A method for manufacturing a high-strength sheet having improved formability segregation accord which the chemical composition of the steel contains, in percent by weight: 0.1%≦C≦0.4% 4.2%≦Mn≦8.0% 1%≦Si≦3% 0.2%≦Mo≦0.5% the remainder being Fc and unavoidable impurities, the method comprising the steps of annealing a rolled sheet made of said steel by soaking it at an annealing temperature AT higher than the Ac3 transformation point of the steel, quenching the sheet by cooling it down to a quenching temperature QT between the Ms and Mf transformation points in order to obtain a final structure containing at least 65% of martensite and at least 20% of residual austenite, the sum of the ferrite and bainite contents being less than 10%, heating the sheet up to an overaging temperature PT between 300° C. and 500° C. and maintaining it at said temperature for a time Pt greater than 10s and cooling the sheet down to the ambient temperature. Sheet obtained.