ArcelorMittal S.A.
Very high strength martensitic steel or part and method of fabrication

Abstract:

The present invention provides a method for the fabrication of a steel sheet with a completely martensitic structure which has an average lath size of less than 1 micrometer and an average elongation factor of the laths is between 2 and 5. The elongation factor of a lath is defined as a maximum dimension l.sub.max divided by and a minimum dimension l.sub.min. The steel sheet has a yield stress greater than 1300 MPa and a mechanical strength greater than (3220(C)+958) megapascals. A composition of a semi-finished steel product includes, expressed in percent by weight, is, 0.15%.ltoreq.C.ltoreq.0.40%, 1.5%.ltoreq.Mn.ltoreq.3%, 0.005%.ltoreq.Si.ltoreq.2%, 0.005%.ltoreq.Al.ltoreq.0.1%, 1.8%.ltoreq.Cr.ltoreq.4%, 0%.ltoreq.Mo.ltoreq.2%, whereby: 2.7% 0.5 (Mn)+(Cr)+3(Mo).ltoreq.5.7%, S.ltoreq.0.05%, P.ltoreq.0.1%, optionally: 0%.ltoreq.Nb.ltoreq.0.050%, 0.01%.ltoreq.Ti.ltoreq.0.1%, 0.0005%.ltoreq.B.ltoreq.0.005%, 0.0005%.ltoreq.Ca.ltoreq.0.005%. The semi-finished product is reheated to a temperature T.sub.1 in the range between 1050.degree. C. and 1250.degree. C., then subjected to a roughing rolling at a temperature T.sub.2 in the range between 1000 and 880.degree. C., with a cumulative rate of reduction .epsilon..sub.a greater than 30%, to obtain a sheet with a completely recrystallized austenitic structure with an average grain size less than 40 micrometers and preferably less than 5 micrometers. The sheet is then partially cooled to prevent a transformation of the austenite at a rate V.sub.R1 greater than 2.degree. C./s to a temperature T.sub.3 between 600.degree. C. and 400.degree. C. in the metastable austenitic range, and subjected to a finishing hot rolling at the temperature T.sub.3 of the partially cooled sheet, with a cumulative rate of reduction .epsilon..sub.b greater than 30% to obtain a sheet that is then cooled at a rate V.sub.R2 which is greater than the critical martensitic quenching rate.