Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced

Abstract

A process for manufacturing a cold-rolled steel sheet with a strength of at least 1200 MPa and an elongation at break greater than 10%. A steel is provided having a microstructure comprising 65 to 90% bainite. A semifinished product is cast from the steel and heated to a temperature greater than 1150° C. The semifinished product is hot rolled to obtain a hot-rolled sheet; the coiled and pickled. Cold-rolling occurs with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet; and then reheating occurs at a rate Vbetween 5 and 15° C./s up to a temperature Tbetween Ac3 and Ac3+20° C. and held at said temperature Tfor a time tbetween 50 and 150 s. The sheet is cooled at a rate Vgreater than 40° C./s but below 100° C./s down to a temperature Tbetween (M−30° C. and M+30° C.). The sheet is maintained at temperature Tfor a time tbetween 150 and 350 s, and then cooled at a rate Vof less than 30° C./s down to an ambient temperature.