Method for manufacturing a steel sheet with a completely martensitic structure having a mean lath size of less than 1 micrometre, the mean elongation factor of said laths being between 2 and 5, it being understood that the elongation factor of a lath with a maximum dimension l max and minimum dimension l min is defined by l max l min, with an elastic limit greater than 1300 MPa and a mechanical strength greater than (3220 +958) megapascals, it being understood that designates the carbon content as a percentage by weight of said steel, comprising the successive steps in this order, according to which: - a semi-finished product is procured, the composition of which comprises, the proportions being expressed by weight, 0.15 % ≤ C ≤ 0.40 % 1.5 % ≤ Mn ≤ 3 % 0.005 % ≤ Si ≤ 2 % 0.005 % ≤ Al ≤ 0.1 % 1.8 % ≤ Cr ≤ 4 % 0 % ≤ Mo ≤ 2 % it being understood that 2.7 % ≤ 0.5 Mn + Cr + 3 Mo ≤ 5.7 % S ≤ 0.05 % P ≤ 0.1 % and optionally: 0 % ≤ Nb ≤ 0.050 % 0.01 % ≤ Ti ≤ 0.1 % 0.0005 % ≤ B ≤ 0.005 % 0.0005 % ≤ Ca ≤ 0.005 %, the rest of the composition consisting of iron and unavoidable impurities resulting from the production, - the semi-finished product is heated to a temperature T 1 of between 1050°C and 1250°C, and then - a roughing-down rolling of said heated semi-finished product is carried out, at a temperature T 2 between 1000 and 880°C, with a total degree of reduction ε a greater than 30% so as to obtain a sheet with a completely recrystallised austenitic structure with a mean grain size of less than 40 micrometres and preferably less than 5 micrometres, it being understood that the total reduction rate ε a is defined by: Ln e ia e f a, e ia designating the thickness of the semi-finished product before said roughing-down hot rolling and e fa the thickness of the sheet after said roughing-down rolling, and then - the sheet is cooled not completely to a temperature T 3 of between 600°C and 400°C in the metastable austenitic range, at a rate V R1 greater than 2°C/s, then - a finishing hot rolling is carried out, at said temperature T 3, of the not completely cooled sheet, with a total degree of reduction ε b greater than 30% so as to obtain a sheet, it being understood that said total degree of reduction ε b is defined by: Ln e ib e f b, e ib designating the thickness of the sheet before said finished hot rolling and e fa the thickness of the sheet after said finishing rolling, then - said sheet is cooled at a rate V R2 greater than the critical martensitic hardening rate. - a semi-finished product is procured, the composition of which comprises, the proportions being expressed by weight, 0.15 % ≤ C ≤ 0.40 % 1.5 % ≤ Mn ≤ 3 % 0.005 % ≤ Si ≤ 2 % 0.005 % ≤ Al ≤ 0.1 % 1.8 % ≤ Cr ≤ 4 % 0 % ≤ Mo ≤ 2 % it being understood that 2.7 % ≤ 0.5 Mn + Cr + 3 Mo ≤ 5.7 % S ≤ 0.05 % P ≤ 0.1 % and optionally: 0 % ≤ Nb ≤ 0.050 % 0.01 % ≤ Ti ≤ 0.1 % 0.0005 % ≤ B ≤ 0.005 % 0.0005 % ≤ Ca ≤ 0.005 %, the rest of the composition consisting of iron and unavoidable impurities resulting from the production, - the semi-finished product is heated to a temperature T 1 of between 1050°C and 1250°C, and then - a roughing-down rolling of said heated semi-finished product is carried out, at a temperature T 2 between 1000 and 880°C, with a total degree of reduction ε a greater than 30% so as to obtain a sheet with a completely recrystallised austenitic structure with a mean grain size of less than 40 micrometres and preferably less than 5 micrometres, it being understood that the total reduction rate ε a is defined by: Ln e ia e f a, e ia designating the thickness of the semi-finished product before said roughing-down hot rolling and e fa the thickness of the sheet after said roughing-down rolling, and then - the sheet is cooled not completely to a temperature T 3 of between 600°C and 400°C in the metastable austenitic range, at a rate V R1 greater than 2°C/s, then - a finishing hot rolling is carried out, at said temperature T 3, of the not completely cooled sheet, with a total degree of reduction ε b greater than 30% so as to obtain a sheet, it being understood that said total degree of reduction ε b is defined by: Ln e ib e f b, e ib designating the thickness of the sheet before said finished hot rolling and e fa the thickness of the sheet after said finishing rolling, then - said sheet is cooled at a rate V R2 greater than the critical martensitic hardening rate.