COLD-ROLLED STEEL PLATE COATED WITH ZINC OR A ZINC ALLOY, METHOD FOR MANUFACTURING SAME, AND USE OF SUCH A STEEL PLATE

摘要

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.