Flat product, such as sheet, made of steel having a high yield strength and exhibiting good ductility and process for manufacturing this product

A flat product made of multiphase steel whose chemical composition contains carbon and manganese, wherein said steel has a structure obtained by a process comprising:

• hot-rolling, at a temperature above the austenite transformation temperature (A3), of a steel containing in wt % 0.05 to 0.8% C, 0.2 to 3.0% Mn, Si≦1%, B≦0.100%, Ti, Nb, Zr and V each ≦0.200%, Al≦0.400%, N≦0.100%, P≦0.100%, Cr, Ni and Cu each ≦2.000%, Mo≦0.500%, the balance being Fe and inevitable impurities;
• soaking or tempering at a temperature of between 300° C. and a temperature 50° C. above the eutectoid temperature (A1) for at least 4 hours, so as to form a phase enriched in carbide-forming and/or gammagenic elements in a ferrite matrix;
• heat treating at a temperature above the aforementioned eutectoid temperature (A1) and below the austenite transformation temperature (A3) so as to form islands of austenite enriched with at least one of said carbide-forming and gammagenic elements and/or to enrich the austenite already formed with at least one of said carbide-forming and gammagenic elements;
• and
• subsequently cooling down to room temperature in such a way as to obtain a final product having a ferrite matrix containing islands of residual austenite enriched with said carbide-forming and/or gammagenic elements.