 0.1% Mo. As a result, the R4B temperature of this steel was too low to allow a second annealing in the temperature range desired for enrichment of the austenite with Mn, while the structure (as annealed) contained 10% to 45% ferrite and 55% to 90% austenite. Therefore, even if the steel was annealed at a second annealing temperature TA2 of 775°C, i.e. lower than the second annealing temperatures of the examples according to the invention, the structure contained more than 90% austenite and less than 10% ferrite as annealed, such that the final structure contained less than 10% ferrite. Due to the low ferrite fraction, the HER hole expansion ratio of the R4B example is less than 20%. Examples R5A and R5B were produced from steel containing more than 1.0% Al and no Mo. Example R5A was annealed at 830°C, so that the final structure consists of more than 45% ferrite. Furthermore, due to the absence of Mo in the steel, the product of the surface fraction of ferrite and the average grain size of ferrite is more than 35 m%. As a result, none of the target properties are achieved. R5B was annealed at a higher secondary annealing temperature (860°C), such that the final structure contains less than 45% ferrite. However, the product of the surface fraction of ferrite and the average grain size of ferrite remains greater than 35 m%. As a result, tensile and yield strengths are not achieved, and the sum of YS*UE + TS*TE + TS*HER remains less than 56,000 MPa%. In addition, the inventors evaluated the weldability of steel sheets obtained with the manufacturing conditions described above. In particular, resistance spot welding tests were carried out on some steel sheets. Cold rolled and heat treated steel sheets were resistance spot welded under different test conditions with a welding force of 400 daN. Cross-sectional tensile tests were performed and the alpha value was determined. Here, the alpha value represents the maximum load in the cross test divided by the weld diameter and thickness. It is a normalized load for resistance spot welding expressed in daN / mm2. The parameters and results are reported in Table VI, where: - "Test conditions" refers to cold-rolled and heat-treated steel sheet on which resistance spot welding was performed, "Alpha" indicates the alpha value, i.e. the maximum load in the test divided by the weld diameter and thickness, expressed in daN / mm2. Table VI Alpha Test Conditions (daN / mm2) I1B 81 I2B 75 I3B 51 I4A 75 R1A 49 R2A 49 R3A 70 R4A 48 R5A 85 Examples I1B, I2B, I3B and I4A were made from steels having the composition according to the invention, and were produced under the production conditions according to the invention. As a result, the resistance spot welds produced by resistance spot welding these sheets have high ductility, which is characterized by an alpha value of at least 50 daN / mm2 before any post-weld heat treatment. Cold rolled and heat treated steel sheets according to the invention and manufactured according to the invention can be used with profiles for the manufacture of structural and safety parts of vehicles.

Claims

CLAIMS 1.-Cold-rolled and heat-treated steel sheet, made of a steel having a composition comprising, by weight percent: 0.10% ≤ C ≤ 0.25% 3.5% ≤ Mn ≤ 6.0% 0.5% ≤ Si ≤ 2.0% 0.3% ≤ Al ≤ 1.2% with Si+Al ≥ 0.8% 0.10% ≤ Mo ≤ 0.50% S ≤ 0.010% P ≤ 0.020% N ≤ 0.008% and optionally one or more elements selected from amongst Cr, Ti, Nb, V and B, such that: 0.01% ≤ Cr ≤ 1.0% 0.010% ≤ Ti ≤ 0.080% 0.010% ≤ Nb ≤ 0.080% 0.010% ≤ V ≤ 0.30% 0.0005% ≤ B ≤ 0.004%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said cold-rolled steel sheet having a microstructure consisting of , in surface fraction: - between 10% and 45% of ferrite, having an average grain size of at most 1.3  m, the product of the surface fraction of ferrite by the average grain size of the ferrite being of at most 35  m%, - between 8% and 30% of retained austenite, said retained austenite having an Mn content higher than 1.1*Mn%, Mn% designating the Mn content of the steel, - at most 8% of fresh martensite, - at most 2.5% of cementite and - partitioned martensite.2.- The cold-rolled and heat-treated steel sheet according to claim 1, wherein the retained austenite has an average C content of at least 0.4%.3.- The cold-rolled and heat-treated steel sheet according to any one of claims 1 or 2, wherein said fresh martensite and said partitioned martensite comprise carbides, the surface density of carbides whose surface area is higher than 10x60nm2 being lower than to 0.05*106 / mm2 .4.- The cold-rolled and heat-treated steel sheet according to any one of claims 1 to 3, wherein the retained austenite is in the shape of islands, the islands of retained austenite having an average size lower than 500 nm.5.- The cold-rolled and annealed steel sheet according to any one of claims 1 to 4, wherein the structure comprises at most 0.3% of cementite, the cementite particles, if any, having an average size lower than 50 nm.6 .- The cold-rolled and heat-treated steel sheet according to any one of claims 1 to 5, wherein the cold-rolled and heat-treated steel sheet has a yield strength YS comprised between 1000 MPa and 1300 MPa, a tensile strength TS comprised between 1200 MPa and 1600 MPa, a uniform elongation UE of at least 10%, a total elongation TE, measured according to ISO standard ISO 6892-1, of at least 14%, a hole expansion ratio HER, measured according to the ISO standard 16630:2009, of at least 20%, and a sum of the product of the yield strength YS by the uniform elongation UE (YS*UE), the product of the tensile strength TS by the total elongation TE (TS*TE), and the product of the tensile strength TS by the hole expansion ratio HER (TS x HER), YS*UE + TS*TE + TS*HER, of at least 56000 MPa%.7.- The c old-rolled and heat-treated steel sheet according to any one of claims 1 to 6, wherein the c old-rolled and heat-treated steel sheet is coated with Zn or a Zn alloy.8.- The c old-rolled and heat-treated steel sheet according to any one of claims 1 to 6, wherein the c old-rolled and heat-treated steel sheet is coated with Al or a Al alloy.9.- The c old-rolled and heat-treated steel sheet according to any one of claims 1 to 8, wherein the steel has a carbon equivalent Ceq lower than 0.4%, the carbon equivalent being defined as Ceq = C+Si% / 55 + Cr% / 20 + Mn% / 19 – Al% / 18+ 2.2*P% – 3.24*B% - 0.133Mn% * Mo%.10.- Resistance spot welded joint of at least two steel sheets, wherein at least one of said two steel sheets is a cold-rolled and heat-treated steel sheet according to any one of claims 1 to 9.11.- Resistance spot welded joint according to claim 10, having an alpha value, before any post welding heat treatment, of at least 50 daN / mm2 .12.- Resistance spot welded joint according to any one of claims 10 or 11, having an alpha value of at least 70 daN / mm2 .13.-Method for manufacturing a cold-rolled and heat-treated steel sheet, comprising the following successive steps: - casting a steel so as to obtain a slab, said steel having a composition comprising, by weight percent: 0.10% ≤ C ≤ 0.25% 3.5% ≤ Mn ≤ 6.0% 0.5% ≤ Si ≤ 2.0% 0.3% ≤ Al ≤ 1.2% with Si+Al ≥ 0.8% 0.10% ≤ Mo ≤ 0.50% S ≤ 0.010% P ≤ 0.020% N ≤ 0.008% and optionally one or more elements selected from amongst Cr, Ti, Nb, V and B, such that: 0.01% ≤ Cr ≤ 1.0% 0.010% ≤ Ti ≤ 0.080% 0.010% ≤ Nb ≤ 0.080% 0.010% ≤ V ≤ 0.30% 0.0005% ≤ B ≤ 0.004%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, - reheating the slab at a temperature Treheat comprised between 1150°C and 1300°C, - hot rolling the reheated slab at a temperature higher than Ar3 to obtain a hot rolled steel sheet, - coiling the hot rolled steel sheet at a coiling temperature Tcoil comprised between 20°C and 600°C, - annealing the hot rolled steel sheet at a first annealing temperature TA1 comprised between 500°C and TA1max , TA1max being the temperature at which at most 30% of austenite is created upon heating, the hot-rolled steel sheet being maintained at said first annealing temperature TA1 for a time tA1 comprised between 3 s and 50000 s, to obtain a hot-rolled and annealed steel sheet, - cold rolling the hot-rolled and annealed steel sheet so as to obtain a cold rolled steel sheet, - reheating the cold-rolled steel sheet to a second annealing temperature TA2 comprised between Ae1 and Ae3 and maintaining the cold-rolled steel sheet at the second annealing temperature TA2 for a holding time tA comprised between 30 s and 500 s, so as to obtain, upon annealing, a structure comprising between 55 and 90% of austenite and between 10% and 45% of ferrite, - quenching the cold-rolled steel sheet at a cooling rate Vc comprised between 1°C / s and 100°C / s, to a quenching temperature QT comprised between 20°C and Ms-50°C, - reheating the cold-rolled steel sheet to a partitioning temperature TP comprised between 350°C and 500°C, and maintaining the cold-rolled steel sheet at said partitioning temperature TP for a partitioning time tP comprised between 3 s and 1000 s, - cooling the cold-rolled steel sheet to the room temperature, to obtain a cold-rolled and heat treated steel sheet.14.- The method according to claim 13, wherein the hot-rolled and annealed steel sheet has a structure consisting of, in surface fraction: - at least 67% of ferrite, with an average grain size lower than 4 µm, - at most 30% of retained austenite, - at most 2% of fresh martensite, and - at most 3% of cementite,.15.- The method according to any one of claims 13 or 14, wherein the cold-rolled and heat treated steel sheet has a microstructure consisting of , in surface fraction: - between 10% and 45% of ferrite, having an average grain size of at most 1.3  m, the product of the surface fraction of ferrite by the average grain size of the ferrite being of at most 35  m%, - between 8% and 30% of retained austenite, said retained austenite having an Mn content higher than 1.1*Mn%, Mn% designating the Mn content of the steel, - at most 8% of fresh martensite, - at most 2.5% of cementite and - partitioned martensite.16.- The method according to claim 15, wherein the retained austenite has an average C content of at least 0.4%.17.- The method according to any one of claims 13 to 16, wherein the annealing performed on the hot-rolled steel sheet is a batch annealing, the first annealing temperature TA1 being comprised between 500°C and 670°C, the hot-rolled steel sheet being maintained at said first annealing temperature TA1 for a time comprised between 1000 s and 50000 s.18.- The method according to claim 17, wherein the hot-rolled and annealed steel sheet has a microstructure consisting of , in surface fraction: - at least 75% of ferrite with an average grain size lower than 4 µm , - at most 10% of retained austenite - at most 2% of fresh martensite, and - at most 3% of cementite, said retained austenite having a Mn content higher than 1.5*Mn%, Mn% designating the Mn content of the steel.19.- The method according to any one of claims 13 to 16, wherein the annealing performed on the hot-rolled steel sheet is a continuous annealing, the first annealing temperature TA1 being comprised between 650°C and a maximal continuous annealing temperature TICAmax, which is the temperature at which 30% of austenite is created upon heating, the hot-rolled steel sheet being maintained at said first annealing temperature TA1 for a time comprised between 3 s and 500 s.20.- The method according to claim 19, wherein the hot-rolled and annealed steel sheet has a structure consisting of, in surface fraction: - at least 67% of ferrite, with an average grain size lower than 4 µm, - at most 30% of austenite, - at most 2% of fresh martensite, and - at most 1% of cementite, the cementite particles, if any, having an average size lower than 150 nm.21.- The method according to any one of claims 19 or 20 , wherein the cold-rolled and heat treated steel sheet has a microstructure consisting of , in surface fraction: - between 10% and 45% of ferrite, having an average grain size of at most 1.3  m, the product of the surface fraction of ferrite by the average grain size of the ferrite being of at most 35  m%, - between 8% and 30% of retained austenite, said retained austenite having an Mn content higher than 1.1*Mn%, Mn% designating the Mn content of the steel, - at most 8% of fresh martensite, - at most 0.3% of cementite, the cementite particles, if any, having an average size lower than 50 nm, and - partitioned martensite .22.- The method according to any one of claims 13 to 21, wherein, between the maintaining at the partitioning temperature TP and the cooling to the room temperature, the cold-rolled steel sheet is hot-dip coated in a bath.23.- The method according to any one of claims 13 to 21, wherein, after the maintaining of the cold-rolled sheet at the partitioning temperature TP , the cold-rolled steel sheet is cooled to the room temperature.24.- The method according claim 23, wherein after the step of cooling down the cold-rolled steel sheet to the room temperature, the cold-rolled and heat treated steel sheet is coated by an electrochemical method or through a vacuum coating process.25.- The method according to any one of claims 22 or 24, wherein the cold-rolled and heat treated steel sheet is coated with Zn or a Zn alloy.26.- The method according to any one of claims 22 or 24, wherein the c old-rolled and heat-treated steel sheet is coated with Al or a Al alloy.27.- The method according to any one of claims 13 to 26, wherein the steel has a carbon equivalent Ceq lower than 0.4%, the carbon equivalent being defined as Ceq = C+Si% / 55 + Cr% / 20 + Mn% / 19 – Al% / 18+ 2.2*P% – 3.24*B% - 0.133Mn% * Mo%.28.- A process for producing a spot welded joint of at least two steel sheets, comprising the steps of: - providing a cold-rolled and heat-treated steel sheet according to any one of claims 1 to 9 or produced by a method according to any one of claims 13 to 27, - providing a second steel sheet, - spot welding the cold-rolled and heat-treated steel sheet to the second steel sheet.29.- The process according to claim 28, wherein the second steel sheet is a cold-rolled and heat-treated steel sheet according to any one of claims 1 to 9 or produced by a method according to any one of claims 13 to 27.