Abstract: The present invention provides a “TRIP effect” cold-rolled and annealed steel sheet which has improved formability and weldability, strength between 780 and 900 MPa and elongation at fracture greater than 19%, the composition of which includes the following elements in percentages expressed by weight: 0.17%?C?0.25%, 1.5%?Mn?2%, 0.50%?Si?1%, 0.50%?Al?1.2%, whereby Si+Al?1.30%, the remainder of the composition including iron and the inevitable impurities resulting from processing. The microstructure of the sheet includes 65 to 85% ferrite and 15 to 35% islands of martensite and residual austenite. The average size of these islands of martensite and residual austenite is less than 1.3 micrometers and their shape factor is less than 3.