EP2707515B1 - Producing method for very high yield strength martensitic steel sheet and steel sheet obtained - Google Patents

Description

The invention relates to a method of manufacturing steel sheets with a thickness of less than 3 millimeters with a totally martensitic structure with a mechanical strength greater than that which could be obtained by a simple quenching treatment with martensitic quenching, and resistance properties. mechanical and elongation allowing their application to the manufacture of energy absorbing parts in motor vehicles.

In some applications, it is sought to produce parts from steel sheet with very high mechanical strength. This type of combination is particularly desirable in the automotive industry where significant vehicle lightening is sought. This can be achieved in particular by the use of steels with very high mechanical properties whose microstructure is totally martensitic. Anti-intrusion parts, structure or participating in the safety of motor vehicles such as: bumper cross members, door or center pillar reinforcements, wheel arms, require for example such characteristics. Their thickness is preferably less than 3 millimeters.

We seek to obtain sheets with an even higher mechanical strength. It is well known the possibility of increasing the mechanical strength of a steel martensitic structure by means of a carbon addition. However, this higher carbon content reduces the weldability of the sheets or parts made from these sheets, and increases the risk of cracking due to the presence of hydrogen.

It is therefore sought to have a method of manufacturing steel sheets not having the above disadvantages, which would have a higher tensile strength of more than 50 MPa to that which could be obtained through austenitization followed by a simple martensitic quenching of the steel in question. The inventors have demonstrated that, for carbon contents ranging from 0.15 to 0.40% by weight, the tensile strength Rm of sheets, steels manufactured by total austenitization followed of a simple martensitic quench, practically depended only on the carbon content and was connected to it with a very good precision, according to the expression (1): Rm (megapascals) = 3220 (C) + 908.

In this expression, (C) denotes the carbon content of the steel, expressed as a percentage by weight. Given the carbon content C of a steel, a method of manufacture is thus sought which makes it possible to obtain an ultimate tensile strength of 50 MPa at expression (1), ie a strength greater than 3220 ( C) + 958 MPa for this steel. It seeks to have a method for the manufacture of sheet with a very high yield strength, that is greater than 1300 MPa. It is also sought to have a method for the manufacture of directly usable sheets, that is to say without the imperative need of a tempering treatment after quenching.

These sheets must be weldable by the usual methods and do not include expensive additions of alloying elements.

The present invention aims to solve the problems mentioned above. It aims in particular at providing sheets with a thickness of less than 3 millimeters with a yield strength greater than 1300 MPa, tensile strength, expressed in megapascals, greater than (3220 (C) +958) MPa, and preferably a total elongation of greater than 3%.

• on approvisionne un demi-produit d'acier dont la composition comprend, les teneurs étant exprimées en poids : 0,15% ≤ C ≤0,40%, 1,5%≤ Mn ≤ 3%, 0,005% ≤ Si ≤ 2%, 0,005%≤ Al ≤ 0,1%, S ≤ 0,05%, P≤ 0,1%, 0,025%≤ Nb≤0,1% et optionnellement : 0,01%≤ Ti≤0,1%, 0%≤ Cr≤ 4%, 0%≤ Mo ≤2%, 0,0005% ≤ B ≤ 0,005%, 0,0005% ≤ Ca ≤ 0,005%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration.
• on réchauffe le demi-produit à une température T₁ comprise entre 1050°C et 1250°C, puis
• on effectue un laminage de dégrossissage du demi-produit réchauffé, à une température T₂ comprise entre 1050 et 1150°C, avec un taux de réduction ε_a cumulé supérieur à 100% de façon à obtenir une tôle avec une structure austénitique non totalement recristallisée de taille moyenne de grain inférieure à 40 micromètres, puis

• on refroidit non complètement la tôle jusqu'à une température T₃ comprise entre 970°C et Ar3+30°C, de façon à éviter une transformation de l'austénite, à une vitesse V_R1 supérieure à 2°C/s, puis
• on effectue un laminage à chaud de finition à la température T₃, de la tôle non complètement refroidie, avec un taux de réduction cumulé ε_b supérieur à 50% de façon à obtenir une tôle, d'épaisseur inférieure à 3 millimètres puis
• on refroidit la tôle à une vitesse V_R2 supérieure à la vitesse critique de trempe martensitique.

For this purpose, the subject of the invention is a method of manufacturing a thickness of less than 3 millimeters of totally martensitic structure with a yield strength greater than 1300 MPa, comprising the successive steps and in this order in which:

• supplying a semi-finished steel product whose composition comprises, the contents being expressed by weight: 0.15% ≤ C ≤ 0.40%, 1.5% ≤ Mn ≤ 3%, 0.005% ≤ Si ≤ 2% , 0.005% ≤ Al ≤ 0.1%, S ≤ 0.05%, P ≤ 0.1%, 0.025% ≤ Nb ≤ 0.1% and optionally: 0.01% ≤ Ti ≤ 0.1%, 0 % ≤ Cr≤4%, 0% ≤ Mo ≤2%, 0.0005% ≤ B ≤ 0.005%, 0.0005% ≤ Ca ≤ 0.005%, the remainder of the composition consisting of iron and unavoidable impurities resulting of elaboration.
• the semi-finished product is heated to a temperature T ₁ of between 1050 ° C. and 1250 ° C., and then
• the heat-treated semi-product is rolled at a temperature T ₂ of between 1050 and 1150 ° C. with a reduction ratio ε _a cumulative greater than 100% so as to obtain a sheet with a non-totally recrystallized austenitic structure with a mean grain size of less than 40 microns, then

• the sheet is cooled to a temperature T ₃ between 970 ° C. and Ar 3 + 30 ° C. so as to avoid a transformation of the austenite, at a speed V _R1 greater than 2 ° C./s, then
• a final hot rolling is carried out at the temperature T ₃ of the sheet which is not completely cooled, with a cumulative reduction ratio ε _b greater than 50% so as to obtain a sheet having a thickness of less than 3 millimeters then
• the sheet is cooled to a speed V _R2 greater than the critical speed of martensitic quenching.

In a preferred embodiment, the average size of austenitic grains is less than 5 micrometers.

Preferably, the sheet is subjected to a subsequent thermal treatment of tempering at a temperature T ₄ of between 150 and 600 ° C. for a period of between 5 and 30 minutes.

The subject of the invention is also a sheet of steel with a thickness of less than 3 millimeters and no yield strength greater than 1300 MPa, obtained by a method according to one of the above methods of manufacture, with a totally reduced structure. martensitic, having an average slat size of less than 1.2 micrometers, the average elongation factor of slats being between 2 and 5.

The subject of the invention is also a steel sheet having a thickness of less than 3 millimeters obtained by the process with the above-mentioned tempering treatment, the steel having a totally martensitic structure with an average slat size of less than 1.2. micrometer, the average elongation factor of the slats being between 2 and 5.

• Lorsque la teneur en carbone de l'acier est inférieure à 0,15% en poids, la trempabilité de l'acier est insuffisante et il n'est pas possible d'obtenir une structure totalement martensitique compte tenu du procédé mis en oeuvre.
• Lorsque cette teneur est supérieure à 0,40%, les joints soudés réalisés à partir de ces tôles ou de ces pièces présentent une ténacité insuffisante. La teneur optimale en carbone pour la mise en oeuvre de l'invention est comprise entre 0,16 et 0,28%.

The composition of the steels used in the process according to the invention will now be detailed:

• When the carbon content of the steel is less than 0.15% by weight, the hardenability of the steel is insufficient and it is not possible to obtain a totally martensitic structure taking into account the process used.
• When this content is greater than 0.40%, welded joints made from these sheets or these parts have insufficient toughness. The optimum carbon content for the implementation of the invention is between 0.16 and 0.28%.

Manganese lowers the initial formation temperature of martensite and slows the decomposition of austenite. In order to obtain sufficient effects, the manganese content must not be less than 1.5%. Moreover, when the manganese content exceeds 3%, segregated zones are present in excessive amount which impairs the implementation of the invention. A preferred range for the implementation of the invention is 1.8 to 2.5% Mn.

The silicon content must be greater than 0.005% so as to participate in the deoxidation of the steel in the liquid phase. The silicon must not exceed 2% by weight because of the formation of surface oxides which significantly reduce the coating ability, in the case where it would be desirable to coat the sheet by passing through a metal coating bath, in particular by continuous galvanizing.

The aluminum content of the steel according to the invention is not less than 0.005% so as to obtain sufficient deoxidation of the steel in the liquid state. When the aluminum content is greater than 0.1% by weight, casting problems can occur. It is also possible to form inclusions of alumina in too large quantities or sizes which play a detrimental role on toughness.

The sulfur and phosphorus contents of the steel are respectively limited to 0.05 and 0.1% in order to avoid a reduction in the ductility or toughness of the parts or sheets produced according to the invention.

The steel also contains niobium in an amount between 0.025 and 0.1%, and optionally titanium in an amount between 0.01 and 0.1%. These additions of niobium and optionally titanium allow the implementation of the process according to the invention by delaying the recrystallization of the austenite at high temperature and allow to obtain a sufficiently fine grain size at high temperature.

Chromium and molybdenum are very effective elements for delaying the transformation of austenite and can be used optionally for the implementation of the invention. These elements have the effect of separating the ferrito-pearlitic and bainitic transformation domains, the transformation Ferritic-pearlitic occurring at temperatures above the bainitic transformation. These transformation domains are then in the form of two distinct "noses" in an isothermal transformation diagram (Transformation-Temperature-Time)

The chromium content must be less than or equal to 4%. Beyond this content, its effect on the quenchability is practically saturated; an additional addition is then expensive without corresponding beneficial effect.

However, the molybdenum content must not exceed 2% because of its excessive cost.

As an option, the steel can also contain boron: indeed, the significant deformation of the austenite can accelerate the conversion to ferrite on cooling, a phenomenon that should be avoided. Addition of boron in an amount of between 0.0005 and 0.005% by weight makes it possible to guard against early ferritic transformation.

As an option, the steel may also contain calcium in an amount between 0.0005 and 0.005%: by combining with oxygen and sulfur, calcium prevents the formation of large inclusions that are harmful to the ductility of the sheets or parts thus manufactured.

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

Afin d'être statistiquement représentative, cette observation porte sur au moins 1000 lattes martensitiques. Le facteur d'allongement moyen $\frac{l\max }{l\min }$

est ensuite déterminé pour l'ensemble de ces lattes observées.The steel sheets manufactured according to the invention are characterized by a totally martensitic slatted structure of great fineness: due to the specific thermomechanical cycle and composition, the average size of the martensitic slats is less than 1.2 micrometers and their average elongation factor is between 2 and 5. These microstructural characteristics are determined, for example, by observing the microstructure by Scanning Electron Microscopy by means of a field effect gun ("MEB-FEG" technique) at a magnification. greater than 1200x, coupled to an EBSD detector ("Electron Backscatter Diffraction"). It is defined that two contiguous slats are distinct when their disorientation is greater than 5 degrees. The average slat size is defined by the known intercepts method: the average size of slats intercepted by defined lines is evaluated. random with respect to the microstructure. The measurement is performed on at least 1000 martensitic slats in order to obtain a representative average value. The morphology of the individual slats is then determined by image analysis using software known in itself: the maximum I _max and minimum I _min dimension of each martensitic slat and its elongation factor are determined. $\frac{l\max }{l\min }\mathrm{.}$

In order to be statistically representative, this observation concerns at least 1000 martensitic slats. The average elongation factor $\frac{l\max }{l\min }$

is then determined for all of these slats observed.

• On approvisionne tout d'abord un demi-produit d'acier dont la composition a été exposée ci-dessus. Ce demi-produit peut se présenter par exemple sous forme de brame issue de coulée continue, de brame mince, ou de lingot. A titre d'exemple indicatif, une brame de coulée continue a une épaisseur de l'ordre de 200mm, une brame mince une épaisseur de l'ordre de 50-80mm.
• On réchauffe ce demi-produit à une température T₁ comprise entre 1050°C et 1250°C. La température T₁ est supérieure à A_c3, température de transformation totale en austénite au chauffage. Ce réchauffage permet donc d'obtenir une austénitisation complète de l'acier ainsi que la dissolution d'éventuels carbonitrures de niobium existant dans le demi-produit. Cette étape de réchauffage permet également de réaliser les différentes opérations ultérieures de laminage à chaud qui vont être présentées : on effectue un laminage dit de dégrossissage du demi-produit : ce laminage de dégrossissage est effectué à une température T₂ comprise entre 1050 et 1150°C. Le taux de réduction cumulé des différentes étapes de laminage au dégrossissage est noté ε_a. Si e_ia désigne l'épaisseur du demi-produit avant le laminage à chaud de dégrossissage et e_fa l'épaisseur de la tôle après ce laminage, on définit le taux de réduction cumulé par ${\mathrm{\epsilon }}_{\mathrm{a}}=\mathrm{Ln}\frac{e_{ia}}{e_{f_a}}\mathrm{.}$
  
  Selon l'invention, le taux de réduction ε_a doit être supérieur à 100%, c'est-à-dire supérieur à 1. Dans ces conditions de laminage, la présence de niobium, et optionnellement de titane, retarde la recristallisation et permet d'obtenir une austénite non totalement recristallisée à haute température. La taille moyenne de grain austénitique ainsi obtenue est inférieure à 40 micromètres, voire à 5 micromètres lorsque la teneur en niobium est comprise entre 0,030 et 0,050%. Cette taille de grain peut être mesurée par exemple grâce à des essais où l'on trempe directement après laminage la tôle. On observe ensuite une coupe polie et attaquée de celle-ci, l'attaque étant effectuée grâce à un réactif connu en lui-même, tel que par exemple le réactif de Béchet-Beaujard qui révèle les anciens joints de grains austénitiques.
• On refroidit ensuite non complètement, c'est à dire jusqu'à une température intermédiaire T₃, la tôle à une vitesse V_R1 supérieure à 2°C/s, de façon à éviter une transformation et une éventuelle recristallisation de l'austénite puis on effectue un laminage à chaud de finition de la tôle avec un taux de réduction cumulé ε_b supérieur à 50%. Si e_i2 désigne l'épaisseur de la tôle avant le laminage de finition et e_f2 l'épaisseur de la tôle après ce laminage, on définit le taux de réduction cumulé par ${\mathrm{\epsilon }}_{\mathrm{b}}=\mathrm{Ln}\frac{e_{ib}}{e_{fb}}\mathrm{.}$
  
  Ce laminage de finition est effectué à une température T₃ comprise entre 970 et Ar3+30°C, Ar3 désignant la température de début de transformation de l'austénite au refroidissement. Ceci permet d'obtenir à l'issue du laminage de finition une austénite déformée à grains fins, celle-ci n'ayant pas tendance à recristalliser. On refroidit ensuite cette tôle à une vitesse V_R2 supérieure à la vitesse de trempe critique martensitique et l'on obtient ainsi une tôle caractérisée par une structure martensitique très fine dont les propriétés mécaniques sont supérieures à celles que l'on peut obtenir par un simple traitement thermique de trempe.

The process for manufacturing hot-rolled sheets according to the invention comprises the following steps:

• First, a semi-finished steel product, the composition of which has been described above, is supplied. This semi-finished product may for example be in the form of slab from continuous casting, thin slab or ingot. As an indicative example, a continuous casting slab has a thickness of about 200 mm, a thin slab a thickness of about 50-80 mm.
• This half-product is heated to a temperature T ₁ of between 1050 ° C. and 1250 ° C. The temperature T ₁ is greater than A _c3 , the total conversion temperature to austenite during heating. This reheating thus makes it possible to obtain a complete austenitization of the steel as well as the dissolution of any possible niobium carbonitrides in the semi-finished product. This reheating step also makes it possible to carry out the various subsequent hot rolling operations which will be presented: a so-called roughing operation of the semi-finished product is carried out: this roughing rolling is carried out at a temperature T ₂ of between 1050 and 1150 ° C. vs. The cumulative reduction rate of the various stages of rolling at roughing is noted ε _a . If e _ia denotes the thickness of the semi-finished product before the hot rolling of roughing and e _fa the thickness of the sheet after this rolling, the reduction rate cumulated by ${\mathrm{\epsilon }}_{\mathrm{at}}=\mathrm{Ln}\frac{e_{i\mathrm{at}}}{e_{f_{\mathrm{at}}}}\mathrm{.}$
  
  According to the invention, the reduction rate ε _a must be greater than 100%, that is to say greater than 1. Under these rolling conditions, the presence of niobium, and optionally titanium, delays the recrystallization and allows to obtain austenite not completely recrystallized at high temperature. The average austenitic grain size thus obtained is less than 40 micrometers, or even 5 micrometers when the niobium content is between 0.030 and 0.050%. This grain size can be measured, for example, by means of tests in which the sheet is quenched directly after rolling. A polished and etched section thereof is then observed, the attack being carried out using a reagent known in itself, such as, for example, the Béchet-Beaujard reagent which reveals the old austenitic grain boundaries.
• Then is cooled not completely, that is to say up to an intermediate temperature T ₃ , the sheet at a speed V _R1 greater than 2 ° C / s, so as to avoid a transformation and a possible recrystallization of the austenite then the sheet is hot-rolled with a cumulative reduction ratio ε _b greater than 50%. If e _i2 denotes the thickness of the sheet before the finish rolling and e _f2 the thickness of the sheet after this rolling, the cumulative reduction ratio is defined by ${\mathrm{\epsilon }}_{\mathrm{b}}=\mathrm{Ln}\frac{e_{ib}}{e_{fb}}\mathrm{.}$
  
  This finishing rolling is carried out at a temperature T ₃ of between 970 and Ar3 + 30 ° C, Ar3 denoting the starting temperature of transformation from austenite to cooling. This makes it possible to obtain, at the end of the finishing lamination, a deformed austenite with fine grains, the latter having no tendency to recrystallize. This sheet is then cooled to a speed V _R2 greater than the martensitic critical quenching speed and a sheet is thus obtained characterized by a very fine martensitic structure whose mechanical properties are greater than those which can be obtained by a simple quenching heat treatment.

Although the above method describes the manufacture of sheets, ie flat products, from slabs, the invention is not limited to this geometry and to this type of product, and can also be adapted the manufacture of long products, bars, profiles, by successive stages of hot deformation.

The steel sheets may be used as such or subjected to a heat treatment of tempered temperature T ₄ between 150 and 600 ° C for a period of between 5 and 30 minutes. This treatment of income generally has the effect of increasing the ductility at the price of a decrease of the limit of elasticity and the resistance. The inventors have however demonstrated that the method according to the invention, which gives a tensile strength of at least 50 MPa higher than that obtained after conventional quenching, retained this advantage even after a tempering treatment with temperatures ranging from from 150 to 600 ° C. The fineness characteristics of the microstructure are preserved by this income treatment.

By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention.

Example:

Steel semi-finished products have been supplied whose compositions, expressed in contents by weight (%) are as follows: VS mn Yes Cr MB al S P Nb Ti B It AT 0.27 1.91 0.01 0.01 0.01 0.03 0,003 0,020 0,042 0,010 0.0016 0,001 B 0.198 1.94 0.01 1,909 0.01 0.03 0,003 0,020 0,003 0.012 0.0014 0.0004 The underlined values are not in accordance with the invention

Semi-finished products 31 mm thick were heated and held for 30 minutes at a temperature T ₁ of 1250 ° C. and then subjected to rolling in 4 passes at a temperature T ₂ of 1100 ° C. with a cumulative reduction rate ε ₁ 164%, up to a thickness of 6mm. At this stage, at high temperature after roughing, the structure is totally austenitic, not completely recrystallized with an average grain size of 30 microns. The sheets thus obtained were then cooled at a rate of 3 ° C./s up to a temperature T ₃ of between 955 ° C. and 840 ° C., the latter temperature being equal to Ar 3 + 60 ° C. The sheets were rolled in this temperature range in 5 passes with a cumulative reduction rate ε _b of 76%, ie up to a thickness of 2.8 mm, then cooled. then to room temperature with a speed of 80 ° C / sec so as to obtain a completely martensitic microstructure.

By comparison, steel sheets of the above composition were heated at a temperature of 1250 ° C., held for 30 minutes at this temperature and then cooled with water so as to obtain a completely martensitic microstructure (reference condition).

By means of tensile tests, the yield strength Re, the breaking strength Rm and the total elongation A have been determined for sheets obtained by these different methods of manufacture. The estimated value of the resistance after single martensitic hardening (3220 (C) +908 (MPa)) and the difference ΔRm between this estimated value and the actually measured resistance were also included. Steel Trial Reduction temperature T ₃ (° C) Re (MPa) Rm (MPa) AT (%) 3220 (C) +908 (MPa) ΔRm (MPa) AT A1 955 1410 1840 5.2 1777 63 A2 860 1584 1949 4.9 1777 172 B B1 840 1270 1692 6.5 1545 147 B2 Without 1223 1576 6.9 1545 31 Test conditions and mechanical results obtained
Underlined values: not in accordance with the invention

Steel B does not contain enough niobium: it does not reach a yield strength of 1300 MPa, both after simple martensitic quenching (test B2) and in the case of rolling with roughing and finishing at temperature. T ₃ (test B1)

In the case of the B2 test (simple martensitic quenching), it is observed that the value of the estimated resistance (1545 MPa) from expression (1) is close to that determined experimentally (1576 MPa)

The microstructure of the plates obtained by Scanning Electron Microscopy was also observed by means of a field effect gun ("MEB-FEG" technique) and EBSD detector, and quantified the average size. slats of the martensitic structure and their average elongation factor $\frac{l\max }{l\min }\mathrm{.}$

In tests A1 and A2, the method according to the invention makes it possible to obtain a martensitic structure with an average slat size of 0.9 micrometres and an elongation factor of 3. This structure is considerably thinner than that observed after simple martensitic quenching, whose average slat size is of the order of 2 micrometers.

In tests A1 and A2 according to the invention, the values of ΔRm are respectively 63 and 172 MPa respectively. The process according to the invention therefore makes it possible to obtain mechanical strength values significantly greater than those which would be obtained by simple martensitic quenching. In the case of test A2 for example, this increase in resistance (172 MPa) is equivalent to that which would be obtained, according to relation (1), thanks to a simple martensitic quenching applied to steels in which an addition additional 0.05% would have been achieved. Such an increase in the carbon content would however have adverse consequences with respect to the weldability and toughness, whereas the method according to the invention makes it possible to increase the mechanical strength without these disadvantages.

The plates produced according to the invention, because of their lower carbon content, have good weldability by the usual processes, in particular spot resistance welding. They also have good ability to be coated, for example by galvanizing or continuous dipping aluminization.

Thus, the invention allows the manufacture of sheets of thickness less than 3 millimeters or bare or coated with very high mechanical characteristics, under very satisfactory economic conditions.

Claims (5)

1. Method for producing a steel sheet having a thickness of less than 3 millimetres, having a completely martensitic structure and having a yield strength of more than 1300 MPa, comprising the successive steps, in this order, according to which:

   - a steel semi-finished product is provided, the composition of which comprises, the contents being expressed by weight, $$\mathrm{0.15}\%\le \mathrm{C}\le \mathrm{0.40}\%$$

   

   $$\mathrm{1.5}\%\le \mathrm{Mn}\le \mathrm{3}\%$$

   

   $$\mathrm{0.005}\%\le \mathrm{Si}\le \mathrm{2}\%$$

   

   $$\mathrm{0.005}\%\le \mathrm{Al}\le \mathrm{0.1}\%$$

   

   $$\mathrm{S}\le 0.05\%$$

   

   $$\mathrm{P}\le 0.1\%$$

   

   $$\mathrm{0.025}\%\le \mathrm{Nb}\le \mathrm{0.1}\%$$

   

   
   and optionally: $$\mathrm{0.01}\%\le \mathrm{Ti}\le \mathrm{0.1}\%$$

   

   $$\mathrm{0}\%\le \mathrm{Cr}\le \mathrm{4}\%$$

   

   $$\mathrm{0}\%\le \mathrm{Mo}\le \mathrm{2}\%$$

   

   $$0.0005\%\le \mathrm{B}\le \mathrm{0.005}\%$$

   

   $$0.0005\%\le \mathrm{Ca}\le \mathrm{0.005}\%,$$

   

   
   the remainder of the composition consisting of iron and inevitable impurities resulting from production,

   - said semi-finished product is heated to a temperature T₁ between 1050°C and 1250°C, then

   - said heated semi-finished product is rough-rolled at a temperature T₂ between 1050 and 1150°C, with a cumulative reduction rate ε_a greater than 100%, so as to obtain a sheet with an austenitic structure, not fully recrystallized, with an average grain size of less than 40 micrometres, then

   - said sheet is cooled, not completely, to a temperature T₃ between 970°C and Ar3+30°C, at a rate V_R1 greater than 2°C/s, then

   - said sheet, which is not completely cooled, is subjected to hot finish-rolling at said temperature T₃, with a cumulative reduction rate ε_b greater than 50%, so as to obtain a sheet having a thickness of less than 3 millimetres, then

   - said sheet is cooled at a rate V_R2 which is greater than a critical martensitic quenching rate.
2. Method for producing a steel sheet according to claim 1, characterized in that said average austenitic grain size is less than 5 micrometres.
3. Method for producing a steel sheet according to any one of claims 1 or 2, characterized in that said sheet is subjected to a subsequent tempering heat treatment at a temperature T₄ between 150 and 600°C for a duration between 5 and 30 minutes.
4. Steel sheet having a thickness of less than 3 millimetres and having a yield strength of more than 1300 MPa, obtained by a method according to any one of claims 1 or 2, having a completely martensitic structure, having an average lath size of less than 1.2 micrometres, the average elongation factor of said laths being between 2 and 5.
5. Steel sheet obtained by a method according to claim 3, having a completely martensitic structure, having an average lath size of less than 1.2 micrometres, the average elongation factor of said laths being between 2 and 5.