ArcelorMittal S.A.
High Strength Steel Sheet Having Excellent Formability and a Method of Manufacturing the Steel Sheet

Abstract:

A cold rolled and heat treated steel sheet is provided. The steel sheet includes a composition including 0.17%.ltoreq.carbon.ltoreq.0.24%, 1.9%.ltoreq.manganese.ltoreq.2.2%, 0.5%.ltoreq.aluminum.ltoreq.1.2%, 0.5%.ltoreq.silicon.ltoreq.1%, Si+Al.gtoreq.1.3%, 0.05%.ltoreq.chromium.ltoreq.0.2%, 0.015%.ltoreq.niobium.ltoreq.0.03%, sulphur.ltoreq.0.003%, phosphorus.ltoreq.0.03% and optionally 0.005%.ltoreq.titanium.ltoreq.0.05%, 0.001%.ltoreq.molybdenum.ltoreq.0.05%, a remainder of the composition including iron and the unavoidable impurities. A microstructure of the coated steel sheet includes, in area fraction, 10 to 20% residual austenite, said austenite phase having a carbon content between 0.9 to 1.1%, 40 to 55% polygonal ferrite, 15 to 40% granular bainite and at least 5% tempered martensite, a sum of tempered martensite and residual austenite being between 20 to 30%. A method is also provided.