Hot Rolled Flat Steel Product Consisting of a Complex-Phase Steel with a Largely Bainitic Microstructure and Method for Manufacturing Such a Flat Steel Product

摘要

A flat steel product and a method of making a flat steel product having a hole expansion of at least 60%, a yield strength of at least 660 MPa, a tensile strength of at least 760 MPa, and an elongation at break of at least 10%. The flat steel product is made from a complex-phase steel, which includes (in wt %) C: 0.01-0.1%, Si: 0.1-0.45%, Mn: 1-2.5%, Al: 0.005-0.05%, Cr: 0.5-1%, Mo: 0.05-0.15%, Nb: 0.01-0.1%, Ti: 0.05-0.2%, N: 0.001-0.009%, P: 0.02%, S: 0.005%, Cu: ≤0.1 % , Mg: ≤0.0005 % , O: 0.01 % , optionally one or more of Ni, B, V, Ca, Zr, Ta, W, REM, and Co, where Ni: ≤1%, B: ≤0.005%, V: ≤0.3%, Ca: 0.0005-0.005%, Zr, Ta, W: in total ≤2%, REM: 0.0005-0.05%, and Co: 1%, and iron and unavoidable impurities as the remainder, where % Ti(48/14)% N+(48/32)% S and % Nb(93/12)% C+(45/14)% N+(45/32)% S. The structure of the flat steel product includes (in area %) 80% bainite, 15% ferrite, 15% martensite, cementite, and 5 vol % retained austenite.