ArcelorMittal S.A.
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.degree. 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 V.sub.c between 5 and 15.degree. C./s up to a temperature T.sub.1 between Ac3 and Ac3+20.degree. C. and held at said temperature T.sub.1 for a time t.sub.1 between 50 and 150 s. The sheet is cooled at a rate V.sub.R1 greater than 40.degree. C./s but below 100.degree. C./s down to a temperature T.sub.2 between (M.sub.s-30.degree. C. and M.sub.s+30.degree. C.). The sheet is maintained at temperature T.sub.2 for a time t.sub.2 between 150 and 350 s, and then cooled at a rate V.sub.R2 of less than 30.degree. C./s down to an ambient temperature.