STEEL SHEET HAVING VERY HIGH MECHANICAL PROPERTIES OF STRENGTH AND DUCTILITY, MANUFACTURING METHOD AND USE OF SUCH SHEETS

The invention relates to the manufacture of a cold-rolled steel sheet having a strength of more than 900 MPa, an elastic limit of more than 700 MPa and distributed elongation of more than 12 %, the composition of which includes, the contents being expressed as weight percentages: 0.26 % ≤ C ≤ 0.45 %, 1.0 % ≤ Mn ≤ 3.0 %, 1.0 % ≤ Si ≤ 3.0 %, Al ≤ 0.10 %, 0 % ≤ Cr ≤ 1.5 %, S ≤ 0.005 %, P ≤ 0.020 %, Nb ≤ 0.1 %, Ti ≤ 0.020 %, V ≤ 0.015 %, N ≤ 0.01%, the remainder of the composition being iron and inevitable impurities resulting from the production process. The microstructure, given in surface proportions, comprises 13 % to 25 % residual austenite and 13 % to 30 % MA islands, the remainder consisting of bainite and possibly ferrite. La invención se refiere a la fabricación de una hoja de acero laminada en frío con resistencia superior o igual a 900 MPa, con un límite elástico superior a 700 MPa y con un alargamiento uniforme superior a 12% cuya composición comprende contenidos que se expresan en porcentaje de peso: 0.26% = C = 0.45%, 1.0% = Mn = 3.0%, 1.0% = Si = 3.0%, Al = 0.1%, 0% = Gr = 1.5%, S = 0.005%, P = 0.020%, Nb = 0.1%, Ti = 0.02%, y = 0.015%, N = 0.01%, el resto de la composición siendo constituido de hierro e impurezas inevitables que resultan del proceso. La microestructura es constituida, en fracciones de superficie, de 13 a 25% de austenita residual, de 13 a 30% de islotes de MA, el restante siendo constituido de bainita y eventualmente de ferrita.